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The structure of part of a DNA double helix

Deoxyribonucleic acid (DNA) is a that contains the instructions used in the and functioning of all known and some . The main role of DNA is the long-term storage of . DNA is often compared to a set of or a recipe, or a code, since it contains the instructions needed to construct other components of , such as and molecules. The DNA segments that carry this genetic information are called , but other DNA sequences have structural purposes, or are involved in regulating the use of this genetic information.

Chemically, DNA consists of two long of simple units called , with backbones made of sugars and phosphate groups joined by bonds. These two strands run in opposite directions to each other and are therefore . Attached to each sugar is one of four types of molecules called . It is the sequence of these four bases along the backbone that encodes information. This information is read using the , which specifies the sequence of the within proteins. The code is read by copying stretches of DNA into the related nucleic acid RNA, in a process called .

Within cells, DNA is organized into structures called . These chromosomes are duplicated before cells , in a process called . (, , , and ) store their DNA inside the , while in ( and ) it is found in the cell's . Within the chromosomes, proteins such as compact and organize DNA. These compact structures guide the interactions between DNA and other proteins, helping control which parts of the DNA are transcribed.

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The chemical structure of DNA. are shown as dotted lines.

DNA is a long made from repeating units called .<ref name=Alberts></ref><ref name=Butler>. 14–15.</ref> The DNA chain is 22 to 26  wide (2.2 to 2.6 ), and one nucleotide unit is 3.3 Å (0.33 nm) long.<ref></ref> Although each individual repeating unit is very small, DNA polymers can be enormous molecules containing millions of nucleotides. For instance, the largest human chromosome, chromosome number 1, is approximately 220 million long.<ref></ref>

In living organisms, DNA does not usually exist as a single molecule, but instead as a tightly-associated pair of molecules.<ref name=Watson></ref><ref name=berg>Berg J., Tymoczko J. and Stryer L. (2002) Biochemistry. W. H. Freeman and Company </ref> These two long strands entwine like vines, in the shape of a . The nucleotide repeats contain both the segment of 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 and a base linked to a sugar and one or more phosphate groups is called a . If multiple nucleotides are linked together, as in DNA, this polymer is called a .<ref name=IUPAC> IUPAC-IUB Commission on Biochemical Nomenclature (CBN), Accessed 03 Jan 2006</ref>

The backbone of the DNA strand is made from alternating and residues.<ref name=Ghosh></ref> The sugar in DNA is 2-deoxyribose, which is a (five-) sugar. The sugars are joined together by phosphate groups that form between the third and fifth carbon of adjacent sugar rings. These asymmetric 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 DNA strands are referred to as the (five prime) and (three prime) ends, with the 5' end being that with a terminal phosphate group and the 3' end that with a terminal hydroxyl group. One of the major differences between DNA and RNA is the sugar, with 2-deoxyribose being replaced by the alternative pentose sugar in RNA.<ref name=berg/>

The DNA double helix is stabilized by between the bases attached to the two strands. The four bases found in DNA are (abbreviated A), (C), (G) and (T). These four bases are attached to the sugar/phosphate to form the complete nucleotide, as shown for adenosine monophosphate.

These bases are classified into two types; adenine and guanine are fused five- and six-membered called , while cytosine and thymine are six-membered rings called .<ref name=berg/> A fifth pyrimidine base, called (U), usually takes the place of thymine in RNA and differs from thymine by lacking a on its ring. Uracil is not usually found in DNA, occurring only as a breakdown product of cytosine.

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Animation of the structure of a section of DNA. The bases lie horizontally between the two spiraling strands. <ref>Created from </ref>

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, the major groove, is 22 Å wide and the other, the minor groove, is 12 Å wide.<ref></ref> 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 that can bind to specific sequences in double-stranded DNA usually make contacts to the sides of the bases exposed in the major groove.<ref></ref>

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Each type of base on one strand forms a bond with just one type of base on the other strand. This is called complementary . Here, purines form to pyrimidines, with A bonding only to T, and C bonding only to G. This arrangement of two nucleotides binding together across the double helix is called a base pair. The double helix is also stabilized by the and , which are not influenced by the sequence of the DNA.<ref></ref> As hydrogen bonds are not , 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 a mechanical force or high .<ref></ref> As a result of this complementarity, all the information in the double-stranded sequence of a DNA helix is duplicated on each strand, which is vital in DNA replication. Indeed, this reversible and specific interaction between complementary base pairs is critical for all the functions of DNA in living organisms.<ref name=Alberts/>

Top, a GC base pair with three . Bottom, an AT base pair with two hydrogen bonds. Non-covalent hydrogen bonds between the pairs are shown as dashed lines.

