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For a non-technical introduction to the topic, see Introduction to genetics. DNA_sentence_0

For other uses, see DNA (disambiguation). DNA_sentence_1

Deoxyribonucleic acid (/diːˈɒksɪˌraɪboʊnjuːˌkliːɪk, -ˌkleɪ-/ (listen); DNA) is a molecule composed of two polynucleotide chains that coil around each other to form a double helix carrying genetic instructions for the development, functioning, growth and reproduction of all known organisms and many viruses. DNA_sentence_2

DNA and ribonucleic acid (RNA) are nucleic acids. DNA_sentence_3

Alongside proteins, lipids and complex carbohydrates (polysaccharides), nucleic acids are one of the four major types of macromolecules that are essential for all known forms of life. DNA_sentence_4

The two DNA strands are known as polynucleotides as they are composed of simpler monomeric units called nucleotides. DNA_sentence_5

Each nucleotide is composed of one of four nitrogen-containing nucleobases (cytosine [C], guanine [G], adenine [A] or thymine [T]), a sugar called deoxyribose, and a phosphate group. DNA_sentence_6

The nucleotides are joined to one another in a chain by covalent bonds (known as the phospho-diester linkage) between the sugar of one nucleotide and the phosphate of the next, resulting in an alternating sugar-phosphate backbone. DNA_sentence_7

The nitrogenous bases of the two separate polynucleotide strands are bound together, according to base pairing rules (A with T and C with G), with hydrogen bonds to make double-stranded DNA. DNA_sentence_8

The complementary nitrogenous bases are divided into two groups, pyrimidines and purines. DNA_sentence_9

In DNA, the pyrimidines are thymine and cytosine; the purines are adenine and guanine. DNA_sentence_10

Both strands of double-stranded DNA store the same biological information. DNA_sentence_11

This information is replicated as and when the two strands separate. DNA_sentence_12

A large part of DNA (more than 98% for humans) is non-coding, meaning that these sections do not serve as patterns for protein sequences. DNA_sentence_13

The two strands of DNA run in opposite directions to each other and are thus antiparallel. DNA_sentence_14

Attached to each sugar is one of four types of nucleobases (informally, bases). DNA_sentence_15

It is the sequence of these four nucleobases along the backbone that encodes genetic information. DNA_sentence_16

RNA strands are created using DNA strands as a template in a process called transcription, where DNA bases are exchanged for their corresponding bases except in the case of thymine (T), for which RNA substitutes uracil (U). DNA_sentence_17

Under the genetic code, these RNA strands specify the sequence of amino acids within proteins in a process called translation. DNA_sentence_18

Within eukaryotic cells, DNA is organized into long structures called chromosomes. DNA_sentence_19

Before typical cell division, these chromosomes are duplicated in the process of DNA replication, providing a complete set of chromosomes for each daughter cell. DNA_sentence_20

Eukaryotic organisms (animals, plants, fungi and protists) store most of their DNA inside the cell nucleus as nuclear DNA, and some in the mitochondria as mitochondrial DNA or in chloroplasts as chloroplast DNA. DNA_sentence_21

In contrast, prokaryotes (bacteria and archaea) store their DNA only in the cytoplasm, in circular chromosomes. DNA_sentence_22

Within eukaryotic chromosomes, chromatin proteins, such as histones, compact and organize DNA. DNA_sentence_23

These compacting structures guide the interactions between DNA and other proteins, helping control which parts of the DNA are transcribed. DNA_sentence_24

Properties DNA_section_0

DNA is a long polymer made from repeating units called nucleotides, each of which is usually symbolized by a single letter: either A, T, C, or G. The structure of DNA is dynamic along its length, being capable of coiling into tight loops and other shapes. DNA_sentence_25

In all species it is composed of two helical chains, bound to each other by hydrogen bonds. DNA_sentence_26

Both chains are coiled around the same axis, and have the same pitch of 34 angstroms (Å) (3.4 nanometres). DNA_sentence_27

The pair of chains has a radius of 10 angstroms (1.0 nanometre). DNA_sentence_28

According to another study, when measured in a different solution, the DNA chain measured 22 to 26 angstroms wide (2.2 to 2.6 nanometres), and one nucleotide unit measured 3.3 Å (0.33 nm) long. DNA_sentence_29

Although each individual nucleotide is very small, a DNA polymer can be very large and may contain hundreds of millions of nucleotides, such as in chromosome 1. DNA_sentence_30

Chromosome 1 is the largest human chromosome with approximately 220 million base pairs, and would be 85 mm long if straightened. DNA_sentence_31

DNA does not usually exist as a single strand, but instead as a pair of strands that are held tightly together. DNA_sentence_32

These two long strands coil around each other, in the shape of a double helix. DNA_sentence_33

The nucleotide contains both a segment of the backbone of the molecule (which holds the chain together) and a nucleobase (which interacts with the other DNA strand in the helix). DNA_sentence_34

A nucleobase linked to a sugar is called a nucleoside, and a base linked to a sugar and to one or more phosphate groups is called a nucleotide. DNA_sentence_35

A biopolymer comprising multiple linked nucleotides (as in DNA) is called a polynucleotide. DNA_sentence_36

The backbone of the DNA strand is made from alternating phosphate and sugar groups. DNA_sentence_37

The sugar in DNA is 2-deoxyribose, which is a pentose (five-carbon) sugar. DNA_sentence_38

The sugars are joined together by phosphate groups that form phosphodiester bonds between the third and fifth carbon atoms of adjacent sugar rings. DNA_sentence_39

These are known as the 3′-end (three prime end), and 5′-end (five prime end) carbons, the prime symbol being used to distinguish these carbon atoms from those of the base to which the deoxyribose forms a glycosidic bond. DNA_sentence_40

Therefore, any DNA strand normally has one end at which there is a phosphate group attached to the 5′ carbon of a ribose (the 5′ phosphoryl) and another end at which there is a free hydroxyl group attached to the 3′ carbon of a ribose (the 3′ hydroxyl). DNA_sentence_41

The orientation of the 3′ and 5′ carbons along the sugar-phosphate backbone confers directionality (sometimes called polarity) to each DNA strand. DNA_sentence_42

In a nucleic acid double helix, the direction of the nucleotides in one strand is opposite to their direction in the other strand: the strands are antiparallel. DNA_sentence_43

The asymmetric ends of DNA strands are said to have a directionality of five prime end (5′ ), and three prime end (3′), with the 5′ end having a terminal phosphate group and the 3′ end a terminal hydroxyl group. DNA_sentence_44

One major difference between DNA and RNA is the sugar, with the 2-deoxyribose in DNA being replaced by the alternative pentose sugar ribose in RNA. DNA_sentence_45

The DNA double helix is stabilized primarily by two forces: hydrogen bonds between nucleotides and base-stacking interactions among aromatic nucleobases. DNA_sentence_46

