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DNA

  (dē'ĕn-ā')

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DNA

double-helix DNA model A. adenine T. thymine C. cytosine G. guanine D. deoxyribose P. phosphate (Academy Artworks)

A nucleic acid that carries the genetic information in the cell and is capable of self-replication and synthesis of RNA. DNA consists of two long chains of nucleotides twisted into a double helix and joined by hydrogen bonds between the complementary bases adenine and thymine or cytosine and guanine. The sequence of nucleotides determines individual hereditary characteristics.

[D(EOXYRIBO)N(UCLEIC) A(CID).]

The abbreviation stands for deoxyribonucleic acid, a double-stranded nucleic acid, in which the two strands twist together to form a helix. The strands consist of sugar and phosphate groups, the sugars being attached to a base — adenine, thymine, guanine, or cytosine. In DNA the bases pair to form a ladder-like structure, with adenine paired with thymine and guanine with cytosine. DNA forms the basis of inheritance in all organisms, except viruses, the DNA code being sufficient to build and control the organism. DNA is located in the nucleus of all cells; it is the substance of the chromosomes that separate out from the nucleus when cells divide, and it carries the genes, each of which is a segment of a DNA molecule. A small fraction of total DNA is present in mitochondria that codes for a few mitochondrial proteins. This DNA is passed down the female line from the mitochondria contained in the ovum.

— Alan W. Cuthbert

Bibliography

• Watson, J. D. (1968). The double helix. Weidenfeld and Nicolson, London

See cell; genetics, human.

Deoxyribonucleic acid, the genetic material in the nuclei of all cells. Chemically it is a polymer of deoxyribonucleotides; the purine bases adenine and guanine, and the pyrimidine bases thymidine and cytidine, linked to deoxyribose phosphate. The sugar-phosphates form a double-stranded helix, with the bases paired internally. See also nucleic acids.

DNA (deoxyribonucleic acid) was discovered in the late 1800s, but its role as the material of heredity was not elucidated for fifty years after that. It occupies a central and critical role in the cell as the genetic information in which all the information required to duplicate and maintain the organism. All information necessary to maintain and propagate life is contained within a linear array of four simple bases: adenine, guanine, thymine, and cytosine.

DNA was first described as a monotonously uniform helix, generally called B-DNA. However, we now know that DNA can adopt many different shapes and conformations. Moreover, many of these alternative shapes have biological importance. Thus, the DNA is not simply an informational repository, from which information flows through RNA into proteins. Rather, structural information exists within the specific sequence patterns of the bases. This structural information dictates the interaction of DNA with proteins to carry out processes of DNA replication, transcription into RNA, and repair of errors or damage to the DNA.

The Components of Dna

DNA is composed of purine (adenine and guanine) and pyrimidine (cytosine and thymine) bases, each connected through a ribose sugar to a phosphate backbone. Many variations are possible in the chemical structure of the bases and the sugar, and in the structural relationship of the base to the sugar that result in differences in helical shape and form. The most common DNA helix, B-DNA, is a double helix of two DNA strands with about 10.5 base pairs per helical turn.

Bases and Base Pairs

The four bases found in DNA are shown in Figures 1 and 2. The purines and pyrimidines are the informational molecules of the genetic blueprint for the cell. The two sides of the helix are held together by hydrogen bonds between base pairs. Hydrogen bonds are weak attractions between a hydrogen atom on one side and an oxygen or nitrogen atom on the other. Hydrogen atoms of amino groups serve as the hydrogen bond donor while the carbonyl oxygens and ring nitrogens serve as hydrogen bond acceptors. The specific location of hydrogen bond donor and acceptor groups gives the bases their specificity for hydrogen bonding in unique pairs. Thymine (T) pairs with adenine (A) through two hydrogen bonds, and cytosine (C) pairs with guanine (G) through three hydrogen bonds (Figure 2). T does not normally pair with G, nor does C normally pair with A.

