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Deoxyribonucleic acid (DNA) is a nucleic acid that contains the genetic instructions for the development and function of living things. All known cellular life and some viruses contain DNA. In eukaryotes such as animals and plants, the DNA is stored inside the cell nucleus, while in prokaryotes such as bacteria and archaea, the DNA is in the cell's cytoplasm.

The main role of DNA in the cell is the long-term storage of information. It is often compared to a blueprint, since it contains the information to construct other components of the cell, such as proteins and RNA molecules. The DNA segments that encode for proteins and RNA are called genes. However, not all DNA belongs to a gene; some DNA serves a structural purpose or is involved in regulating the expression of genes. Unlike enzymes, DNA does not act directly on other molecules; rather, various proteins and RNA molecules act on DNA and copy it in DNA replication, or transcribe it into RNA.

DNA is a long polymer of simple units called nucleotides, which are held together by a backbone made of sugars and phosphate groups. This backbone carries four types of molecules called bases and it is the sequence of these four bases that encodes information. The major function of DNA is to encode the sequence of amino acid residues in proteins, using the genetic code. To read the genetic code, cells make a copy of a stretch of DNA in the nucleic acid RNA. These RNA copies can then used to direct protein synthesis, but they can also be used directly as parts of ribosomes or spliceosomes.

Physical and chemical properties

Structure

DNA is a long polymer made from repeating units called nucleotides. The DNA chain is 22 to 26 angstroms wide and one nucleotide unit is 3.3 angstroms long. Although these repeating units are very small, DNA polymers can be enormous molecules containing millions of nucleotides. For instance, the largest human chromosome is 220 million base pairs long.

In living organisms, DNA does not usually exist as a single molecule, but instead as a tightly-associated pair of molecules. These two long strands entwine like vines, in the shape of a double helix (see the illustration above). The nucleotide repeats contain both the backbone of the molecule, which holds the chain together, and a base, which interacts with the other DNA strand in the helix. In general, a base linked to a sugar is called a nucleoside and a base linked to a sugar and one or more phosphate groups is called a nucleotide. If multiple nucleotides are linked together, as in DNA, this polymer can be referred to as a polynucleotide.

The backbone of the DNA strand is made from alternating sugars and phosphates. The sugar in DNA is the pentose (five carbon) sugar 2-deoxyribose. The sugars are joined together by phosphate groups that join the third and fifth carbon atoms in the sugar rings. These asymmetric bonds mean a strand of DNA has a direction. In a double helix the direction of the nucleotides in one strand is opposite to their direction in the other strand. This arrangement of DNA strands is called antiparallel. The asymmetric ends of a strand of DNA bases are referred to as the 5' (five prime) and 3' (three prime) ends. The major difference between DNA and RNA is the sugar, with 2-deoxyribose being replaced by the alternative pentose sugar ribose in RNA.

The DNA double helix is held together by hydrogen bonds between the bases attached to the two strands. These nucleotides contains the backbone of the molecule, which holds the chain together, and a base, which interacts with the other DNA strand in the helix. The bases are adenine (abbreviated A), cytosine (C), guanine (G) and thymine (T). These four bases are shown below and are attached to the sugar/phosphate to form the complete nucleotide, as shown for adenosine monophosphate.

Bases found in DNA and the structure of a nucleotide

The double helix is a right-handed spiral. As the DNA strands wind around each other, they leave gaps between each set of phosphate backbones, revealing the sides of the bases inside (see animation). There are two of these grooves twisting around the surface of the double helix: one groove is 22 angstroms wide and the other 12 angstroms wide. The larger groove is called the major groove, while the smaller, narrower groove is called the minor groove. The narrowness of the minor groove means that the edges of the bases are more accessible in the major groove. As a result, proteins like transcription factors that can bind to specific sequences in double-stranded DNA usually read the sequence by making contacts to the sides of the bases exposed in the major groove.

