science

For a non-technical introduction to the topic, see [|Introduction to genetics]. For other uses, see [|DNA (disambiguation)]. http://www.youtube.com/watch?v=ZXt4pDVb2W0&NR=1

structure structure structure The structure of the DNA [|double helix]. The [|atoms] in the structure are colour coded by [|element] and the detailed structure of two base pairs is shown in the bottom right. http://www.youtube.com/watch?v=ZXt4pDVb2W0&NR=1 The structure of part of a DNA [|double helix] DNA consists of two long [|polymers] of simple units called [|nucleotides], with [|backbones] made of [|sugars] and [|phosphate] groups joined by [|ester] bonds. These two strands run in opposite directions to each other and are therefore [|anti-parallel]. Attached to each sugar is one of four types of molecules called [|nucleobases] (informally, //bases//). It is the [|sequence] of these four nucleobases along the backbone that encodes information. This information is read using the [|genetic code], which specifies the sequence of the [|amino acids] within proteins. The code is read by copying stretches of DNA into the related nucleic acid RNA in a process called [|transcription]. Within cells DNA is organized into long structures called [|chromosomes]. During [|cell division] these chromosomes are duplicated in the process of [|DNA replication], providing each cell its own complete set of chromosomes. [|Eukaryotic organisms] ( [|animals], [|plants] , [|fungi] , and [|protists] ) store most of their DNA inside the [|cell nucleus] and some of their DNA in [|organelles] , such as [|mitochondria] or [|chloroplasts]. [|[1]] In contrast, [|prokaryotes] ( [|bacteria] and [|archaea] ) store their DNA only in the [|cytoplasm]. Within the chromosomes, [|chromatin] proteins such as [|histones] compact and organize DNA. These compact structures guide the interactions between DNA and other proteins, helping control which parts of the DNA are transcribed. [ [|hide] ] *  [|1 Properties]
 * Deoxyribonucleic acid** ( [|/diˌɒksiˌraɪbɵ.njuːˌkleɪ.ɨk ˈæsɪd/]  ( [[image:http://upload.wikimedia.org/wikipedia/commons/thumb/2/21/Speaker_Icon.svg/13px-Speaker_Icon.svg.png width="13" height="13" link="http://en.wikipedia.org/wiki/File:En-us-Deoxyribonucleic_acid.ogg"]] [|listen] ) ; **DNA**) is a [|nucleic acid] that contains the [|genetic] instructions used in the development and functioning of all known living [|organisms] (with the exception of [|RNA viruses] ). The DNA segments that carry this genetic information are called [|genes], but other DNA sequences have structural purposes, or are involved in regulating the use of this genetic information. Along with RNA and [|proteins] , DNA is one of the three major [|macromolecules] that are essential for all known forms of life.
 * == Contents ==
 * [|1.1 Grooves]
 * [|1.2 Base pairing]
 * [|1.3 Sense and antisense]
 * [|1.4 Supercoiling]
 * [|1.5 Alternate DNA structures]
 * [|1.6 Alternate DNA chemistry]
 * [|1.7 Quadruplex structures]
 * [|1.8 Branched DNA]
 * [|1.9 Vibration]
 * [|2 Chemical modifications]
 * [|2.1 Base modifications]
 * [|2.2 Damage]
 * [|3 Biological functions]
 * [|3.1 Genes and genomes]
 * [|3.2 Transcription and translation]
 * [|3.3 Replication]
 * [|4 Interactions with proteins]
 * [|4.1 DNA-binding proteins]
 * [|4.2 DNA-modifying enzymes]
 * [|4.2.1 Nucleases and ligases]
 * [|4.2.2 Topoisomerases and helicases]
 * [|4.2.3 Polymerases]
 * [|5 Genetic recombination]
 * [|6 Evolution]
 * [|7 Uses in technology]
 * [|7.1 Genetic engineering]
 * [|7.2 Forensics]
 * [|7.3 Bioinformatics]
 * [|7.4 DNA nanotechnology]
 * [|7.5 History and anthropology]
 * [|8 History of DNA research]
 * [|9 See also]
 * [|10 References]
 * [|11 Further reading]
 * [|12 External links] ||