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, left). 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 stronger-interacting strands, while short helices with high AT content have weaker-interacting strands.<ref></ref> In biology, parts of the DNA double helix that need to separate easily, such as the TATAAT in some , tend to have a high AT content, making the strands easier to pull apart.<ref></ref> In the laboratory, the strength of this interaction can be measured by finding the temperature required to break the hydrogen bonds, their (also called Tm value). When all the base pairs in a DNA double helix melt, the strands separate and exist in solution as two entirely independent molecules. These single-stranded DNA molecules have no single common shape, but some conformations are more stable than others.<ref></ref>

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A DNA sequence is called "sense" if its sequence is the same as that of a copy that is translated into protein.<ref> JCBN/NC-IUB Newsletter 1989, Accessed 07 May 2008</ref> The sequence on the opposite strand is called the "antisense" sequence. Both sense and antisense sequences can exist on different parts of the same strand of DNA (i.e. both strands contain both sense and antisense sequences). In both prokaryotes and eukaryotes, antisense RNA sequences are produced, but the functions of these RNAs are not entirely clear.<ref></ref> One proposal is that antisense RNAs are involved in regulating through RNA-RNA base pairing.<ref></ref>

A few DNA sequences in prokaryotes and eukaryotes, and more in and , blur the distinction between sense and antisense strands by having overlapping genes.<ref></ref> In these cases, some DNA sequences do double duty, encoding one protein when read along one strand, and a second protein when read in the opposite direction along the other strand. In , this overlap may be involved in the regulation of gene transcription,<ref></ref> while in viruses, overlapping genes increase the amount of information that can be encoded within the small viral genome.<ref></ref>

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DNA can be twisted like a rope in a process called . With DNA in its "relaxed" state, a strand usually 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.<ref></ref> 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 that is introduced by called .<ref name=Champoux></ref> These enzymes are also needed to relieve the twisting stresses introduced into DNA strands during processes such as and .<ref name=Wang></ref>

From left to right, the structures of A, B and Z DNA

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DNA exists in many possible .<ref name=Ghosh/> However, only , B-DNA, and have been observed in organisms. Which conformation DNA adopts depends on the sequence of the DNA, the amount and direction of supercoiling, chemical modifications of the bases and also solution conditions, such as the concentration of and .<ref></ref> Of these three conformations, the "B" form described above is most common under the conditions found in cells.<ref></ref> The two alternative double-helical forms of DNA differ in their geometry and dimensions.

The A form is a wider right-handed spiral, with a shallow, wide minor groove and a narrower, deeper major groove. The A form occurs under non-physiological conditions in dehydrated samples of DNA, while in the cell it may be produced in hybrid pairings of DNA and RNA strands, as well as in enzyme-DNA complexes.<ref></ref><ref></ref> Segments of DNA where the bases have been chemically-modified by may undergo a larger change in conformation and adopt the . Here, the strands turn about the helical axis in a left-handed spiral, the opposite of the more common B form.<ref></ref> These unusual structures can be recognized by specific Z-DNA binding proteins and may be involved in the regulation of transcription.<ref></ref>

Structure of a DNA quadruplex formed by repeats. The conformation of the DNA backbone diverges significantly from the typical helical structure<ref>Created from </ref>