Base pairing

There are two types of bases in DNA, adenine and guanine are fused five and six-membered heterocyclic compounds called purines, while cytosine and thymine are six-membered rings called pyrimidines. A fifth pyrimidine base called uracil (U), replaces thymine in RNA and differs from thymine by lacking a methyl group on its ring. Uracil is normally only found in DNA as a breakdown product of cytosine, but a very rare exception to this rule is a bacterial virus called PBS1 that contains uracil in its DNA.

Each type of base on one strand forms a bond with just one type of base on the other strand. This is called complementary base pairing. Here, purines form hydrogen bonds to pyrimidines, with A bonding only to T, and C bonding only to G. As a result of this complementarity, the whole double-stranded sequence can be described by the sequence on one of the strands. This arrangement of two nucleotides joined together across the double helix is called a base pair. In a double helix, the two strands are also held together by forces generated by the hydrophobic effect and pi stacking, but these forces are not affected by the sequence of the DNA. As hydrogen bonds are not covalent they can be broken and rejoined relatively easily. The two strands of DNA in a double helix can therefore be pulled apart like a zipper, either by mechanical force or high temperatures. This reversible and specific interaction between complementary base pairs is critical for all the functions of DNA in living organisms.

The two types of base pairs form different numbers of hydrogen bonds, AT forming two hydrogen bonds, and GC forming three hydrogen bonds (see figures, right). The GC base-pair is therefore stronger than the AT base pair. As a result, it is both the percentage of GC base pairs and the overall length of a DNA double helix that determine the strength of the association between the two strands of DNA. Long DNA helices with a high GC content have strongly interacting strands, while short helices with high AT content have weakly interacting strands. Parts of the DNA double helix that need to separate easily, such as the TATAAT Pribnow box in promoters, tend to have sequences with a high AT content, making the strands easier to pull apart. In the laboratory, the strength of this interaction can be measured by finding the temperature required to break the hydrogen bonds, their melting temperature (also called T_m value).

Sense and antisense

DNA is copied into RNA by RNA polymerase enzymes that only work in the 5' to 3' direction. A DNA sequence is called "sense" if its sequence is copied by these enzymes and then translated into protein. The sequence on the opposite strand is complementary to the sense sequence and is therefore called the "antisense" sequence. Both sense and antisense sequences can exist on the same strand of DNA. In both prokaryotes and eukaryotes, antisense sequences are transcribed but the functions of these RNAs are not entirely clear. One proposal is that antisense RNAs are involved in regulating gene expression through RNA-RNA base pairing.

A few DNA sequences in prokaryotes and eukaryotes, and more in plasmids and viruses, blur the distinction made above between sense and antisense strands by having overlapping genes. In these cases, some DNA sequences do double duty, encoding one protein when read 5' to 3' along one strand, and a second protein when read in the opposite direction (still 5' to 3') along the other strand. In bacteria, this overlap may be involved in the regulation of gene transcription. While in viruses, overlapping genes increase the amount of information that can be encoded within the small viral genome.

Single-stranded DNA

In some viruses DNA appears in a non-helical, single-stranded form. Because many of the DNA repair mechanisms of cells work only on paired bases, viruses that carry single-stranded DNA genomes mutate more frequently than they would otherwise. As a result, such species may adapt more rapidly to avoid extinction. The result would not be so favorable in more complicated and more slowly replicating organisms, however, which may explain why only viruses carry single-stranded DNA. These viruses presumably also benefit from the lower cost of replicating one strand versus two.

Supercoiling

DNA can be twisted like a rope in a process called DNA supercoiling. Normally, with DNA in its "relaxed" state a strand circles the axis of the double helix once every 10.4 base pairs, but if the DNA is twisted the strands become more tightly or more loosely wound. If the DNA is twisted in the direction of the helix this is positive supercoiling and the bases are held more tightly together. If they are twisted in the opposite direction this is negative supercoiling and the bases come apart more easily. In nature, most DNA has slight negative supercoiling, as a result of the action of enzymes called topoisomerases. These enzymes are also needed to relieve the twisting stresses introduced into DNA strands during processes such as transcription and DNA replication.

