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  Biology, Genetics, and Evolution  
  Biology is the science of life: the study of millions of different organisms that share that phenomenon—their structure, functioning, interrelationships, and origins. All are able or have the potential to grow, reproduce, move, and respond to such stimuli as light, heat, and sound; and all are sustained by the processes of nutrition, respiration, and excretion.  
  For all organisms—apart from viruses—the basic unit of life is the cell, each of which also possesses the essential characteristics of life, including the potential to move and reproduce.  
  Origins of Life  
  Life probably originated in the primitive oceans. The original atmosphere, 4 billion years ago, consisted of carbon dioxide, nitrogen, and water. Laboratory experiments have shown that more complex organic molecules, such as c0016-01.gifamino acids and nucleotides, can be produced from these ingredients by passing electric sparks through a mixture. The climate of the early atmosphere was probably very violent, with lightning a common feature, and these conditions could have resulted in the oceans becoming rich in organic molecules, producing the so-called "primeval soup." These molecules may then have organized themselves into clusters capable of reproducing and eventually developing into simple cells. Soon after life developed, photosynthesis would have become the primary source of energy for life. By this process, life would have substantially affected the chemistry of the atmosphere and, in turn, that of its own environment. Once the atmosphere had changed to its present composition, life could only be created by the replication of living organisms (a process called biogenesis).  
  Biology Timeline
  Chronology of important developments in the biological sciences. It includes items from the mentioning of hand pollination of date palms in 1800 B.C. to the Nobel prize award for the discovery of site-directed mutagenesis in 1993.  
  The Cell  
  The cell is the basic structural unit of life. It is the smallest unit capable of independent existence that can reproduce itself exactly. All living organisms—with the exception of viruses—are composed of one or more cells. Single-cell organisms such as bacteria, protozoa, and other microorganisms are termed unicellular, while plants and animals that contain many cells are termed multicellular. Highly complex organisms such as human beings consist of billions of cells, all of which are adapted to carry out specific functions—for instance, groups of these specialized cells are organized into tissues and organs.  
  eukaryote and prokaryote cells  
  Although cells may differ widely in size, appearance, and function, their essential features are similar. Each is composed of a mass of jellylike substance called cytoplasm, surrounded by a membrane. The cytoplasm contains (ribosomes, which carry out protein synthesis, and c0016-01.gifDNA, the coded instructions for the behavior and reproduction of the cell.  
  In eukaryote cells (those of protozoa, fungi, plants, and animals) the DNA is organized into chromosomes and is contained within a clearly defined nucleus, which is surrounded by a double membrane. (Some cells, however, such as mammalian red blood cells, lose their nuclei as they mature.) The cytoplasm also contains other membrane-bound structures called organelles, such as mitochondria and chloroplasts, which carry out specific functions.  
  In prokaryote cells (those of bacteria and cyanobac-  




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  cell structure Typical plant and animal cell. Plant and animal cells share many structures,
such as ribosomes, mitochondria, and chromosomes, but they also have notable
differences: plant cells have chloroplasts, a large vacuole, and a cellulose cell wall.
Animal cells do not have a rigid cell wall but have an outside cell membrane only.
  teria, or blue-green algae) the DNA forms a simple loop and there is no nucleus. The prokaryotic cell also lacks organelles, though it possesses many ribosomes.  
  eukaryote cell structure  
  Each eukaryote has a surrounding membrane, which is a thin layer of protein and fat that restricts the flow of substances in and out of the cell and encloses the c0016-01.gifcytoplasm, a jellylike material containing the c0016-01.gifnucleus and other structures (organelles) such as mitochondria.  
  In general, plant cells differ from animal cells in that the membrane is surrounded by a cell wall made of cellulose. They also have larger vacuoles (fluid-filled pouches) and contain chloroplasts that convert light energy to chemical energy for the synthesis of glucose. (A fuller description of structures unique to plant cells is given in the Plant Kingdom chapter.)  
  membrane The membrane is a thin, continuous layer, made up of fat (phospholipid) and protein molecules, that encloses a cell or organelles within a cell. Small molecules, such as water and sugars, can pass through the cell membrane by (osmosis and (diffusion. Large molecules, such as proteins, are transported across the membrane via special channels, a process often involving the input of energy (see (active transport). The Golgi apparatus within the cell is thought to produce certain membranes.  
  In cell organelles, enzymes may be attached to the membrane at specific positions, often alongside other enzymes involved in the same process, like workers at a conveyor belt. Thus membranes help to make cellular processes more efficient.  
  cytoplasm This is the part of the cell outside the c0016-01.gifnucleus. Strictly speaking it includes all the organelles, but often cytoplasm refers to the jellylike matter in which the organelles are embedded (correctly termed the cytosol).  
  The cytoplasm is permeated with a matrix of protein microfilaments and tubules that, together, form the cytoskeleton. This gives the cell a definite shape, keeps the organelles in position, transports vital substances around the cell, and may also be involved in cell movement (cytoplasmic streaming).  
  In many cells, the cytoplasm is made up of two parts: the ectoplasm (or plasmagel), a dense gelatinous outer layer concerned with cell movement, and the endoplasm (or plasmasol), a more fluid inner part where most of the organelles are found.  
  nucleus The nucleus is usually the largest and most prominent structure in the eukaryote cell. Its function is to house and pass on the genetic information to future generations of cells, and to direct and control the activities of the cell according to its own genetic instructions.  
  The nucleus is bounded by a double membrane, or nuclear envelope, with numerous pores that provide channels of communication between the nucleus and the rest of the cell. The nucleoplasm inside contains the genetic material (DNA.  
  When the cell is not dividing, the DNA is dispersed in the form of chromatin, but during cell division (mitosis or meiosis), it becomes coiled and folded into compact bodies called chromosomes, which will carry the genetic code to the next generation. In each species, the number of chromosomes in a cell is  




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  chromosome The 23 pairs of chromosomes of a normal human male.  
  constant—for example, in humans it is 46 (23 in ova and sperm).  
  Also in the nucleoplasm are dense, spherical bodies called nucleoli. These contain protein and ribosomal (RNA, and play a role in the manufacture of (ribosomes (structures responsible for synthesizing protein). They disappear during cell division.  
  Cells Alive
  Lively and attractive collection of microscopic and computer-generated images of living cells and microorganisms. It includes sections on HIV infection, penicillin, and how antibodies are made.  
  ribosome The ribosomes are the protein-making machinery of the cell. They are small, dense bodies composed of protein and a special form of RNA, called ribosomal RNA. They receive messenger RNA (copied from the DNA) and amino acids (the components of protein), and "translate" the chemically coded instructions on the messenger RNA to link the amino acids in the order required to make a strand of a particular protein (see (protein synthesis).  
  Ribosomes are located on the endoplasmic reticulum (ER)—creating rough endoplasmic reticulum—or they may be found within other organelles or free in the cytoplasm.  
  endoplasmic reticulum Endoplasmic reticulum (ER) is a membranous system that forms compartments within the cell. It stores and transports proteins within cells and also carries various enzymes needed for the synthesis of fats. Rough endoplasmic reticulum has ribosomes (sites of protein synthesis) attached to its surface.  
  Under the electron microscope, ER looks like a series of channels and vesicles, but it is in fact a large, sealed, baglike structure crumpled and folded into a convoluted mass. The interior of the "bag," the ER lumen, stores various proteins needed elsewhere in the cell, then organizes them into transport vesicles formed by a small piece of ER membrane budding from the main membrane.  
  Golgi apparatus The Golgi apparatus, or Golgi body, is a stack of flattened membranous sacs called cisternae. Many proteins and other molecules travel through the Golgi apparatus on their way from the endoplasmic reticulum (ER) to other parts of the cell. Inside the Golgi apparatus, they are modified or fused with other molecules, and then transported away in vesicles (tiny, membrane-bound spheres) that bud off from the tips of the cisternae.  
  The vesicles may be secretory, migrating to the cell membrane to release their contents to the outside, or they may remain within the cytoplasm, serving as compartments for enzyme reactions (as in, for example, lysosomes).  
  lysosome Lysosomes are tiny, membrane-bound sacs, or vesicles, responsible for intracellular digestion. They contain a number of enzymes—known collectively as lysozyme—that can break down proteins and other biological substances. In single-celled organisms they are responsible for digesting foodstuffs, and in white blood cells they destroy ingested bacteria. Lysosomes also play a role in cell death (apoptosis), breaking down the structures of the cell into small fragments that are easily digestible by surrounding cells.  
  mitochondria Mitochondria (singular "mitochondrion") are sometimes called the "powerhouses" of the cell as they are the sites of most of the processes involved in aerobic (respiration, and—in nonphotosynthesizing cells at least—are responsible for producing most of the cell's ATP (the energy-rich molecule that powers cellular activities).  




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  They are rodlike organelles bound in a double membrane, the inner of which is folded into projections called cristae. The cristae are covered with proteins that play a role in the electron transport chain (the last stage of respiration), while the viscous matrix inside the mitochondrion contains the enzymes involved in the Krebs cycle (the penultimate stage).  
  Mitochondria are thought to be derived from freeliving bacteria that, at a very early stage in the history of life, invaded larger cells and took up a symbiotic way of life inside. Each still contains its own small loop of DNA, called mitochondrial DNA, and new mitochondria arise by the division of existing ones.  
  cilia and flagella Cilia (singular "cilium") and flagella (singular 'flagellum') are small, hairlike organelles found on the surface of certain cells. They are locomotory structures that create motion by beating back and forth. Both are made up of cylinders of protein tubules arranged in a characteristic 'nine plus two' format (nine pairs of tubules around the circumference and two separate central tubules) and covered in cell membrane.  
  Cilia and flagella differ from each other in that cilia are shorter and occur in large groups, where their action is coordinated so that they beat in a wave. Flagella are longer, occur singly or in pairs, and have a more complex, snakelike action.  
  Some single-celled organisms move by means of cilia. In mammals they are found in the cells lining the upper respiratory tract, where their wavelike movements waft particles of dust and debris toward the exterior. They also move food in the digestive tracts of some invertebrates.  
  Flagella are the motile organs of certain protozoa and single-celled algae, and of the sperm cells of multicellular organisms. Water movement inside sponges is also produced by flagella.  
  cell division  
  This is the process by which a cell divides to form new cells, either by mitosis, which is associated with growth, cell replacement, or repair; or meiosis, which is associated with sexual reproduction. Both forms involve the duplication of DNA and the splitting of the nucleus.  
  mitosis The stages of mitosis, the process of cell division that takes place when
a plant or animal cell divides for growth or repair. The two daughter cells each
receive the same number of chromosomes as were in the original cell.




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  Dictionary of Cell Biology
  Searchable database of more than 5,000 terms frequently encountered in reading modern biology literature. The dictionary can be searched as a whole, or in sections such as ''disease," "cytoskeleton," and "nucleus, genes, and DNA."  
  mitosis Mitosis is the process of cell division by which identical daughter cells are produced. During mitosis the DNA is duplicated and the chromosome number doubled, so new cells contain the same amount of DNA as the original cell.  
  The genetic material of eukaryotic cells is carried on a number of chromosomes. To control movements of chromosomes during cell division so that both new cells get the correct number, a system of protein tubules, known as the spindle, organizes the chromosomes into position in the middle of the cell before they replicate. The spindle then controls the movement of chromosomes as the cell goes through the stages of division: prophase, metaphase, anaphase, and telophase.  
  To remember the phases of mitosis:  
  Parrots meet and talk!  
  (Prophase, metaphase, anaphase, telophase)  


  meiosis Meiosis results in the number of chromosomes in the daughter cells being halved. It only occurs in eukaryote cells, and is essential to sexual reproduction because it allows the genes of two parents to be combined without the total number of chromosomes increasing.  
  Meiosis is the stage in the life cycle where genetic variation arises. This is due to recombination mechanisms, which "shuffle" the genetic material, thus increasing genetic variation in the offspring. The two main mechanisms are: crossing over, in which chromosome pairs twist around each other and exchange corresponding segments, and the random reassortment of chromosomes that occurs when each gamete (sperm or egg) receives only one of each chromosome pair.  
  In sexually reproducing diploid animals (having two sets of chromosomes per cell), meiosis occurs during  
  meiosis Meiosis is a type of cell division that produces gametes (sex cells, sperm and
egg). This sequence shows an animal cell but only four chromosomes are present
in the parent cell (1). There are two stages in the division process. In the first
stage (2–6), the chromosomes come together in pairs and exchange genetic material.
This is called crossing over. In the second stage (7–9), the cell divides to produce
four gamete cells, each with only one copy of each chromosome from the parent cell.




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  formation of the gametes (sex cells, sperm and egg), so that the gametes are haploid (having only one set of chromosomes). When the gametes unite during fertilization the diploid condition is restored. In plants meiosis occurs just before spore formation. Thus the spores are haploid and in lower plants such as mosses they develop into a haploid plant called a gametophyte which produces the gametes.  
  The Molecules of Life  
  Cells contain many simple, inorganic chemicals, including water, which provide a favorable environment for biochemical reactions and make up 60%–65% of living matter; and inorganic ions, such as those of potassium, sodium, and chlorine, which play an essential role in many cell processes. However, the molecules that form the main functional units of living matter are large, complex organic molecules—they are natural polymers formed by linking together many smaller organic building blocks, or monomers. These macromolecules act as structural components—for example, the cell membrane is made up of proteins and lipids, the nucleus of proteins and nucleic acids; or they may provide sources of energy, store genetic information, or speed up biological reactions. They can be classified into four families: proteins, polysaccharides, lipids, and nucleic acids.  
  Proteins are complex, biologically important substances composed of long chains of amino acids that are twisted or folded in characteristic shapes. They are essential to all living organisms. As enzymes they regulate all aspects of metabolism. Structural proteins  
Amino Acids
name formula
glycine CH2(NH2).COOH
alanine CH3CH.(NH2).COOH
phenylalanine C6H5CH2CH.(NH2).COOH
tyrosine C6H4OH.CH2CH.(NH2).COOH
valine (CH3)2CH.CH.(NH2).COOH
leucine (CH3)2CH.CH2CH.(NH2).COOH
iso-leucine (CH3).CH2CH(CH3)CH.(NH2).C OOH
serine CH2OH.CH.(NH2).COOH
threonine CH3CHOH.CH.(NH2).COOH
cysteine CH.CH2CH.(NH2).COOH
methionine CH3.S.(CH2)2CH.(NH2).COOH
asparagine NH2CO.CH2CH.(NH2).COOH
glutamine NH2CH.(CH2)2(CO.NH2).COOH
lysine NH2CH3CH.(NH2).COOH
arginine NH2C(NH).NH(CH2)3CH.(NH2).COOH
aspartic acid COOH.CH2CH.(NH2).COOH
glutamic acid COOH.(CH2)2CH.(NH2).COOH
histidine C3H3N2.CH2CH.(NH2).COOH
trytophan C4.NH.CH2CH2CH.(NH2).COOH
proline NH.(CH2)3CH.COOH


  such as keratin and collagen make up the skin, claws, bones, tendons, and ligaments; muscle proteins produce movement; hemoglobin transports oxygen; and membrane proteins regulate the movement of substances into and out of cells.  
  The three-dimensional shapes of proteins are so complex, and so specific to their functioning, that a hierarchy of primary, secondary, tertiary, and even quaternary structures has been created to describe them.  
  protein A protein molecule is a long chain of amino acids linked by peptide
bonds. The properties of a protein are determined by the order, or sequence,
of amino acids in its molecule, and by the three-dimensional structure of the
molecular chain. The chain folds and twists, often forming a spiral shape.




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  Molecular Expressions: The Amino Acid Collection
  Fascinating collection of images showing what all the known amino acids look like when photographed through a microscope. There is also a detailed article about the different amino acids.  
  amino acids and primary structure Amino acids are water-soluble organic molecules mainly composed of carbon, oxygen, hydrogen, and nitrogen, containing both a basic amino group (NH2) and an acidic carboxyl (COOH) group. They are the structural units, or monomers, of proteins—linked together by means of peptide bonds between the amino group of one amino acid and the carboxyl group of the next to form long, unbranching chains called polypeptides.  
  The creation of polypeptides takes place in the (ribosomes of the cell, where amino acids are joined together in a specific order, according to instructions from the DNA (see (protein synthesis). The primary structure of proteins is the amino acid sequence within their constituent polypeptides. It is this that will determine the shape and properties of the final protein.  
  All proteins are made up of the same 20 amino acids (although other types of amino acid do occur infrequently in nature). Eight of these, the essential amino acids, cannot be synthesized by humans and must be obtained from the diet. Children need a further two amino acids that are not essential for adults. Other animals also need some preformed amino acids in their diet, but green plants can manufacture all the amino acids they need from simpler molecules, relying on energy from the sun and minerals (including nitrates) from the soil.  
  Amino Acids
  Small but interesting site giving the names and chemical structures of all the amino acids. The information is available in both English and German.  
  structure The secondary structure describes the initial repeated folding or organization of the polypeptide chain, which takes place in the rough (endoplasmic reticulum and (Golgi apparatus of the cell. It is basically of two types: the alpha helix or the beta-pleated sheet. In the alpha helix a single polypeptide coils up into a regular helix, or spiral, that is cross-linked at intervals by hydrogen bonds. In the beta-pleated sheet polypeptides are lined up in parallel and cross-linked to form sheets, with the zigzagging of the peptide bonds giving the impression of pleats.  
  Fibrous proteins, which need to be strong and insoluble, are predominantly secondary in their structure. For example, structural proteins such as keratin and collagen have considerable alpha-helical content, while silk fibroin, a major constituent of silk, is composed of beta-pleated sheets.  
  tertiary structure When a protein has adopted its secondary structure, it will fold, coil, or organize itself still further into a complex tertiary structure, such as a helix, sphere, rod, or globule. For example, a length of protein arranged alpha-helically may coil back on itself to create a further helix.  
  The driving forces behind this are the formation of bonds such as disulfide bridges, hydrogen bonds, and ionic bonds between amino acids, and—most importantly—the tendency for hydrophobic, or water-repelling, amino acids to keep away from the external, aqueous environment. Thus, the protein will fold up with hydrophobic groups in the center of the protein, and hydrophilic, or water-loving, groups on the outside.  
  Globular proteins are predominantly tertiary—and sometimes quaternary—in their structure. For example, albumin—a storage protein important in maintaining the blood's osmotic pressure—must be small, compact, and spherical in order to travel in the bloodstream. Hence it has considerable tertiary structure coiling it up into a compact unit. (Enzymes, which have to maintain a highly specific shape in order to function, are always tertiary or quaternary in structure.  
  quaternary structure Quaternary structure is simply a packing together of two or more tertiary-stage subunits to form a protein. For example, hemoglobin—the oxygen-carrying protein in red blood cells—is composed of four globular subunits held together with a weak hydrogen bond.  
  Protein Data Bank WWW Home Page
  Expanding scientific site that contains a fully searchable database of molecule images. This is useful for students as well as more in-depth researchers.  
  Polysaccharides are long-chain polymers made up of hundreds or thousands of linked simple sugars (monosaccharides) such as glucose. They act as energy-rich food stores in plants (starch) and animals (glycogen), and have structural roles in the plant cell wall (cellulose, pectin) and in the tough outer skeleton of insects and similar creatures (chitin). Polysaccharides and sugars are all types of carbohydrate.  