Deoxyribose Sugar

In DNA the bases are connected to a β-D-2-deoxyribose sugar with a hydrogen atom at the 2′ ("two prime") position. The sugar is a very dynamic part of the DNA molecule. Unlike the nucleotide bases, which are planar and rigid, the sugar ring is easily bent and twisted into various conformations (which exist in different structural forms of DNA). In canonical B-DNA, the accepted and most common form of DNA, the sugar configuration is known as C2′ endo.

Nucleosides and Nucleotides

The term "nucleoside" refers to a base and sugar. "Nucleotide," on the other hand, refers to the base, sugar, and phosphate group (Figure 1). A bond, called the glycosidic bond, holds the base to the sugar and the 3′-5′ ("three prime-five prime") phosphodiester bond holds the individual nucleotides together. Nucleotides are joined from the 3′ carbon of the sugar in one nucleotide to the 5′ carbon of the sugar of the adjacent nucleotide. The 3′ and the 5′ ends are chemically very distinct and have different reactive properties. During DNA replication, new nucleotides are added only to the 3′ OH end of a DNA strand. This fact has important implications for replication.

The Structure of Double-Stranded Dna

As mentioned above, the two individual strands are held together by hydrogen bonds between individual T·A and C·G base pairs. In DNA, the distance between the atoms involved is 2.8 to 2.95 angstroms (10⁻¹⁰ meters). While individually weak, the large number of hydrogen bonds along a DNA chain provides sufficient stability to hold the two strands together.

The stabilization of duplex (double-stranded) DNA is also dependent on base stacking. The planar, rigid bases stack on top of one another, much like a stack of coins. Since the two purine.pyrimidine pairs (A.T and C.G) have the same width, the bases stack in a rather uniform fashion. Stacking near the center of the helix affords protection from chemical and environmental attack. Both hydrophobic interactions and van der Waal's forces hold bases together in stacking interactions. About half the stability of the DNA helix comes from hydrogen bonding, while base stacking provides much of the rest.

Double-stranded DNA in its canonical B-form is a right-handed helix formed by two individual DNA strands aligned in an antiparallel fashion (a right-handed helix, when viewed on end, twists clockwise going away from the viewer). Antiparallel DNA has the two strands organized in the opposite polarity, with one strand oriented in the 5′-3′ direction and the other oriented in the 3′-5′ direction.

In the right-handed B-DNA double helix, the stacked base pairs are separated by about 3.24 angstroms with 10.5 base pairs forming one helical turn (360°), which is 35.7 angstroms in length. Two successive base pairs, therefore, are rotated about 34.3° with respect to each other. The width of the helix is 20 angstroms. An idealized model of the double helix is shown in Figure 3. As can be seen, the organization of the bases creates a major groove and a minor groove.

Adenine and thymine are said to be complementary, as are cytosine and guanine. Complementary means "matching opposite." The shapes and charges of adeninne and thymine complement each other, so that they attract one another and link up (as do cytosine and guanine). Indeed, one entire strand of duplex DNA is complementary to the opposing strand. During replication, the two strands unwind, and each serves as a template for formation of new complementary strand, so that replication ends with two exact double-stranded copies.

Alternative Dna Conformations

While the vast majority of the DNA exists in the canonical B-DNA form, DNA can adopt an amazing array of alternative structures. This is the result of certain particular sequence arrangements of DNA and, in many cases, energy in the DNA double helix from DNA supercoiling, the property of DNA in which the double helix, in a high-energy state, becomes twisted around itself. Some alternative DNA conformations identified are shown in Figure 4.

Unwound Dna

Since A·T base pairs contain two hydrogen bonds and C·G base pairs contain three, A+T-rich tracts are less thermally stable that C+G-rich tracts in DNA. Under denaturing conditions (heat or alkali), the DNA begins to "melt" (separate), and unwound regions of DNA will form, and it is the A+T-rich sequences that melt first. In addition, in the presence of superhelical energy (a high-energy state of DNA resulting from its supercoiling, which is the natural form of DNA in the chromosomes of most organisms), A+T-rich regions can unwind and remain unwound under conditions normally found in the cell. Such sites often provide places for DNA replication proteins to enter DNA to begin the process of chromosome duplication.