The four bases found in DNA are adenine (A), cytosine (C), guanine (G) and thymine (T). DNA_sentence_47

These four bases are attached to the sugar-phosphate to form the complete nucleotide, as shown for adenosine monophosphate. DNA_sentence_48

Adenine pairs with thymine and guanine pairs with cytosine, forming A-T and G-C base pairs. DNA_sentence_49

Nucleobase classification DNA_section_1

The nucleobases are classified into two types: the purines, A and G, which are fused five- and six-membered heterocyclic compounds, and the pyrimidines, the six-membered rings C and T. A fifth pyrimidine nucleobase, uracil (U), usually takes the place of thymine in RNA and differs from thymine by lacking a methyl group on its ring. DNA_sentence_50

In addition to RNA and DNA, many artificial nucleic acid analogues have been created to study the properties of nucleic acids, or for use in biotechnology. DNA_sentence_51

Non-canonical bases DNA_section_2

Modified bases occur in DNA. DNA_sentence_52

The first of these recognised was 5-methylcytosine, which was found in the genome of Mycobacterium tuberculosis in 1925. DNA_sentence_53

The reason for the presence of these noncanonical bases in bacterial viruses (bacteriophages) is to avoid the restriction enzymes present in bacteria. DNA_sentence_54

This enzyme system acts at least in part as a molecular immune system protecting bacteria from infection by viruses. DNA_sentence_55

Modifications of the bases cytosine and adenine, the more common and modified DNA bases, plays vital roles in the epigenetic control of gene expression in plants and animals. DNA_sentence_56

Listing of non-canonical bases found in DNA DNA_section_3

A number of non canonical bases are known to occur in DNA. DNA_sentence_57

Most of these are modifications of the canonical bases plus uracil. DNA_sentence_58


  • Modified AdenosineDNA_item_0_0
    • N6-carbamoyl-methyladenineDNA_item_0_1
    • N6-methyadenineDNA_item_0_2
  • Modified GuanineDNA_item_0_3
    • 7-DeazaguanineDNA_item_0_4
    • 7-MethylguanineDNA_item_0_5
  • Modified CytosineDNA_item_0_6
    • N4-MethylcytosineDNA_item_0_7
    • 5-CarboxylcytosineDNA_item_0_8
    • 5-FormylcytosineDNA_item_0_9
    • 5-GlycosylhydroxymethylcytosineDNA_item_0_10
    • 5-HydroxycytosineDNA_item_0_11
    • 5-MethylcytosineDNA_item_0_12
  • Modified ThymidineDNA_item_0_13
    • α-GlutamythymidineDNA_item_0_14
    • α-PutrescinylthymineDNA_item_0_15
  • Uracil and modificationsDNA_item_0_16
    • Base JDNA_item_0_17
    • UracilDNA_item_0_18
    • 5-DihydroxypentauracilDNA_item_0_19
    • 5-HydroxymethyldeoxyuracilDNA_item_0_20
  • OthersDNA_item_0_21
    • DeoxyarchaeosineDNA_item_0_22
    • 2,6-DiaminopurineDNA_item_0_23

Grooves DNA_section_4

Twin helical strands form the DNA backbone. DNA_sentence_59

Another double helix may be found tracing the spaces, or grooves, between the strands. DNA_sentence_60

These voids are adjacent to the base pairs and may provide a binding site. DNA_sentence_61

As the strands are not symmetrically located with respect to each other, the grooves are unequally sized. DNA_sentence_62

One groove, the major groove, is 22 angstroms (Å) wide and the other, the minor groove, is 12 Å wide. DNA_sentence_63

The width of the major groove means that the edges of the bases are more accessible in the major groove than in the minor groove. DNA_sentence_64

As a result, proteins such as transcription factors that can bind to specific sequences in double-stranded DNA usually make contact with the sides of the bases exposed in the major groove. DNA_sentence_65

This situation varies in unusual conformations of DNA within the cell (see below), but the major and minor grooves are always named to reflect the differences in size that would be seen if the DNA is twisted back into the ordinary B form. DNA_sentence_66

Base pairing DNA_section_5

Further information: Base pair DNA_sentence_67

In a DNA double helix, each type of nucleobase on one strand bonds with just one type of nucleobase on the other strand. DNA_sentence_68

This is called complementary base pairing. DNA_sentence_69

Purines form hydrogen bonds to pyrimidines, with adenine bonding only to thymine in two hydrogen bonds, and cytosine bonding only to guanine in three hydrogen bonds. DNA_sentence_70

This arrangement of two nucleotides binding together across the double helix is called a Watson-Crick base pair. DNA_sentence_71

DNA with high GC-content is more stable than DNA with low GC-content. DNA_sentence_72

A Hoogsteen base pair is a rare variation of base-pairing. DNA_sentence_73

As hydrogen bonds are not covalent, they can be broken and rejoined relatively easily. DNA_sentence_74

The two strands of DNA in a double helix can thus be pulled apart like a zipper, either by a mechanical force or high temperature. DNA_sentence_75

As a result of this base pair complementarity, all the information in the double-stranded sequence of a DNA helix is duplicated on each strand, which is vital in DNA replication. DNA_sentence_76

This reversible and specific interaction between complementary base pairs is critical for all the functions of DNA in organisms. DNA_sentence_77

As noted above, most DNA molecules are actually two polymer strands, bound together in a helical fashion by noncovalent bonds; this double-stranded (dsDNA) structure is maintained largely by the intrastrand base stacking interactions, which are strongest for G,C stacks. DNA_sentence_78

The two strands can come apart—a process known as melting—to form two single-stranded DNA (ssDNA) molecules. DNA_sentence_79

Melting occurs at high temperature, low salt and high pH (low pH also melts DNA, but since DNA is unstable due to acid depurination, low pH is rarely used). DNA_sentence_80

The stability of the dsDNA form depends not only on the GC-content (% G,C basepairs) but also on sequence (since stacking is sequence specific) and also length (longer molecules are more stable). DNA_sentence_81

The stability can be measured in various ways; a common way is the "melting temperature", which is the temperature at which 50% of the ds molecules are converted to ss molecules; melting temperature is dependent on ionic strength and the concentration of DNA. DNA_sentence_82

As a result, it is both the percentage of GC base pairs and the overall length of a DNA double helix that determines the strength of the association between the two strands of DNA. DNA_sentence_83

Long DNA helices with a high GC-content have stronger-interacting strands, while short helices with high AT content have weaker-interacting strands. DNA_sentence_84

In biology, parts of the DNA double helix that need to separate easily, such as the TATAAT Pribnow box in some promoters, tend to have a high AT content, making the strands easier to pull apart. DNA_sentence_85