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  polysaccharide A molecule of the polysaccharide glycogen
(animal starch) is formed from linked glucose (C
6H12O6) molecules.
A typical glycogen molecule has 100–1,000 glucose units.
  monosaccharide Monosaccharides are the simplest form of carbohydrate: single sugar molecules that form the basic structural units, or monomers, of polysaccharides. They are soluble compounds, some with a sweet taste. Monosaccharides have the general formula CnH2nOn, where n may be any number from three to seven. Monosaccharides that have five carbons in their molecules are called pentoses—they include ribose (a constituent of RNA), arabinose, and xylose. Hexoses have six carbons in their molecules and include glucose, galactose, and fructose.  
  Glucose is the most abundant monosaccharide, present in the blood and manufactured by green plants during photosynthesis. The anaerobic respiration reactions inside cells involves the oxidation of glucose to produce ATP, the "energy molecule" used to drive many of the body's biochemical reactions.  
  disaccharide Disaccharides are soluble sugars made up of two monosaccharide units linked by a glycosidic bond. For example, sucrose, or cane sugar, consists of glucose and fructose; lactose, or milk sugar, consists of glucose and galactose; while maltose, found in germinating seeds, consists of two glucose units.  
  polysaccharide formation Polysaccharides are made up of many monosaccharides—often of only a single type—joined by glycosidic links. For example, glycogen, starch, and cellulose are all polymers of glucose, though they are distinctly different in form and function: glycogen and starch being globular, storage polysaccharides, while cellulose is a fibrous, structural material.  
  The difference is due to the frequency and position of the glycosidic links between monosaccharides, and whether the sugar units are joined together in long chains or in branching structures.  
  Lipids are a diverse group of biological molecules that are soluble in alcohol but not in water. They may be divided into complex lipids, which are all esters of fatty acids, and simple lipids, which do not contain fatty acids. Lipids are the chief constituents of plant and animal waxes, fats, and oils.  
  complex lipid The fatty acids that characterize complex lipids are organic compounds consisting of a hydrocarbon chain, up to 24 carbon atoms long, with a carboxyl group (–COOH) at one end. They are produced in the small intestine when fat is digested. Fatty acids may be described as saturated (having only single bonds between their carbon atoms) or unsaturated (having one or more double bonds between their carbon atoms).  
  triglyceride The molecular structure of typical fat. The molecule consists
of three fatty acid molecules linked to a molecule of glycerol.




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  The most familiar of the complex lipids are the triglycerides, which make up the fats and oils of plants and animals. They are formed as a result of a condensation reaction between a glycerol molecule and three fatty acid molecules.  
  Phospholipids consist of the same glycerol backbone found in triglycerides, but with one of the fatty acids replaced with a hydrophilic (water-loving) phosphate group. They are major components of cell membranes, forming sandwichlike sheets two molecules thick, with the hydrophilic phosphate "heads" aligned outward toward the aqueous cytoplasm and the hydrophobic fatty-acid "tails" aligning themselves inward toward the middle of the sandwich.  
  simple lipid Simple lipids do not have a fatty-acid component; they include sterols and steroids.  
  Sterols are solid, cyclic, unsaturated alcohols, with a complex structure that includes four carbon rings. They include cholesterol—a white, crystalline substance that is an integral part of all cell membranes and is a component of lipoproteins, which transport fats and fatty acids in the blood.  
  Steroids are derived from sterols, but lack their alcohol (-OH) group. They include the sex hormones (such as testosterone), the corticosteroid hormones produced by the adrenal gland, bile acids, and cholesterol.  
  nucleic acids  
  These are complex organic acids made up of a long chain of nucleotides, found in the nucleus and sometimes the cytoplasm of the living cell. The two types, known as DNA (deoxyribonucleic acid) and RNA (ribonucleic acid), form the basis of heredity. Each nucleotide is made up of a sugar (deoxyribose or ribose), a phosphate group, and one of four purine or pyrimidine bases. The order of the bases along the nucleic acid strand contains the genetic code.  
  DNA DNA, or deoxyribonucleic acid, is a complex giant molecule that contains, in chemically coded form, the information needed for a cell to make proteins. It is a ladderlike double-stranded nucleic acid that forms the basis of genetic inheritance in all organisms, except for a few viruses that have only c0016-01.gifRNA. Within the cell, DNA is organized into dense structures called chromosomes and, in eukaryotes (organisms other than bacteria), it is found only in the cell nucleus.  
  DNA is made up of two chains of nucleotide subunits, with each nucleotide containing either a purine (adenine or guanine) or pyrimidine (cytosine or  
  DNA How the DNA molecule divides. The DNA
molecule consists of two strands wrapped around
each other in a spiral or helix. The main strands
consist of alternate sugar (S) and phosphate (P)
groups, and attached to each sugar is a nitrogenous
base—adenine (A), cytosine (C), guanine (G), or
thymine (T). The sequence of bases carries the
genetic code which specifies the characteristics
of offspring. The strands are held together by
weak bonds between the bases, cytosine to
guanine, and adenine to thymine. The weak
bonds allow the strands to split apart, allowing
new bases to attach, forming another double strand.




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  Crick, Francis Harry Compton
  Profile of the life and achievements of the pioneer molecular biologist. It traces his upbringing and education and how he brought his knowledge of X-ray diffraction to his work with James Watson in unraveling the structure of DNA. There is a photo, a listing of Crick's major books and articles, and a bibliography.  
  thymine) base. The bases link up with each other (adenine linking with thymine, and cytosine with guanine) to form base pairs that connect the two strands of the DNA molecule like the rungs of a twisted ladder.  
  The specific way in which the pairs form means that the base sequence is preserved from generation to generation. Hereditary information is stored as a specific sequence of bases. A set of three bases—known as a codon—acts as a blueprint for the manufacture of a particular c0016-01.gifamino acid, the subunit of a protein molecule.  
DNA nucleotides
  To remember classes of nucleotides:  
  In DNA there are four nucleotides (cytosine, thymine, adenine, guanine) divided into two classes (pyrimidines and purines). The easiest way to remember which class they belong to is that the pyrimidines contain the letter y (cytosine and thymine).  


  The information encoded by the codons is transcribed (see protein synthesis) by messenger RNA and is then translated into amino acids in the ribosomes and cytoplasm. The sequence of codons determines the precise order in which amino acids are linked up during manufacture and, therefore, the kind of protein that is to be produced. Because proteins are the chief structural molecules of living matter and, as enzymes, regulate all aspects of metabolism, it may be seen that the genetic code is effectively responsible for building and controlling the whole organism.  
  Molecular Expressions: The DNA Collection
  Spectacular gallery of DNA photographic representations in the laboratory as well as in vivo. This site also has links to several other sites offering photographs through a microscope of various substances, including computer chips and various pharmaceutical substances.  
  RNA, or ribonucleic acid, is responsible for translating the genetic information encoded in the c0016-01.gifDNA into proteins (see c0016-01.gifprotein synthesis). It is usually single-stranded, unlike the double-stranded DNA, and consists of a large number of nucleotides strung together, each of which comprises the sugar ribose, a phosphate group, and one of four bases—uracil (replacing the thymine of DNA), cytosine, adenine, or guanine.  
  RNA occurs in three major forms, each with a different function in the synthesis of protein molecules. Messenger RNA (mRNA) copies a section of the DNA's genetic code in a process called transcription and then acts as a template for the assembly of amino acids to make proteins. Each codon (a set of three bases) on the mRNA molecule is matched up with the corresponding amino acid, in accordance with the genetic code. This process (translation) takes place in the ribosomes, which are made up of proteins and ribosomal RNA (rRNA). Transfer RNA (tRNA) is responsible for combining with specific amino acids in the cytoplasm, and then transporting them to the ribosomes to be matched up with the mRNA.  
  Although RNA is normally associated only with the process of protein synthesis, it makes up the hereditary material itself in some viruses, such as retroviruses.  
  To remember that although DNA and RNA are both nucleic acids, they do different jobs in the cell:  
  DNA delivers the blueprint, RNA reads it  


  The Chemical Processes of Life  
  Living organisms require many hundreds of interrelated chemical processes to enable them to grow and function—the sum of which can be described as an organism's metabolism. It involves a constant alternation between building up complex molecules (anabolism) and breaking them down (catabolism). For example, green plants build up complex organic substances from water, carbon dioxide, and mineral salts (photosynthesis, described in the Plant Kingdom chapter); animals partially break down complex organic substances, ingested as food, and subsequently resynthesize them for use in their own bodies (described in the Human Body chapter); and, within cells, complex molecules are broken down to release their energy in the process of c0016-01.gifrespiration.  
  Complex processes such as respiration can be described as metabolic pathways, in which the  




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Discover of the Structure of DNA
  By Julian Rowe  
the first announcement
"We wish to suggest a structure for the salt of deoxyribose nucleic acid (DNA). This structure has novel features which are of considerable biological interest." So began a 900-word article that was published in the journal Nature in April 1953. Its authors were British molecular biologist Francis Crick (1916–) and U.S. biochemist James Watson (1928– ).
The article described the correct structure of DNA, a discovery that many scientists have called the most important since Austrian botanist and monk Gregor Mendel (1822–1884) laid the foundations of the science of genetics.
DNA is the molecule of heredity, and by knowing its structure, scientists can see exactly how forms of life are transmitted from one generation to the next.
the problem of inheritance
The story of DNA really begins with British naturalist Charles Darwin (1809–1882). When, in November 1859, he published On the Origin of Species by Means of Natural Selection outlining his theory of evolution, he was unable to explain exactly how inheritance came about. For at that time it was believed that offspring inherited an average of the features of their parents. If this were so, as Darwin's critics pointed out, any remarkable features produced in a living organism by evolutionary processes would, in the natural course of events, soon disappear.
The work of Gregor Mendel, only rediscovered 18 years after Darwin's death, provided a clear demonstration that inheritance was not a "blending" process at all. His description of the mathematical basis to genetics followed years of careful plant-breeding experiments. He concluded that each of the features he studied, such as color or stem length, was determined by two "factors" of inheritance, one coming from each parent. Each egg or sperm cell contained only one factor of each pair. In this way a particular factor, say for the color red, would be preserved through subsequent generations.
Today, we call Mendel's factors genes. Through the work of many scientists, it came to be realized that genes are part of the chromosomes located in the nucleus of living cells and that DNA, rather than protein as was first thought, was a hereditary material.
the double helix
In the early 1950s, scientists realized that X-ray crystallography, a method of using X-rays to obtain an exact picture of the atoms in a molecule, could be successfully applied to the large and complex molecules found in living cells.
It had been known since 1946 that genes consist of DNA. At King's College, London, New Zealand-British biophysicist Maurice Wilkins (1916–) had been using X-ray crystallography to examine the structure of DNA, together with his colleague, British X-ray crystallographer Rosalind Franklin (1920–1958), and had made considerable progress.
While in Copenhagen, U.S. scientist James Watson had realized that one of the major unresolved problems of biology was the precise structure of DNA. In 1952, he came as a young postdoctoral student to join the Medical Research Council Unit at the Cavendish Laboratory, Cambridge, England, where Francis Crick was already working. Convinced that a gene must be some kind of molecule, the two scientists set to work on DNA. Helped by the work of Wilkins, they were able to build an accurate model of DNA. They showed that DNA had a double helical structure, rather like a spiral staircase. Because the molecule of DNA was made from two strands, they envisioned that as a cell divides, the strands unravel, and each could serve as a template as new DNA was formed in the resulting daughter cells. Their model also explained how genetic information might be coded in the sequence of the simpler molecules of which DNA is comprised. Here for the first time was a complete insight into the basis of heredity. James Watson commented that this result was "too pretty not to be true!"
cracking the code
Later, working with South African-British molecular biologist Sidney Brenner (1927– ), Crick went on to work out the genetic code, and so ascribe a precise function to each specific region of the molecule of DNA. These triumphant results created a tremendous flurry of scientific activity around the world. The pioneering work of Crick, Wilkins, and Watson was recognized in the award of the Nobel Prize for Physiology or Medicine in 1962.
The unraveling of the structure of DNA lead to a new scientific discipline, molecular biology, and laid the foundation stones for genetic engineering—a powerful new technique that is revolutionizing biology, medicine, and food production through the


  product of one reaction forms the substrate for another. In each case, the reaction will be accelerated, or catalyzed, by the action of a specific enzyme.  
  Enzymes are biological catalysts produced in cells, and capable of speeding up the chemical reactions necessary for life. They are large, complex proteins, and are highly specific—each chemical reaction requiring its own particular enzyme. The enzyme's specificity arises from its active site, an area with a shape corresponding to part of the molecule with which it reacts (the  




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  substrate). The enzyme and the substrate slot together forming an enzyme—substrate complex that allows the reaction to take place, after which the enzyme falls away unaltered.  
  The activity and efficiency of enzymes are influenced by various factors, including temperature and pH conditions. Temperatures above 60°C/140°F damage (denature) the intricate structure of enzymes, causing reactions to cease. Each enzyme operates best within a specific pH range, and is denatured by excessive acidity or alkalinity.  
  Respiration employs about 70 different enzymes to act as catalysts. Digestive enzymes include amylases (which digest starch), lipases (which digest fats), and proteases (which digest protein). Other enzymes play a part in the replication of c0016-01.gifDNA when a cell divides; the production of hormones; and the control of movement of substances into and out of cells.  
  transport across cell membranes  
  The ability to move molecules and ions into and out of cells and cellular structures as required is of vital importance in metabolic reactions. Such transportation may rely on passive processes such as diffusion, or it may involve the expenditure of energy.  
  diffusion Diffusion is the spontaneous movement of molecules or ions in a fluid from a region in which they are at a high concentration to a region of lower concentration, until a uniform concentration is achieved throughout. In biological systems it plays an essential role in the transport, over short distances, of molecules such as nutrients, respiratory gases, and neurotransmitters. It provides the means by which small molecules pass into and out of individual cells and microorganisms, such as amoebas, that possess no circulatory system.  
  osmosis Osmosis is the movement of water molecules through a semipermeable membrane from a region in which they are at a high concentration to a region where they are at a lower concentration. Many cell membranes behave as semipermeable membranes, and osmosis is a vital mechanism in the transport of fluids in living organisms—for example, in the transport of water from the roots up the stems of plants.  
  active transport Active transport requires the expenditure of energy to move molecules or ions across a membrane against a concentration gradient (that is, from where they are at a low concentration to where they are at a higher concentration). The molecule or ion becomes attached to a carrier protein straddling the cell membrane, which then—it is thought—alters its configuration as a result of energy input from ATP to carry the molecule across the membrane and release it on the other side.  
  Active transport differs therefore from diffusion and osmosis, which are both passive processes requiring no input of energy.  
  This is the process by which food molecules are broken down to release their energy in a form that the organism can use to drive other metabolic processes. The resultant energy is packaged in the energy-carrying molecules ATP (adenosine triphosphate).  
  aerobic respiration Respiration that involves the presence of oxygen is called aerobic respiration. It is the more common form (the other is anaerobic respiration) and takes place largely in the mitochondria of the cell. Glucose is the usual substrate (though other food substances such as fatty acids and pentose sugars are also broken down). The process may be summarized as follows:  
  Krebs cycle The purpose of the Krebs (or citric acid) cycle is to complete the
biochemical breakdown of food to produce energy-rich molecules, which the
organism can use to fuel work. Acetyl coenzyme A (acetyl CoA)–produced
by the breakdown of sugars, fatty acids, and some amino acids—reacts with
oxaloacetic acid to produce citric acid, which is then converted in a series of
enzyme-catalyzed steps back to oxaloacetic acid. In the process, molecules of
carbon dioxide and water are given off, and the precursors of the energy-rich
molecules ATP are formed. (The numbers in the diagram indicate the number
of carbon atoms in the principal compounds.)