Cruciform Structures

DNA sequences are said to be palindromic when they contain inverted repeat symmetry, as in the sequence GGAATTAATTCC, reading from the 5′ to the 3′ end. Palindromic sequences can form intramolecular bonds (within a single strand), rather than the normal intermolecular (between the two complementary strands), hydrogen bonds. To form cruciforms ("cross-shaped"), the DNA must form a small unwound structure, and then base pairs must begin to form within each individual strand, thus forming the four-stranded cruciform structure.

Slipped-Strand Dna

Slipped-strand DNA structures can form within direct repeat DNA sequences, such as (CTG)_n·(CAG)_n and (CGG)_n·(CCG)_n (where "n" denotes a variable number of repetitions). They form following denaturation, after the strands become unwound, and during renaturation, when the strands come back together. To form slipped-strand DNA, the opposite strands come together in an out-of-alignment fashion, during renaturation. Expansion of such triplet repeats are features of certain neurological diseases.

Intermolecular Triplex Dna

Three-stranded, or triplex DNA, can form within tracts of polypurine.polypyrimidine sequence, such as (GAA)_n·(TTC)_n. Purines, with their two-ring structures, have the potential to form hydrogen bonds with a second base, even while base paired in the canonical A·T and G·C configurations. This second type of base pair is called a Hoogsteen base pair, and it can form in the major groove (the top of the base pair representations in Figure 2). Pyrimidines can only pair with a single other base, and thus a long Pu·Py tract must be present for triplex DNA formation. The important factor for triplex DNA formation is the presence of an extended purine tract in a single DNA strand. The third-strand base-pairing code is as follows: A can pair with A or T; G can pair with a protonated C (C⁺) or G.

Intramolecular Triplex Dna

When a Pu·Py tract exists that has mirror repeat symmetry (5′ GAAGAG-GAGAAG 3′), an intramolecular triplex can form, in which half of the Pu.Py tract unwinds and one strand wraps into the major groove, forming a triplex. The structure in Figure 4 shows the pyrimidine strand (CTT) pairing with the purine strand (GAA) of a canonical DNA duplex. In an intramolecular triplex, one strand of the unwound region remains unpaired, as shown.

Quadruplex Dna

DNA sequences containing runs of G·C base pairs can form quadruplex, or four-stranded DNA, in which the four DNA strands are held together by Hoogsteen hydrogen bonds between all four guanines. The four guanines are aligned in a plane, and the successive rings of guanines are stacked one upon another.

Left-Handed Z-Dna

Alternating runs of (CG)_n·(CG)_n or (TG)_n·(CA)_n dinucleotides in DNA, under superhelical tension or high salt (more than 3 M NaCl) (M, moles per liter) can adopt a left-handed helix called Z-DNA. In this form, the two DNA strands become wrapped in a left-handed helix, which is the opposite sense to that of canonical B-DNA. This can occur within a small region of a larger right-handed B-DNA molecule, creating two junctions at the B-Z transition region.

Curved Dna

DNA containing tracts of (A)_3-4·(T)_3-4 (that is, runs of three or four bases of A in one strand and a similar run of T in the other) spaced at 10-base pair intervals can adopt a curved helix structure.

In summary, DNA can exist in a very stable, right-handed double helix, in which the genetic information is very stable. Certain DNA sequences can also adopt alternative conformations, some of which are important regulatory signals involved in the genetic expression or replication of the DNA.

Bibliography

Sinden, Richard R. DNA Structure and Function. San Diego: Academic Press, 1994.

—Richard R. Sinden

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DNA double helix. A. Molecular model of DNA. The molecules include (1) hydrogen, (2) oxygen (3) … (credit: © Merriam-Webster Inc.)