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At the ends of the linear chromosomes are specialized regions of DNA called . The main function of these regions is to allow the cell to replicate chromosome ends using the enzyme , as the enzymes that normally replicate DNA cannot copy the extreme 3′ ends of chromosomes.<ref name=Greider></ref> These specialized chromosome caps also help protect the DNA ends, and stop the systems in the cell from treating them as damage to be corrected.<ref name=Nugent></ref> In human cells, telomeres are usually lengths of single-stranded DNA containing several thousand repeats of a simple TTAGGG sequence.<ref></ref>

These guanine-rich sequences may stabilize chromosome ends by forming structures of stacked sets of four-base units, rather than the usual base pairs found in other DNA molecules. Here, four guanine bases form a flat plate and these flat four-base units then stack on top of each other, to form a stable structure.<ref name=Burge></ref> These structures are stabilized by hydrogen bonding between the edges of the bases and of a metal ion in the centre of each four-base unit.<ref></ref> Other structures can also be formed, with the central set of four bases coming from either a single strand folded around the bases, or several different parallel strands, each contributing one base to the central structure.

In addition to these stacked structures, telomeres also form large loop structures called telomere loops, or T-loops. Here, the single-stranded DNA curls around in a long circle stabilized by telomere-binding proteins.<ref></ref> At the very end of the T-loop, the single-stranded telomere DNA is held onto a region of double-stranded DNA by the telomere strand disrupting the double-helical DNA and base pairing to one of the two strands. This structure is called a displacement loop or .<ref name=Burge/>

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Structure of cytosine with and without the 5-methyl group. After deamination the 5-methylcytosine has the same structure as thymine

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The expression of genes is influenced by how the DNA is packaged in chromosomes, in a structure called . Base modifications can be involved in packaging, with regions of that have low or no gene expression usually containing high levels of of bases. For example, cytosine methylation, produces , which is important for .<ref></ref> The average level of methylation varies between organisms - the worm lacks cytosine methylation, while have higher levels, with up to 1% of their DNA containing 5-methylcytosine.<ref></ref> Despite the importance of 5-methylcytosine, it can to leave a thymine base, methylated cytosines are therefore particularly prone to .<ref></ref> Other base modifications include adenine methylation in bacteria and the of uracil to produce the "J-base" in .<ref></ref><ref></ref>

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, the major mutagen in , in an adduct to DNA<ref>Created from </ref>

DNA can be damaged by many different sorts of , which change the DNA sequence. Mutagens include , and also high-energy such as light and . The type of DNA damage produced depends on the type of mutagen. For example, UV light can damage DNA by producing , which are cross-links between pyrimidine bases.<ref>,</ref> On the other hand, oxidants such as or produce multiple forms of damage, including base modifications, particularly of guanosine, and double-strand breaks.<ref></ref> In each human cell, about 500 bases suffer oxidative damage per day.<ref></ref><ref></ref> Of these oxidative lesions, the most dangerous are double-strand breaks, as these are difficult to repair and can produce , and from the DNA sequence, as well as .<ref></ref>

Many mutagens fit into the space between two adjacent base pairs, this is called . Most intercalators are and planar molecules, and include , , and . In order for an intercalator to fit between base pairs, the bases must separate, distorting the DNA strands by unwinding of the double helix. This inhibits both transcription and DNA replication, causing toxicity and mutations. As a result, DNA intercalators are often , and , , and are well-known examples.<ref></ref><ref></ref><ref></ref> Nevertheless, due to their ability to inhibit DNA transcription and replication, these toxins are also used in to inhibit rapidly-growing cells.<ref></ref>

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DNA usually occurs as linear in eukaryotes, and circular chromosomes in prokaryotes. The set of chromosomes in a cell makes up its ; the has approximately 3 billion base pairs of DNA arranged into 46 chromosomes.<ref></ref> The information carried by DNA is held in the of pieces of DNA called . of genetic information in genes is achieved via complementary base pairing. For example, in transcription, when a cell uses the information in a gene, the DNA sequence is copied into a complementary RNA sequence through the attraction between the DNA and the correct RNA nucleotides. Usually, this RNA copy is then used to make a matching protein sequence in a process called which depends on the same interaction between RNA nucleotides. Alternatively, a cell may simply copy its genetic information in a process called DNA replication. The details of these functions are covered in other articles; here we focus on the interactions between DNA and other molecules that mediate the function of the genome.