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  However, respiration takes place in a number of stages, each of which involves several steps, which proceed by coupling energy-yielding reactions to those that consume energy and are catalyzed by the presence of specific enzymes.  
  The first stage, glycolysis, does not require oxygen and is therefore a form of anaerobic respiration. It takes place in the cell cytoplasm and is responsible for converting glucose to acetyl coenzyme A (acetyl CoA), with a net energy gain of two molecules of ATP for each molecule of glucose.  
  The next stage, the Krebs cycle (or citric acid or tricarboxylic acid cycle) takes place in the mitochondrial matrix. The acetyl CoA molecules created by glycolysis diffuse from the cytoplasm to the mitochondria, and are oxidized there to carbon dioxide in a complex series of steps that also result in the release of a further four molecules of ATP.  
  The last stage, the electron transport chain, is the main energy-producing stage and takes place on the mitochondrial walls. Much of the energy released from glucose during glycolysis and the Krebs cycle remains trapped within intermediate products (reduced nucleotides such as NADH, reduced nicotinamide adenine dinucleotide). The electron transport chain brings about the oxidation of the reduced nucleotides, shuttling the resultant electrons along a chain of carriers to bring about the reduction of molecular oxygen to water. It also brings about the phosphorylation of ADP molecules (adenosine diphosphate) to create a further 32 molecules of ATP.  
  The whole process therefore produces a total of 38 molecules of ATP for each molecule of glucose.  
  anaerobic respiration Anaerobic respiration takes place in the absence of oxygen. It begins with the process of glycolysis described above, but the acetyl CoA produced is then reduced either to alcohol (as in fermentation by yeast fungi) or to lactic acid (as in muscle cells experiencing oxygen debt). Only the glycolysis stage yields ATP; therefore, the net energy gain per molecule of glucose is only two molecules of ATP (compared with 38 molecules for aerobic respiration).  
  protein synthesis  
  This is the process by which cells manufacture the (proteins essential to their functioning, according to instructions encoded in the (DNA. Proteins play a vital role, most importantly as enzymes, which accelerate and control metabolic processes. The structure of proteins, which depends on the exact sequence of their amino-acid building blocks, is closely linked to their correct functioning. It is the genetic information carried in the DNA that controls that sequence.  
  protein synthesis and the genetic code Each of the nucleotide subunits of DNA is attached to a molecule called a base, which may be either adenine, guanine, thymine, or cytosine. A set of three consecutive bases—known as a codon—specifies a particular amino acid. A section of DNA that contains the sequence of codons required to encode a particular protein is called a gene.  
  Geneticists identify the codons by the initial letters of their constituent bases—for example, the base sequence of codon CAG is cytosine—adenine—guanine. Because there are four different bases, there must be 4 x 4 x 4 = 64 different codons. Proteins are usually made up of only 20 different amino acids, so many amino acids have more than one codon (for example, GGT, GGC, GGA, and GGG all code for the same amino acid, glycine).  
  transcription In order for the information represented by the DNA codons to be converted to amino acids, it must be transcribed to another form—messenger RNA—that can pass from the cell nucleus to the protein-making machinery in the cytoplasm.  
  In a section of DNA that represents a particular protein, the two strands of DNA unwind and separate. One of the strands then acts as a template for the production of messenger (RNA (mRNA). An enzyme moves along it, pairing the exposed bases with mRNA nucleotides that have complementary bases (cytosine in the DNA will pair with guanine nucleotides and vice versa, thymine will pair with adenine nucleotides, and adenine with uracil). In this way, a "mirror image" of the information in the DNA is copied onto the mRNA. The mRNA nucleotides link together to form an mRNA strand, which then detaches itself from the DNA and passes out of the nucleus toward the (ribosomes in the cytoplasm. This is where the information carried by the mRNA will be translated into amino acids and, ultimately, the protein.  
  translation This is the process by which the information encoded as a sequence of bases in mRNA is transformed into the precise sequence of amino acids that makes up a polypeptide chain (the precursor of a protein molecule). It takes place in the ribosomes, tiny bodies that are either dispersed in the cytoplasm or attached to the (endoplasmic reticulum (a membranous system within the cytoplasm).  
  During translation, a ribosome binds to an mRNA strand, and then moves along its length. Small, hairpin-shaped strands of RNA, called transfer RNA  




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  (tRNA), bind to specific amino acids in the cytoplasm and bring these to the ribosome. They possess a special triplet of bases, called an anticodon, on their looped end that determines the amino acid that they will collect and also the codon on the mRNA to which they will bring that amino acid.  
  When the ribosome encounters a codon on the mRNA, the tRNA molecule carrying the correct amino acid is brought into position. The ribosome connects the new amino acid to the preceding amino acid with a peptide bond, and so the polypeptide chain is built up. When completed, the polypeptide is released by the ribosome into the endoplasmic reticulum or cytoplasm, where it may fold itself automatically into a protein or it may experience further modification.  
  Genetics is the branch of biology that is concerned with the study of heredity and variation; it attempts to explain how characteristics of living organisms are passed on from one generation to the next.  
  history of genetics  
  The science of genetics was initiated by the work of Austrian biologist Gregor Mendel whose experiments with the cross-breeding (hybridization) of peas showed that the inheritance of characteristics and traits takes place by means of discrete "particles" (which would later be identified as c0016-01.gifgenes). These are present in the cells of all organisms, and are now recognized as being the basic units of heredity. Every organism possesses a genotype (a set of variable genes) and a phenotype (characteristics produced by certain genes). Modern geneticists investigate the structure, function, and transmission of genes.  
  Before the publication of Mendel's work in 1865, it had been assumed that the characteristics of both parents were blended during inheritance, but Mendel showed that the genes remain intact, although their combinations change. Since Mendel, the study of genetics has advanced greatly, first through breeding experiments and light-microscope observations (classical genetics), later by means of biochemical and electron microscope studies (molecular genetics).  
  In 1944 Canadian-born bacteriologist Oswald Avery, together with his colleagues at the Rockefeller Institute, Colin McLeod and Maclyn McCarthy, showed that the genetic material was deoxyribonucleic acid (c0016-01.gifDNA), and not protein as was previously thought. A further breakthrough was made in 1953 when James Watson and Francis Crick published their molecular model for the structure of DNA, the double helix, based on X-ray diffraction photographs. The following decade saw the cracking of the genetic code. The genetic code is said to be universal since the same code applies to all organisms from bacteria and viruses to higher plants and animals, including humans. Today the deliberate manipulation of genes by biochemical techniques, or c0016-01.gifgenetic engineering, is commonplace.  
  Hefty resource for anyone interested in Gregor Mendel, the origins of classical genetics, and the history and literature of science. View or download Mendel's original paper, with hypertext links to glossaries, biographical information, and exercises, or look up the essays, timeline, bibliography, and statistical tools.  
  The nucleus of each cell of every organism contains a number of chromosomes—long threads made of DNA and protein. The number of chromosomes in each cell is constant for and characteristic of a particular species. Within the DNA of these chromosomes are the genes—units of inherited material that determine the characteristics of each organism, or parts of organisms, and allow these characteristics to be transmitted down from generation to generation. Each gene carries the chemically coded instructions for the production of a particular polypeptide, which in turn forms a protein (see (protein synthesis); it is these proteins that determine the form and function of the organism.  
  Located on the 23 pairs of chromosomes of each human cell are some 80,000 different genes, which determine features such as hair and skin color. The presence or absence of specific genes can be a contributing factor in diseases such as hemophilia and cystic fibrosis. Each chromosome of a chromosome pair carries genes for the same characteristics in the same place, or locus. These two kinds of genes defining alternative characteristics are called c0016-01.gifalleles. If the two alleles match, they are said to be homozygous; if they differ, they are described as heterozygous. Some alleles are dominant, and others are recessive; a  
  Natural History of Genetics
  Through a combination of scientific experts and teachers, this site offers an accessible and well-designed introduction to genetics. It includes several guided projects with experiments and explanations aimed initially at young teenage children. However, this site also includes "intermediate" and "expert" sections allowing this page to be used by a wide variety of ages and levels of expertise. In addition to the experiments, the site also includes sections on such topics as "core genetics," "teacher workshops,'' and "fun stuff."  




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  dominant allele masks the effects of its recessive partner. In other words, the dominant allele is expressed, and the recessive allele is not. A trait that results from a recessive allele is evident only in an individual that has two recessive alleles for that trait.  
  Human Genome Project  
  The Human Genome Project is a research scheme, begun in 1988, to map the complete nucleotide (see (nucleic acid) sequence of human DNA. There are approximately 80,000 different genes in the human genome, and one gene may contain more than 2 million nucleotides. The program aims to collect 10–15,000 genetic specimens from 722 ethnic groups whose genetic make-up is to be preserved for future use and study. The knowledge gained is expected to help prevent or treat many crippling and lethal diseases, but there are potential ethical problems associated with knowledge of an individual's genetic make-up, and fears that it will lead to genetic discrimination.  
  Only 3% of the genome had been sequenced by mid 1998 but plans were announced to complete a "rough draft" of 95% of the genome over the next three years. The target date for sequencing the whole genome is 2005, though a private US company announced plans to sequence the entire genome by 2001.  
  The Human Genome Organization (HUGO) coordinating the project expects to spend $1 billion over the first five years, making this the largest research project ever undertaken in the life sciences. Work is being carried out in more than 20 centres around the world.  
  Concern that, for example, knowledge of an individual's genes may make that person an unacceptable insurance risk has led to planned legislation on genome privacy in the United States, and 3% of HUGO's funds have been set aside for researching and reporting on the ethical implications of the project.  
  gene sequencing Each strand of DNA carries a sequence of chemical building blocks, the nucleotides. There are only four different types, but the number of possible combinations is immense. The different combinations of nucleotides produce different proteins in the cell, and thus determine the structure of the body and its individual variations. To establish the nucleotide sequence, DNA strands are broken into fragments, which are duplicated (by being introduced into cells of yeast or the bacterium Escherichia coli) and distributed to the research centers.  
  Genes account for only a small amount of the DNA sequence. Over 90% of DNA appears not to have any function, although it is perfectly replicated each time the cell divides, and handed on to the next generation. Many higher organisms have large amounts of redundant DNA and it may be that this is an advantage, in that there is a pool of DNA available to form new genes if an old one is lost by mutation.  
  Human Genome Project Information
  U.S.-based site devoted to this mammoth project—with news, progress reports, a molecular genetics primer, and links to other relevant sites.  
  genetic engineering  
  This is the deliberate manipulation of genetic material by biochemical techniques. It is often achieved by the introduction of new DNA, usually by means of a virus or plasmid. This can be for pure research, gene therapy (curing or alleviating inherited diseases or defects), or to breed functionally specific plants, animals, or bacteria. These organisms with a foreign gene added are said to be transgenic. At the beginning of 1995 more than 60 plant species had been genetically engineered, and nearly 3,000 transgenic crops had been field-tested.  
  practical uses In genetic engineering, the splicing and reconciliation of genes is used to increase knowledge of cell function and reproduction, but it can also achieve practical ends. For example, plants grown for food could be given the ability to fix nitrogen, found in some bacteria, and so reduce the need for expensive fertilizers, or simple bacteria may be modified to produce rare drugs. A foreign gene can be inserted into laboratory cultures of bacteria to generate commercial biological products, such as synthetic insulin, hepatitis-B vaccine, and interferon. Gene splicing was invented in 1973 by the U.S. scientists Stanley Cohen and Herbert Boyer, and patented in the United States in 1984.  
  new developments Developments in genetic engineering have led to the production of growth hormone, and a number of other bone-marrow stimulating hormones. New strains of animals have also been produced; a new strain of mouse was patented in the United States in 1989 (the application was rejected in the European Patent Office). A vaccine against a sheep parasite (a larval tapeworm) has been developed by  
  Your Genes, Your Choices: Exploring the Issues Raised by Genetic Research
  Illustrated electronic book which describes the science of genetic research, as well as the ethical, legal, and social issues that it raises. Detailed and informative in itself, the site also contains an extensive bibliography.  




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  genetic engineering; most existing vaccines protect against bacteria and viruses.  
  The first genetically engineered food went on sale in 1994; the "Flavr Savr" tomato, produced by the U.S. biotechnology company Calgene, was available in California and Chicago.  
  Evolution is the slow, gradual process by which life has developed from single-celled organisms into the multiplicity of animal and plant life, extinct and existing, that inhabits the earth. The development of the concept of evolution is usually associated with the English naturalist Charles Darwin, who attributed the main role in evolutionary change to natural selection acting on randomly occurring, hereditary variations to produce adaptations that make the organism better suited to its environment. These hereditary variations are now known to be produced by spontaneous changes, or mutations, in the genetic material, coupled with the recombination of that genetic material during sexual reproduction.  
  Evolutionary changes have not taken place in a linear manner, but as a branching process of descent from a common ancestor; for example, mammals and the now extinct synapsid reptiles share an ancestor that lived about 225 million years ago, while humans and chimpanzees share an ancestor that lived 5–10 million years ago. Of the 1.5 million identifiable species now existing on earth, every one is the result of a long line of extinct species. Bacteria are among the earliest known species of life on earth and are still evolving today.  
  natural and sexual selection  
  Evolution depends on the presence, within a population, of alleles—or inheritable variations in the (genes—that confer a selective advantage on the individuals possessing them. That is, they are more likely—under the natural pressures of predation, disease, and competition—to succeed than individuals that do not possess these alleles, and so the alleles become more prevalent in the population. The phrase "survival of the fittest" is misleading since it implies the death of the "unfit" individuals. From an evolutionary point of view, reproductive success is much more important than survival since if one type regularly leaves more offspring than another, the frequency of the more fertile type in the population is bound to increase. Reproductive success depends on many things including general vigor, the length of the reproductive period, and the ability to mate successfully (sexual selection).  
  The accumulated effect of natural selection is to produce adaptations such as the insulating coat of a polar bear or the spadelike forelimbs of a mole. Natural selection usually takes place over many years, but in fast-breeding organisms it can occur rapidly, for example the spread of antibiotic resistance in some bacteria.  
  sexual selection Sexual selection is a process similar to natural selection but relating exclusively to success in finding a mate for the purpose of sexual reproduction and producing offspring. Sexual selection occurs when one sex (usually but not always the female) invests more effort in producing young than the other. Members of the other sex compete for access to this limited resource (usually males competing for the chance to mate with females).  
  Sexual selection often favors features that increase a male's attractiveness to females (such as the pheasant's tail) or enable males to fight with one another (such as a deer's antlers). More subtly, it can produce hormonal effects by which the male makes the female unreceptive to other males, causes the abortion of fetuses already conceived, or removes the sperm of males who have already mated with a female.  
  Over the course of time, natural and sexual selection and, perhaps, chance genetic drift may cause a population to diverge so widely from its ancestral population that they are no longer able to interbreed and may therefore be considered a separate species. One cause of speciation is the geographical separation of populations, followed by reproductive isolation. Another cause is assortative mating—selective mating between individuals that are genetically related or have similar characteristics. If sufficiently consistent, assortative mating can theoretically result in the evolution of new species without geographical isolation.  
  adaptive radiation  
  Adaptive radiation is the formation of several species, with adaptations to different ways of life, from a single ancestral type. Adaptive radiation is likely to occur whenever members of a species migrate to a new habitat with unoccupied ecological niches. It is thought that the lack of competition in such niches allows sections of the migrant population to develop new adaptations, and eventually to become new species.  
  The colonization of newly formed volcanic islands has led to the development of many unique species. The 13 species of Darwin's finch on the Galápagos Islands, for example, are probably descended from a single species from the South American mainland. The parent stock evolved into different species that now occupy a range of diverse niches.  