In the laboratory, the strength of this interaction can be measured by finding the temperature necessary to break half of the hydrogen bonds, their melting temperature (also called Tm value). DNA_sentence_86

When all the base pairs in a DNA double helix melt, the strands separate and exist in solution as two entirely independent molecules. DNA_sentence_87

These single-stranded DNA molecules have no single common shape, but some conformations are more stable than others. DNA_sentence_88

Sense and antisense DNA_section_6

Further information: Sense (molecular biology) DNA_sentence_89

A DNA sequence is called a "sense" sequence if it is the same as that of a messenger RNA copy that is translated into protein. DNA_sentence_90

The sequence on the opposite strand is called the "antisense" sequence. DNA_sentence_91

Both sense and antisense sequences can exist on different parts of the same strand of DNA (i.e. both strands can contain both sense and antisense sequences). DNA_sentence_92

In both prokaryotes and eukaryotes, antisense RNA sequences are produced, but the functions of these RNAs are not entirely clear. DNA_sentence_93

One proposal is that antisense RNAs are involved in regulating gene expression through RNA-RNA base pairing. DNA_sentence_94

A few DNA sequences in prokaryotes and eukaryotes, and more in plasmids and viruses, blur the distinction between sense and antisense strands by having overlapping genes. DNA_sentence_95

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. DNA_sentence_96

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. DNA_sentence_97

Supercoiling DNA_section_7

Further information: DNA supercoil DNA_sentence_98

DNA can be twisted like a rope in a process called DNA supercoiling. DNA_sentence_99

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. DNA_sentence_100

If the DNA is twisted in the direction of the helix, this is positive supercoiling, and the bases are held more tightly together. DNA_sentence_101

If they are twisted in the opposite direction, this is negative supercoiling, and the bases come apart more easily. DNA_sentence_102

In nature, most DNA has slight negative supercoiling that is introduced by enzymes called topoisomerases. DNA_sentence_103

These enzymes are also needed to relieve the twisting stresses introduced into DNA strands during processes such as transcription and DNA replication. DNA_sentence_104

Alternative DNA structures DNA_section_8

Further information: Molecular Structure of Nucleic Acids: A Structure for Deoxyribose Nucleic Acid, Molecular models of DNA, and DNA structure DNA_sentence_105

DNA exists in many possible conformations that include A-DNA, B-DNA, and Z-DNA forms, although, only B-DNA and Z-DNA have been directly observed in functional organisms. DNA_sentence_106

The conformation that DNA adopts depends on the hydration level, DNA sequence, the amount and direction of supercoiling, chemical modifications of the bases, the type and concentration of metal ions, and the presence of polyamines in solution. DNA_sentence_107

The first published reports of A-DNA X-ray diffraction patterns—and also B-DNA—used analyses based on Patterson transforms that provided only a limited amount of structural information for oriented fibers of DNA. DNA_sentence_108

An alternative analysis was then proposed by Wilkins et al., in 1953, for the in vivo B-DNA X-ray diffraction-scattering patterns of highly hydrated DNA fibers in terms of squares of Bessel functions. DNA_sentence_109

In the same journal, James Watson and Francis Crick presented their molecular modeling analysis of the DNA X-ray diffraction patterns to suggest that the structure was a double-helix. DNA_sentence_110

Although the B-DNA form is most common under the conditions found in cells, it is not a well-defined conformation but a family of related DNA conformations that occur at the high hydration levels present in cells. DNA_sentence_111

Their corresponding X-ray diffraction and scattering patterns are characteristic of molecular paracrystals with a significant degree of disorder. DNA_sentence_112

Compared to B-DNA, the A-DNA form is a wider right-handed spiral, with a shallow, wide minor groove and a narrower, deeper major groove. DNA_sentence_113

The A form occurs under non-physiological conditions in partly dehydrated samples of DNA, while in the cell it may be produced in hybrid pairings of DNA and RNA strands, and in enzyme-DNA complexes. DNA_sentence_114

Segments of DNA where the bases have been chemically modified by methylation may undergo a larger change in conformation and adopt the Z form. DNA_sentence_115

Here, the strands turn about the helical axis in a left-handed spiral, the opposite of the more common B form. DNA_sentence_116

These unusual structures can be recognized by specific Z-DNA binding proteins and may be involved in the regulation of transcription. DNA_sentence_117

A 2020 study concluded that DNA turned right-handed due to ionization by cosmic rays. DNA_sentence_118

Alternative DNA chemistry DNA_section_9

For many years, exobiologists have proposed the existence of a shadow biosphere, a postulated microbial biosphere of Earth that uses radically different biochemical and molecular processes than currently known life. DNA_sentence_119

One of the proposals was the existence of lifeforms that use arsenic instead of phosphorus in DNA. DNA_sentence_120

A report in 2010 of the possibility in the bacterium GFAJ-1, was announced, though the research was disputed, and evidence suggests the bacterium actively prevents the incorporation of arsenic into the DNA backbone and other biomolecules. DNA_sentence_121

Quadruplex structures DNA_section_10

Further information: G-quadruplex DNA_sentence_122

At the ends of the linear chromosomes are specialized regions of DNA called telomeres. DNA_sentence_123

The main function of these regions is to allow the cell to replicate chromosome ends using the enzyme telomerase, as the enzymes that normally replicate DNA cannot copy the extreme 3′ ends of chromosomes. DNA_sentence_124

These specialized chromosome caps also help protect the DNA ends, and stop the DNA repair systems in the cell from treating them as damage to be corrected. DNA_sentence_125

In human cells, telomeres are usually lengths of single-stranded DNA containing several thousand repeats of a simple TTAGGG sequence. DNA_sentence_126

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. DNA_sentence_127

Here, four guanine bases, known as a guanine tetrad, form a flat plate. DNA_sentence_128

These flat four-base units then stack on top of each other to form a stable G-quadruplex structure. DNA_sentence_129

These structures are stabilized by hydrogen bonding between the edges of the bases and chelation of a metal ion in the centre of each four-base unit. DNA_sentence_130

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. DNA_sentence_131

In addition to these stacked structures, telomeres also form large loop structures called telomere loops, or T-loops. DNA_sentence_132

Here, the single-stranded DNA curls around in a long circle stabilized by telomere-binding proteins. DNA_sentence_133

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. DNA_sentence_134

This triple-stranded structure is called a displacement loop or D-loop. DNA_sentence_135

Branched DNA DNA_section_11

Further information: Branched DNA and DNA nanotechnology DNA_sentence_136

In DNA, fraying occurs when non-complementary regions exist at the end of an otherwise complementary double-strand of DNA. DNA_sentence_137

However, branched DNA can occur if a third strand of DNA is introduced and contains adjoining regions able to hybridize with the frayed regions of the pre-existing double-strand. DNA_sentence_138

Although the simplest example of branched DNA involves only three strands of DNA, complexes involving additional strands and multiple branches are also possible. DNA_sentence_139