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Genomic DNA is located in the of eukaryotes, as well as small amounts in and . In prokaryotes, the DNA is held within an irregularly shaped body in the cytoplasm called the .<ref></ref> The genetic information in a genome is held within genes, and the complete set of this information in an organism is called its . A gene is a unit of and is a region of DNA that influences a particular characteristic in an organism. Genes contain an that can be transcribed, as well as such as and , which control the transcription of the open reading frame.

In many , only a small fraction of the total sequence of the encodes protein. For example, only about 1.5% of the human genome consists of protein-coding , with over 50% of human DNA consisting of non-coding .<ref></ref> The reasons for the presence of so much in eukaryotic genomes and the extraordinary differences in , or , among species represent a long-standing puzzle known as the "."<ref></ref> However, DNA sequences that do not code protein may still encode functional molecules, which are involved in the regulation of gene expression.<ref></ref>

(blue) producing a mRNA (green) from a DNA template (orange).<ref>Created from </ref>

Some non-coding DNA sequences play structural roles in chromosomes. and typically contain few genes, but are important for the function and stability of chromosomes.<ref name=Nugent/><ref></ref> An abundant form of non-coding DNA in humans are , which are copies of genes that have been disabled by mutation.<ref></ref> These sequences are usually just molecular , although they can occasionally serve as raw genetic material for the creation of new genes through the process of and .<ref></ref>

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A gene is a sequence of DNA that contains genetic information and can influence the of an organism. Within a gene, the sequence of bases along a DNA strand defines a sequence, which then defines one or more protein sequences. The relationship between the nucleotide sequences of genes and the sequences of proteins is determined by the rules of , known collectively as the . The genetic code consists of three-letter 'words' called codons formed from a sequence of three nucleotides (e.g. ACT, CAG, TTT).

In transcription, the codons of a gene are copied into messenger RNA by . This RNA copy is then decoded by a that reads the RNA sequence by base-pairing the messenger RNA to , which carries amino acids. Since there are 4 bases in 3-letter combinations, there are 64 possible codons (43 combinations). These encode the twenty , giving most amino acids more than one possible codon. There are also three 'stop' or 'nonsense' codons signifying the end of the coding region; these are the TAA, TGA and TAG codons.

DNA replication. The double helix is unwound by a and . Next, one produces the copy. Another DNA polymerase binds to the . This enzyme makes discontinuous segments (called ) before joins them together.

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is essential for an organism to grow, but when a cell divides it must replicate the DNA in its genome so that the two daughter cells have the same genetic information as their parent. The double-stranded structure of DNA provides a simple mechanism for . Here, the two strands are separated and then each strand's complementary DNA sequence is recreated by an called . This enzyme makes the complementary strand by finding the correct base through complementary base pairing, and bonding it onto the original strand. As DNA polymerases can only extend a DNA strand in a 5′ to 3′ direction, different mechanisms are used to copy the antiparallel strands of the double helix.<ref></ref> In this way, the base on the old strand dictates which base appears on the new strand, and the cell ends up with a perfect copy of its DNA.

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All the functions of DNA depend on interactions with proteins. These protein interactions can be non-specific, or the protein can bind specifically to a single DNA sequence. Enzymes can also bind to DNA and of these, the polymerases that copy the DNA base sequence in transcription and DNA replication are particularly important.

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Interaction of DNA with (shown in white, top). These proteins' basic amino acids (below left, blue) bind to the acidic phosphate groups on DNA (below right, red).