Properties
Chemical structure of DNA. [|Hydrogen bonds] shown as dotted lines. DNA is a long [|polymer] made from repeating units called [|nucleotides]. [|[2]] [|[3]] [|[4]] As first discovered by [|James D. Watson] and [|Francis Crick], the structure of DNA of all species comprises two helical chains each coiled round the same axis, and each with a pitch of 34 [|Ångströms] (3.4 [|nanometres] ) and a radius of 10 [|Ångströms] (1.0 [|nanometres] ). [|[5]] According to another study, when measured in a particular solution, the DNA chain measured 22 to 26 [|Ångströms] wide (2.2 to 2.6 [|nanometres] ), and one nucleotide unit measured 3.3 Å (0.33 nm) long. [|[6]] Although each individual repeating unit is very small, DNA polymers can be very large molecules containing millions of nucleotides. For instance, the largest [|human chromosome], chromosome number 1, is approximately 220 million [|base pairs] long. [|[7]] In living organisms DNA does not usually exist as a single molecule, but instead as a pair of molecules that are held tightly together. [|[5]] [|[8]] These two long strands entwine like vines, in the shape of a [|double helix]. The nucleotide repeats contain both the 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. A nucleobase linked to a sugar is called a [|nucleoside] and a base linked to a sugar and one or more phosphate groups is called a [|nucleotide]. Polymers comprising multiple linked nucleotides (as in DNA) is called a [|polynucleotide]. [|[9]] The backbone of the DNA strand is made from alternating [|phosphate] and [|sugar] residues. [|[10]] The sugar in DNA is [|2-deoxyribose], which is a [|pentose] (five- [|carbon] ) sugar. The sugars are joined together by phosphate groups that form [|phosphodiester bonds] between the third and fifth carbon [|atoms] of adjacent sugar rings. These asymmetric [|bonds] mean a strand of DNA has a direction. In a double helix the direction of the nucleotides in one strand is opposite to their direction in the other strand: the strands are //antiparallel//. The asymmetric ends of DNA strands are called the [|5′] (//five prime//) and [|3′] (//three prime//) ends, with the 5' end having a terminal phosphate group and the 3' end a terminal hydroxyl group. 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. [|[8]] A section of DNA. The bases lie horizontally between the two spiraling strands. [|[11]] Animated version at. The DNA double helix is stabilized primarily by two forces: [|hydrogen bonds] between nucleotides and [|base-stacking] interactions among the [|aromatic] nucleobases. [|[12]] In the aqueous environment of the cell, the conjugated [|π bonds] of nucleotide bases align perpendicular to the axis of the DNA molecule, minimizing their interaction with the [|solvation shell] and therefore, the [|Gibbs free energy]. The four bases found in DNA are [|adenine] (abbreviated A), [|cytosine] (C), [|guanine] (G) and [|thymine] (T). These four bases are attached to the sugar/phosphate to form the complete nucleotide, as shown for [|adenosine monophosphate]. The nucleobases are classified into two types: the [|purines], A and G, being fused five- and six-membered [|heterocyclic compounds] , and the [|pyrimidines] , the six-membered rings C and T. [|[8]] 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. Uracil is not usually found in DNA, occurring only as a breakdown product of cytosine. In addition to RNA and DNA a large number of artificial [|nucleic acid analogues] have also been created to study the proprieties of nucleic acids, or for use in biotechnology. [|[13]] Major and minor grooves of DNA. Minor groove is a binding site for the dye [|Hoechst 33258].

Grooves
Twin helical strands form the DNA backbone. Another double helix may be found by tracing the spaces, or grooves, between the strands. These voids are adjacent to the base pairs and may provide a [|binding site]. As the strands are not directly opposite each other, the grooves are unequally sized. One groove, the major groove, is 22 Å wide and the other, the minor groove, is 12 Å wide. [|[14]] The narrowness of the minor groove means that the edges of the bases are more accessible in the major groove. As a result, proteins like [|transcription factors] that can bind to specific sequences in double-stranded DNA usually make contacts to the sides of the bases exposed in the major groove. [|[15]] 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.