Branched DNA can be used in nanotechnology to construct geometric shapes, see the section on uses in technology below. DNA_sentence_140

Artificial bases DNA_section_12

Main article: Nucleic acid analogue DNA_sentence_141

Several artificial nucleobases have been synthesized, and successfully incorporated in the eight-base DNA analogue named Hachimoji DNA. DNA_sentence_142

Dubbed S, B, P, and Z, these artificial bases are capable of bonding with each other in a predictable way (S–B and P–Z), maintain the double helix structure of DNA, and be transcribed to RNA. DNA_sentence_143

Their existence implies that there is nothing special about the four natural nucleobases that evolved on Earth. DNA_sentence_144

Chemical modifications and altered DNA packaging DNA_section_13

Base modifications and DNA packaging DNA_section_14

Further information: DNA methylation and Chromatin remodeling DNA_sentence_145

The expression of genes is influenced by how the DNA is packaged in chromosomes, in a structure called chromatin. DNA_sentence_146

Base modifications can be involved in packaging, with regions that have low or no gene expression usually containing high levels of methylation of cytosine bases. DNA_sentence_147

DNA packaging and its influence on gene expression can also occur by covalent modifications of the histone protein core around which DNA is wrapped in the chromatin structure or else by remodeling carried out by chromatin remodeling complexes (see Chromatin remodeling). DNA_sentence_148

There is, further, crosstalk between DNA methylation and histone modification, so they can coordinately affect chromatin and gene expression. DNA_sentence_149

For one example, cytosine methylation produces 5-methylcytosine, which is important for X-inactivation of chromosomes. DNA_sentence_150

The average level of methylation varies between organisms—the worm Caenorhabditis elegans lacks cytosine methylation, while vertebrates have higher levels, with up to 1% of their DNA containing 5-methylcytosine. DNA_sentence_151

Despite the importance of 5-methylcytosine, it can deaminate to leave a thymine base, so methylated cytosines are particularly prone to mutations. DNA_sentence_152

Other base modifications include adenine methylation in bacteria, the presence of 5-hydroxymethylcytosine in the brain, and the glycosylation of uracil to produce the "J-base" in kinetoplastids. DNA_sentence_153

Damage DNA_section_15

Further information: DNA damage (naturally occurring), Mutation, and DNA damage theory of aging DNA_sentence_154

DNA can be damaged by many sorts of mutagens, which change the DNA sequence. DNA_sentence_155

Mutagens include oxidizing agents, alkylating agents and also high-energy electromagnetic radiation such as ultraviolet light and X-rays. DNA_sentence_156

The type of DNA damage produced depends on the type of mutagen. DNA_sentence_157

For example, UV light can damage DNA by producing thymine dimers, which are cross-links between pyrimidine bases. DNA_sentence_158

On the other hand, oxidants such as free radicals or hydrogen peroxide produce multiple forms of damage, including base modifications, particularly of guanosine, and double-strand breaks. DNA_sentence_159

A typical human cell contains about 150,000 bases that have suffered oxidative damage. DNA_sentence_160

Of these oxidative lesions, the most dangerous are double-strand breaks, as these are difficult to repair and can produce point mutations, insertions, deletions from the DNA sequence, and chromosomal translocations. DNA_sentence_161

These mutations can cause cancer. DNA_sentence_162

Because of inherent limits in the DNA repair mechanisms, if humans lived long enough, they would all eventually develop cancer. DNA_sentence_163

DNA damages that are naturally occurring, due to normal cellular processes that produce reactive oxygen species, the hydrolytic activities of cellular water, etc., also occur frequently. DNA_sentence_164

Although most of these damages are repaired, in any cell some DNA damage may remain despite the action of repair processes. DNA_sentence_165

These remaining DNA damages accumulate with age in mammalian postmitotic tissues. DNA_sentence_166

This accumulation appears to be an important underlying cause of aging. DNA_sentence_167

Many mutagens fit into the space between two adjacent base pairs, this is called intercalation. DNA_sentence_168

Most intercalators are aromatic and planar molecules; examples include ethidium bromide, acridines, daunomycin, and doxorubicin. DNA_sentence_169

For an intercalator to fit between base pairs, the bases must separate, distorting the DNA strands by unwinding of the double helix. DNA_sentence_170

This inhibits both transcription and DNA replication, causing toxicity and mutations. DNA_sentence_171

As a result, DNA intercalators may be carcinogens, and in the case of thalidomide, a teratogen. DNA_sentence_172

Others such as benzo[a]pyrene diol epoxide and aflatoxin form DNA adducts that induce errors in replication. DNA_sentence_173

Nevertheless, due to their ability to inhibit DNA transcription and replication, other similar toxins are also used in chemotherapy to inhibit rapidly growing cancer cells. DNA_sentence_174

Biological functions DNA_section_16

DNA usually occurs as linear chromosomes in eukaryotes, and circular chromosomes in prokaryotes. DNA_sentence_175

The set of chromosomes in a cell makes up its genome; the human genome has approximately 3 billion base pairs of DNA arranged into 46 chromosomes. DNA_sentence_176

The information carried by DNA is held in the sequence of pieces of DNA called genes. DNA_sentence_177

Transmission of genetic information in genes is achieved via complementary base pairing. DNA_sentence_178

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. DNA_sentence_179

Usually, this RNA copy is then used to make a matching protein sequence in a process called translation, which depends on the same interaction between RNA nucleotides. DNA_sentence_180

In alternative fashion, a cell may simply copy its genetic information in a process called DNA replication. DNA_sentence_181

The details of these functions are covered in other articles; here the focus is on the interactions between DNA and other molecules that mediate the function of the genome. DNA_sentence_182

Genes and genomes DNA_section_17

Further information: Cell nucleus, Chromatin, Chromosome, Gene, and Noncoding DNA DNA_sentence_183

Genomic DNA is tightly and orderly packed in the process called DNA condensation, to fit the small available volumes of the cell. DNA_sentence_184

In eukaryotes, DNA is located in the cell nucleus, with small amounts in mitochondria and chloroplasts. DNA_sentence_185

In prokaryotes, the DNA is held within an irregularly shaped body in the cytoplasm called the nucleoid. DNA_sentence_186

The genetic information in a genome is held within genes, and the complete set of this information in an organism is called its genotype. DNA_sentence_187

A gene is a unit of heredity and is a region of DNA that influences a particular characteristic in an organism. DNA_sentence_188

Genes contain an open reading frame that can be transcribed, and regulatory sequences such as promoters and enhancers, which control transcription of the open reading frame. DNA_sentence_189

In many species, only a small fraction of the total sequence of the genome encodes protein. DNA_sentence_190

For example, only about 1.5% of the human genome consists of protein-coding exons, with over 50% of human DNA consisting of non-coding repetitive sequences. DNA_sentence_191