Structural proteins that bind DNA are well-understood examples of non-specific DNA-protein interactions. Within chromosomes, DNA is held in complexes with structural proteins. These proteins organize the DNA into a compact structure called . In eukaryotes this structure involves DNA binding to a complex of small basic proteins called , while in prokaryotes multiple types of proteins are involved.<ref></ref><ref></ref> The histones form a disk-shaped complex called a , which contains two complete turns of double-stranded DNA wrapped around its surface. These non-specific interactions are formed through basic residues in the histones making to the acidic sugar-phosphate backbone of the DNA, and are therefore largely independent of the base sequence.<ref></ref> Chemical modifications of these basic amino acid residues include , and .<ref></ref> These chemical changes alter the strength of the interaction between the DNA and the histones, making the DNA more or less accessible to and changing the rate of transcription.<ref></ref> Other non-specific DNA-binding proteins in chromatin include the high-mobility group proteins, which bind to bent or distorted DNA.<ref></ref> These proteins are important in bending arrays of nucleosomes and arranging them into the larger structures that make up chromosomes.<ref></ref>

A distinct group of DNA-binding proteins are the DNA-binding proteins that specifically bind single-stranded DNA. In humans, replication protein A is the best-understood member of this family and is used in processes where the double helix is separated, including DNA replication, recombination and DNA repair.<ref></ref> These binding proteins seem to stabilize single-stranded DNA and protect it from forming or being degraded by .

The lambda repressor transcription factor bound to its DNA target<ref>Created from </ref>

In contrast, other proteins have evolved to bind particular DNA sequences. The most intensively-studied of these are the various , which are proteins that regulate transcription. Each transcription factor binds to one particular set of DNA sequences and activates or inhibits the transcription of genes that have these sequences close to their promoters. The transcription factors do this in two ways. Firstly, they can bind the RNA polymerase responsible for transcription, either directly or through other mediator proteins; this locates the polymerase at the promoter and allows it to begin transcription.<ref></ref> Alternatively, transcription factors can bind that modify the histones at the promoter; this will change the accessibility of the DNA template to the polymerase.<ref></ref>

As these DNA targets can occur throughout an organism's genome, changes in the activity of one type of transcription factor can affect thousands of genes.<ref></ref> Consequently, these proteins are often the targets of the processes that control responses to environmental changes or cellular differentiation and development. The specificity of these transcription factors' interactions with DNA come from the proteins making multiple contacts to the edges of the DNA bases, allowing them to "read" the DNA sequence. Most of these base-interactions are made in the major groove, where the bases are most accessible.<ref></ref>

The (green) in a complex with its substrate DNA<ref>Created from </ref>

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Nucleases and ligases

are that cut DNA strands by catalyzing the of the . Nucleases that hydrolyse nucleotides from the ends of DNA strands are called , while cut within strands. The most frequently-used nucleases in are the , which cut DNA at specific sequences. For instance, the EcoRV enzyme shown to the left recognizes the 6-base sequence 5′-GAT|ATC-3′ and makes a cut at the vertical line. In nature, these enzymes protect against infection by digesting the phage DNA when it enters the bacterial cell, acting as part of the .<ref></ref> In technology, these sequence-specific nucleases are used in and .

Enzymes called can rejoin cut or broken DNA strands.<ref name=Doherty></ref> Ligases are particularly important in DNA replication, as they join together the short segments of DNA produced at the into a complete copy of the DNA template. They are also used in and .<ref name=Doherty/>

Topoisomerases and helicases

are enzymes with both nuclease and ligase activity. These proteins change the amount of in DNA. Some of these enzyme work by cutting the DNA helix and allowing one section to rotate, thereby reducing its level of supercoiling; the enzyme then seals the DNA break.<ref name=Champoux/> Other types of these enzymes are capable of cutting one DNA helix and then passing a second strand of DNA through this break, before rejoining the helix.<ref></ref> Topoisomerases are required for many processes involving DNA, such as DNA replication and transcription.<ref name=Wang/>

are proteins that are a type of . They use the chemical energy in , predominantly , to break hydrogen bonds between bases and unwind the DNA double helix into single strands.<ref></ref> These enzymes are essential for most processes where enzymes need to access the DNA bases.

Polymerases

are that synthesize polynucleotide chains from . The sequence of their products are copies of existing polynucleotide chains - which are called templates. These enzymes function by adding nucleotides onto the 3′ of the previous nucleotide in a DNA strand. Consequently, all polymerases work in a 5′ to 3′ direction.<ref name=Joyce></ref> In the of these enzymes, the incoming nucleoside triphosphate base-pairs to the template: this allows polymerases to accurately synthesize the complementary strand of their template. Polymerases are classified according to the type of template that they use.