One of two types of nucleic acid (the other is RNA); a complex organic compound found in all living cells and many viruses. It is the chemical substance of genes. Its structure, with two strands wound around each other in a double helix to resemble a twisted ladder, was first described (1953) by Francis Crick and James D. Watson. Each strand is a long chain (polymer) of repeating nucleotides: adenine (A), guanine (G), cytosine (C), and thymine (T). The two strands contain complementary information: A forms hydrogen bonds (see hydrogen bonding) only with T, C only with G. When DNA is copied in the cell, the strands separate and each serves as a template for assembling a new complementary strand; this is the key to stable heredity. DNA in cells is organized into dense protein-DNA complexes (see nucleoprotein) called chromosomes. In eukaryotes these are in the nucleus, and DNA also occurs in mitochondria and chloroplasts (if any). Prokaryotes have a single circular chromosome in the cytoplasm. Some prokaryotes and a few eukaryotes have DNA outside the chromosomes in plasmids. See also Rosalind Franklin; genetic engineering; mutation; Maurice Wilkins.

For more information on DNA, visit Britannica.com.

Dna (deoxyribonucleic acid) is a nucleic acid that carries genetic information. The study of DNA launched the science of Molecular Biology, transformed the study of genetics, and led to the cracking of the biochemical code of life. Understanding DNA has facilitated Genetic Engineering, the genetic manipulation of various organisms; has enabled cloning, the asexual reproduction of identical copies of genes and organisms; has allowed for genetic fingerprinting, the identification of an individual by the distinctive patterns of his or her DNA; and made possible the use of Genetics to predict, diagnose, prevent, and treat disease.

Discovering Dna

In the late nineteenth century, biologists noticed structural differences between the two main cellular regions, the nucleus and the cytoplasm. The nucleus attracted attention because short, stringy objects appeared, doubled, then disappeared during the process of cell division. Scientists began to suspect that these objects, dubbed chromosomes, might govern heredity. To understand the operation of the nucleus and the chromosomes, scientists needed to determine their chemical composition.

Swiss physiologist Friedrich Miescher first isolated "nuclein"—DNA—from the nuclei of human pus cells in 1869. Although he recognized nuclein as distinct from other well-known organic compounds like fats, proteins, and carbohydrates, Miescher remained unsure about its hereditary potential. Nuclein was renamed nucleic acid in 1889, and for the next forty years, biologists debated the purpose of the compound.

In 1929, Phoebus Aaron Levene, working with yeast at New York's Rockefeller Institute, described the basic chemistry of DNA. Levene noted that phosphorus bonded to a sugar (either ribose or deoxyribose, giving rise to the two major nucleic acids, RNA and DNA), and supported one of four chemical "bases" in a structure he called a nucleotide. Levene insisted that nucleotides only joined in four-unit-long chains, molecules too simple to transmit hereditary information.

Levene's conclusions remained axiomatic until 1944, when Oswald Avery, a scientist at the Rockefeller Institute, laid the groundwork for the field of molecular genetics. Avery continued the 1920s-era research of British biologist Fred Griffiths, who worked with pneumococci, the bacteria responsible for pneumonia. Griffiths had found that pneumococci occurred in two forms, the disease-causing S-pneumococci, and the harmless R-pneumococci. Griffiths mixed dead S-type bacteria with live R-type bacteria. When rats were inoculated with the mixture, they developed pneumonia. Apparently, Griffiths concluded, something had transformed the harmless R-type bacteria into their virulent cousin. Avery surmised that the transforming agent must be a molecule that contained genetic information. Avery shocked himself, and the scientific community, when he isolated the transforming agent and found that it was DNA, thereby establishing the molecular basis of heredity.

Dna's Molecular Structure

Erwin Chargaff, a biochemist at Columbia University, confirmed and refined Avery's conclusion that DNA was complex enough to carry genetic information. In 1950, Chargaff reported that DNA exhibited a phenomenon he dubbed a complementary relationship. The four DNA bases—adenine, cytosine, guanine, and thymine (A, C, G, T, identified earlier by Levene)—appeared to be paired. That is, any given sample of DNA contained equal amounts of G and C, and equal amounts of A and T; guanine was the complement to cytosine, as adenine was to thymine. Chargaff also discovered that the ratio of GC to AT differed widely among different organisms. Rather than Levene's short molecules, DNA could now be reconceived as a gigantic macromolecule, composed of varying ratios of the base complements strung together. Thus, the length of DNA differed between organisms.