The reasons for the presence of so much noncoding DNA in eukaryotic genomes and the extraordinary differences in genome size, or C-value, among species, represent a long-standing puzzle known as the "C-value enigma". DNA_sentence_192

However, some DNA sequences that do not code protein may still encode functional non-coding RNA molecules, which are involved in the regulation of gene expression. DNA_sentence_193

Some noncoding DNA sequences play structural roles in chromosomes. DNA_sentence_194

Telomeres and centromeres typically contain few genes but are important for the function and stability of chromosomes. DNA_sentence_195

An abundant form of noncoding DNA in humans are pseudogenes, which are copies of genes that have been disabled by mutation. DNA_sentence_196

These sequences are usually just molecular fossils, although they can occasionally serve as raw genetic material for the creation of new genes through the process of gene duplication and divergence. DNA_sentence_197

Transcription and translation DNA_section_18

Further information: Genetic code, Transcription (genetics), and Protein biosynthesis DNA_sentence_198

A gene is a sequence of DNA that contains genetic information and can influence the phenotype of an organism. DNA_sentence_199

Within a gene, the sequence of bases along a DNA strand defines a messenger RNA sequence, which then defines one or more protein sequences. DNA_sentence_200

The relationship between the nucleotide sequences of genes and the amino-acid sequences of proteins is determined by the rules of translation, known collectively as the genetic code. DNA_sentence_201

The genetic code consists of three-letter 'words' called codons formed from a sequence of three nucleotides (e.g. ACT, CAG, TTT). DNA_sentence_202

In transcription, the codons of a gene are copied into messenger RNA by RNA polymerase. DNA_sentence_203

This RNA copy is then decoded by a ribosome that reads the RNA sequence by base-pairing the messenger RNA to transfer RNA, which carries amino acids. DNA_sentence_204

Since there are 4 bases in 3-letter combinations, there are 64 possible codons (4 combinations). DNA_sentence_205

These encode the twenty standard amino acids, giving most amino acids more than one possible codon. DNA_sentence_206

There are also three 'stop' or 'nonsense' codons signifying the end of the coding region; these are the TAA, TGA, and TAG codons. DNA_sentence_207

Replication DNA_section_19

Further information: DNA replication DNA_sentence_208

Cell division 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. DNA_sentence_209

The double-stranded structure of DNA provides a simple mechanism for DNA replication. DNA_sentence_210

Here, the two strands are separated and then each strand's complementary DNA sequence is recreated by an enzyme called DNA polymerase. DNA_sentence_211

This enzyme makes the complementary strand by finding the correct base through complementary base pairing and bonding it onto the original strand. DNA_sentence_212

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. DNA_sentence_213

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. DNA_sentence_214

Extracellular nucleic acids DNA_section_20

Naked extracellular DNA (eDNA), most of it released by cell death, is nearly ubiquitous in the environment. DNA_sentence_215

Its concentration in soil may be as high as 2 μg/L, and its concentration in natural aquatic environments may be as high at 88 μg/L. DNA_sentence_216

Various possible functions have been proposed for eDNA: it may be involved in horizontal gene transfer; it may provide nutrients; and it may act as a buffer to recruit or titrate ions or antibiotics. DNA_sentence_217

Extracellular DNA acts as a functional extracellular matrix component in the biofilms of several bacterial species. DNA_sentence_218

It may act as a recognition factor to regulate the attachment and dispersal of specific cell types in the biofilm; it may contribute to biofilm formation; and it may contribute to the biofilm's physical strength and resistance to biological stress. DNA_sentence_219

Cell-free fetal DNA is found in the blood of the mother, and can be sequenced to determine a great deal of information about the developing fetus. DNA_sentence_220

Under the name of environmental DNA eDNA has seen increased use in the natural sciences as a survey tool for ecology, monitoring the movements and presence of species in water, air, or on land, and assessing an area's biodiversity. DNA_sentence_221

Interactions with proteins DNA_section_21

All the functions of DNA depend on interactions with proteins. DNA_sentence_222

These protein interactions can be non-specific, or the protein can bind specifically to a single DNA sequence. DNA_sentence_223

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. DNA_sentence_224

DNA-binding proteins DNA_section_22

Further information: DNA-binding protein DNA_sentence_225

Structural proteins that bind DNA are well-understood examples of non-specific DNA-protein interactions. DNA_sentence_226

Within chromosomes, DNA is held in complexes with structural proteins. DNA_sentence_227

These proteins organize the DNA into a compact structure called chromatin. DNA_sentence_228

In eukaryotes, this structure involves DNA binding to a complex of small basic proteins called histones, while in prokaryotes multiple types of proteins are involved. DNA_sentence_229

The histones form a disk-shaped complex called a nucleosome, which contains two complete turns of double-stranded DNA wrapped around its surface. DNA_sentence_230

These non-specific interactions are formed through basic residues in the histones, making ionic bonds to the acidic sugar-phosphate backbone of the DNA, and are thus largely independent of the base sequence. DNA_sentence_231

Chemical modifications of these basic amino acid residues include methylation, phosphorylation, and acetylation. DNA_sentence_232

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_sentence_233

Other non-specific DNA-binding proteins in chromatin include the high-mobility group proteins, which bind to bent or distorted DNA. DNA_sentence_234

These proteins are important in bending arrays of nucleosomes and arranging them into the larger structures that make up chromosomes. DNA_sentence_235

A distinct group of DNA-binding proteins is the DNA-binding proteins that specifically bind single-stranded DNA. DNA_sentence_236

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. DNA_sentence_237

These binding proteins seem to stabilize single-stranded DNA and protect it from forming stem-loops or being degraded by nucleases. DNA_sentence_238

In contrast, other proteins have evolved to bind to particular DNA sequences. DNA_sentence_239

The most intensively studied of these are the various transcription factors, which are proteins that regulate transcription. DNA_sentence_240

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. DNA_sentence_241

The transcription factors do this in two ways. DNA_sentence_242

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. DNA_sentence_243

Alternatively, transcription factors can bind enzymes that modify the histones at the promoter. DNA_sentence_244

This changes the accessibility of the DNA template to the polymerase. DNA_sentence_245

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. DNA_sentence_246

Consequently, these proteins are often the targets of the signal transduction processes that control responses to environmental changes or cellular differentiation and development. DNA_sentence_247

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. DNA_sentence_248

Most of these base-interactions are made in the major groove, where the bases are most accessible. DNA_sentence_249

DNA-modifying enzymes DNA_section_23

Nucleases and ligases DNA_section_24

Nucleases are enzymes that cut DNA strands by catalyzing the hydrolysis of the phosphodiester bonds. DNA_sentence_250

Nucleases that hydrolyse nucleotides from the ends of DNA strands are called exonucleases, while endonucleases cut within strands. DNA_sentence_251