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  evolutionary theory  
  The idea of evolutionary change can be traced as far back as Lucretius in the 1st century B.C., but it did not gain wide acceptance until the 19th century, following the work of Scottish geologist Charles Lyell, French naturalist Jean Baptiste Lamarck (who suggested that characteristics acquired during an organism's lifetime could be inherited by its offspring), and Charles Darwin (in conjunction with Welsh naturalist Alfred Russel Wallace).  
  The current theory of evolution, Neo-Darwinism, combines Darwin's theory of natural selection with the principles of (genetics. Although neither the general concept of evolution nor the importance of natural selection is doubted by biologists, but there still remains dispute over other possible mechanisms involved in evolutionary change. Chance, for example, may play a large part in deciding which genes become characteristic of a population—a phenomenon called genetic drift. It is now also clear that evolutionary change does not always occur at a constant rate, but that the process can have long periods of relative stability interspersed with periods of rapid change. This has led to new theories, such as the punctuated equilibrium model.  
  Darwin, Charles
  Complete text of Darwin's seminal works On the Origin of Species and Voyage of the Beagle.  
  evolution and creationism  
  Some religions deny the theory of evolution, considering it conflicts with their belief that God created all things (creationism). But most scientists accept that there is overwhelming evidence that the diversity of life arose by a process of evolutionary divergence and not by individual acts of divine creation. There are several lines of evidence for this: the fossil record, the existence of similarities—or homologies—between different groups of organisms, embryology, and geographical distribution.  
  Center for Scientific Creation
  Dedicated to researching the case for creation, rather than evolution, as the origin of living species. Most of the evidence is gathered in a book which is heavily promoted on this page. However, the entire book is available online.  
  fossils, homologies, and embryos Most organisms decompose quite rapidly after death, but sometimes a plant or animal is preserved, usually by being buried soon after death, occasionally by freezing. Burial is generally in peat or mud, although it can also be in volcanic ash or amber (fossilized tree resin)—some remarkably well-preserved fossils of small animals have been found in amber. Even after burial the soft tissues of an organism may decompose so that only the skeleton of an animal or the woody parts of a plant become fossilized.  
  Evolutionist, The
  Online magazine devoted to evolutionary ideas which includes features, interviews, and comment.  
  The dating of fossils presents great difficulties. In the first instance they are dated according to the stratum, or layer, in which they are found, and correlated with fossils in the same layer, but radiometric methods have been developed for estimating the absolute ages of fossils. These depend on the fact that natural radioactive isotopes, such as those of carbon and potassium, decay at constant rates so that the amount of isotope remaining in a specimen is proportional to the length of time that has elapsed since its formation or deposition.  
  Although the fossil record does not actually prove the theory of evolution, a study of a series of fossils can provide a visual record of the evolution of individual species, such as the horse, and their adaptation to changing environments. Certain link fossils provide evidence of a link between species; an example of this is the Archeopteryx, which was a birdlike animal with teeth and this fossil provides corroboration of the reptile ancestry of birds.  
  Evolution: Theory and History
  Dedicated to the study of the history and theories associated with evolution, this site explores topics on classification, taxonomy, and dinosaur discoveries, and then looks at the key figures in the field and reviews their contributions.  
  Additional evidence for evolution is found in homologous structures. For instance, a comparison of the limb bones of several different kinds of vertebrate indicates striking similarities in their construction. Such structures are termed homologous and their existence suggests that all these animals have evolved from a common ancestor.  
  Embryology can also provide important clues as to the ancestry of a group. The vertebrate animals all show very similar embryonic development, and all have embryonic gill slits even though the mature animal has lungs and breathes air. This is taken as evidence that the vertebrates all evolved from an aquatic ancestor  




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  breathing through gills like modern fish and the tadpole stage of amphibians. The human embryo has a small tail, the coccyx, which becomes vestigial (functionless)—possible evidence of a common ancestor with a tail.  
  evolution of humans  
  The African apes (gorilla and chimpanzee) have been shown by anatomical and molecular comparisons to be the closest living relatives of humans. The oldest known hominids (of the human group), the australopithecines, found in Africa, date from 3.5 to 4.4 million years ago. The first to use tools came 2 million years later, and the first humanoids to use fire and move out of Africa appeared 1.7 million years ago. Neanderthals were not our direct ancestors. Modern humans are all believed to descend from one African female of 200,000 years ago, although there is a rival theory that humans evolved in different parts of the world simultaneously.  
  Miocene apes Genetic studies indicate that the last common ancestor between chimpanzees and humans lived 5 to 10 million years ago. There are only fragmentary remains of ape and hominid fossils from this period. Dispute continues over the hominid status of Ramapithecus, the jaws and teeth of which have been found in India and Kenya in late Miocene deposits, dated between 14 and 10 million years. The lower jaw of a fossil ape found in the Otavi Mountains, Namibia, comes from deposits dated between 10 and 15 million years ago, and is similar to finds from East Africa and Turkey. It is thought to be close to the initial divergence of the great apes and humans.  
  australopithecines Bones of the earliest known human ancestor, a hominid named Australopithecus ramidus were found in Ethiopia in 1994 and dated as 4.4 million years old. Australopithecus afarensis, found in Ethiopia and Kenya, date from 3.9 to 4.4 million years ago. These hominids walked upright and they were either direct ancestors or an offshoot of the line that led to modern humans. They may have been the ancestors of Homo habilis (considered by some to be a species of Australopithecus), who appeared about 2 million years later, had slightly larger bodies and brains, and were probably the first to use stone tools. Also living in Africa at the same time was Australopithecus africanus, a possibly carnivorous hominid, and Australopithecus robustus, a hominid with robust bones, large teeth, heavy jaws, and thought to be a vegetarian. They are not generally considered to be our ancestors.  
  Homo erectus Over 1.7 million years ago, Homo erectus, believed by some to be descended from Homo habilis, appeared in Africa. Homo erectus had prominent brow ridges, a flattened cranium, with the widest part of the skull low down, and jaws with a rounded tooth row, but the chin, characteristic of modern humans, is lacking. They also had much larger brains (900–1,200 cu cm), and were probably the first to use fire and the first to move out of Africa. Their remains are found as far afield as China, western Asia, Spain, and southern Britain. Modern humans Homo sapiens sapiens and the Neanderthals Homo sapiens neanderthalensis are probably descended from Homo erectus.  
  Neanderthals Neanderthals were large brained and heavily built, probably adapted to the cold conditions of the ice ages. They lived in Europe and the Middle East, and disappeared about 40,000 years ago, leaving Homo sapiens sapiens as the only remaining species of the hominid group. Possible intermediate forms between Neanderthals and Homo sapiens sapiens have been found at Mount Carmel in Israel and at Broken Hill in Zambia, but it seems that Homo sapiens sapiens appeared in Europe quite rapidly and either wiped out the Neanderthals or interbred with them.  
  modern humans There are currently two major views of human evolution: the "out of Africa" model, according to which Homo sapiens sapiens emerged from Homo erectus, or a descendant species, in Africa and then spread throughout the world; and the multiregional model, according to which selection pressures led to the emergence of similar advanced types of Homo sapiens sapiens from Homo erectus in different parts of the world at around the same time. Analysis of DNA in recent human populations suggests that Homo sapiens sapiens originated about 200,000 years ago in Africa from a single female ancestor, "Eve". The oldest known fossils of Homo sapiens sapiens also come from Africa, dating from 150,000–100,000 years ago. Separation of human populations would have occurred later, with separation of Asian, European, and Australian populations taking place between 100,000 and 50,000 years ago.  
  Humans are distinguished from apes by the complexity of their brain and its size relative to body size; by their small jaw, which is situated under the face and is correlated with reduction in the size of the anterior teeth, especially the canines, which no longer project beyond the tooth row; by their bipedalism, which affects the position of the head on the vertebral column, the lumbar and cervical curvature of the vertebral column, and the structure of the pelvis, knee joint, and foot; by their complex language; and by their elaborate culture.  




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  The broad characteristics of human behavior are a continuation of primate behavior rather than a departure from it. For example, tool use, once a criterion for human status, has been found regularly in gorillas, orang-utans, and chimpanzees, and sporadically in baboons and macaques. Chimpanzees even make tools. In hominid evolution manual dexterity has increased so that more precise tools can be made. Cooperation in hunting, also once thought to be a unique human characteristic, has been found in chimpanzees, and some gorillas and chimpanzees have been taught to use sign language to communicate.  
  Fossil Hominids FAQ
  Basic information about hominid species, the most important hominid fossils, and creationist arguments, plus links to related sites.  
  Classification and Nomenclature  
  This is the arrangement of organisms into a hierarchy of groups on the basis of their similarities. The basic grouping is a species, several of which may constitute a genus, which in turn are grouped into families, and so on up through orders, classes, phyla (in plants, sometimes called divisions), to kingdoms.  
  The oldest method of classification, called phenetic classification, aims to classify organisms on the basis of as many as possible of their observable characteristics: their morphology, anatomy, physiology, and so on. Greek philosopher Theophrastus adopted this method in the 4th century B.C., when he classified plants into trees, shrubs, undershrubs, and herbs. Awareness of evolutionary theory, however, led to the development of phylogenetic classification, which aims to classify groups of organisms on the basis of their common ancestry and their genetic relationship. In practice, most present-day systems of classification compromise between the two approaches.  
  Journey into Phylogenetic Systematics
  Online exhibition about evolutionary theory with a specific emphasis on phylogenetic classification: the way that biologists reconstruct the pattern of events that has led to the distribution and diversity of life. The site provides an introduction to the philosophy, methodology, and implication of cladistic analysis, with a separate section on the need for cladistics.  
  Cladistics is a controversial phylogenetic method that uses a formal step-by-step procedure for objectively assessing the extent to which organisms share particular characters with a common ancestor, and for assigning them to taxonomic groups called clades.  
  five-kingdom system  
  The kingdom is the primary division in biological classification. At one time, only two kingdoms were recognized: animals and plants. Today most biologists prefer a five-kingdom system, even though it still involves grouping together organisms that are probably unrelated. One widely accepted scheme is as follows: Kingdom Animalia (all multicellular animals); Kingdom Plantae (all plants, excluding seaweeds and other algae); Kingdom Fungi (all fungi, including the unicellular yeasts, but not slime molds); Kingdom Protista or Protoctista (multicellular algae, protozoa, diatoms, dinoflagellates, slime molds, and various  
The Five Kingdoms of Living Things
Kingdom Main features of organisms Number of species
Monera1 all are bacteria; single-celled; prokaryotic (lack a membrane-bound nucleus); autotrophic (photosynthesis and chemosynthesis) and heterotrophic; all reproduce asexually, some also reproduce sexually >10,000
Protista single-celled or multicellular; eukaryotic (have a membrane-bound nucleus and membrane-bound organelles); autotrophic (photosynthesis in algae and Euglenoids) and heterotrophic; may reproduce asexually or sexually >65,000
Fungi single-celled and multicellular; eukaryotic; heterotrophic; form spores at all stages of their life cycle; usually reproduce asexually, many reproduce sexually by conjugation about 100,000
Plantae all are multicellular; eukaryotic; most are autotrophic (via photosynthesis); reproduce sexually; in some life cycle includes an alternation of generations (a haploid gametophyte stage and a diploid sporophyte stage) about 500,000
Animalia all are multicellular; eukaryotic; all are heterotrophic; reproduce sexually; develop from a blastula; most have tissues organized into organs >795,000
1 The Kingdom Monera is sometimes called the Kingdom Prokaryotae.





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Evolution: Out of Africa and the Eve Hypothesis
  By Chris Stringer  
Most paleoanthropologists recognize the existence of two human species during the last million years—Homo erectus, now extinct, and Homo sapiens, the species which includes recent or "modern" humans. In general, they believe that Homo erectus was the ancestor of Homo sapiens.
How did the transition occur?
the multiregional model
There are two opposing views. The multiregional model says that Homo erectus gave rise to Homo sapiens across its whole range which, about 700,000 years ago, included Africa, China, Java (Indonesia), and, probably, Europe.
Homo erectus, following an African origin about 1.7 million years ago, dispersed around the Old World, developing the regional variation that lies at the roots of modern "racial variation. Particular features in a given region persisted in the local descendant populations of today.
For example, Chinese Homo erectus specimens had the same flat faces, with prominent cheekbones, as modern Oriental populations. Javanese Homo erectus had robustly built cheekbones and faces that jutted out from the braincase, characteristics found in modern Australian Aborigines. No definite representatives of Homo erectus have yet been discovered in Europe. Here, the fossil record does not extend back as far as those of Africa and eastern Asia, although a possible Homo erectus jawbone more than a million years old was recently excavated in Georgia.
Nevertheless, the multiregional model claims that European Homo erectus did exist, and evolved into a primitive form of Homo sapiens. Evolution in turn produced the Neanderthals: the ancestors of modern Europeans. Features of continuity in this European lineage include prominent noses and midfaces.
genetic continuity
The multiregional model was first described in detail by Franz Weidenreich, a German palaeoanthropologist. It was developed further by the American Carleton Coon, who tended to regard the regional lineages as genetically separate. Most recently, the model has become associated with such researchers as Milford Wolpoff (United States) and Alan Thorne (Australia), who have re-emphasized the importance of gene flow between the regional lines. In fact, they regard the continuity in time and space between the various forms of Homo erectus and their regional descendants to be so complete that they should be regarded as representing only one species—Homo sapiens.
the opposing view
The opposing view is that Homo sapiens had a restricted origin in time and space. This is an old idea. Early in the 20th century, workers such as Marcellin Boule (France) and Arthur Keith (U.K.) believed that the lineage of Homo sapiens was very ancient, having developed in parallel with that of Homo erectus and the Neanderthals. However, much of the fossil evidence used to support their ideas has been re-evaluated, and few workers now accept the idea of a very ancient and separate origin for modern Homo sapiens.
the Garden of Eden
Modern proponents of this approach focus on a recent and restricted origin for modern Homo sapiens. This was dubbed the "Garden of Eden" or "Noah's Ark" model by the U.S. anthropologist William Howells in 1976 because of the idea that all modern human variation had a localized origin from one center. Howells did not specify the centre of origin, but research since 1976 points to Africa as especially important in modern human origins.
The consequent "Out of Africa" model claims that Homo erectus evolved into modern Homo sapiens in Africa about 100,00–150,000 years ago. Part of the African stock of early modern humans spread from the continent into adjoining regions and eventually reached Australia, Europe, and the Americas (probably by 45,000, 40,000, and 15,000 years ago respectively). Regional ("racial") variation only developed during and after the dispersal, so that there is no continuity of regional features between Homo erectus and present counterparts in the same regions.
Like the multiregional model, this view accepts that Homo erectus evolved into new forms of human in inhabited regions outside Africa, but argues that these non-African lineages became extinct without evolving into modern humans. Some, such as the Neanderthals, were displaced and then replaced by the spread of modern humans into their regions.
 . . . and an African Eve?
In 1987, research on the genetic material called mitochondrial DNA (mtDNA) in living humans led to the reconstruction of a hypothetical female ancestor for all present-day humanity. This "Eve" was believed to have lived in Africa about 200,000 years ago. Recent re-examination of the "Eve" research has cast doubt on this hypothesis, but further support for an "Out of Africa" model has come from genetic studies of nuclear DNA, which also point to a relatively recent African origin for present-day Homo sapiens.
Studies of fossil material of the last 50,000 years also seem to indicate that many "racial" features in the human skeleton have developed only over the last 30,000 years, in line with the "Out of Africa" model, and at odds with the million-year times-pan one would expect from the multiregional model.





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  other lower organisms with eukaryotic cells); and Kingdom Monera or Prokaryotae (the prokaryotes: bacteria and cyanobacteria, or blue-green algae). The first four of these kingdoms make up the eukaryotes.  
  When only two kingdoms were recognized, any organism with a rigid cell wall was a plant, and so bacteria and fungi were considered plants, despite their many differences. Other organisms, such as the photosynthetic flagellates (euglenoids), were claimed by both kingdoms. The unsatisfactory nature of the two-kingdom system became evident during the 19th century, and the biologist Ernst Haeckel was among the first to try to reform it. High-power microscopes have revealed more about the structure of cells; it has become clear that there is a fundamental difference between cells without a nucleus (prokaryotes) and those with a nucleus (eukaryotes). However, these differences are larger than those between animals and higher plants, and are unsuitable for use as kingdoms. At present there is no agreement on how many kingdoms there are in the natural world. Some schemes have as many as 20.  
  binomial system of nomenclature  
  This is the system by which all organisms are identified by a two-part Latinized name. Devised by the biologist Linnaeus, it is also known as the Linnaean system. The first name is capitalized and identifies the genus; the second identifies the species within that genus.  
  Usually the names are descriptive. Thus, the name of the dog, Canis familiaris, means the "familiar species of the dog genus," Canis being Latin for "dog." Each species is defined by an officially designated type specimen housed at a particular museum. The rules for naming organisms in this way are specified in a number of International Codes of Taxonomic Nomenclature administered by two International Commissions on Nomenclature, one zoological and one botanical.  
  Biology, Genetics, and Evolution Chronology  
Biology, Genetics, and Evolution Chronology
c. 570 B.C. Greek philosopher Anaximander argues that life evolved from the sea, and that land animals are descendants of sea animals—the first evolutionary theory.
c. 560 B.C. Greek philosopher Xenophanes correctly recognizes the nature of fossils when he suggests that fossil seashells are the result of a great flood that buried them in the mud.
1665 English scientist Robert Hooke publishes Micrographia, the first serious scientific work on microscopy, describing the function of the microscope, and coining the name "cells" to describe cavities he has found in the structure of cork.
1674 Dutch microscopist Anton van Leeuwenhoek develops the single-lens microscope, and begins a series of important discoveries by observing protozoa.
1683 Leeuwenhoek is the first to observe bacteria.
1735 In his Systema Naturae/System of Nature, Swedish botanist Carolus Linnaeus introduces a system for classifying plants by genus and species—a taxonomy that will survive the upheavals of evolutionism and remains in use today.
1735 In his Telliamed, French scientist Benoit de Maillet puts forward an evolutionary hypothesis.
1745 French naturalist Pierre de Maupertuis attacks the currently favoured theory of reproduction, that the sperm contain a miniature version of the adult. He argues that characteristics of both parents influence the offspring.
1751 French naturalist Georges Buffon is condemned by the Sorbonne (University of Paris, France) for supporting the idea of evolution in his Natural History. He is forced to recant, declaring the biblical account of creation correct.
1758 Linnaeus applies the binomial taxonomy he developed for plant classification to animal species.
1780 French chemist Antoine-Laurent Lavoisier demonstrates that respiration is a form of combustion.
1794 English naturalist and physician Erasmus Darwin publishes Zoonomia, or the Laws of Organic Life, expressing his ideas on evolution (which he assumes has an environmental cause).
1802 French biologist Jean-Baptiste de Lamarck is the first to use the term "biology."
1809 Lamarck theorizes that organs improve with use and degenerate with disuse and that these environmentally adapted traits are inheritable.
1817 French scientist Georges Cuvier breaks away from the view that animals can be arranged in a linear sequence leading to humans and argues instead that they should be classified according to their anatomical organization.
1831 Scottish botanist Robert Brown discovers the nucleus in plant cells.
1831 English naturalist Charles Darwin undertakes a five-year voyage, to South America and the Pacific, as naturalist on the Beagle. The voyage convinces him that species have evolved