Base pairing
Further information: [|Base pair] In a DNA double helix, each type of nucleobase on one strand normally interacts with just one type of nucleobase on the other strand. This is called complementary [|base pairing]. Here, purines form [|hydrogen bonds] to pyrimidines, with A bonding only to T, and C bonding only to G. This arrangement of two nucleotides binding together across the double helix is called a base pair. As hydrogen bonds are not [|covalent], they can be broken and rejoined relatively easily. The two strands of DNA in a double helix can therefore be pulled apart like a zipper, either by a mechanical force or high [|temperature]. [|[16]] 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. [|[3]]
 * [[image:http://upload.wikimedia.org/wikipedia/commons/thumb/d/d7/GC_DNA_base_pair.svg/282px-GC_DNA_base_pair.svg.png width="282" height="177" caption="GC DNA base pair.svg" link="http://en.wikipedia.org/wiki/File:GC_DNA_base_pair.svg"]] ||

Top, a **GC** base pair with three [|hydrogen bonds]. 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). DNA with high [|GC-content] is more stable than DNA with low GC-content. Although it is often stated that this is due to the added stability of an additional hydrogen bond, this is incorrect. DNA with high GC-content is more stable due to intra-strand base stacking interactions. As noted above, most DNA molecules are actually two polymer strands, bound together in a helical fashion by noncovalent bonds; this double stranded structure (dsDNA) is maintained largely by the intrastrand base stacking interactions, which are strongest for G,C stacks. The two strands can come apart - a process known as melting - to form two ss DNA molecules. Melting occurs when conditions favor ssDNA; such conditions are 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). 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). 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. 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. [|[17]] 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. [|[18]] In the laboratory, the strength of this interaction can be measured by finding the temperature required to break the hydrogen bonds, their [|melting temperature] (also called //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 (//ssDNA//) have no single common shape, but some conformations are more stable than others. [|[19]]
 * [[image:http://upload.wikimedia.org/wikipedia/commons/thumb/9/91/AT_DNA_base_pair.svg/282px-AT_DNA_base_pair.svg.png width="282" height="151" caption="AT DNA base pair.svg" link="http://en.wikipedia.org/wiki/File:AT_DNA_base_pair.svg"]] ||

Sense and antisense
Further information: [|Sense (molecular biology)] A DNA sequence is called "sense" if its sequence is the same as that of a [|messenger RNA] copy that is translated into protein. [|[20]] 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. [|[21]] One proposal is that antisense RNAs are involved in regulating [|gene expression] through RNA-RNA base pairing. [|[22]] 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. [|[23]] 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 [|bacteria], this overlap may be involved in the regulation of gene transcription, [|[24]] while in viruses, overlapping genes increase the amount of information that can be encoded within the small viral genome. [|[25]]

Supercoiling
Further information: [|DNA supercoil] DNA can be twisted like a rope in a process called [|DNA supercoiling]. 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. [|[26]] 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 [|enzymes] called [|topoisomerases]. [|[27]] These enzymes are also needed to relieve the twisting stresses introduced into DNA strands during processes such as [|transcription] and [|DNA replication]. [|[28]] From left to right, the structures of A, B and Z DNA

Alternate DNA structures
Further information: [|Molecular Structure of Nucleic Acids: A Structure for Deoxyribose Nucleic Acid], [|Molecular models of DNA] , and [|DNA structure] 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. [|[10]] 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], as well as the presence of [|polyamines] in solution. [|[29]] 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. [|[30]] [|[31]] An alternate 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]. [|[32]] In the same journal, [|James D. 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. [|[5]] Although the `B-DNA form' is most common under the conditions found in cells, [|[33]] it is not a well-defined conformation but a family of related DNA conformations [|[34]] that occur at the high hydration levels present in living cells. Their corresponding X-ray diffraction and scattering patterns are characteristic of molecular [|paracrystals] with a significant degree of disorder. [|[35]] [|[36]] 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. The A form occurs under non-physiological conditions in partially 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. [|[37]] [|[38]] Segments of DNA where the bases have been chemically modified by [|methylation] may undergo a larger change in conformation and adopt the [|Z form]. Here, the strands turn about the helical axis in a [|left-handed] spiral, the opposite of the more common B form. [|[39]] These unusual structures can be recognized by specific Z-DNA binding proteins and may be involved in the regulation of transcription. [|[40]]