The most frequently used nucleases in molecular biology are the restriction endonucleases, which cut DNA at specific sequences. DNA_sentence_252

For instance, the EcoRV enzyme shown to the left recognizes the 6-base sequence 5′-GATATC-3′ and makes a cut at the horizontal line. DNA_sentence_253

In nature, these enzymes protect bacteria against phage infection by digesting the phage DNA when it enters the bacterial cell, acting as part of the restriction modification system. DNA_sentence_254

In technology, these sequence-specific nucleases are used in molecular cloning and DNA fingerprinting. DNA_sentence_255

Enzymes called DNA ligases can rejoin cut or broken DNA strands. DNA_sentence_256

Ligases are particularly important in lagging strand DNA replication, as they join together the short segments of DNA produced at the replication fork into a complete copy of the DNA template. DNA_sentence_257

They are also used in DNA repair and genetic recombination. DNA_sentence_258

Topoisomerases and helicases DNA_section_25

Topoisomerases are enzymes with both nuclease and ligase activity. DNA_sentence_259

These proteins change the amount of supercoiling in DNA. DNA_sentence_260

Some of these enzymes 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. DNA_sentence_261

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. DNA_sentence_262

Topoisomerases are required for many processes involving DNA, such as DNA replication and transcription. DNA_sentence_263

Helicases are proteins that are a type of molecular motor. DNA_sentence_264

They use the chemical energy in nucleoside triphosphates, predominantly adenosine triphosphate (ATP), to break hydrogen bonds between bases and unwind the DNA double helix into single strands. DNA_sentence_265

These enzymes are essential for most processes where enzymes need to access the DNA bases. DNA_sentence_266

Polymerases DNA_section_26

Polymerases are enzymes that synthesize polynucleotide chains from nucleoside triphosphates. DNA_sentence_267

The sequence of their products is created based on existing polynucleotide chains—which are called templates. DNA_sentence_268

These enzymes function by repeatedly adding a nucleotide to the 3′ hydroxyl group at the end of the growing polynucleotide chain. DNA_sentence_269

As a consequence, all polymerases work in a 5′ to 3′ direction. DNA_sentence_270

In the active site of these enzymes, the incoming nucleoside triphosphate base-pairs to the template: this allows polymerases to accurately synthesize the complementary strand of their template. DNA_sentence_271

Polymerases are classified according to the type of template that they use. DNA_sentence_272

In DNA replication, DNA-dependent DNA polymerases make copies of DNA polynucleotide chains. DNA_sentence_273

To preserve biological information, it is essential that the sequence of bases in each copy are precisely complementary to the sequence of bases in the template strand. DNA_sentence_274

Many DNA polymerases have a proofreading activity. DNA_sentence_275

Here, the polymerase recognizes the occasional mistakes in the synthesis reaction by the lack of base pairing between the mismatched nucleotides. DNA_sentence_276

If a mismatch is detected, a 3′ to 5′ exonuclease activity is activated and the incorrect base removed. DNA_sentence_277

In most organisms, DNA polymerases function in a large complex called the replisome that contains multiple accessory subunits, such as the DNA clamp or helicases. DNA_sentence_278

RNA-dependent DNA polymerases are a specialized class of polymerases that copy the sequence of an RNA strand into DNA. DNA_sentence_279

They include reverse transcriptase, which is a viral enzyme involved in the infection of cells by retroviruses, and telomerase, which is required for the replication of telomeres. DNA_sentence_280

For example, HIV reverse transcriptase is an enzyme for AIDS virus replication. DNA_sentence_281

Telomerase is an unusual polymerase because it contains its own RNA template as part of its structure. DNA_sentence_282

It synthesizes telomeres at the ends of chromosomes. DNA_sentence_283

Telomeres prevent fusion of the ends of neighboring chromosomes and protect chromosome ends from damage. DNA_sentence_284

Transcription is carried out by a DNA-dependent RNA polymerase that copies the sequence of a DNA strand into RNA. DNA_sentence_285

To begin transcribing a gene, the RNA polymerase binds to a sequence of DNA called a promoter and separates the DNA strands. DNA_sentence_286

It then copies the gene sequence into a messenger RNA transcript until it reaches a region of DNA called the terminator, where it halts and detaches from the DNA. DNA_sentence_287

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. DNA_sentence_288

Genetic recombination DNA_section_27

Further information: Genetic recombination DNA_sentence_289

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". DNA_sentence_290

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 in chromosomal crossover which occurs during sexual reproduction, when genetic recombination occurs. DNA_sentence_291

Chromosomal crossover is when two DNA helices break, swap a section and then rejoin. DNA_sentence_292

Recombination allows chromosomes to exchange genetic information and produces new combinations of genes, which increases the efficiency of natural selection and can be important in the rapid evolution of new proteins. DNA_sentence_293

Genetic recombination can also be involved in DNA repair, particularly in the cell's response to double-strand breaks. DNA_sentence_294

The most common form of chromosomal crossover is homologous recombination, where the two chromosomes involved share very similar sequences. DNA_sentence_295

Non-homologous recombination can be damaging to cells, as it can produce chromosomal translocations and genetic abnormalities. DNA_sentence_296

The recombination reaction is catalyzed by enzymes known as recombinases, such as RAD51. DNA_sentence_297

The first step in recombination is a double-stranded break caused by either an endonuclease or damage to the DNA. DNA_sentence_298

A series of steps catalyzed in part by the recombinase then leads to joining of the two helices by at least one Holliday junction, in which a segment of a single strand in each helix is annealed to the complementary strand in the other helix. DNA_sentence_299

The Holliday junction is a tetrahedral junction structure that can be moved along the pair of chromosomes, swapping one strand for another. DNA_sentence_300

The recombination reaction is then halted by cleavage of the junction and re-ligation of the released DNA. DNA_sentence_301

Only strands of like polarity exchange DNA during recombination. DNA_sentence_302

There are two types of cleavage: east-west cleavage and north–south cleavage. DNA_sentence_303

The north–south cleavage nicks both strands of DNA, while the east–west cleavage has one strand of DNA intact. DNA_sentence_304

The formation of a Holliday junction during recombination makes it possible for genetic diversity, genes to exchange on chromosomes, and expression of wild-type viral genomes. DNA_sentence_305

Evolution DNA_section_28

Further information: RNA world hypothesis DNA_sentence_306

DNA contains the genetic information that allows all forms of life to function, grow and reproduce. DNA_sentence_307

However, it is unclear how long in the 4-billion-year history of life DNA has performed this function, as it has been proposed that the earliest forms of life may have used RNA as their genetic material. DNA_sentence_308

RNA may have acted as the central part of early cell metabolism as it can both transmit genetic information and carry out catalysis as part of ribozymes. DNA_sentence_309