In DNA replication, a DNA-dependent makes a DNA copy of a DNA sequence. Accuracy is vital in this process, so many of these polymerases have a activity. Here, the polymerase recognizes the occasional mistakes in the synthesis reaction by the lack of base pairing between the mismatched nucleotides. If a mismatch is detected, a 3′ to 5′ activity is activated and the incorrect base removed.<ref></ref> In most organisms DNA polymerases function in a large complex called the that contains multiple accessory subunits, such as the or .<ref></ref>

RNA-dependent DNA polymerases are a specialized class of polymerases that copy the sequence of an RNA strand into DNA. They include , which is a enzyme involved in the infection of cells by , and , which is required for the replication of telomeres.<ref></ref><ref name=Greider/> Telomerase is an unusual polymerase because it contains its own RNA template as part of its structure.<ref name=Nugent/>

Transcription is carried out by a DNA-dependent that copies the sequence of a DNA strand into RNA. To begin transcribing a gene, the RNA polymerase binds to a sequence of DNA called a promoter and separates the DNA strands. It then copies the gene sequence into a transcript until it reaches a region of DNA called the , where it halts and detaches from the DNA. As with human DNA-dependent DNA polymerases, RNA polymerase II, the enzyme that transcribes most of the genes in the human genome, operates as part of a large protein complex with multiple regulatory and accessory subunits.<ref></ref>

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Structure of the intermediate in . The four separate DNA strands are coloured red, blue, green and yellow.<ref>Created from </ref>

Recombination involves the breakage and rejoining of two chromosomes (M and F) to produce two re-arranged chromosomes (C1 and C2).

A DNA helix usually does not interact with other segments of DNA, and in human cells the different chromosomes even occupy separate areas in the nucleus called "chromosome territories".<ref></ref> This physical separation of different chromosomes is important for the ability of DNA to function as a stable repository for information, as one of the few times chromosomes interact is during when they . Chromosomal crossover is when two DNA helices break, swap a section and then rejoin.

Recombination allows chromosomes to exchange genetic information and produces new combinations of genes, which increases the efficiency of and can be important in the rapid evolution of new proteins.<ref></ref> Genetic recombination can also be involved in DNA repair, particularly in the cell's response to double-strand breaks.<ref></ref>

The most common form of chromosomal crossover is , where the two chromosomes involved share very similar sequences. Non-homologous recombination can be damaging to cells, as it can produce and genetic abnormalities. The recombination reaction is catalyzed by enzymes known as recombinases, such as .<ref></ref> The first step in recombination is a double-stranded break either caused by an or damage to the DNA.<ref></ref> A series of steps catalyzed in part by the recombinase then leads to joining of the two helices by at least one , in which a segment of a single strand in each helix is annealed to the complementary strand in the other helix. The Holliday junction is a tetrahedral junction structure that can be moved along the pair of chromosomes, swapping one strand for another. The recombination reaction is then halted by cleavage of the junction and re-ligation of the released DNA.<ref></ref>

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DNA contains the genetic information that allows all modern living things to function, grow and reproduce. However, it is unclear how long in the 4-billion-year DNA has performed this function, as it has been proposed that the earliest forms of life may have used RNA as their genetic material.<ref name=Joyce></ref><ref></ref> RNA may have acted as the central part of early cell metabolism as it can both transmit genetic information and carry out as part of .<ref></ref> This ancient where nucleic acid would have been used for both catalysis and genetics may have influenced the of the current genetic code based on four nucleotide bases. This would occur since the number of unique bases in such an organism is a trade-off between a small number of bases increasing replication accuracy and a large number of bases increasing the catalytic efficiency of ribozymes.<ref></ref>

Unfortunately, there is no direct evidence of ancient genetic systems, as recovery of DNA from most fossils is impossible. This is because DNA will survive in the environment for less than one million years and slowly degrades into short fragments in solution.<ref></ref> Claims for older DNA have been made, most notably a report of the isolation of a viable bacterium from a salt crystal 250-million years old,<ref></ref> but these claims are controversial.<ref></ref><ref></ref>