Alternate DNA chemistry
For a number of 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. One of the proposals was the existence of lifeforms that use [|arsenic instead of phosphorus in DNA]. A December 2010 [|NASA] press conference stated that the [|bacterium] [|GFAJ-1], which has evolved in an arsenic-rich environment, is the first terrestrial lifeform found which may have this ability. The bacterium was found in [|Mono Lake], east of [|Yosemite National Park]. GFAJ-1 is a [|rod] -shaped [|extremophile] bacterium in the family [|Halomonadaceae] that, when starved of [|phosphorus], may be capable of incorporating the usually poisonous element [|arsenic] in its DNA. [|[41]] This discovery may lend weight to the long-standing idea that [|extraterrestrial life] could have a [|different chemical makeup] from life on [|Earth]. [|[41]] [|[42]] The research was carried out by a team led by [|Felisa Wolfe-Simon], a [|geomicrobiologist] and geobiochemist, a Postdoctoral Fellow of the [|NASA Astrobiology Institute] with [|Arizona State University]. This finding has, however, faced strong criticism from the scientific community; scientists have argued that there is no evidence that arsenic is actually incorporated into biomolecules. [|[42]] [|[43]] Independent conformation of this finding has also not yet been possible.

Quadruplex structures
Further information: [|G-quadruplex] At the ends of the linear chromosomes are specialized regions of DNA called [|telomeres]. 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. [|[44]] 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. [|[45]] In [|human cells], telomeres are usually lengths of single-stranded DNA containing several thousand repeats of a simple TTAGGG sequence. [|[46]] DNA quadruplex formed by [|telomere] repeats. The looped conformation of the DNA backbone is very different from the typical DNA helix. [|[47]] 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 // [|G-quadruplex] // structure. [|[48]] 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. [|[49]] 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. [|[50]] 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 [|triple-stranded] structure is called a displacement loop or [|D-loop]. [|[48]] [|Branched DNA] can form networks containing multiple branches.
 * [[image:http://upload.wikimedia.org/wikipedia/commons/thumb/9/9b/Branch-dna.png/75px-Branch-dna.png width="75" height="49" caption="Branch-dna.png" link="http://en.wikipedia.org/wiki/File:Branch-dna.png"]] || [[image:http://upload.wikimedia.org/wikipedia/commons/thumb/5/55/Multi-branch-dna.png/95px-Multi-branch-dna.png width="95" height="114" caption="Multi-branch-dna.png" link="http://en.wikipedia.org/wiki/File:Multi-branch-dna.png"]] ||
 * Single branch || Multiple branches ||

Branched DNA
Further information: [|Branched DNA] and [|DNA nanotechnology] In DNA [|fraying] occurs when non-complementary regions exist at the end of an otherwise complementary double-strand of DNA. 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. Although the simplest example of branched DNA involves only three strands of DNA, complexes involving additional strands and multiple branches are also possible. [|[51]] Branched DNA can be used in [|nanotechnology] to construct geometric shapes, see the section on [|uses in technology] below.

Vibration
DNA may carry out [|low-frequency] collective motion as observed by the [|Raman spectroscopy] [|[52]] [|[53]] and analyzed with a quasi-continuum model. [|[54]] [|[55]]

Chemical modifications
Structure of cytosine with and without the 5-methyl group. [|Deamination] converts 5-methylcytosine into thymine.
 * [[image:http://upload.wikimedia.org/wikipedia/commons/thumb/d/dd/Cytosin.svg/75px-Cytosin.svg.png width="75" height="102" caption="Cytosin.svg" link="http://en.wikipedia.org/wiki/File:Cytosin.svg"]] || [[image:http://upload.wikimedia.org/wikipedia/commons/thumb/3/3a/5-Methylcytosine.svg/95px-5-Methylcytosine.svg.png width="95" height="83" caption="5-Methylcytosine.svg" link="http://en.wikipedia.org/wiki/File:5-Methylcytosine.svg"]] || [[image:http://upload.wikimedia.org/wikipedia/commons/thumb/1/15/Thymin.svg/97px-Thymin.svg.png width="97" height="83" caption="Thymin.svg" link="http://en.wikipedia.org/wiki/File:Thymin.svg"]] ||
 * [|cytosine] || [|5-methylcytosine] || [|thymine] ||