This ancient RNA world where nucleic acid would have been used for both catalysis and genetics may have influenced the evolution of the current genetic code based on four nucleotide bases. DNA_sentence_310

This would occur, since the number of different 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. DNA_sentence_311

However, there is no direct evidence of ancient genetic systems, as recovery of DNA from most fossils is impossible because DNA survives in the environment for less than one million years, and slowly degrades into short fragments in solution. DNA_sentence_312

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, but these claims are controversial. DNA_sentence_313

Building blocks of DNA (adenine, guanine, and related organic molecules) may have been formed extraterrestrially in outer space. DNA_sentence_314

Complex DNA and RNA organic compounds of life, including uracil, cytosine, and thymine, have also been formed in the laboratory under conditions mimicking those found in outer space, using starting chemicals, such as pyrimidine, found in meteorites. DNA_sentence_315

Pyrimidine, like polycyclic aromatic hydrocarbons (PAHs), the most carbon-rich chemical found in the universe, may have been formed in red giants or in interstellar cosmic dust and gas clouds. DNA_sentence_316

Uses in technology DNA_section_29

Genetic engineering DNA_section_30

Further information: Molecular biology, Nucleic acid methods, and Genetic engineering DNA_sentence_317

Methods have been developed to purify DNA from organisms, such as phenol-chloroform extraction, and to manipulate it in the laboratory, such as restriction digests and the polymerase chain reaction. DNA_sentence_318

Modern biology and biochemistry make intensive use of these techniques in recombinant DNA technology. DNA_sentence_319

Recombinant DNA is a man-made DNA sequence that has been assembled from other DNA sequences. DNA_sentence_320

They can be transformed into organisms in the form of plasmids or in the appropriate format, by using a viral vector. DNA_sentence_321

The genetically modified organisms produced can be used to produce products such as recombinant proteins, used in medical research, or be grown in agriculture. DNA_sentence_322

DNA profiling DNA_section_31

Further information: DNA profiling DNA_sentence_323

Forensic scientists can use DNA in blood, semen, skin, saliva or hair found at a crime scene to identify a matching DNA of an individual, such as a perpetrator. DNA_sentence_324

This process is formally termed DNA profiling, also called DNA fingerprinting. DNA_sentence_325

In DNA profiling, the lengths of variable sections of repetitive DNA, such as short tandem repeats and minisatellites, are compared between people. DNA_sentence_326

This method is usually an extremely reliable technique for identifying a matching DNA. DNA_sentence_327

However, identification can be complicated if the scene is contaminated with DNA from several people. DNA_sentence_328

DNA profiling was developed in 1984 by British geneticist Sir Alec Jeffreys, and first used in forensic science to convict Colin Pitchfork in the 1988 Enderby murders case. DNA_sentence_329

The development of forensic science and the ability to now obtain genetic matching on minute samples of blood, skin, saliva, or hair has led to re-examining many cases. DNA_sentence_330

Evidence can now be uncovered that was scientifically impossible at the time of the original examination. DNA_sentence_331

Combined with the removal of the double jeopardy law in some places, this can allow cases to be reopened where prior trials have failed to produce sufficient evidence to convince a jury. DNA_sentence_332

People charged with serious crimes may be required to provide a sample of DNA for matching purposes. DNA_sentence_333

The most obvious defense to DNA matches obtained forensically is to claim that cross-contamination of evidence has occurred. DNA_sentence_334

This has resulted in meticulous strict handling procedures with new cases of serious crime. DNA_sentence_335

DNA profiling is also used successfully to positively identify victims of mass casualty incidents, bodies or body parts in serious accidents, and individual victims in mass war graves, via matching to family members. DNA_sentence_336

DNA profiling is also used in DNA paternity testing to determine if someone is the biological parent or grandparent of a child with the probability of parentage is typically 99.99% when the alleged parent is biologically related to the child. DNA_sentence_337

Normal DNA sequencing methods happen after birth, but there are new methods to test paternity while a mother is still pregnant. DNA_sentence_338

DNA enzymes or catalytic DNA DNA_section_32

Further information: Deoxyribozyme DNA_sentence_339

Deoxyribozymes, also called DNAzymes or catalytic DNA, were first discovered in 1994. DNA_sentence_340

They are mostly single stranded DNA sequences isolated from a large pool of random DNA sequences through a combinatorial approach called in vitro selection or systematic evolution of ligands by exponential enrichment (SELEX). DNA_sentence_341

DNAzymes catalyze variety of chemical reactions including RNA-DNA cleavage, RNA-DNA ligation, amino acids phosphorylation-dephosphorylation, carbon-carbon bond formation, etc. DNAzymes can enhance catalytic rate of chemical reactions up to 100,000,000,000-fold over the uncatalyzed reaction. DNA_sentence_342

The most extensively studied class of DNAzymes is RNA-cleaving types which have been used to detect different metal ions and designing therapeutic agents. DNA_sentence_343

Several metal-specific DNAzymes have been reported including the GR-5 DNAzyme (lead-specific), the CA1-3 DNAzymes (copper-specific), the 39E DNAzyme (uranyl-specific) and the NaA43 DNAzyme (sodium-specific). DNA_sentence_344

The NaA43 DNAzyme, which is reported to be more than 10,000-fold selective for sodium over other metal ions, was used to make a real-time sodium sensor in cells. DNA_sentence_345

Bioinformatics DNA_section_33

Further information: Bioinformatics DNA_sentence_346

Bioinformatics involves the development of techniques to store, data mine, search and manipulate biological data, including DNA nucleic acid sequence data. DNA_sentence_347

These have led to widely applied advances in computer science, especially string searching algorithms, machine learning, and database theory. DNA_sentence_348

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. DNA_sentence_349

The DNA sequence may be aligned with other DNA sequences to identify homologous sequences and locate the specific mutations that make them distinct. DNA_sentence_350

These techniques, especially multiple sequence alignment, are used in studying phylogenetic relationships and protein function. DNA_sentence_351

Data sets representing entire genomes' worth of DNA sequences, such as those produced by the Human Genome Project, are difficult to use without the annotations that identify the locations of genes and regulatory elements on each chromosome. DNA_sentence_352

Regions of DNA sequence that have the characteristic patterns associated with protein- or RNA-coding genes can be identified by gene finding algorithms, which allow researchers to predict the presence of particular gene products and their possible functions in an organism even before they have been isolated experimentally. DNA_sentence_353

Entire genomes may also be compared, which can shed light on the evolutionary history of particular organism and permit the examination of complex evolutionary events. DNA_sentence_354

DNA nanotechnology DNA_section_34

Further information: DNA nanotechnology DNA_sentence_355

DNA nanotechnology uses the unique molecular recognition properties of DNA and other nucleic acids to create self-assembling branched DNA complexes with useful properties. DNA_sentence_356