Alternative helix geometries

The DNA helix can assume one of three slightly different geometries, of which the "B" form described above is most common under the conditions found in cells. The two other known double-helical forms of DNA, called A and Z, differ modestly in their geometry and dimensions. The A form is a wider right-handed spiral, with a shallow and wide minor groove and a narrower and deeper major groove. The A form occurs in dehydrated samples of DNA, such as those used in x-ray crystallography experiments, and possibly in hybrid pairings of DNA and RNA strands. Segments of DNA where the bases have been methylated may undergo a larger change in conformation and adopt the Z geometry. Here, the strands turn about the helical axis in a left-handed spiral, a mirror image of the B form.

Biological function

File:Nucleosome (opposites attracts).JPG

DNA contains the genetic information that is inherited by an organism. This information is encoded by the sequence of base pairs in the DNA of the organism's genome. This DNA contains genes, areas that regulate genes, and areas that either have no function, or a function yet unknown. Genes are the units of heredity and can be loosely viewed as the organism's "cookbook" or "blueprint". The regulation of these genes can involve changes in the structure of the DNA, including chemical modification of the bases or changes in the packaging of the DNA strand in the nucleus.

Location and organisation in cells

DNA is located in the cell nucleus of eukaryotes, as well as small amounts in mitochondria and chloroplasts. In prokaryotes, the DNA is held within an irregularly shaped body in the cytoplasm called the nucleoid. The DNA is usually in linear chromosomes in eukaryotes, and circular chromosomes in prokaryotes. In the human genome, there is approximately 3 billion base pairs of DNA arranged into 46 chromosomes. Within these chromosomes, DNA is held in complexes between DNA and proteins. These proteins organize the DNA into a compact structure called chromatin. In eukaryotes this structure is based on the interaction of DNA with conserved proteins called histones, while in prokaryotes multiple types of proteins are involved.

The genetic code

Within a gene, the sequence of nucleotides along a DNA strand defines a messenger RNA sequence which then defines a protein, that an organism is liable to manufacture or "express" at one or several points in its life using the information of the sequence. The relationship between the nucleotide sequence and the amino-acid sequence of the protein is determined by simple cellular rules of translation, known collectively as the genetic code. The genetic code consists of three-letter 'words' (termed a codon) formed from a sequence of three nucleotides (e.g. ACT, CAG, TTT). These codons can then be translated with messenger RNA and then transfer RNA, with a codon corresponding to a particular amino acid. There are 64 possible codons (4 bases in 3 places ${\displaystyle 4^{3}}$) that encode 20 amino acids. Most amino acids, therefore, have more than one possible codon. There are also three 'stop' or 'nonsense' codons signifying the end of the coding region, namely the UAA, UGA and UAG codons.

Non-coding DNA

In many species, only a small fraction of the total sequence of the genome appears to encode protein. For example, only about 1.5% of the human genome consists of protein-coding exons. The function of the rest is a matter of speculation. It is known that certain nucleotide sequences specify affinity for DNA binding proteins, which play a wide variety of vital roles, in particular through control of replication and transcription. These sequences are frequently called regulatory sequences, and researchers assume that so far they have identified only a tiny fraction of the total that exist. "Junk DNA" represents sequences that do not yet appear to contain genes or to have a function. The reasons for the presence of so much non-coding DNA in eukaryotic genomes and the extraordinary differences in genome size ("C-value") among species represent a long-standing puzzle in DNA research known as the "C-value enigma".