Even as biochemists described DNA's chemistry, molecular physicists attempted to determine DNA's shape. Using a process called X-ray crystallography, chemist Rosalind Franklin and physicist Maurice Wilkins, working together at King's College London in the early 1950s, debated whether DNA had a helical shape. Initial measurements indicated a single helix, but later experiments left Franklin and Wilkins undecided between a double and a triple helix. Both Chargaff and Franklin were one step away from solving the riddle of DNA's structure. Chargaff understood base complementarity but not its relation to molecular structure; Franklin understood general structure but not how complementarity necessitated a double helix.

In 1952, an iconoclastic research team composed of an American geneticist, James Watson, and a British physicist, Francis Crick, resolved the debate and unlocked DNA's secret. The men used scale-model atoms to construct a model of the DNA molecule. Watson and Crick initially posited a helical structure, but with the bases radiating outward from a dense central helix. After meeting with Chargaff, Watson and Crick learned that the GC and AT ratios could indicate chemical bonds; hydrogen atoms could bond the guanine and cytosine, but could not bond either base to adenine or thymine. The inverse also proved true, since hydrogen could bond adenine to thymine. Watson and Crick assumed these weak chemical links and made models of the nucleotide base pairs GC and AT. They then stacked the base-pair models one atop the other, and saw that the phosphate and sugar components of each nucleotide bonded to form two chains with one chain spinning "up" the molecule, the other spinning "down" the opposite side. The resulting DNA model resembled a spiral staircase—the famous double helix.

Watson and Crick described their findings in an epochal 1953 paper published in the journal Nature. Watson and Crick had actually solved two knotty problems simultaneously: the structure of DNA and how DNA replicated itself in cell division—an idea they elaborated in a second path breaking paper in Nature. If one split the long DNA molecule at the hydrogen bonds between the bases, then each half provided a framework for assembling its counterpart, creating two complete molecules—the doubling of chromosomes during cell division. Although it would take another thirty years for crystallographic confirmation of the double helix, Crick, Watson, and Rosalind Franklin's collaborator Maurice Wilkins shared the 1962 Nobel Prize in physiology or medicine (Franklin had died in 1958). The study of molecular genetics exploded in the wake of Watson and Crick's discovery.

Once scientists understood the structure of DNA molecules, they focused on decoding the DNA in chromosomes—determining which base combinations created structural genes (those genes responsible for manufacturing amino acids, the building blocks of life) and which combinations created regulator genes (those that trigger the operation of structural genes). Between 1961 and 1966, Marshall Nirenberg and Heinrich Matthaei, working at the National Institutes of Health, cracked the genetic code. By 1967, scientists had a complete listing of the sixty-four three-base variations that controlled the production of life's essential twenty amino acids. Researchers, however, still lacked a genetic map precisely locating specific genes on individual chromosomes. Using enzymes to break apart or splice together nucleic acids, American scientists, like David Baltimore, helped develop recombinant DNA or genetic engineering technology in the 1970s and 1980s.

Genetic engineering paved the way for genetic map-ping and increased genetic control, raising a host of political and ethical concerns. The contours of this debate have shifted with the expansion of genetic knowledge. In the 1970s, activists protested genetic engineering and scientists decried for-profit science; thirty years later, protesters organized to fight the marketing of genetically modified foods as scientists bickered over the ethics of cloning humans. Further knowledge about DNA offers both promises and problems that will only be resolved by the cooperative effort of people in many fields—medicine, law, ethics, social policy, and the humanities—not just molecular biology.

Dna and American Culture

Like atomic technology, increased understanding of DNA and genetics has had both intended and unintended consequences, and it has captured the public imagination. The popular media readily communicated the simplicity and elegance of DNA's structure and action to nonscientists. Unfortunately, media coverage of advances in DNA technology has often obscured the biological complexity of these developments. Oversimplifications in the media, left uncorrected by scientists, have allowed DNA to be invoked as a symbol for everything from inanimate objects to the absolute essence of human potential.