DNA is thus used as a structural material rather than as a carrier of biological information. DNA_sentence_357

This has led to the creation of two-dimensional periodic lattices (both tile-based and using the DNA origami method) and three-dimensional structures in the shapes of polyhedra. DNA_sentence_358

Nanomechanical devices and algorithmic self-assembly have also been demonstrated, and these DNA structures have been used to template the arrangement of other molecules such as gold nanoparticles and streptavidin proteins. DNA_sentence_359

History and anthropology DNA_section_35

Further information: Phylogenetics and Genetic genealogy DNA_sentence_360

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 phylogeny. DNA_sentence_361

This field of phylogenetics is a powerful tool in evolutionary biology. DNA_sentence_362

If DNA sequences within a species are compared, population geneticists can learn the history of particular populations. DNA_sentence_363

This can be used in studies ranging from ecological genetics to anthropology. DNA_sentence_364

Information storage DNA_section_36

Main article: DNA digital data storage DNA_sentence_365

DNA as a storage device for information has enormous potential since it has much higher storage density compared to electronic devices. DNA_sentence_366

However, high costs, extremely slow read and write times (memory latency), and insufficient reliability has prevented its practical use. DNA_sentence_367

History DNA_section_37

Further information: History of molecular biology DNA_sentence_368

DNA was first isolated by the Swiss physician Friedrich Miescher who, in 1869, discovered a microscopic substance in the pus of discarded surgical bandages. DNA_sentence_369

As it resided in the nuclei of cells, he called it "nuclein". DNA_sentence_370

In 1878, Albrecht Kossel isolated the non-protein component of "nuclein", nucleic acid, and later isolated its five primary nucleobases. DNA_sentence_371

In 1909, Phoebus Levene identified the base, sugar, and phosphate nucleotide unit of the RNA (then named "yeast nucleic acid"). DNA_sentence_372

In 1929, Levene identified deoxyribose sugar in "thymus nucleic acid" (DNA). DNA_sentence_373

Levene suggested that DNA consisted of a string of four nucleotide units linked together through the phosphate groups ("tetranucleotide hypothesis"). DNA_sentence_374

Levene thought the chain was short and the bases repeated in a fixed order. DNA_sentence_375

In 1927, Nikolai Koltsov proposed that inherited traits would be inherited via a "giant hereditary molecule" made up of "two mirror strands that would replicate in a semi-conservative fashion using each strand as a template". DNA_sentence_376

In 1928, Frederick Griffith in his experiment discovered that traits of the "smooth" form of Pneumococcus could be transferred to the "rough" form of the same bacteria by mixing killed "smooth" bacteria with the live "rough" form. DNA_sentence_377

This system provided the first clear suggestion that DNA carries genetic information. DNA_sentence_378

In 1933, while studying virgin sea urchin eggs, Jean Brachet suggested that DNA is found in the cell nucleus and that RNA is present exclusively in the cytoplasm. DNA_sentence_379

At the time, "yeast nucleic acid" (RNA) was thought to occur only in plants, while "thymus nucleic acid" (DNA) only in animals. DNA_sentence_380

The latter was thought to be a tetramer, with the function of buffering cellular pH. DNA_sentence_381

In 1937, William Astbury produced the first X-ray diffraction patterns that showed that DNA had a regular structure. DNA_sentence_382

In 1943, Oswald Avery, along with co-workers Colin MacLeod and Maclyn McCarty, identified DNA as the transforming principle, supporting Griffith's suggestion (Avery–MacLeod–McCarty experiment). DNA_sentence_383

DNA's role in heredity was confirmed in 1952 when Alfred Hershey and Martha Chase in the Hershey–Chase experiment showed that DNA is the genetic material of the enterobacteria phage T2. DNA_sentence_384

Late in 1951, Francis Crick started working with James Watson at the Cavendish Laboratory within the University of Cambridge. DNA_sentence_385

In February 1953, Linus Pauling and Robert Corey proposed a model for nucleic acids containing three intertwined chains, with the phosphates near the axis, and the bases on the outside. DNA_sentence_386

In May 1952, Raymond Gosling a graduate student working under the supervision of Rosalind Franklin took an X-ray diffraction image, labeled as "Photo 51", at high hydration levels of DNA. DNA_sentence_387

This photo was given to Watson and Crick by Maurice Wilkins and was critical to their obtaining the correct structure of DNA. DNA_sentence_388

Franklin told Crick and Watson that the backbones had to be on the outside. DNA_sentence_389

Before then, Linus Pauling, and Watson and Crick, had erroneous models with the chains inside and the bases pointing outwards. DNA_sentence_390

Her identification of the space group for DNA crystals revealed to Crick that the two DNA strands were antiparallel. DNA_sentence_391

In February 1953, Watson and Crick completed their model, which is now accepted as the first correct model of the double-helix of DNA. DNA_sentence_392

On 28 February 1953 Crick interrupted patrons' lunchtime at The Eagle pub in Cambridge to announce that he and Watson had "discovered the secret of life". DNA_sentence_393

In the 25 April 1953 issue of the journal Nature, were published a series of five articles giving the Watson and Crick double-helix structure DNA, and evidence supporting it. DNA_sentence_394

The structure was reported in a letter titled "MOLECULAR STRUCTURE OF NUCLEIC ACIDS A Structure for Deoxyribose Nucleic Acid", in which they said, "It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material." DNA_sentence_395

Followed by a letter from Franklin and Gosling, which was the first publication of their own X-ray diffraction data, and of their original analysis method. DNA_sentence_396

Then followed a letter by Wilkins, and two of his colleagues, which contained an analysis of in vivo B-DNA X-ray patterns, and supported the presence in vivo of the Watson and Crick structure. DNA_sentence_397

In 1962, after Franklin's death, Watson, Crick, and Wilkins jointly received the Nobel Prize in Physiology or Medicine. DNA_sentence_398

Nobel Prizes are awarded only to living recipients. DNA_sentence_399

A debate continues about who should receive credit for the discovery. DNA_sentence_400

In an influential presentation in 1957, Crick laid out the central dogma of molecular biology, which foretold the relationship between DNA, RNA, and proteins, and articulated the "adaptor hypothesis". DNA_sentence_401

Final confirmation of the replication mechanism that was implied by the double-helical structure followed in 1958 through the Meselson–Stahl experiment. DNA_sentence_402

Further work by Crick and co-workers showed that the genetic code was based on non-overlapping triplets of bases, called codons, allowing Har Gobind Khorana, Robert W. Holley, and Marshall Warren Nirenberg to decipher the genetic code. DNA_sentence_403

These findings represent the birth of molecular biology. DNA_sentence_404

See also DNA_section_38

Credits to the contents of this page go to the authors of the corresponding Wikipedia page: en.wikipedia.org/wiki/DNA.