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  gradually but he waits over 20 years to publish his findings.
1838 Dutch chemist Gerard Johann Mulder coins the word "protein."
1838 German botanist Matthias Jakob Schleiden recognizes that cells are the fundamental units of all plant life. He is thus the first to formulate cell theory.
1838 German chemist Justus von Liebig demonstrates that animal heat is due to respiration.
1839 German physiologist Theodor Schwann argues that all animals and plants are composed of cells. Along with Matthias Schleiden, he thus founds modern cell theory.
1840 Swiss embryologist Rudolf Albert von Kölliker identifies spermatozoa as cells.
1845 German zoologist Carl von Siebold describes the unicellular nature of protozoa and describes the function of the cilia.
1846 German botanist Hugo von Mohl uses the word protoplasm to describe the main living substance in a cell. It leads to the development of cell physiology.
1852 German physician Robert Remak discovers that the growth of tissues involves both the multiplication and division of cells.
1856 German naturalist Johann Fuhrott discovers the first fossil remains of a Neanderthal in Quaternary bed in Feldhofen Cave near Hochdal Cave above the Neander Valley, Germany. They cause immediate debate about whether they are the remains of ancient humans or the deformed bones of a modern human.
1858 English chemists W. H. Perkin and B. F. Duppa synthesize glycine, the first amino acid to be manufactured.
1858 English naturalists Charles Darwin and Alfred Russel Wallace contribute a joint paper on the variation of species to the Linnaean Society of London, England, stating their conclusions about natural selection and evolution.
1859 Charles Darwin publishes On the Origin of Species by Natural Selection, which expounds his theory of evolution by natural selection, and by implication denies the truth of biblical creation and God's hand in Nature. It sells out immediately and revolutionizes biology.
1860 At the Oxford meeting of the British Association, Bishop Samuel Wilberforce and biologist Thomas Henry Huxley debate creationism versus evolutionism.
1861 German zoologist Max Schultze defines the cell as consisting of protoplasm and a nucleus, a structure he recognizes as fundamental in both plants and animals.
1864 English philosopher Herbert Spencer coins the term "survival of the fittest" in Principles of Biology.
1865 Austrian monk and botanist Gregor Mendel publishes a paper that outlines the fundamental laws of heredity.
1866 German embryologist Ernst Haeckel proposes a third category of living beings intermediate between plants and animals. Called Protista, it consists mostly of microscopic organisms such as protozoans, algae, and fungi.
1868 French geologist Louis Lartet is the first to discover the skeletal remains of anatomically modern humans, in a cave near Co-Magnon, France. They are 35,000 years old.
1869 Swiss biochemist Johann Miescher discovers a nitrogen and phosphorous material in cell nuclei that he calls nuclein but which is now known as the genetic material DNA.
1876 German cytologist Eduard Adolf Strasburger describes the process of mitosis.
1880 German cytologist Eduard Adolf Strasburger announces that new cell nuclei arise from the division of old nuclei.
1882 German chemist Emil Hermann Fischer shows that proteins are polymers, or large molecules, comprised of amino acids.
1886 German biologist August Friedrich Leopold Weismann states that reproductive cells, or "germ plasm" cells, remain unchanged from generation to generation, and that they contain some hereditary substance which is now known as chromosomes, DNA, and genes.
1892 Dutch geneticist Hugo Marie de Vries, through a program of plant breeding, establishes the same laws of heredity discovered by Gregor Mendel in 1865.
1892 German embryologist and anatomist Oscar Hertwig establishes the science of cytology by suggesting that the processes that go on inside the cell are reflections of organismic processes.
1892 Russian microbiologist Dimitry losifovich Ivanovsky publishes "On Two Diseases of Tobacco" in which he announces that mosaic disease in tobacco is caused by microorganisms too small to be seen through a microscope. Now known as viruses, his discovery pioneers the science of virology.
1894 Dutch anatomist Marie Eugène Dubois announces the discovery, in Java, of the remains of the first specimen of Homo erectus ("upright man"), which he calls Pithecanthropus erectus, and which has a cranial capacity of 900 cc and is 0.5 to 1 million years old.
1899 The amino acid cystine is discovered to be a component of protein.
1900 Dutch geneticist Hugo Marie de Vries, German botanist Carl Erich Correns, and Austrian botanist Erich Tschermak von Seysenegg, simultaneously and independently rediscover the Austrian monk Gregor Mendel's 1865 work on heredity.
1901 English biochemist Frederick Gowland Hopkins isolates the amino acid tryptophan.





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1902 German chemists Emil Fischer and Franz Hofmeister discover that proteins are polypeptides consisting of amino acids.
1902–04 U.S. geneticist Walter Sutton and the German zoologist Theodor Boveri found the chromosomal theory of inheritance when they show that cell division is connected with heredity.
1905 Danish botanist Wilhelm Johannsen introduces the terms "genotype" and "phenotype" to explain how genetically identical plants differ in external characteristics.
1906 British biochemists Arthur Harden and William Young discover catalysis among enzymes.
1906 English biologist William Bateson introduces the term "genetics."
1907 German chemist Emil Fischer describes the synthesis of amino acid chains in proteins.
1907 The Heidelberg jaw is discovered in a sand pit at Mauer, Germany. Belonging to Homo erectus, it is the oldest European hominid fossil discovered to date and thought to be 400,000 years old.
1909 Danish botanist Wilhelm Ludvig Johannsen introduces the term "gene."
1909 English biologist William Bateson publishes Mendel's Principles of Genetics, which introduces Mendelian genetics to the English-speaking world.
1909 German botanist Carl Correns shows that certain hereditary characteristics of plants are determined by factors in the cytoplasm of the female sex cell. It is the first example of non-Mendelian heredity.
1909 Russian-born U.S. chemist Phoebus Levene discovers D-ribose, the five-carbon sugar that forms the basis of RNA.
1910 U.S. geneticist Thomas Hunt Morgan discovers that certain inherited characteristics of the fruit fly Drosophila melanogaster are sex linked. He later argues that because all sex-related characteristics are inherited together they are linearly arranged on the X-chromosome.
1914 German biochemist Fritz Albert Lepmann explains the role of adenosine triphosphate (ATP) as the carrier of chemical energy from the oxidation of food to the energy consumption processes in the cells.
1915 U.S. geneticists Thomas Hunt Morgan, Alfred Sturtevant, Calvin Bridges, and Hermann Muller publish The Mechanism of Mendelian Heredity, which outlines their work on the fruit fly Drosophila demonstrating that genes can be mapped on chromosomes.
1924 Australian-born South African anthropologist Raymond Dart discovers the skull of an early hominid at Tuang, Botswana, which he calls Australopithecus africanus. It is now believed to be one of the oldest human ancestors.
1927 Canadian anthropologist Davidson Black discovers the first specimens of "Beijing man" (Sinanthropous pekinensis), a species of Homo erectus believed to be 300,000 to 400,000 years old, at Choukoutien, China.
1927 U.S. geneticist Hermann Muller uses X-rays to cause mutations in the fruit fly. It permits a greater understanding of the mechanisms of variation.
1930 English geneticist Ronald Fisher publishes The Genetical Theory of Natural Selection in which he synthesizes Mendelian genetics and Darwinian evolution.
1932 U.S. anthropologist George Edward Lewis discovers the jaw of a Miocene ape Ramapithecus in the Siwalik Hills of India. Living about 8–15 million years ago, it is thought to be the oldest human ancestor.
1935 U.S. biochemist Wendell Meredith Stanley shows that viruses are not submicroscopic organisms but are proteinaceous in nature.
1936 Fossil remains of Pithecanthropus (now Homo erectus) found in Java indicate that Homo erectus lived in the area 500,000–1 million years ago.
1937 German-born British biochemist Hans Krebs describes the citric acid cycle in cells, which converts sugars, fats, and proteins into carbon dioxide, water, and energy—the "Krebs cycle."
1944 The role of deoxyribonucleic acid (DNA) in genetic inheritance is first demonstrated by U.S. bacteriologist Oswald Avery, U.S. biologist Colin MacLeod, and U.S. biologist Maclyn McCarthy; it opens the door to the elucidation of the genetic code.
1946 U.S. biologists Max Delbrück and Alfred D Hershey discover recombinant DNA (deoxyribonucleic acid) when they observe that genetic material from different viruses can combine to create new viruses.
1946 U.S. geneticists Joshua Lederberg and Edward Lawrie Tatum pioneer the field of bacterial genetics with their discovery that sexual reproduction occurs in the bacterium Escherichia coli.
1948 Proconsul africanus is discovered by Kenyan anthropologist Louis Leakey in Kenya. It is a Miocene ape that is a possible ancestor of both apes and monkeys.
1948 U.S. biologist Alfred Mirsky discovers ribonucleic acid (RNA) in chromosomes.
1952 English biophysicist Rosalind Franklin uses X-ray diffraction to study the structure of DNA. She suggests that its sugar-phosphate backbone is on the outside—an important clue that leads to the elucidation of the structure of DNA the following year.
1953 U.S. biochemist Stanley Lloyd Miller shows that amino acids can be formed when simulated lightning is passed through containers of water, methane, ammonia, and hydrogen—conditions under which life may have arisen.





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1953 English molecular biologist Francis Crick and U.S. biologist James Watson announce the discovery of the double helix structure of DNA, the basic material of heredity. They also theorize that if the strands are separated then each can form the template for the synthesis of an identical DNA molecule. It is perhaps the most important discovery in biology.
1954 Russian-born U.S. cosmologist George Gamow suggests that the genetic code consists of the order of nucleotide triplets in the DNA molecule.
1955 U.S. geneticists Joshua Lederberg and Norton Zinder discover that some viruses carry part of the chromosome of one bacterium to another; called transduction it becomes an important tool in genetics research.
1956 Romanian-born U.S. biologist George Palade discovers ribosomes, which contain RNA (ribonucleic acid).
1956 Spanish-born U.S. molecular biologist Severo Ochoa discovers polynucleotide phosphorylase, the enzyme responsible for the synthesis of RNA (Ribonucleic acid), which allows him to synthesize RNA.
1956 U.S. biochemist Arthur Kornberg, using radioactively tagged nucleotides, discovers that the bacteria Escherichia coli uses an enzyme, now known as DNA polymerase, to replicate DNA (deoxyribonucleic acid). It allows him to synthesize DNA in the test tube.
1956 U.S. biologists Mahlon Hoagland and Paul Zamecnik discover transfer RNA (ribonucleic acid) which transfers amino acids, the building blocks of proteins, to the correct site on the messenger RNA.
1961 English molecular biologist Francis Crick and South African chemist Sydney Brenner discover that each base triplet on the DNA strand codes for a specific amino acid in a protein molecule.
1961 French biochemists François Jacob and Jacques Monod discover messenger ribonucleic acid (mRNA), which transfers genetic information to the ribosomes, where proteins are synthesized.
1961 Kenyan anthropologist Louis Leakey and English anthropologist Mary Leakey find the first fossilized remains of Homo habilis (''Handy Man") at Olduvai Gorge, Tanganyika (modern Tanzania). Makers of Oldowan stone tools—the oldest stone tools—they lived 1.15 to 1.7 million years ago.
1967 U.S. biochemist Marshall Nirenberg establishes that mammals, amphibians, and bacteria all share a common genetic code.
1967 U.S. scientist Charles Caskey and associates demonstrate that identical forms of messenger RNA produce the same amino acids in a variety of living beings, showing that the genetic code is common to all life forms.
1969 U.S. geneticist Jonathan Beckwith and associates at the Harvard Medical School isolate a single gene for the first time.
1970 U.S. biochemists Howard Temin and David Baltimore separately discover the enzyme reverse transcriptase, which allows some cancer viruses to transfer their RNA to the DNA of their hosts turning them cancerous—a reversal of the common pattern in which genetic information always passes from DNA to RNA.
1970 U.S. geneticist Hamilton Smith discovers type II restriction enzyme that breaks the DNA strand at predictable places, making it an invaluable tool in recombinant DNA technology.
1970 Indian-born U.S. biochemist Har Gobind Khorana assembles an artificial yeast gene from its chemical components.
1972 U.S. paleontologists Stephen Jay Gould and Nils Eldridge propose the punctuated equilibrium model—the idea that evolution progresses in fits and starts rather than at a uniform rate.
1973 Two fossil skulls, both about 1.85 million years old, are discovered at Koobi Fora, Kenya; they have features typical of Australopithecus boisei as well as Homo habilis, confounding classification of early hominid species.
1973 U.S. biochemists Stanley Cohen and Herbert Boyer develop the technique of recombinant DNA (deoxyribonucleic acid). Strands of DNA are cut by restriction enzymes from one species and then inserted into the DNA of another; this marks the beginning of genetic engineering.
1974 U.S. anthropologists Donald Johanson and Maurice Taieb discover the 3.2 million-years-old remains of "Lucy," an adult female hominid classified as Australopithecus afarensis, at Hadar in Ethiopia. About 40% of her skeleton is found and it indicates that she was bipedal.
1975 Kenyan field worker Bernard Ngeneo discovers a Homo erectus skull at Koobi Fora, Kenya, which is estimated to be 1.7 million years old; discovered in the same stratum as Australopithecus boisei, it puts an end to the single species hypothesis, the idea that there has never been more than one hominid species at any point in history.
1975 The gel-transfer hybridization technique for the detection of specific DNA (deoxyribonucleic acid) sequences is developed; it is a key development in genetic engineering.
1976 The first oncogene (cancer-inducing gene) is discovered by U.S. scientists Harold E. Varmus and J. Michael Bishop.
1976 U.S. biochemist Herbert Boyer and venture capitalist Robert Swanson found Genentech in San Francisco, California, the world's first genetic engineering company.
1976 Har Gobind Khorana and his colleagues





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  announce the construction of the first artificial gene to function naturally when inserted into a bacterial cell. This is a major breakthrough in genetic engineering.
1977 English biochemist Frederick Sanger describes the full sequence of 5,386 bases in the DNA (deoxyribonucleic acid) to virus phiX174 in Cambridge, England; the first sequencing of an entire genome.
1977 U.S. scientist Herbert Boyer, of the firm Genentech, fuses a segment of human DNA (deoxyribonucleic acid) into the bacterium Escherichia coli which begins to produce the human protein somatostatin; this is the first commercially produced genetically engineered product.
1981 The U.S. Food and Drug Administration grants permission to Eli Lilley and Co. to market insulin produced by bacteria, the first genetically engineered product to go on sale.
1981 U.S. geneticists J. W. Gordon and F. H. Ruddle of the University of Ohio inject genes from one animal into the fertilized egg of a mouse that develops into mice with the foreign gene in many of the cells; the gene is then passed on to their offspring creating permanently altered (transgenic) animals; it is the first transfer of a gene from one animal species to another.
1982 The Swedish firm Kabivitrum manufactures human growth hormone using genetically engineered bacteria.
1982 U.S. firm Applied Biosystems markets an automated gene sequencer that can sequence 18,000 DNA bases a day, compared with a few hundred a year by hand in the 1970s.
1983 U.S. biologists Andrew Murray and Jack Szostak create the first artificial chromosome; it is grafted onto a yeast cell.
1983 U.S. biochemist Kary Mullis invents the polymerase chain reaction (PCR); a method of multiplying genes or known sections of the DNA molecule a million times without the need for the living cell.
1984 Robert Sinsheimer, the chancellor of the University of California at Santa Cruz, California, proposes that all human genes be mapped; the proposal eventually leads to the development of the Human Genome Project.
1987 The first genetically altered bacteria are released into the environment in the United States; they protect crops against frost.
1987 The U.S. Patent and Trademark Office announces its intention to allow the patenting of animals produced by genetic engineering.
1987 The New York Times announces Dr. Helen Donis-Keller's mapping of all 23 pairs of human chromosomes, allowing the location of specific genes for the prevention and treatment of genetic disorders.
1988 Fossil remains of a modern Homo sapiens are discovered in Israel, dated about 92,000 years ago; they suggest modern humans appeared twice as early as previously thought.
1988 The Human Genome Organization (HUGO) is established in Washington, D.C., United States; scientists announce a project to compile a complete "map" of human genes.
1990 A four-year-old girl in the United States has the gene for adenosine deaminase inserted into her DNA (deoxyribonucleic acid); she is the first human to receive gene therapy.
1991 British geneticists Peter Goodfellow and Robin Lovell-Badge discover the gene on the Y chromosome that determines sex.
1992 The U.S. biotechnology company Agracetus patents transgenic cotton, which has had a foreign gene added to it by genetic engineering.
1992 U.S. biologist Philip Leder receives a patent for the first genetically engineered animal, the oncomouse, which is sensitive to carcinogens.
1994 Bones of the earliest known human ancestor, a hominid named Australopithecus ramidus, are found in Ethiopia and dated at 4.4 million years old.
1994 The first genetically engineered food goes on sale in California and Chicago, Illinois. The "Flavr Savr" tomato is produced by the U.S. biotechnology company Calgene.
1996 U.S. geneticists clone two rhesus monkeys from embryo cells.
1997 Scottish researcher Ian Wilmut of the Roslin Institute in Edinburgh, Scotland, announces that British geneticists have cloned an adult sheep.
1997 Spanish paleoanthropologist Bermúdez de Castro and his team discover the fossilized remains of six individuals belonging to a new human species in a cave in Spain's Atepuerca Mountains. Named Homo antecessor, and about 780,000 years old (the only human fossils found in Europe of that age), they possess a face like Homo sapiens and a jaw and brow similar to the Neanderthals, and are believed to be the ancestors of both modern humans and Neanderthals.
1997 U.S. geneticist Huntington F. Wilard constructs the first artificial human chromosome.
1997 Teams of researchers from Germany and the United States use mitochondrial DNA (deoxyribonucleic acid) extracted from the original Neanderthal fossils, discovered in the Neander Valley near Düsseldorf, Germany, in 1856, to confirm that Neanderthals and modern humans diverged evolutionarily about 600,000 years ago.
1998 Doctors meeting at the World Medical Association's conference in Hamburg, Germany, call for a worldwide ban on human cloning. U.S. president Clinton calls for legislation banning cloning the following day.