Base modifications
Further information: [|DNA methylation] The expression of genes is influenced by how the DNA is packaged in chromosomes, in a structure called [|chromatin]. 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. For example, cytosine methylation, produces [|5-methylcytosine], which is important for [|X-chromosome inactivation]. [|[56]] 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. [|[57]] Despite the importance of 5-methylcytosine, it can [|deaminate] to leave a thymine base, so methylated cytosines are particularly prone to [|mutations]. [|[58]] Other base modifications include adenine methylation in bacteria, the presence of [|5-hydroxymethylcytosine] in the [|brain], [|[59]] and the [|glycosylation] of uracil to produce the "J-base" in [|kinetoplastids]. [|[60]] [|[61]]

Damage
Further information: [|Mutation] A [|covalent] [|adduct] between a [|metabolically activated] form of [|benzo[//a//pyrene]], the major [|mutagen] in [|tobacco smoke] , and DNA [|[62]] DNA can be damaged by many sorts of [|mutagens], which change the DNA sequence. Mutagens include [|oxidizing agents], [|alkylating agents] and also high-energy [|electromagnetic radiation] such as [|ultraviolet] light and [|X-rays]. The type of DNA damage produced depends on the type of mutagen. For example, UV light can damage DNA by producing [|thymine dimers], which are cross-links between pyrimidine bases. [|[63]] 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. [|[64]] A typical human cell contains about 150,000 bases that have suffered oxidative damage. [|[65]] Of these oxidative lesions, the most dangerous are double-strand breaks, as these are difficult to repair and can produce [|point mutations], [|insertions] and [|deletions] from the DNA sequence, as well as [|chromosomal translocations]. [|[66]] Many mutagens fit into the space between two adjacent base pairs, this is called [|//intercalation//]. Most intercalators are [|aromatic] and planar molecules; examples include [|ethidium bromide], [|daunomycin] , and [|doxorubicin]. 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 [|carcinogens], and [|benzo[//a//pyrene diol epoxide]] , [|acridines] , [|aflatoxin] and [|ethidium bromide] are well-known examples. [|[67]] [|[68]] [|[69]] 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. [|[70]]

Biological functions
DNA usually occurs as linear [|chromosomes] in [|eukaryotes], and circular chromosomes in [|prokaryotes]. 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. [|[71]] The information carried by DNA is held in the [|sequence] of pieces of DNA called [|genes]. [|Transmission] 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 [|translation], which depends on the same interaction between RNA nucleotides. In alternative fashion, 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.

Genes and genomes
Further information: [|Cell nucleus], [|Chromatin] , [|Chromosome] , [|Gene] , [|Noncoding DNA] Genomic DNA is tightly and orderly packed in the process called [|DNA condensation] to fit the small available volumes of the cell. In eukaryotes, DNA is located in the [|cell nucleus], as well as small amounts in [|mitochondria] and [|chloroplasts]. In prokaryotes, the DNA is held within an irregularly shaped body in the cytoplasm called the [|nucleoid]. [|[72]] The genetic information in a genome is held within genes, and the complete set of this information in an organism is called its [|genotype]. A gene is a unit of [|heredity] and is a region of DNA that influences a particular characteristic in an organism. Genes contain an [|open reading frame] that can be transcribed, as well as [|regulatory sequences] such as [|promoters] and [|enhancers], which control the transcription of the open reading frame. In many [|species], only a small fraction of the total sequence of the [|genome] encodes protein. 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]. [|[73]] 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] ". [|[74]] However, DNA sequences that do not code protein may still encode functional [|non-coding RNA] molecules, which are involved in the [|regulation of gene expression]. [|[75]] [|T7 RNA polymerase] (blue) producing a mRNA (green) from a DNA template (orange). [|[76]] Some noncoding DNA sequences play structural roles in chromosomes. [|Telomeres] and [|centromeres] typically contain few genes, but are important for the function and stability of chromosomes. [|[45]] [|[77]] An abundant form of noncoding DNA in humans are [|pseudogenes], which are copies of genes that have been disabled by mutation. [|[78]] 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]. [|[79]]

Transcription and translation
Further information: [|Genetic code], [|Transcription (genetics)] , [|Protein biosynthesis] A gene is a sequence of DNA that contains genetic information and can influence the [|phenotype] of an organism. Within a gene, the sequence of bases along a DNA strand defines a [|messenger RNA] sequence, which then defines one or more protein sequences. 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]. 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 [|RNA polymerase]. 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. Since there are 4 bases in 3-letter combinations, there are 64 possible codons (43 combinations). These encode the twenty [|standard amino acids], 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 [|helicase] and [|topoisomerase]. Next, one [|DNA polymerase] produces the [|leading strand] copy. Another DNA polymerase binds to the [|lagging strand]. This enzyme makes discontinuous segments (called [|Okazaki fragments] ) before [|DNA ligase] joins them together.