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Methods have been developed to purify DNA from organisms, such as and manipulate it in the laboratory, such as and the . Modern and make intensive use of these techniques in recombinant DNA technology. is a man-made DNA sequence that has been assembled from other DNA sequences. They can be into organisms in the form of or in the appropriate format, by using a .<ref></ref> The organisms produced can be used to produce products such as recombinant , used in medical research,<ref></ref> or be grown in .<ref></ref><ref></ref>

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can use DNA in , , , or found at a crime scene to identify a matching DNA of an individual, such as a perpetrator. This process is called , or more accurately, DNA profiling. In DNA profiling, the lengths of variable sections of repetitive DNA, such as and , are compared between people. This method is usually an extremely reliable technique for identifying a matching DNA.<ref></ref> However, identification can be complicated if the scene is contaminated with DNA from several people.<ref></ref> DNA profiling was developed in 1984 by British geneticist Sir ,<ref></ref> and first used in forensic science to convict Colin Pitchfork in the 1988 case.<ref> Forensic Science Service Accessed 23 Dec 2006</ref>

People convicted of certain types of crimes may be required to provide a sample of DNA for a database. This has helped investigators solve old cases where only a DNA sample was obtained from the scene. DNA profiling can also be used to identify victims of mass casualty incidents.<ref></ref> On the other hand, many convicted people have been released from prison on the basis of DNA techniques, which were not available when a crime had originally been committed.

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involves the manipulation, searching, and of DNA sequence data. The development of techniques to store and search DNA sequences have led to widely-applied advances in , especially , and .<ref></ref> String searching or matching algorithms, which find an occurrence of a sequence of letters inside a larger sequence of letters, were developed to search for specific sequences of nucleotides.<ref>Gusfield, Dan. Algorithms on Strings, Trees, and Sequences: Computer Science and Computational Biology. Cambridge University Press, 15 January 1997. .</ref> In other applications such as , even simple algorithms for this problem usually suffice, but DNA sequences cause these algorithms to exhibit near-worst-case behaviour due to their small number of distinct characters. The related problem of aims to identify sequences and locate the specific that make them distinct. These techniques, especially , are used in studying relationships and protein function.<ref></ref> Data sets representing entire genomes' worth of DNA sequences, such as those produced by the , are difficult to use without annotations, which label the locations of genes and regulatory elements on each chromosome. Regions of DNA sequence that have the characteristic patterns associated with protein- or RNA-coding genes can be identified by algorithms, which allow researchers to predict the presence of particular in an organism even before they have been isolated experimentally.<ref name="Mount"></ref>

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The DNA structure at left (schematic shown) will self-assemble into the structure visualized by at right. is the field which seeks to design nanoscale structures using the properties of DNA molecules. Image from Strong, 2004.

DNA nanotechnology uses the unique properties of DNA and other nucleic acids to create self-assembling branched DNA complexes with useful properties. DNA is thus used as a structural material rather than as a carrier of biological information. This has led to the creation of two-dimensional periodic lattices (both tile-based as well as using the "" method) as well as three-dimensional structures in the shapes of . and have also been demonstrated, and these DNA structures have been used to template the arrangement of other molecules such as and proteins.

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Because DNA collects mutations over time, which are then inherited, it contains historical information and by comparing DNA sequences, geneticists can infer the evolutionary history of organisms, their .<ref></ref> This field of phylogenetics is a powerful tool in . If DNA sequences within a species are compared, can learn the history of particular populations. This can be used in studies ranging from to ; for example, DNA evidence is being used to try to identify the .<ref>Lost Tribes of Israel, , PBS airdate: 22 February 2000. Transcript available from (last accessed on 4 March 2006)</ref><ref>Kleiman, Yaakov. aish.com (January 13, 2000). Accessed 4 March 2006.</ref>

DNA has also been used to look at modern family relationships, such as establishing family relationships between the descendants of and . 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 matched relatives of the guilty individual.<ref>Bhattacharya, Shaoni. newscientist.com (20 April 2004). Accessed 22 Dec 06</ref>