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  Avery, Oswald Theodore (1877–1955) Canadian-born U.S. bacteriologist. His work on transformation in bacteria established in 1944 that DNA is responsible for the transmission of heritable characteristics. Avery proved conclusively that DNA was the transforming principle responsible for the development of polysaccharide capsules in unencapsulated bacteria that had been in contact with dead, encapsulated bacteria. This implicated DNA as the basic genetic material of the cell.  
  Bateson, William (1861–1926) English geneticist. Bateson was one of the founders of the science of genetics (a term he introduced), and a leading proponent of Austrian biologist Gregor Mendel's work on heredity. In Material for the Study of Variation (1894) Bateson put forward his theory of discontinuity to explain the long process of evolution. According to this theory, species do not develop in a predictable sequence of very gradual changes but instead evolve in a series of discontinuous jumps. Mendel's work, which he translated and championed, provided him with supportive evidence. Bateson also carried out breeding experiments, and showed that certain traits are consistently inherited together; this phenomenon (called linkage) is now known to result from genes being situated close together on the same chromosome.  
  Beadle, George Wells (1903–1989) U.S. biologist. In 1958 he shared a Nobel prize with Edward L c0016-01.gifTatum and Joshua c0016-01.gifLederberg for his work in biochemical genetics, forming the "one-gene–one-enzyme" hypothesis (a single gene codes for a single kind of enzyme). This concept found wide applications in biology and virtually created the science of biochemical genetics.  
  Berg, Paul (1926– ) U.S. molecular biologist. In 1972, using gene-splicing techniques developed by others, Berg spliced and combined into a single hybrid the c0016-01.gifDNA from an animal tumor virus (SV40) and the DNA from a bacterial virus. For his work on recombinant DNA he shared the 1980 Nobel Prize for Chemistry.  
  In 1956 Berg identified an RNA molecule (later known as a transfer RNA) that is specific to the amino acid methionine. He then perfected a method for making bacteria accept genes from other bacteria. This genetic engineering can be extremely useful for creating strains of bacteria to manufacture specific substances, such as interferon. But there are also dangers: a new, highly virulent pathogenic microorganism might accidentally be created, for example. Berg has therefore advocated restrictions on genetic engineering research.  
  Brenner, Sidney (1927– ) South African scientist, one of the pioneers of genetic engineering. Brenner discovered messenger c0016-01.gifRNA (a link between c0016-01.gifDNA and the c0016-01.gifribosomes in which proteins are synthesized) in 1960.  
  Brenner became engaged in one of the most elaborate efforts in anatomy ever attempted: investigating the nervous system of nematode worms and comparing the nervous systems of different mutant forms of the animal. About a hundred genes are involved in constructing the nervous system of a nematode and most of the mutations that occur affect the overall design of a section of the nervous system.  
  Crick, Francis Harry Compton (1916– ) English molecular biologist. From 1949 he researched the molecular structure of c0016-01.gifDNA, and the means whereby characteristics are transmitted from one generation to another. For this work he was awarded a Nobel prize (with Maurice c0016-01.gifWilkins and James c0016-01.gifWatson) in 1962.  
  Using Wilkins's and others' discoveries, Crick and Watson postulated that DNA consists of a double helix consisting of two parallel chains of alternate sugar and phosphate groups linked by pairs of organic bases. They built molecular models which also explained how genetic information could be coded—in the sequence of organic bases. Crick and Watson published their work on the proposed structure of DNA in 1953. Their model is now generally accepted as correct.  
  Darwin, Charles Robert (1809–1882) English naturalist who developed the modern theory of c0016-01.gifevolution and proposed, with Alfred Russel c0016-01.gifWallace, the principle of c0016-01.gifnatural selection.  
  After research in South America and the Galápagos Islands as naturalist on HMS Beagle during 1831–36, Darwin published On the Origin of Species by Means of Natural Selection or the Preservation of Favoured Races in the Struggle for Life (1859). This book explained the evolutionary process through the principles of natural selection and aroused bitter controversy because it disagreed with the literal interpretation of the Book of Genesis in the Bible.  
  On the Origin of Species also refuted earlier evolutionary theories, such as those of French naturalist J. B. de c0016-01.gifLamarck. Darwin himself played little part in the debates, but his Descent of Man (1871) added fuel to the theological discussion, in which English scientist T. H. c0016-01.gifHuxley and German zoologist Ernst c0016-01.gifHaeckel took leading parts.  
  Darwin's theory of natural selection concerned the variation existing between members of a sexually reproducing population. Those members with variations better fitted to the environment would be more likely to survive and breed, subsequently passing on these favorable characteristics to their offspring. He avoided the issue of human evolution, however, remarking at the end of The Origin of Species that "much light will be thrown on the origin of man and his history." It was not until his publication of The Descent of Man and Selection in Relation to Sex (1871) that Darwin argued that people evolved just like other organisms.  
  Dawkins, (Clinton) Richard (1941– ) British zoologist whose book The Selfish Gene (1976) popularized the theories of sociobiology (social behavior in humans and animals in the context of evolution). In The Blind Watchmaker (1986) he explained the modern theory of evolution.  
  In The Selfish Gene he argued that genes—not individuals, populations, or species—are the driving force of evolution. He suggested an analogous system of cultural transmission in human societies, and proposed the term "mimeme," abbreviated to "meme," as the unit of such a scheme. He considered the idea of God to be a meme with a high survival value.  
  Dobzhansky, Theodosius (1900–1975), originally Feodosy Grigorevich Dobrzhansky, Ukrainian-born U.S. geneticist who established evolutionary genetics as an independent discipline. He showed that genetic variability between individuals of the same species is very high and that this diversity is vital to the process of evolution.  




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  His book Genetics and the Origin of Species (1937) was the first significant synthesis of Darwinian evolutionary theory and Mendelian genetics. Dobzhansky also proved that there is a period when speciation is only partly complete and during which several races coexist.  
  Fischer, Edmond (1920– ) U.S. biochemist who shared the 1992 Nobel Prize for Physiology or Medicine with Edwin Krebs for isolating and describing the action of the enzymes responsible for reversible protein phosphorylation. Reversible phosphorylation is the attachment or detachment of phosphate groups to or from proteins in cells. It is at the heart of a wide range of biological processes ranging from muscle contraction to the regulation of genes.  
  Fisher, Ronald Aylmer (1890–1962) English statistician and geneticist. He modernized Charles Darwin's theory of evolution, thus securing the key biological concept of genetic change by natural selection. Fisher developed several new statistical techniques and, applying his methods to genetics, published The Genetical Theory of Natural Selection (1930).  
  This classic work established that the discoveries of the geneticist Gregor c0016-01.gifMendel could be shown to support Darwin's theory of evolution.  
  Franklin, Rosalind Elsie (1920–1958) English biophysicist whose research on X-ray diffraction of c0016-01.gifDNA crystals helped Francis c0016-01.gifCrick and James D c0016-01.gifWatson to deduce the chemical structure of DNA.  
  Gould, Stephen Jay (1941– ) U.S. paleontologist and writer. In 1972 he proposed the theory of punctuated equilibrium, suggesting that the evolution of species did not occur at a steady rate but could suddenly accelerate, with rapid change occurring over a few hundred thousand years. His books include Ever Since Darwin (1977), The Panda's Thumb (1980), The Flamingo's Smile (1985), and Wonderful Life (1990).  
  Hoagland, Mahlon Bush (1921– ) U.S. biochemist who was the first to isolate transfer RNA (tRNA), a nucleic acid that plays an essential part in intracellular protein synthesis.  
  In the late 1950s Hoagland isolated various types of tRNA molecules from cytoplasm and demonstrated that each type of tRNA can combine with only one specific amino acid. Each tRNA molecule has as part of its structure a characteristic triplet of nitrogenous bases that links to a complementary triplet on a messenger RNA (mRNA) molecule. A number of these reactions occur on the ribosome, building up a protein one amino acid at a time.  
  Holley, Robert William (1922– ) U.S. biochemist who established the existence of transfer RNA (tRNA) and its function. For this work he shared the 1968 Nobel Prize for Physiology or Medicine. At Cornell University Holley obtained evidence for the role of tRNAs as acceptors of activated amino acids. In 1958 he succeeded in isolating the alanine-, tyrosine-, and valene-specific tRNAs from baker's yeast, and eventually Holley and his colleagues succeeded in solving the entire nucleotide sequence of this RNA.  
  Jacob, François (1920– ) biochemist who, with Jacques c0016-01.gifMonod and André Lwoff, pioneered research into molecular genetics and showed how the production of proteins from c0016-01.gifDNA is controlled. They shared the Nobel Prize for Physiology or Medicine in 1965.  
  Jacob began his work on the control of gene action in 1958, working with Lwoff and Monod. It was known that the types of proteins produced in an organism are controlled by DNA, and Jacob focused his research on how the amount of protein is controlled. He performed a series of experiments in which he cultured the bacterium Escherichia coli in various mediums to discover the effect of the medium on enzyme production. He and his team found that there were three types of gene concerned with the production of each specific protein.  
  Khorana, Har Gobind (1922– ) Indian-born U.S. biochemist who in 1976 led the team that first synthesized a biologically active c0016-01.gifgene. In 1968 he shared the Nobel Prize for Physiology or Medicine for research on the chemistry of the genetic code and its function in protein synthesis. Khorana's work provides much of the basis for gene therapy and biotechnology.  
  Khorana systematically synthesized every possible combination of the genetic signals from the four nucleotides known to be involved in determining the genetic code. He showed that a pattern of three nucleotides, called a triplet, specifies a particular amino acid (the building blocks of proteins). He further discovered that some of the triplets provided punctuation marks in the code, marking the beginning and end points of protein synthesis.  
  Kornberg, Arthur (1918– ) U.S. biochemist. In 1956 he discovered the enzyme DNA-polymerase, which enabled molecules of the genetic material DNA to be synthesized for the first time. For this work he shared the 1959 Nobel Prize for Physiology or Medicine. By 1967 he had synthesized a biologically active artificial viral DNA.  
  Krebs, Hans Adolf (1900–1981) German-born British biochemist. He discovered the citric acid cycle, also known as the Krebs cycle, the final pathway by which food molecules are converted into energy in living tissues. For this work he shared the 1953 Nobel Prize for Physiology or Medicine. Knighted in 1958.  
  Krebs first became interested in the process by which the body degrades amino acids. He discovered that nitrogen atoms are the first to be removed (deamination) and are then excreted as urea in the urine. He then investigated the processes involved in the production of urea from the removed nitrogen atoms, and by 1932 he had worked out the basic steps in the urea cycle.  
  Lamarck, Jean Baptiste de (1744–1829) French naturalist. His theory of evolution, known as Lamarckism, was based on the idea that acquired characteristics (changes acquired in an individual's lifetime) are inherited by the offspring, and that organisms have an intrinsic urge to evolve into better-adapted forms. Philosophie zoologique/Zoological Philosophy (1809) tried to show that various parts of the body developed because they were necessary, or disappeared because of disuse when variations in the environment caused a change in habit. If these body changes were inherited over many generations, new species would eventually be produced.  
  Lederberg, Joshua (1925– ) U.S. geneticist who showed that bacteria can reproduce sexually, combining genetic material so that offspring possess characteristics of both parent organisms. In 1958 he shared the Nobel Prize for Physiology or Medicine with George c0016-01.gifBeadle and Edward c0016-01.gifTatum.  




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  Lederberg is a pioneer of genetic engineering, a science that relies on the possibility of artificially shuffling genes from cell to cell. He realized in 1952 that bacteriophages, viruses which invade bacteria, can transfer genes from one bacterium to another, a discovery that led to the deliberate insertion by scientists of foreign genes into bacterial cells.  
  Lehninger, Albert L(ester) (1917–1986) U.S. biochemist. An authority on cellular energy systems, he made a major contribution to enzymology, the bioenergetics of normal and cancer cells, and the results of calcification. In 1948 Lehninger and E P Kennedy discovered that cellular organelles called mitochondria are the main sites of cell respiration.  
  McClintock, Barbara (1902–1992) U.S. geneticist who discovered jumping c0016-01.gifgenes (genes that can change their position on a chromosome from generation to generation). This would explain how originally identical cells take on specialized functions as skin, muscle, bone, and nerve, and also how evolution could give rise to the multiplicity of species. She was awarded the Nobel Prize for Physiology or Medicine in 1983.  
  McClintock's discovery that genes are not stable overturned one of the main tenets of heredity laid down by Gregor c0016-01.gifMendel. It had enormous implications and explained, for example, how resistance to antibiotic drugs can be transmitted between entirely different bacterial types.  
  Mendel, Gregor Johann (1822–1884) Austrian biologist, founder of c0016-01.gifgenetics. His experiments with successive generations of peas gave the basis for his theory of particulate inheritance rather than blending, involving dominant and recessive characters; see c0016-01.gifMendelism. His results, published 1865–69, remained unrecognized until the early 20th century.  
  Mendel formulated two laws now recognized as fundamental laws of heredity: the law of segregation and the law of independent assortment of characters. Mendel concluded that each parent plant contributes a "factor" to its offspring that determines a particular trait and that the pairs of factors in the offspring do not give rise to a blend of traits.  
  Monod, Jacques Lucien (1910–1976) French biochemist who shared the 1965 Nobel Prize for Physiology or Medicine with his coworkers André Lwoff and François c0016-01.gifJacob for research in genetics and microbiology.  
  Working on the way in which genes control intracellular metabolism in microorganisms, Monod and his colleagues postulated the existence of a class of genes (which they called operons) that regulate the activities of the genes that actually control the synthesis of enzymes within the cell. They further hypothesized that the operons suppress the activities of the enzyme-synthesizing genes by affecting the synthesis of messenger c0016-01.gifRNA.  
  Morgan, Thomas Hunt (1866–1945) U.S. geneticist who helped establish that the c0016-01.gifgenes are located on the chromosomes, discovered sex chromosomes, and invented the techniques of genetic mapping. He was the first to work on the fruit fly Drosophila, which has since become a major subject of genetic studies. He was awarded the Nobel Prize for Physiology or Medicine in 1933.  
  Following the rediscovery of Austrian scientist Gregor c0016-01.gifMendel's work, Morgan became interested in the mechanisms involved in heredity, and in 1908 he began his research on the genetics of Drosophila. From his findings he postulated that certain characteristics are sex linked, that the X chromosome carries several discrete hereditary units (genes), and that the genes are linearly arranged on chromosomes. He also demonstrated that sex-linked characters are not invariably inherited together, from which he developed the concept of crossing over and the associated idea that the extent of crossing over is a measure of the spatial separation of genes on chromosomes.  
  Nirenberg, Marshall Warren (1927– ) U.S. biochemist who shared the 1968 Nobel Prize for Physiology or Medicine for his work in deciphering the chemistry of the genetic code. Nirenberg became interested in the way in which the nitrogen bases—adenine (A), cytosine (C), guanine (G), and thymine (T)—specify a particular amino acid. To simplify the task of identifying the RNA triplet (codon) responsible for each amino acid, he used a simple synthetic RNA polymer. He found that certain amino acids could be specified by more than one codon, and that some triplets did not specify an amino acid at all. These "nonsense" triplets signified the beginning or the end of a sequence. He then worked on finding the orders of the letters in the triplets, and obtained unambiguous results for 60 of the possible codons.  
  Ochoa, Severo (1905–1993) Spanish-born U.S. biochemist who discovered an enzyme able to assemble units of the nucleic acid RNA in 1955. For his work toward the synthesis of RNA, Ochoa shared the 1959 Nobel Prize for Physiology or Medicine. Ochoa's early work concerned biochemical pathways in the human body, especially those involving carbon dioxide, but his main research was into nucleic acids and how their nucleotide units are linked, either singly (as in RNA) or to form two helically wound strands (as in DNA). In 1955 Ochoa obtained an enzyme from bacteria that was capable of joining together similar nucleotide units to form a nucleic acid, a type of artificial RNA. Nucleic acids containing exactly similar nucleotide units do not occur naturally, but the method of synthesis used by Ochoa was the same as that employed by a living cell.  
  Sanger, Frederick (1918– ) English biochemist. He was the first person to win a Nobel Prize for Chemistry twice: the first in 1958 for determining the structure of the hormone insulin, and the second in 1980 for work on the chemical structure of genes. Sanger's second Nobel prize was shared with two U.S. scientists, Paul Berg and Walter Gilbert, for establishing methods of determining the sequence of nucleotides strung together along strands of RNA and DNA. He also worked out the structures of various enzymes and other proteins.  
  Schleiden, Matthias Jakob (1804–1881) German botanist who identified the fundamental units of living organisms when, in 1838, he announced that the various parts of plants consist of cells or derivatives of cells. This was extended to animals by Theodor Schwann the following year. The existence of cells had been discovered by British physicist Robert Hooke in 1665, but Schleiden was the first to recognize their importance. He also noted the role of the nucleus in cell division, and the active movement of intracellular material in plant tissues.  
  Schwann, Theodor (1810–1882) German physiologist who, with Matthias Schleiden, is credited with formulating the cell theory, one of the most fundamental concepts in biology. Schwann also did important work on digestion, fermentation, and the study of tissues.  