DNA's biological power has translated into great cultural power as the image of the double helix entered the iconography of America after 1953. As Dorothy Nellkin and M. Susan Lindee have shown, references to DNA and the power of genetics are ubiquitous in modern culture. Inanimate objects like cars are advertised as having "a genetic advantage." Movies and television dramas have plots that revolve around DNA, genetic technology, and the power of genetics to shape lives. Humorists use DNA as the punch line of jokes to explain the source of human foibles. Consumer and popular culture's appropriation of DNA to signify fine or poor quality has merged with media oversimplifications to give rise to a new wave of hereditarian thinking in American culture.

The DNA technology that revolutionized criminology, genealogy, and medicine convinced many Americans that DNA governed not only people's physical development, but also their psychological and social behavior. Genetic "fingerprints" that allow forensics experts to discern identity from genetic traces left at a crime scene, or that determine ancestralties by sampling tissue from long-dead individuals, have been erroneously touted as foolproof and seem to equate peoples' identities and behavior with their DNA. Genomic research allows scientists to identify genetic markers that indicate increased risk for certain diseases. This development offers hope for preventive medicine, even as it raises the specter of genetic discrimination and renewed attempts to engineer a eugenic master race. In the beginning of the twenty-first century, more scientists began to remind Americans that DNA operates within a nested series of environments—nuclear, cellular, organismic, ecological, and social—and these conditions affect DNA's operation and its expression. While DNA remains a powerful cultural symbol, people invoke it in increasingly complex ways that more accurately reflect how DNA actually influences life.

Without question, in the 131 years spanning Miescher's isolation of nuclein, Crick and Watson's discovery of DNA's structure, and the completion of the human genome, biologists have revolutionized humanity's understanding of, and control over, life itself. American contributions to molecular biology rank with the harnessing of atomic fission and the landing of men on the moon as signal scientific and technological achievements.

Bibliography

Chargaff, Erwin. Heraclitean Fire?: Sketches from a Life before Nature. New York: Rockefeller University Press, 1978. Bitter but provocative.

Judson, Horace Freeland. The Eighth Day of Creation: Makers of the Revolution in Biology. New York: Simon and Schuster, 1979. Readable history of molecular biology.

Kay, Lily E. Who Wrote the Book of Life?: A History of the Genetic Code. Stanford, Calif.: Stanford University Press, 2000.

Kevles, Daniel J., and Leroy Hood, eds. The Code of Codes: Scientific and Social Issues in the Human Genome Project. Cambridge, Mass.: Harvard University Press, 1992.

Lagerkvist, Ulf. DNA Pioneers and Their Legacy. New Haven, Conn.: Yale University Press, 1998.

Nelkin, Dorothy, and M. Susan Lindee. The DNA Mystique: The Gene as Cultural Icon. New York: W. H. Freeman, 1995. Excellent cultural interpretation of DNA in the 1990s.

Watson, James D. The Double-Helix. New York: Atheneum, 1968. Crotchety account of discovery.

Watson, James D., and F. H. C. Crick. "Molecular Structure of Nucleic Acid: A Structure for Deoxyribonucleic Acid." Nature 171 (1953): 737–738.

—Gregory Michael Dorr

Because of the uniqueness of every human's DNA and the ubiquity of DNA in cells, this genetic molecule has become an important tool for the identification of individuals, both in forensics and security applications. Deoxyribonucleic acid (DNA) consists of two twisted strands of polymers, made up of mononucleotide units. Each nucleotide is composed of three separate parts: a 2-deoxyribose sugar ("2-deoxy-" because the hydroxyl or -OH group of the ribose sugar is missing from the second carbon position on the sugar ring), a phosphate, and one of the four bases: adenine (A), guanine (G), cytosine (C), thymine (T). The deoxyribose sugar and phosphate are linked by phosphodiester bridges in such a way as to form an unbranched polynucleotide chain. According to the Watson-Crick model, which was published in 1953, the DNA molecule consists of two such polynucleotide chains which are complementary but not identical and which spiral around an imaginary common axis. The two strands are antiparallel, meaning that the phosphodiester links between the deoxyribose units read in opposite directions designated 5' to 3' on one chain and 3' to 5' on the other. The bases, which are perpendicular to the helix axis, protrude at regular intervals from the two spiral sugar phosphate strands, and reach into the interior of the helix. The strands are annealed together by hydrogen bonds between the bases of opposite strands and for correct annealing to occur a purine (adenine or guanine) on one strand must pair with a pyrimidine (thymine or cytosine) on the other. Within the constraints of the double helix, hydrogen bonds can only form between adenine and thymine (A:T) and between guanine and cytosine (G:C). Through this pairing, the arrangement of bases along one strand determines that of the other and the genetic information is thus coded in these base sequences.