Some DNA sequences play structural roles in chromosomes. Telomeres and centromeres typically contain few (if any) protein-coding genes, but are important for the function and stability of chromosomes. Some genes code for "RNA genes" (see tRNA and rRNA). Some RNA genes code for transcripts that function as regulatory RNAs (see siRNA) that influence the function of other RNA molecules. The intron-exon structure of some genes (such as immunoglobin and protocadeherin genes) is important for allowing alternative splicing of pre-mRNA which allows several different proteins to be made from the same gene. Indeed, the 34,000 human genes encode some 100,000 proteins. Some non-coding DNA represents pseudogenes, which have been hypothesized to serve as raw genetic material for the creation of new genes through the process of gene duplication and divergence. Some non-coding DNA provided hot-spots for duplication of short DNA regions; such sequence duplication has been the major form of genetic change in the human lineage (see evidence from the Chimpanzee Genome Project).

Sequence also determines a DNA segment's susceptibility to cleavage by restriction enzymes, an important tool in genetic engineering. The position of cleavage sites throughout an individual's genome determines one kind of an individual's "DNA fingerprint".

Base and histone modification

The expression of genes is influenced by modifications of the bases in DNA and the histones around which the DNA is wrapped. The most common base modification is cytosine methylation, and is associated with reduced expression of a sequence of DNA. Other base modifications include adenine methylation in bacteria and the glycosylation of uracil to produce the "J-base" in kinetoplastids. The modifications of histones are more complicated, with residues in these proteins being altered by methylation, phosphorylation and acetylation. These chemical changes alter the strength of the interaction between the DNA and the histones, making the DNA more or less accessible to transcription factors and changing the rate of transcription.

DNA-protein interactions

All the processes described above depend on the ability of proteins to interact with DNA.

Replication and inheritance

Replication

The double-stranded structure of DNA provides a mechanism for DNA replication: the two strands are separated, and then each strand's complement is recreated by exposing the strand to a mixture of the four bases. An enzyme makes the complement strand by finding the correct base in the mixture and bonding it with the original strand. All such DNA polymerases work in a 5 prime to 3 prime direction. In this way, the base on the old strand dictates which base appears on the new strand, and the cell ends up with an extra copy of its DNA.

Inheritance

DNA is responsible for the genetic inheritance of most inherited traits. In humans, these traits range from hair colour to disease susceptibility. The genetic information encoded by an organism's DNA is encoded within its genome. During cell division, this DNA is replicated, and during reproduction is transmitted to offspring.

In eukaryotic cells most of the DNA is located in the cell nucleus, usually packaged into linear chromosomes that are passed to daughter cells during cell division. In contrast, in prokaryotes, the DNA is in the cytosol (not separated by a nuclear envelope) and is usually circular. Some eukaryotic organelles, such as chloroplasts and mitochondria, also have DNA that is similar to prokaryotic DNA, reflecting the fact that these organelles evolved from prokaryotes. DNA is thought to have originated approximately 3.5 to 4.6 billion years ago.

In humans, the mother's mitochondrial DNA together with 23 chromosomes from each parent combine to form the genome of a zygote, the fertilized egg. As a result, with certain exceptions such as red blood cells, most human cells contain 23 pairs of chromosomes, together with mitochondrial DNA inherited from the mother. Lineage studies can be done because mitochondrial DNA only comes from the mother, and the Y chromosome only comes from the father.

Mutation and repair

A cell's machinery separates the DNA double helix, and uses each DNA strand as a template for synthesizing a new strand which is nearly identical to the previous strand. Errors that occur in the synthesis are called mutations. Mutations are the results of the cells' attempts to repair chemical imperfections in this process, where a base is accidentally skipped, inserted, or incorrectly copied, or the chain is trimmed, or added to. On rare occasions, wrong pairing can happen, when thymine goes into its enol form or cytosine goes into its imino form. Mutations can also occur after chemical damage (through mutagens), light (UV damage), or through other more complicated gene swapping events.