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  Sturtevant, Alfred Henry (1891–1970) U.S. geneticist who–working with U.S. biologist Thomas Morgan–was the first, in 1911, to map the position of genes on a chromosome.  
  Sturtevant developed methods for mapping gene positions on the chromosomes of mutant Drosophila (fruit flies). In 1911 he produced the first gene map ever derived, showing the positioning of five genes on a Drosophila X chromosome: white-eyed, vermilion-eyed, rudimentary wings, small wings, and yellow body.  
  Sutton, Walter S(tanborough) (1877–1916) U.S. geneticist and surgeon. He used grasshopper cells to prove that chromosomal behavior in meiosis is responsible for observed Mendelian phenomena, an achievement still recognized as classic.  
  Tatum, Edward Lawrie (1909–1975) U.S. microbiologist. For his work on biochemical c0016-01.gifgenetics, he shared the 1958 Nobel Prize for Physiology or Medicine with his coworkers George c0016-01.gifBeadle and Joshua c0016-01.gifLederberg.  
  Beadle and Tatum used X-rays to cause mutations in bread mold, studying particularly the changes in the enzymes of the various mutant strains. This led them to conclude that for each enzyme there is a corresponding gene. From 1945, with Lederberg, Tatum applied the same technique to bacteria and showed that genetic information can be passed from one bacterium to another. The discovery that a form of sexual reproduction can occur in bacteria led to extensive use of these organisms in genetic research.  
  Wallace, Alfred Russel (1823–1913) Welsh naturalist who collected animal and plant specimens in South America and Southeast Asia, and independently arrived at a theory of evolution by natural selection similar to that proposed by Charles c0016-01.gifDarwin.  
  In 1858 Wallace wrote an essay outlining his ideas on evolution and sent it to Darwin, who had not yet published his. Together they presented a paper to the Linnaean Society that year. Wallace's section, entitled "On the Tendency of Varieties to Depart Indefinitely from the Original Type," described the survival of the fittest.  
  Although both thought that the human race had evolved to its present physical form by natural selection, Wallace was of the opinion that humans' higher mental capabilities had arisen from some "metabiological" agency.  
  Warburg, Otto Heinrich (1883–1970) German biochemist who in 1923 devised a manometer (pressure gauge) sensitive enough to measure oxygen uptake of respiring tissue. By measuring the rate at which cells absorb oxygen under differing conditions, he was able to show that enzymes called cytochromes enable cells to process oxygen. He was awarded the Nobel Prize for Physiology or Medicine in 1931.  
  Watson, James Dewey (1928– ) U.S. biologist. His research on the molecular structure of c0016-01.gifDNA and the genetic code, in collaboration with Francis c0016-01.gifCrick, earned him a shared Nobel prize in 1962. Based on earlier works, they were able to show that DNA formed a double helix of two spiral strands held together by base pairs.  
  Crick and Watson published their work on the proposed structure of DNA in 1953, and explained how genetic information could be coded.  
  Crick and Watson envisioned DNA replication occurring by a parting of the two strands of the double helix, each organic base thus exposed linking with a nucleotide (from the free nucleotides within a cell) bearing the complementary base. Thus two complete DNA molecules would eventually be formed by this step-by-step linking of nucleotides, with each of the new DNA molecules comprising one strand from the original DNA and one new strand.  
  Weismann, August Friedrich Leopold (1834–1914) German biologist, one of the founders of c0016-01.gifgenetics. He postulated that every living organism contains a special hereditary substance, the "germ plasm," and in 1892 he proposed that changes to the body do not in turn cause an alteration of the genetic material.  
  This "central dogma" of biology remains of vital importance to biologists supporting the Darwinian theory of evolution. If the genetic material can be altered only by chance mutation and recombination, then the Lamarckian view that acquired bodily changes can subsequently be inherited becomes obsolete.  
  Wilkins, Maurice Hugh Frederick (1916– ) New Zealandborn British molecular biologist. In 1962 he shared the Nobel Prize for Physiology or Medicine with Francis c0016-01.gifCrick and James c0016-01.gifWatson for his work on the molecular structure of nucleic acids, particularly c0016-01.gifDNA, using X-ray diffraction. Studying the X-ray diffraction pattern of DNA, he discovered that the molecule has a double helical structure and passed on his findings to Crick and Watson.  
  Wright, Sewall (1889–1988) U.S. geneticist and statistician. During the 1920s he helped modernize Charles c0016-01.gifDarwin's theory of evolution, using statistics to model the behavior of populations of c0016-01.gifgenes. Wright's work on genetic drift centred on a phenomenon occurring in small isolated colonies where the chance disappearance of some types of gene leads to evolution without the influence of natural selection.  
  acetyl coenzyme A
compound active in processes of metabolism. It is a heat-stable coenzyme with an acetyl group (–COCH
3) attached by sulfur linkage. This linkage is a high-energy bond and the acetyl group can easily be donated to other compounds. Acetyl groups donated in this way play an important part in glucose breakdown as well as in fatty acid and steroid synthesis.
  acquired character
feature of the body that develops during the lifetime of an individual, usually as a result of repeated use or disuse, such as the enlarged muscles of a weightlifter.
  active transport
in cells, the use of energy to move substances, usually molecules or ions, across a membrane.




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any change in the structure or function of an organism that allows it to survive and reproduce more effectively in its environment. In c0016-01.gifevolution, adaptation is thought to occur as a result of random variation in the genetic makeup of organisms coupled with c0016-01.gifnatural selection. Species become extinct when they are no longer adapted to their environment—for instance, if the climate suddenly becomes colder.
  adaptive radiation
in evolution, the formation of several species, with c0016-01.gifadaptations to different ways of life, from a single ancestral type.
  adenosine triphosphate
compound present in cells. See c0016-01.gifATP.
abbreviation for adenosine diphosphate, the chemical product formed in cells when c0016-01.gifATP breaks down to release energy.
one of two or more alternative forms of a c0016-01.gifgene at a given position (locus) on a chromosome, caused by a difference in the c0016-01.gifDNA. Blue and brown eyes in humans are determined by different alleles of the gene for eye color.
  amino acid
building block of proteins; a soluble organic molecule, mainly composed of carbon, oxygen, hydrogen, and nitrogen, containing both a basic amino group (NH
2) and an acidic carboxyl (COOH) group.
process of building up body tissue, promoted by the influence of certain hormones. It is the constructive side of c0016-01.gifmetabolism, as opposed to c0016-01.gifcatabolism.
term describing a structure that has a similar function to a structure in another organism, but not a similar evolutionary path. For example, the wings of bees and of birds have the same purpose—to give powered flight—but have different origins. Compare c0016-01.gifhomologous.
  artificial selection
the selective breeding of individuals that exhibit the particular characteristics that a plant or animal breeder wishes to develop.
abbreviation for adenosine triphosphate, a nucleotide molecule found in all cells. It can yield large amounts of energy, and is used to drive the thousands of biological processes needed to sustain life, growth, movement, and reproduction. Green plants use light energy to manufacture ATP as part of the process of photosynthesis. In animals, ATP is formed by the breakdown of glucose molecules, usually obtained from the carbohydrate component of a diet, in a series of reactions termed c0016-01.gifrespiration. It is the driving force behind muscle contraction and the synthesis of complex molecules needed by individual cells.
any c0016-01.gifchromosome in the cell other than a sex chromosome. Autosomes are of the same number and kind in both males and females of a given species.
  base pair
in biochemistry, the linkage of two base (purine or pyrimidine) molecules in c0016-01.gifDNA. Bases are found in nucleotides, and form the basis of the genetic code.
science concerned with the chemistry of living organisms: the structure and reactions of proteins (such as enzymes), nucleic acids, carbohydrates, and lipids.
synthesis of organic chemicals from simple inorganic ones by living cells—for example, the conversion of carbon dioxide and water to glucose by plants during photosynthesis.
the destructive part of c0016-01.gifmetabolism where living tissue is changed into energy and waste products. It is the opposite of c0016-01.gifanabolism.
substance that alters the speed of, or makes possible, a chemical or biochemical reaction but remains unchanged at the end of the reaction. c0016-01.gifEnzymes are natural biochemical catalysts. In practice most catalysts are used to speed up reactions.
  cell division
the process by which a cell divides, either c0016-01.gifmeiosis, associated with sexual reproduction, or c0016-01.gifmitosis, associated with growth, cell replacement, or repair. Both forms involve the duplication of DNA and the splitting of the nucleus.
  cell membrane, or plasma membrane,
thin layer of protein and fat surrounding cells that controls substances passing between the cytoplasm and the intercellular space. The cell membrane is semipermeable, allowing some substances to pass through and some not.
  central dogma
in genetics and evolution, the fundamental belief that c0016-01.gifgenes can affect the nature of the physical body, but that changes in the body (c0016-01.gifacquired character, for example, through use or accident) cannot be translated into changes in the genes.
structure found in the c0016-01.gifcells of animals that plays a role in the processes of c0016-01.gifmeiosis and c0016-01.gifmitosis (cell division).
part of the c0016-01.gifchromosome where there are no c0016-01.gifgenes. Under the microscope, it usually appears as a constriction in the strand of the chromosome, and is the point at which the spindle fibers are attached during c0016-01.gifmeiosis and c0016-01.gifmitosis (cell division).
cell body that contains the c0016-01.gifcentrioles. During cell division the centrosomes organize the microtubules to form the spindle that divides the chromosomes into daughter cells.
structure in a cell nucleus that carries the c0016-01.gifgenes. Each chromosome consists of one very long strand of DNA, coiled and folded to produce a compact body. The point on a chromosome where a particular gene occurs is known as its locus. Most higher organisms have two copies of each chromosome, together known as a homologous pair (they are c0016-01.gifdiploid) but some have only one (they are c0016-01.gifhaploid).
singular cilium, small hairlike organs on the surface of some cells, particularly the cells lining the upper respiratory tract.
in genetics, the segment of c0016-01.gifDNA that is required to synthesize a complete polypeptide chain. It is the molecular equivalent of a c0016-01.gifgene.
  citric acid cycle
another term for the c0016-01.gifKrebs cycle.
in classification, a group of related c0016-01.giforders. For example, all mammals belong to the class Mammalia and all birds to the class Aves. Related classes are grouped together in a c0016-01.gifphylum.




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an exact replica. In genetics, any one of a group of genetically identical cells or organisms. An identical twin is a clone; so, too, are bacteria living in the same colony.
in genetics, the failure of a pair of alleles, controlling a particular characteristic, to show the classic recessive-dominant relationship. Instead, aspects of both alleles may show in the phenotype.
in genetics, a triplet of bases (see c0016-01.gifbase pair) in a molecule of DNA or RNA that directs the placement of a particular amino acid during the process of protein (polypeptide) synthesis. There are 64 codons in the c0016-01.gifgenetic code.
evolution of those structures and behaviors within a species that can best be understood in relation to another species. For example, insects and flowering plants have evolved together: insects have produced mouthparts suitable for collecting pollen or drinking nectar, and plants have developed chemicals and flowers that will attract insects to them.
  concentration gradient
change in the concentration of a substance from one area to another.
  continuous variation
the slight difference of an individual character, such as height, across a sample of the population. Although there are very tall and very short humans, there are also many people with an intermediate height. The same applies to weight. Continuous variation can result from the genetic make-up of a population, or from environmental influences, or from a combination of the two.
  convergent evolution
the independent evolution of similar structures in species (or other taxonomic groups) that are not closely related, as a result of living in a similar way. Thus, birds and bats have wings, not because they are descended from a common winged ancestor, but because their respective ancestors independently evolved flight.
  crossing over
exchange of genetic material that occurs during c0016-01.gifmeiosis. While the chromosomes are lying alongside each other in pairs, each partner may twist around the other and exchange corresponding chromosomal segments. It is a form of genetic c0016-01.gifrecombination, which increases variation and thus provides the raw material of evolution.
the part of the cell outside the c0016-01.gifnucleus. Strictly speaking, this includes all the c0016-01.giforganelles (mitochondria, chloroplasts, and so on), but often cytoplasm refers to the jellylike matter in which the organelles are embedded.
matrix of protein filaments and tubules that occurs within the cytoplasm. It gives the cell a definite shape, transports vital substances around the cell, and may also be involved in cell movement.
irreversible changes occurring in the structure of proteins such as enzymes, usually caused by changes in pH or temperature, by radiation or chemical treatments. An example is the heating of egg albumen resulting in solid egg white.
  deoxyribonucleic acid
full name of c0016-01.gifDNA.
spontaneous and random movement of molecules or particles in a fluid (gas or liquid) from a region in which they are at a high concentration to a region of lower concentration, until a uniform concentration is achieved throughout.
  dihybrid inheritance
in genetics, a pattern of inheritance observed when two characteristics are studied in succeeding generations. The first experiments of this type, as well as in c0016-01.gifmonohybrid inheritance, were carried out by Austrian biologist Gregor Mendel using pea plants.
having paired c0016-01.gifchromosomes in each cell. In sexually reproducing species, one set is derived from each parent, the c0016-01.gifgametes, or sex cells, of each parent being c0016-01.gifhaploid (having only one set of chromosomes) due to c0016-01.gifmeiosis (reduction cell division).
sugar made up of two monosaccharides or simple sugars. Sucrose, C
12H22O11, or table sugar, is a disaccharide.
abbreviation for deoxyribonucleic acid, giant, complex molecule that contains, in chemically coded form, the information needed for a cell to make proteins. DNA is a ladderlike double-stranded c0016-01.gifnucleic acid, and is organized into c0016-01.gifchromosomes.
in genetics, the masking of one allele (an alternative form of a gene) by another allele. For example, if a c0016-01.gifheterozygous person has one allele for blue eyes and one for brown eyes, his or her eye color will be brown. The allele for blue eyes is described as c0016-01.gifrecessive and the allele for brown eyes as dominant.
  endoplasmic reticulum (ER)
a membranous system of tubes, channels, and flattened sacs that form compartments within eukaryotic cells.
in ecology, the sum of conditions affecting a particular organism, including physical surroundings, climate, and influences of other living organisms.
biological c0016-01.gifcatalyst produced in cells, and capable of speeding up the chemical reactions necessary for life.
one of the two major groupings into which all organisms are divided. Included are all organisms, except bacteria and cyanobacteria (blue-green algae), which belong to the c0016-01.gifprokaryote grouping.
in genetics, a sequence of bases in c0016-01.gifDNA that codes for a protein. Exons make up only 2% of the body's total DNA. The remainder is made up of c0016-01.gifintrons. During RNA processing the introns are cut out of the molecule and the exons spliced together.
in classification, a group of related genera (see c0016-01.gifgenus). Family names are not printed in italic (unlike genus and species names), and by convention they all have the ending -idae (animals) or -aceae (plants and fungi). Related families are grouped together in an c0016-01.giforder.
in the broadest sense, a mixture of c0016-01.giflipids—chiefly triglycerides (lipids containing three c0016-01.giffatty acid molecules linked to a molecule of glycerol). More specifically, the term refers to a lipid mixture that is solid at room temperature (20°C); lipid mixtures that are liquid at room temperature are called oils.




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  fatty acid, or carboxylic acid,
organic compound consisting of a hydrocarbon chain, up to 24 carbon atoms long, with a carboxyl group (–COOH) at one end.
in genetic theory, a measure of the success with which a genetically determined character can spread in future generations. By convention, the normal character is assigned a fitness of one, and variants (determined by other c0016-01.gifalleles) are then assigned fitness values relative to this. Those with fitness greater than one will spread more rapidly and will ultimately replace the normal allele; those with fitness less than one will gradually die out.
small hairlike organ on the surface of certain cells.
cell that functions in sexual reproduction by merging with another gamete to form a zygote. Examples of gametes include sperm and egg cells. In most organisms, the gametes are haploid (they contain half the number of chromosomes of the parent), owing to reduction division or c0016-01.gifmeiosis.
unit of inherited material; a strand of c0016-01.gifDNA that encodes a particular protein. In higher organisms, genes are located on the c0016-01.gifchromosomes.
  gene amplification
technique by which selected DNA from a single cell can be duplicated indefinitely until there is a sufficient amount to analyze by conventional genetic techniques.
  gene pool
total sum of c0016-01.gifalleles (variants of c0016-01.gifgenes) possessed by all the members of a given population or species alive at a particular time.
  genetic engineering
deliberate manipulation of genetic material by biochemical techniques. It is often achieved by the introduction of new c0016-01.gifDNA, usually by means of a virus or c0016-01.gifplasmid.
the full complement of c0016-01.gifgenes carried by a single (haploid) set of c0016-01.gifchromosomes. The term may be applied to the genetic information carried by an individual or to the range of genes found in a given species. The human genome is made up of about 80,000 genes.
the particular set of c0016-01.gifalleles (variants of genes) possessed by a given organism. The term is usually used in conjunction with c0016-01.gifphenotype, which is the product of the genotype and all environmental effects.
plural genera, group of c0016-01.gifspecies with many characteristics in common. Thus all doglike species (including dogs, wolves, and jackals) belong to the genus Canis (Latin "dog"). Species of the same genus are thought to be descended from a common ancestor species. Related genera are grouped into c0016-01.giffamilies.
  Golgi apparatus, or Golgi body,
stack of flattened membranous sacs found in the cells of c0016-01.gifeukaryotes.
having a single set of c0016-01.gifchromosomes in each cell. Most higher organisms are c0016-01.gifdiploid—that is, they have two sets—but their gametes (sex cells) are haploid.
the transmission of traits from parent to offspring.
term describing an organism that has two different c0016-01.gifalleles for a given trait. In c0016-01.gifhomozygous organisms, by contrast, both chromosomes carry the same allele.
term describing an organ or structure possessed by members of different taxonomic groups (for example, species, genera, families, orders) that originally derived from the same structure in a common ancestor. The wing of a bat, the arm of a monkey, and the flipper of a seal are homologous because they all derive from the forelimb of an ancestral mammal.
term describing an organism that has two identical c0016-01.gifalleles for a given trait. Individuals homozygous for a trait always breed true; that is, they produce offspring that resemble them in appearance when bred with a genetically similar individual. c0016-01.gifRecessive alleles are only expressed in the homozygous condition. See also c0016-01.gifheterozygous.
  inclusive fitness
in genetics, the success with which a given variant (or allele) of a c0016-01.gifgene is passed on to future generations by a particular individual, after additional copies of the allele in the individual's relatives and their offspring have been taken into account.
  intron, or junk DNA,
in genetics, a sequence of bases in c0016-01.gifDNA that carries no genetic information. Introns make up 98% of DNA (the rest is made up of c0016-01.gifexons). Their function is unknown.
the set of c0016-01.gifchromosomes characteristic of a given species. It is described as the number, shape, and size of the chromosomes in a single cell of an organism. In humans for example, the karyotype consists of 46 chromosomes, in mice 40, crayfish 200, and in fruit flies 8.
in genetics, the association between two or more genes that tend to be inherited together because they are on the same chromosome. The closer together they are on the chromosome, the less likely they are to be separated by crossing over (one of the processes of c0016-01.gifrecombination) and they are then described as being "tightly linked."
membrane-enclosed structure, or organelle, inside a c0016-01.gifcell, principally found in animal cells. Lysosomes contain enzymes that can break down proteins and other biological substances. They play a part in digestion, and in the white blood cells known as phagocytes the lysosome enzymes attack ingested bacteria.
process of cell division in which the number of c0016-01.gifchromosomes in the cell is halved. It only occurs in c0016-01.gifeukaryotic cells, and is part of a life cycle that involves sexual reproduction because it allows the genes of two parents to be combined without the total number of chromosomes increasing.
in genetics, the theory of inheritance originally outlined by Austrian biologist Gregor Mendel. He suggested that, in sexually reproducing species, all characteristics are inherited through indivisible "factors" (now identified with c0016-01.gifgenes) contributed by each parent to its offspring.
the chemical processes of living organisms enabling them to grow and to function. It involves a constant alternation of building up complex molecules (anabolism) and breaking them down (catabolism).
tiny tubes found in almost all cells with a nucleus. They help to define the shape of a cell by forming