The most commonly described DNA structure is that of the right-handed Watson-Crick double helix, also known as B-DNA, which has a diameter of 20Å. The double helix is not symmetrical and has a broad groove and a narrow groove between the chains, known respectively as the major and minor grooves. Adjacent bases are separated by 3.4Å along the helix axis and related by a rotation of 36° which causes the helix structure to repeat after 10 residues on each chain, that is at intervals of 34Å. DNA is, however, a dynamic molecule whose structure can vary and there are two other commonly found DNA conformations, each with slightly different dimensions.

The DNA molecule contains all of the genetic information for every organism. Within a cell, DNA is organized into long strands called chromosomes. Every chromosome contains many thousands of different genes. A gene is a functional segment of DNA that codes for a specific protein. During protein synthesis, a portion of DNA is translated into a complementary strand of ribonucleic acid (RNA), which is further transcribed into a sequence of amino acids. A sequence of three nucleotides is required to code for one amino acid and chains of amino acids are further modified outside the nucleus of the cell into the proteins. There are approximately 50,000 different types of proteins in the human body and they either perform tasks or synthesize molecules required for the biological activity that sustains life. The DNA in every individual, therefore, is the source of information the directs all of the biological functions in the body.

The DNA molecule is inherited by every cell and every individual. In asexual reproduction, the DNA in chromosomes is unwound and duplicated before the cell divides. Both daughter cells receive exact copies of the parent cell's DNA. In sexual reproduction, a portion of the DNA is inherited from both the female and the male parent. In humans, there are 23 pairs of chromosomes in the genome. During meiosis, which forms the sex cells or gametes (the egg in females and the sperm in males), the chromosomal pairs separate and each gamete receives 23 unpaired chromosomes. When a sperm fertilizes an egg, its 23 unpaired chromosomes are paired with the 23 unpaired chromosomes in the egg and the resulting zygote contains a unique set of paired chromosomes.

The molecule that carries genetic information in all living systems (see genetic code). The DNA molecule is formed in the shape of a double helix from a great number of smaller molecules (see nucleotides). The workings of the DNA molecule provide the most fundamental explanation of the laws of genetics.

DNA acts in three important way. First, when a cell divides, the DNA uncoils, and each strand creates a new partner from the surrounding material — a process called replication. The two cells that result from the cell division have the same DNA as the original (see mitosis). Second, in sexual reproduction, each parent contributes one of the two strands in the DNA of the offspring. Third, inside the cell, the DNA governs the production of proteins and other molecules essential to cell function.

Today it is common knowledge that DNA, a nucleic acid, directs the development of cells. Scientists gradually learned about DNA in a curiously twisted fashion that is common in science. For one thing, the discovery of DNA required progress on three separate fronts: cytology (the study of cells through a microscope), genetics, and chemistry.

After Gregor Mendel's laws of heredity were rediscovered in 1900, considerable interest developed in what causes heredity. The fundamental structures involved -- the chromosomes -- had been discovered and studied by Walther Flemming in the 1880s, but no one knew that they were connected to heredity. They were just long thin structures that appeared when cells were stained during cell division. Also, Friederich Miescher had discovered nucleic acids in cell nuclei as early as 1869, but they were not connected either to heredity or to chromosomes -- although Miescher's later discovery that salmon sperm are almost entirely nucleic acid should have been a clue to the connection.