Replication
Further information: [|DNA replication] [|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. The double-stranded structure of DNA provides a simple mechanism for [|DNA replication]. Here, the two strands are separated and then each strand's [|complementary DNA] sequence is recreated by an [|enzyme] called [|DNA polymerase]. 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. [|[80]] 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.

Interactions with proteins
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.

DNA-binding proteins
Further information: [|DNA-binding protein] Interaction of DNA (shown in orange) with [|histones] (shown in blue). These proteins' basic amino acids bind to the acidic phosphate groups on DNA. 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 [|chromatin]. 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. [|[81]] [|[82]] The histones form a disk-shaped complex called a [|nucleosome], 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 [|ionic bonds] to the acidic sugar-phosphate backbone of the DNA, and are therefore largely independent of the base sequence. [|[83]] Chemical modifications of these basic amino acid residues include [|methylation], [|phosphorylation] and [|acetylation]. [|[84]] 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. [|[85]] Other non-specific DNA-binding proteins in chromatin include the high-mobility group proteins, which bind to bent or distorted DNA. [|[86]] These proteins are important in bending arrays of nucleosomes and arranging them into the larger structures that make up chromosomes. [|[87]] 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. [|[88]] These binding proteins seem to stabilize single-stranded DNA and protect it from forming [|stem-loops] or being degraded by [|nucleases]. The lambda repressor [|helix-turn-helix] transcription factor bound to its DNA target [|[89]] In contrast, other proteins have evolved to bind to particular DNA sequences. The most intensively studied of these are the various [|transcription factors], 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. [|[90]] Alternatively, transcription factors can bind [|enzymes] that modify the histones at the promoter; this will change the accessibility of the DNA template to the polymerase. [|[91]] 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. [|[92]] Consequently, these proteins are often the targets of the [|signal transduction] 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. [|[15]] The [|restriction enzyme] [|EcoRV] (green) in a complex with its substrate DNA [|[93]]
 * [[image:http://upload.wikimedia.org/wikipedia/commons/thumb/b/be/Nucleosome1.png/260px-Nucleosome1.png width="260" height="171" caption="Nucleosome1.png" link="http://en.wikipedia.org/wiki/File:Nucleosome1.png"]] ||

Nucleases and ligases
[|Nucleases] are [|enzymes] that cut DNA strands by catalyzing the [|hydrolysis] of the [|phosphodiester bonds]. Nucleases that hydrolyse nucleotides from the ends of DNA strands are called [|exonucleases], while [|endonucleases] cut within strands. The most frequently used nucleases in [|molecular biology] are the [|restriction endonucleases], 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 [|bacteria] against [|phage] infection by digesting the phage DNA when it enters the bacterial cell, acting as part of the [|restriction modification system]. [|[94]] In technology, these sequence-specific nucleases are used in [|molecular cloning] and [|DNA fingerprinting]. Enzymes called [|DNA ligases] can rejoin cut or broken DNA strands. [|[95]] 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. They are also used in [|DNA repair] and [|genetic recombination]. [|[95]]

Topoisomerases and helicases
[|Topoisomerases] are enzymes with both nuclease and ligase activity. These proteins change the amount of [|supercoiling] in DNA. 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. [|[27]] 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. [|[96]] Topoisomerases are required for many processes involving DNA, such as DNA replication and transcription. [|[28]] [|Helicases] are proteins that are a type of [|molecular motor]. They use the chemical energy in [|nucleoside triphosphates], predominantly [|ATP] , to break hydrogen bonds between bases and unwind the DNA double helix into single strands. [|[97]] These enzymes are essential for most processes where enzymes need to access the DNA bases.