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DNA was first isolated by the physician who, in 1869, discovered a microscopic substance in the of discarded surgical bandages. As it resided in the nuclei of cells, he called it "nuclein".<ref></ref> In 1919 this discovery was followed by 's identification of the base, sugar and phosphate nucleotide unit.<ref></ref> Levene suggested that DNA consisted of a string of nucleotide units linked together through the phosphate groups. However, Levene thought the chain was short and the bases repeated in a fixed order. In 1937 produced the first patterns that showed that DNA had a regular structure.<ref></ref>

In 1928, discovered that of the "smooth" form of the Pneumococcus could be transferred to the "rough" form of the same bacteria by mixing killed "smooth" bacteria with the live "rough" form.<ref></ref> This system provided the first clear suggestion that DNA carried genetic information, when , along with coworkers and , identified DNA as the in 1943.<ref></ref> DNA's role in was confirmed in 1952, when and in the showed that DNA is the of the .<ref></ref>

In 1953, based on <ref name=FWPUB></ref> taken by and the information that the bases were paired, and suggested<ref name=FWPUB/> what is now accepted as the first accurate model of in the journal .<ref name=Watson/> Experimental evidence for Watson and Crick's model were published in a series of five articles in the same issue of Nature.<ref name=NatureDNA50>Nature Archives </ref> Of these, and 's paper was the first publication of X-ray diffraction data that supported the Watson and Crick model,<ref name=NatFranGos></ref><ref></ref> this issue also contained an article on DNA structure by and his colleagues.<ref name=NatWilk></ref> In 1962, after Franklin's death, Watson, Crick, and Wilkins jointly received the in .<ref> Nobelprize .org Accessed 22 Dec 06</ref> However, debate continues on who should receive credit for the discovery.<ref></ref>

In an influential presentation in 1957, Crick laid out the , which foretold the relationship between DNA, RNA, and proteins, and articulated the "adaptor hypothesis".<ref>Crick, F.H.C. genome.wellcome.ac.uk (Lecture, 1955). Accessed 22 Dec 2006</ref> Final confirmation of the replication mechanism that was implied by the double-helical structure followed in 1958 through the .<ref></ref> Further work by Crick and coworkers showed that the genetic code was based on non-overlapping triplets of bases, called codons, allowing , and to decipher the genetic code.<ref> Nobelprize.org Accessed 22 Dec 06</ref> These findings represent the birth of .

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  • Clayton, Julie. (Ed.). 50 Years of DNA, Palgrave MacMillan Press, 2003.
  • Judson, Horace Freeland. The Eighth Day of Creation: Makers of the Revolution in Biology, Cold Spring Harbor Laboratory Press, 1996.
  • . The Path to The Double Helix: Discovery of DNA, first published in October 1974 by MacMillan, with foreword by Francis Crick; ; the definitive DNA textbook, revised in 1994, with a 9 page postscript.
  • . Francis Crick: Discoverer of the Genetic Code (Eminent Lives) HarperCollins Publishers; 192 pp, 2006
  • Rose, Steven. The Chemistry of Life, Penguin, .
  • Watson, James D. and Francis H.C. Crick. (PDF). 171, 737–738, 25 April 1953.
  • Watson, James D. DNA: The Secret of Life .
  • Watson, James D. .
  • Watson, James D. "Avoid boring people and other lessons from a life in science" (2007) New York: Random House.
  • Calladine, Chris R.; Drew, Horace R.; Luisi, Ben F. and Travers, Andrew A. Understanding DNA, Elsevier Academic Press, 2003.

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  • at Mandeville Special Collections Library, Geisel Library, University of California, San Diego
  • This site from the Dolan DNA Learning Center included dozens of animations as well as interviews with James Watson and others (requires )
  • Another DNA Learning Center site on DNA, genes, and heredity from Mendel to the human genome project.
  • National Centre for Biotechnology Education
  • ,
  • — watch videos and participate in real-time chat with top scientists
  • DNA from the Beginning Study Guide
  • (2003) Nature 421 (January 23): 402–405.
  • From the official Nobel Prize web site

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