History

First isolation of DNA

Working in the 19th century, biochemists initially isolated DNA and RNA (mixed together) from cell nuclei. They were relatively quick to appreciate the polymeric nature of their "nucleic acid" isolates, but realized only later that nucleotides were of two types--one containing ribose and the other deoxyribose. It was this subsequent discovery that led to the identification and naming of DNA as a substance distinct from RNA.

Friedrich Miescher (1844-1895) discovered a substance he called "nuclein" in 1869. Somewhat later, he isolated a pure sample of the material now known as DNA from the sperm of salmon, and in 1889 his pupil, Richard Altmann, named it "nucleic acid". This substance was found to exist only in the chromosomes.

In 1929 Phoebus Levene at the Rockefeller Institute identified the components (the four bases, the sugar and the phosphate chain) and he showed that the components of DNA were linked in the order phosphate-sugar-base. He called each of these units a nucleotide and suggested the DNA molecule consisted of a string of nucleotide units linked together through the phosphate groups, which are the 'backbone' of the molecule. However Levene thought the chain was short and that the bases repeated in the same fixed order. Torbjorn Caspersson and Einar Hammersten showed that DNA was a polymer.

Chromosomes and inherited traits

Max Delbrück, Nikolai V. Timofeeff-Ressovsky, and Karl G. Zimmer published results in 1935 suggesting that chromosomes are very large molecules the structure of which can be changed by treatment with X-rays, and that by so changing their structure it was possible to change the heritable characteristics governed by those chromosomes. In 1937 William Astbury produced the first X-ray diffraction patterns from DNA. He was not able to propose the correct structure but the patterns showed that DNA had a regular structure and therefore it might be possible to deduce what this structure was.

In 1943, Oswald Theodore Avery and a team of scientists discovered that traits proper to the "smooth" form of the Pneumococcus could be transferred to the "rough" form of the same bacteria merely by making the killed "smooth" (S) form available to the live "rough" (R) form. Quite unexpectedly, the living R Pneumococcus bacteria were transformed into a new strain of the S form, and the transferred S characteristics turned out to be heritable. Avery called the medium of transfer of traits the transforming principle; he identified DNA as the transforming principle, and not protein as previously thought. He essentially redid Frederick Griffith's experiment. In 1953, Alfred Hershey and Martha Chase did an experiment (Hershey-Chase experiment) that showed, in T2 phage, that DNA is the genetic material (Hershey shared the Nobel prize with Luria).

Discovery of the structure of DNA

In the 1950s, three groups made it their goal to determine the structure of DNA. The first group to start was at King's College London and was led by Maurice Wilkins and was later joined by Rosalind Franklin. Another group consisting of Francis Crick and James D. Watson was at Cambridge. A third group was at Caltech and was led by Linus Pauling. Crick and Watson built physical models using metal rods and balls, in which they incorporated the known chemical structures of the nucleotides, as well as the known position of the linkages joining one nucleotide to the next along the polymer. At King's College Maurice Wilkins and Rosalind Franklin examined X-ray diffraction patterns of DNA fibers. Of the three groups, only the London group was able to produce good quality diffraction patterns and thus produce sufficient quantitative data about the structure.

Helix structure

In 1948 Pauling discovered that many proteins included helical (see alpha helix) shapes. Pauling had deduced this structure from X-ray patterns and from attempts to physically model the structures. (Pauling was also later to suggest an incorrect three chain helical structure based on Astbury's data.) Even in the initial diffraction data from DNA by Maurice Wilkins, it was evident that the structure involved helices. But this insight was only a beginning. There remained the questions of how many strands came together, whether this number was the same for every helix, whether the bases pointed toward the helical axis or away, and ultimately what were the explicit angles and coordinates of all the bonds and atoms. Such questions motivated the modeling efforts of Watson and Crick.

Complementary nucleotides

In their modeling, Watson and Crick restricted themselves to what they saw as chemically and biologically reasonable. Still, the breadth of possibilities was very wide. A breakthrough occurred in 1952, when Erwin Chargaff visited Cambridge and inspired Crick with a description of experiments Chargaff had published in 1947. Chargaff had observed that the proportions of the four nucleotides vary between one DNA sample and the next, but that for particular pairs of nucleotides — adenine and thymine, guanine and cytosine — the two nucleotides are always present in equal proportions.