In 1907 Thomas Hunt Morgan, who was somewhat skeptical about genetics, began to use fruit flies in breeding experiments. Within a short time he found that Mendel's laws worked, but also that some inherited characteristics appeared to be linked together. These linkages behaved as if the units of heredity, the genes, lined up in long rows. A suitable long thin part of the cell that could physically contain the genes was the chromosome, as had earlier been suggested on other grounds by August Weismann. By 1911 Morgan was able to show that genes strung along the chromosomes are the agents of heredity.

While this development was occurring on the genetic front, there was also some progress being made in chemistry. In 1909 Phoebus Aaron Theodor Levene was the first to determine that nucleic acids contain a sugar, ribose. Twenty years later, he found that other nucleic acids contain a different sugar, deoxyribose. Hence, there are two types of nucleic acid: ribonucleic acid (RNA) and deoxyribonucleic acid (DNA). Levene also worked out the other compounds that were in RNA and DNA. This chemistry was then explored in detail in the 1930s by Alexander Todd.

Chromosomes, like other cell structures, contain proteins. They also contain DNA. Proteins were known to be complex molecules that are biologically very active, so everyone thought that genes must be proteins -- until 1944 when Oswald Avery and coworkers showed that hereditary characteristics could be induced by pure DNA, without a protein involved.

By the early 1950s a few scientists from different fronts were tackling the problem of understanding DNA. Among these was Linus Pauling, who was at the time probably the most accomplished chemist. In 1951 Pauling, working with Robert Corey, determined that the structure of a class of proteins is a helix, which is a three-dimensional spiral. This was the first determination of the physical structure of a large biological molecule. At about this time, Pauling turned to the study of DNA, hoping to discover its structure as well.

In England, there were several scientists interested in the structure of DNA. Maurice Wilkins and Rosalind Franklin were doing X-ray diffraction studies of DNA in hopes of elucidating its structure. Diffraction studies had proved successful in analyzing crystal structures, and DNA could be crystallized.

Another English scientist interested in the subject was Francis Crick, a 35-year-old graduate student. With an undergraduate degree in physics, he too would have liked to do X-ray diffraction studies; but English custom kept him from competing with Wilkins and Franklin.

A fourth interested scientist was James Watson, an American. Watson was working as a postgraduate student, trying to learn about genetics from studying organisms. But he realized that the solution to the problem was more likely on the chemical front, so he abandoned what he was doing and applied for work in X-ray diffraction. He was lucky to be taken on at the same Cambridge laboratory where Francis Crick was pursuing his degree, not far from London, where Wilkins and Franklin worked.

News of Pauling's discovery of a helical structure in proteins set all the English group (except -- at first -- Franklin) thinking that DNA might be a helix as well. Alec Stokes, who was working with Wilkins, was the first to think DNA might be a helix, an idea he had developed when he first saw the diffraction studies. Wilkins thought it might be several helices twisted together.

Watson and Crick decided to try using the method by which Pauling had found the helix in proteins. He had stuck together models of the subunits of the molecule, rather as one puts a tinker toy set together. The models need to be constructed so that they fit together according to Pauling's theory of the chemical bond. Watson and Crick acquired a copy of Pauling's 1939 book on the chemical bond and came up with a model for DNA of three helices twisted together. But when they showed it to Wilkins and Franklin, Franklin pointed out that it disagreed with her diffraction data and had other deficiencies as well.

Watson gradually established to his and Crick's satisfaction that DNA does have a helical structure. Crick figured out that the bases in DNA are always paired in the same way. Franklin insisted on the correct location of the sugars.

Meanwhile, Pauling produced two versions of his model of DNA. It contained three twisted helices and was clearly wrong. One of the best chemists of the century had made a mistake in his chemistry.

After another false step, Watson finally built a model that incorporated two helices, paired bases, and the sugar structure recommended by Franklin. Crick did calculations that showed that this model was feasible. Wilkins and Franklin produced X-ray diffraction calculations that confirmed the structure. On a visit to Cambridge, Pauling agreed. The true nature of DNA had finally been discovered.