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  scaffolding for cilia and they also form the fibers of mitotic spindle (see c0016-01.gifmitosis).  
singular mitochondrion, membrane-enclosed organelles within eukaryotic cells, containing enzymes responsible for energy production during c0016-01.gifaerobic respiration.
the process of cell division by which identical daughter cells are produced. During mitosis the DNA is duplicated and the chromosome number doubled, so new cells contain the same amount of DNA as the original cell.
  molecular biology
study of the molecular basis of life, including the biochemistry of molecules such as DNA, RNA, and proteins, and the molecular structure and function of the various parts of living cells.
  monohybrid inheritance
pattern of inheritance seen in simple c0016-01.gifgenetics experiments, where the two animals (or two plants) being crossed are genetically identical except for one gene.
or simple sugar, carbohydrate that cannot be hydrolysed (split) into smaller carbohydrate units. Examples are glucose and fructose, both of which have the molecular formula C
change in the genes produced by a change in the c0016-01.gifDNA that makes up the hereditary material of all living organisms. Mutations, the raw material of evolution, result from mistakes during replication (copying) of DNA molecules. Only a few improve the organism's performance and are therefore favoured by c0016-01.gifnatural selection. Mutation rates are increased by certain chemicals and by radiation.
  natural selection
the process whereby gene frequencies in a population change through certain individuals producing more descendants than others because they are better able to survive and reproduce in their environment.
the modern theory of c0016-01.gifevolution, built up since the 1930s by integrating the 19th-century English scientist Charles Darwin's theory of evolution through natural selection with the theory of genetic inheritance founded on the work of the Austrian biologist Gregor Mendel.
  nucleic acid
complex organic acid made up of a long chain of c0016-01.gifnucleotides, present in the nucleus and sometimes the cytoplasm of the living cell. The two types, known as c0016-01.gifDNA (deoxyribonucleic acid) and c0016-01.gifRNA (ribonucleic acid), form the basis of heredity.
structure found in the nucleus of eukaryotic cells. It produces the RNA that makes up the c0016-01.gifribosomes, from instructions in the DNA.
organic compound consisting of a purine (adenine or guanine) or a pyrimidine (thymine, uracil, or cytosine) base linked to a sugar (deoxyribose or ribose) and a phosphate group. c0016-01.gifDNA and c0016-01.gifRNA are made up of long chains of nucleotides.
the central, membrane-enclosed part of a eukaryotic cell, containing the DNA. The nucleus controls the function of the cell by determining which proteins are produced within it.
gene carried by a virus that induces a cell to divide abnormally, giving rise to a cancer. Oncogenes arise from mutations in genes (proto-oncogenes) found in all normal cells. They are usually also found in viruses that are capable of transforming normal cells to tumor cells.
mouse that has a human c0016-01.gifoncogene (gene that can cause certain cancers) implanted into its cells by genetic engineering. Such mice are used to test anticancer treatments and were patented within the United States by Harvard University in 1988, thereby protecting its exclusive rights to produce the animal and profit from its research.
group of genes that are found next to each other on a chromosome, and are turned on and off as an integrated unit. They usually produce enzymes that control different steps in the same biochemical pathway.
in classification, a group of related c0016-01.giffamilies. Related orders are grouped together in a c0016-01.gifclass.
discrete and specialized structure in a living cell; organelles include mitochondria, chloroplasts, lysosomes, ribosomes, and the nucleus.
movement of water through a selectively permeable membrane separating solutions of different concentrations.
molecule comprising two or more c0016-01.gifamino acid molecules (not necessarily different) joined by peptide bonds, whereby the acid group of one acid is linked to the amino group of the other (–CO.NH). The number of amino acid molecules in the peptide is indicated by referring to it as a di-, tri-, or polypeptide (two, three, or many amino acids).
in genetics, visible traits, those actually displayed by an organism. The phenotype is not a direct reflection of the c0016-01.gifgenotype because some alleles are masked by the presence of other, dominant alleles (see c0016-01.gifdominance). The phenotype is further modified by the effects of the environment (for example, poor nutrition stunts growth).
any c0016-01.giflipid consisting of a glycerol backbone, a phosphate group, and two long chains. Phospholipids are found everywhere in living systems as the basis for biological membranes.
plural phyla, major grouping in biological classification. Mammals, birds, reptiles, amphibians, fishes, and tunicates belong to the phylum Chordata; the phylum Mollusca consists of snails, slugs, mussels, clams, squid, and octopuses; the phylum Porifera contains sponges; and the phylum Echinodermata includes starfish, sea urchins, and sea cucumbers. Related phyla are grouped together in a c0016-01.gifkingdom; phyla are subdivided into c0016-01.gifclasses.
process whereby a given gene influences several different observed characteristics of an organism. For example, in the fruit fly Drosophila the vestigial gene reduces the size of wings, modifies the halteres, changes the number of egg strings in the ovaries, and changes the direction of certain bristles.
  polymerase chain reaction (PCR)
technique developed during the 1980s to clone short strands of DNA from the c0016-01.gifgenome of an organism. The aim is to produce enough of the DNA to be able to sequence and identify it.




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in genetics, the coexistence of several distinctly different types in a population (groups of animals of one species). Examples include the different blood groups in humans, different color forms in some butterflies, and snail shell size, length, shape, color, and stripiness.
in genetics, possessing three or more sets of chromosomes in cases where the normal complement is two sets (c0016-01.gifdiploid). Polyploidy arises spontaneously and is common in plants (mainly among flowering plants), but rare in animals. Many crop plants are natural polyploids, including wheat, which has four sets of chromosomes per cell (durum wheat) or six sets (common wheat).
long-chain carbohydrate made up of hundreds or thousands of linked simple sugars (monosaccharides) such as glucose and closely related molecules.
  population genetics
the branch of genetics that studies the way in which the frequencies of different c0016-01.gifalleles (alternative forms of a gene) in populations of organisms change, as a result of natural selection and other processes.
organism whose cells lack nuclei and other organelles (specialized segregated structures such as mitochondria, and chloroplasts). The prokaryotes comprise only the bacteria and cyanobacteria (blue-green algae); all other organisms are (eukaryotes.
complex, biologically important substance composed of amino acids joined by c0016-01.gifpeptide bonds.
  punctuated equilibrium model
evolutionary theory developed by Niles Eldredge and U.S. paleontologist Stephen Jay Gould in 1972 to explain discontinuities in the fossil record. It claims that periods of rapid change alternate with periods of relative stability (stasis), and that the appearance of new lineages is a separate process from the gradual evolution of adaptive changes within a species.
  recessive gene
in genetics, an c0016-01.gifallele (alternative form of a gene) that will show in the c0016-01.gifphenotype (observed characteristics of an organism) only if its partner allele on the paired chromosome is similarly recessive. Such an allele will not show if its partner is dominant, that is if the organism is c0016-01.gifheterozygous for a particular characteristic. Alleles for blue eyes in humans and for shortness in pea plants are recessive.
  recombinant DNA
in genetic engineering, c0016-01.gifDNA formed by splicing together genes from different sources into new combinations.
in genetics, any process that recombines, or ''shuffles," the genetic material, thus increasing genetic variation in the offspring.
production of copies of the genetic material DNA; it occurs during cell division (c0016-01.gifmitosis and c0016-01.gifmeiosis). Most mutations are caused by mistakes during replication.
  restriction enzyme
bacterial c0016-01.gifenzyme that breaks a chain of c0016-01.gifDNA into two pieces at a specific point; used in c0016-01.gifgenetic engineering. The point along the DNA chain at which the enzyme can work is restricted to places where a specific sequence of base pairs occurs. Different restriction enzymes will break a DNA chain at different points. The overlap between the fragments is used in determining the sequence of base pairs in the DNA chain.
  ribonucleic acid
full name of c0016-01.gifRNA.
the protein-making machinery of the cell. Ribosomes are located on the endoplasmic reticulum (ER) of eukaryotic cells, and are made of proteins and a special type of c0016-01.gifRNA, ribosomal RNA.
abbreviation for ribonucleic acid, nucleic acid involved in the process of translating the genetic material c0016-01.gifDNA into proteins. It is usually single-stranded, unlike the double-stranded DNA, and consists of a large number of nucleotides strung together, each of which comprises the sugar ribose, a phosphate group, and one of four bases (uracil, cytosine, adenine, or guanine).
in biochemistry, determining the sequence of chemical subunits within a large molecule. Techniques for sequencing amino acids in proteins were established in the 1950s, insulin being the first for which the sequence was completed. The Human Genome Project is attempting to determine the sequence of the 3 billion base pairs within human c0016-01.gifDNA.
  sex chromosome
chromosome that differs between the sexes and that serves to determine the sex of the individual. In humans, females have two X chromosomes and males have an X and a Y chromosome.
  sex linkage
in genetics, the tendency for certain characteristics to occur exclusively, or predominantly, in one sex only. Human examples include red-green color blindness and hemophilia, both found predominantly in males. In both cases, these characteristics are c0016-01.gifrecessive and are determined by genes on the c0016-01.gifX chromosome.
distinguishable group of organisms that resemble each other or consist of a few distinctive types (as in c0016-01.gifpolymorphism), and that can all interbreed to produce fertile offspring. Species are the lowest level in the system of biological classification.
in biochemistry, a compound or mixture of compounds acted on by an enzyme.
science of naming and identifying species, and determining their degree of relatedness.
process by which the information for the synthesis of a protein is transferred from the c0016-01.gifDNA strand on which it is carried to the messenger c0016-01.gifRNA strand involved in the actual synthesis.
the transfer of genetic material between cells by an infectious mobile genetic element such as a virus. Transduction is used in c0016-01.gifgenetic engineering to produce new varieties of bacteria.
  transgenic organism
plant, animal, bacterium, or other living organism which has had a foreign gene added to it by means of c0016-01.gifgenetic engineering.
the process by which proteins are synthesized. During translation, the information coded as a sequence of nucleotides in messenger c0016-01.gifRNA is transformed into a




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  sequence of amino acids in a peptide chain. The process involves the "translation" of the c0016-01.gifgenetic code. See also c0016-01.giftranscription.  
or jumping gene, segment of DNA able to move within or between chromosomes. Transposons trigger changes in gene expression by shutting off genes or causing insertion c0016-01.gifmutations.
difference between individuals of the same species, found in any sexually reproducing population. Variations may be almost unnoticeable in some cases, obvious in others, and can concern many aspects of the organism. Typically, variations in size, behavior, biochemistry, or coloring may be found. The cause of the variation is genetic (that is, inherited), environmental, or more usually a combination of the two. The origins of variation can be traced to the recombination of the genetic material during the formation of the gametes, and, more rarely, to mutation.
  wild type
in genetics, the naturally occurring gene for a particular character that is typical of most individuals of a given species, as distinct from new genes that arise by mutation.
  X chromosome
the larger of the two sex chromosomes, the smaller being the c0016-01.gifY chromosome. These two chromosomes are involved in sex determination. Females have two X chromosomes, males have an X and a Y. Genes carried on the X chromosome produce the phenomenon of (sex linkage.
  Y chromosome
the smaller of the two sex chromosomes. In male mammals it occurs paired with the other type of sex chromosome (X), which carries far more genes. The Y chromosome is the smallest of all the mammalian chromosomes and is considered to be largely inert (that is, without direct effect on the physical body). There are only 20 genes discovered so far on the human Y chromosome, much fewer than on all other human chromosomes. See also c0016-01.gifsex determination.
  Further Reading  
  Attenborough, David Life on Earth (1979)  
  Baldwin, E. Dynamic Aspects of Biochemistry (1967)  
  Bateson, William Mendel's Principles of Heredity (1909)  
  Bernal, J. D. The Physical Basis of Life (1951)  
  Bloomfield, Molly M. Chemistry and the Living Organism (1991)  
  Brent, Peter Charles Darwin: A Man of Enlarged Curiosity (1981)  
  Brooks, James Origins of Life (1985)  
  Carey, George W. Chemistry and Wonders of the Human Body (1921)  
  Chaplin, Martin F., and Bucke, Christopher Enzyme Technology (1990)  
  Clark, David, and Russell, Lonnie Molecular Biology Made Simple (1997)  
  Clark, R. W. The Survival of Charles Darwin (1985)  
  Cronin, Helena The Ant and the Peacock (1991)  
  Darwin, Charles The Origin of Species (1859)  
  Dawkins, Richard The Blind Watchmaker (1986)  
  de Beer, Gavin Charles Darwin: A Scientific Biography (1963)  
  Dennett, Daniel Darwin's Dangerous Idea (1995)  
  Dyson, Freeman John Origins of Life (1985)  
  Dyson, George Darwin Amongst the Machines (1998)  
  Edey, Maitland A., and Johanson, Donald C. Blueprints (1989)  
  Fichman, M. Alfred Russel Wallace (1981)  
  Fortey, Richard A. Life: An Unauthorized Biography (1997)  
  Gersch, Jack The Matter of Life (1995)  
  Goodwin, Brian How the Leopard Changed Its Spots (1994)  
  Gould, Stephen Jay The Panda's Thumb (1980)  
  Johanson, Don, and Edey, Matt Lucy: The beginnings of humankind (1981)  
  Jones, Steve The Language of the Genes (1993)  
  King, Barry (ed.) Cell Biology (1986)  
  Leakey, Richard The Origin of Humankind (1994)  
  Lehninger, A. L. Principles of Biochemistry (1993)  
  Lewin, Roger Human Evolution: An Illustrated Guide (1993), The Origin of Modern Humans (1993), Bones of Contention (1997), Principles of Human Evolution (1998)  
  Lewontin, Richard The Doctrine of DNA (1991)  
  Mason, S. Chemical Evolution (1993)  
  Maynard Smith, John The Theory of Evolution (1958)  
  Mayr, Ernst, The Growth of Biological Thought (1982)  
  McKinney, H. Lewis Wallace and Natural Selection (1972)  
  Mines, Allan H. Respiratory Physiology (1993)  
  Nelkin, Dorothy, and Lindee, M. Susan The DNA Mystique (1995)  
  Nisbet, Euan George Living Earth: A Short History of Life and Its Home (1991)  
  Orel, V. Mendel (1984)  
  Prange, Henry D. Respiratory Physiology: Understanding Gas Exchange (1996)  
  Ridley, Mark The Problems of Evolution (1986)  
  Ritvo, Harriet The Platypus and the Mermaid and other Figments of the Classifying Imagination (1997)  
  Rose, Steven The Chemistry of Life (1991)  
  Rose, Steven; Kamin, Leon; and Lewontin, Richard Not in our Genes (1984)  




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  Ruse, Michael Darwinism Defended (1982)  
  Russell, Peter J. Genetics (1998)  
  Snedden, Robert Life (1994)  
  Stringer, Chris, and Gamble, Clive In Search of the Neanderthals (1993)  
  Suckling, C. J. Enzyme Chemistry: Impact and Applications (1990)  
  Tattersall, lan (ed.) and others Encyclopaedia of Human Evolution and Prehistory (1988)  
  Tudge, Colin The Engineer in the Garden (1993)  
  Watson, James D. The Double Helix (1968)  
  West, John B. Respiratory Physiology: the Essentials (1995)  
  Wilkie, Tom Perilous Knowledge (1993)  
  Williams, George C. Adaptation and Natural Selection (1966)