Using X-ray diffraction data from Rosalind Franklin and the information that the bases were paired, James D. Watson and Francis Crick arrived at the first accurate model of DNA's molecular structure in 1953. Watson and Crick proposed the central dogma of molecular biology in 1957, describing the process whereby proteins are produced from nucleic DNA. In 1962 Watson, Crick, and Maurice Wilkins jointly received the Nobel Prize for their determination of the structure of DNA.

"Central Dogma"

Watson and Crick's model attracted great interest immediately upon its presentation. Arriving at their conclusion on February 21 1953, Watson and Crick made their first announcement on February 28. In an influential presentation in 1957, Crick laid out the "Central Dogma", which foretold the relationship between DNA, RNA, and proteins, and articulated the "sequence hypothesis." A critical confirmation of the replication mechanism that was implied by the double-helical structure followed in 1958 in the form of the Meselson-Stahl experiment. Work by Crick and coworkers showed that the genetic code was based on non-overlapping triplets of bases, called codons, and Har Gobind Khorana and others deciphered the genetic code not long afterward. These findings represent the birth of molecular biology.

Use of DNA in technology and industry

Forensics

Forensic scientists can use DNA located in blood, semen, skin, saliva or hair left at the scene of a crime to identify a possible suspect, a process called genetic fingerprinting or DNA profiling. In DNA profiling the relative lengths of sections of repetitive DNA, such as short tandem repeats and minisatellites, are compared. DNA profiling was developed in 1984 by British geneticist Sir Alec Jeffreys of the University of Leicester, and was first used to convict Colin Pitchfork in 1988 in the Enderby murders case in Leicestershire, United Kingdom. Many jurisdictions require convicts of certain types of crimes to provide a sample of DNA for inclusion in a computerized database. This has helped investigators solve old cases where the perpetrator was unknown and only a DNA sample was obtained from the scene (particularly in rape cases between strangers). This method is one of the most reliable techniques for identifying a criminal, but is not always perfect, for example if no DNA can be retrieved, or if the scene is contaminated with the DNA of several possible suspects.

DNA and computation

DNA plays an important role in computer science, bioinformatics and computational biology, both as a motivating research problem and as a method of computation in itself. A Sequence profiling tool like Sequerome assists researchers working on sequence data by linking the entire Sequence alignment report (BLAST) to many third party servers/sites that provide highly specific services in sequence manipulations such as restriction enzyme maps, open reading frame analyses for nucleotide sequences, and secondary structure prediction.

Research on string searching algorithms, which find an occurrence of a sequence of letters inside a larger sequence of letters, was motivated in part by DNA research, where it is used to find specific sequences of nucleotides in a large sequence. In other applications such as text editors, even simple algorithms for this problem usually suffice, but DNA sequences cause these algorithms to exhibit near-worst-case behavior due to their small number of distinct characters.

Database theory has been influenced by DNA research, which poses special problems for storing and manipulating DNA sequences. Databases specialized for DNA research are called genomic databases, and must address a number of unique technical challenges associated with the operations of approximate matching, sequence comparison, finding repeating patterns, and homology searching.

In 1994, Leonard Adleman of the University of Southern California made headlines when he discovered a way of solving the directed Hamiltonian path problem, an NP-complete problem, using tools from molecular biology, in particular DNA. The new approach, dubbed DNA computing, has practical advantages over traditional computers in power use, space use, and efficiency, due to its ability to highly parallelize the computation (see parallel computing), although there is labor worth mentioning involved in retrieving the answers. A number of other problems, including simulation of various abstract machines, the boolean satisfiability problem, and the bounded version of the Post correspondence problem, have since been analyzed using DNA computing.

Due to its compactness, DNA also has a theoretical role in cryptography, where in particular it allows unbreakable one-time pads to be efficiently constructed and used.

History and anthropology

Because DNA collects mutations over time, which are then passed down from parent to offspring, it contains information about processes that have occurred in the past, becoming in time ancient DNA. By comparing different DNA sequences, geneticists can attempt to infer the history of organisms.

If DNA sequences from different species are compared, then the resulting family tree, or phylogeny can be used to study the evolution of these species. This field of phylogenetics is a powerful tool in evolutionary biology. If DNA sequences within a species are compared, population geneticists can glean information on the history of particular populations. This can be used in studies ranging from ecological genetics to anthropology (for example, DNA evidence is also being used to try to identify the Ten Lost Tribes of Israel).

DNA has also been used to look at fairly recent issues of family relationships, such as establishing some manner of familial relationship between the descendants of Sally Hemings and the family of Thomas Jefferson. This usage is closely related to the use of DNA in criminal investigations detailed above. Indeed, some criminal investigations have been solved when DNA from crime scenes has fortuitously matched relatives of the guilty individual.

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Further reading

• Robert Olby; "The Path to The Double Helix: Discovery of DNA"; first published in October 1974 by MacMillan, with foreword by Francis Crick; ISBN 046681173 ; the definitive DNA textbook, revised in 1994, with a 9 page postscript.
• Ridley, Matt; Francis Crick: Discoverer of the Genetic Code (Eminent Lives) first published in June 2006 in the USA and then to be in the UK September 2006, by HarperCollins Publishers; 192 pp, ISBN 0-06-082333-X
• Watson, James D. and Francis H.C. Crick. A structure for Deoxyribose Nucleic Acid (PDF). Nature 171, 737 – 738, 25 April 1953.
• Watson, James D. DNA: The Secret of Life ISBN 0-375-41546-7.
• Watson, James D. The Double Helix: A Personal Account of the Discovery of the Structure of DNA (Norton Critical Editions). ISBN 0-393-95075-1
• Chomet, S. (Ed.), DNA Genesis of a Discovery, Newman-Hemisphere Press, London, 1994.
• Miller, Kenneth R., and Levin, Joseph. Biology. Saddle River, New Jersey: Prentice Hall, 2002.
• Judson, Horace Freeland, The Eighth Day of Creation, Touchstone (New York), 1979.

Further reading

• Steven Rose, The Chemistry of Life, Penguin, ISBN 0-14-027273-9. A comprehensive introduction to biochemistry.

External links

• Organ-Targeting Peptides
• DNA from the beginning
• DNA hack: The website for Amateur Genetic Engineering
• on Francis Crick
• First press stories on DNA
• 'Death' of DNA Helix (Crystaline) joke funeral card.
• Double helix: 50 years of DNA, Nature.
• The controversy of Watt & Cricks theory..
• Rosalind Franklin's DNA contributions.
• DNA The Program (in Russian).
• U.S. National DNA Day Watch videos and participate in real-time chat with top scientists
• Genetic Education Modules for Teachers DNA from the Beginning Study Guide
• Talking Glossary of Genetic Terms In Spanish, too
• Linus Pauling and the Race for DNA
• Listen to Francis Crick and James Watson talking on the BBC in 1962, 1972, and 1974
• 17 April, 2003, BBC News: Most ancient DNA ever?
• Using DNA in Genealogical Research
• Genetic Genealogy DNA Testing Information & Resources Help Page
• DNA Interactive (requires Adobe Flash)
• DNA: RCSB PDB Molecule of the Month
• DNA under electron microscope
• Left-handed DNA Hall of Fame
• My First Book About DNA Designed for children to learn more about DNA.
• Template:Dmoz
• DNA Replication and Translation / Cell Biology
• DNA Articles DNA Articles and Information collected from various sources.
• DNA ReplicationAnimation of DNA

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• DNA
• Genetics