Polymerases
[|Polymerases] are [|enzymes] that synthesize polynucleotide chains from [|nucleoside triphosphates]. The sequence of their products are copies of existing polynucleotide chains – which are called //templates//. These enzymes function by adding nucleotides onto the 3′ [|hydroxyl group] of the previous nucleotide in a DNA strand. As a consequence, all polymerases work in a 5′ to 3′ direction. [|[98]] 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. Polymerases are classified according to the type of template that they use. In DNA replication, a DNA-dependent [|DNA polymerase] makes a copy of a DNA sequence. Accuracy is vital in this process, so many of these polymerases have a [|proofreading] 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′ [|exonuclease] activity is activated and the incorrect base removed. [|[99]] 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]. [|[100]] RNA-dependent DNA polymerases are a specialized class of polymerases that copy the sequence of an RNA strand into DNA. 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. [|[44]] [|[101]] Telomerase is an unusual polymerase because it contains its own RNA template as part of its structure. [|[45]] Transcription is carried out by a DNA-dependent [|RNA polymerase] 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 [|messenger RNA] transcript until it reaches a region of DNA called the [|terminator], 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. [|[102]]

Genetic recombination
Structure of the [|Holliday junction] intermediate in [|genetic recombination]. The four separate DNA strands are coloured red, blue, green and yellow. [|[103]] Further information: [|Genetic recombination] 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". [|[104]] 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 [|chromosomal crossover] when they [|recombine]. 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 [|natural selection] and can be important in the rapid evolution of new proteins. [|[105]] Genetic recombination can also be involved in DNA repair, particularly in the cell's response to double-strand breaks. [|[106]] The most common form of chromosomal crossover is [|homologous recombination], where the two chromosomes involved share very similar sequences. Non-homologous recombination can be damaging to cells, as it can produce [|chromosomal translocations] and genetic abnormalities. The recombination reaction is catalyzed by enzymes known as [|recombinases], such as [|RAD51]. [|[107]] The first step in recombination is a double-stranded break either caused by an [|endonuclease] or damage to the DNA. [|[108]] 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. 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. [|[109]]
 * [[image:http://upload.wikimedia.org/wikipedia/commons/thumb/9/9b/Holliday_Junction_cropped.png/250px-Holliday_Junction_cropped.png width="250" height="199" caption="Holliday Junction cropped.png" link="http://en.wikipedia.org/wiki/File:Holliday_Junction_cropped.png"]] ||
 * [[image:http://upload.wikimedia.org/wikipedia/commons/thumb/9/92/Holliday_junction_coloured.png/250px-Holliday_junction_coloured.png width="250" height="250" caption="Holliday junction coloured.png" link="http://en.wikipedia.org/wiki/File:Holliday_junction_coloured.png"]] ||

Evolution
Further information: [|RNA world hypothesis] 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 [|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. [|[98]] [|[110]] 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]. [|[111]] 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. 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. [|[112]] However, 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. [|[113]] 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, [|[114]] but these claims are controversial. [|[115]] [|[116]] On August 8, 2011, a report, based on [|NASA] studies with [|meteorites] found on [|Earth], was published suggesting building blocks of DNA ( [|adenine] , [|guanine] and related [|organic molecules] ) may have been formed extraterrestrially in [|outer space]. [|[117]] [|[118]] [|[119]]

Genetic engineering
Further information: [|Molecular biology], [|nucleic acid methods] and [|genetic engineering] 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]. Modern [|biology] and [|biochemistry] make intensive use of these techniques in recombinant DNA technology. [|Recombinant DNA] is a man-made DNA sequence that has been assembled from other DNA sequences. They can be [|transformed] into organisms in the form of [|plasmids] or in the appropriate format, by using a [|viral vector]. [|[120]] The [|genetically modified] organisms produced can be used to produce products such as recombinant [|proteins], used in [|medical research] , [|[121]] or be grown in [|agriculture]. [|[122]] [|[123]]

Forensics
Further information: [|DNA profiling] [|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. This process is formally termed [|DNA profiling], but may also be called " [|genetic fingerprinting] ". In DNA profiling, the lengths of variable sections of repetitive DNA, such as [|short tandem repeats] and [|minisatellites], are compared between people. This method is usually an extremely reliable technique for identifying a matching DNA. [|[124]] However, identification can be complicated if the scene is contaminated with DNA from several people. [|[125]] DNA profiling was developed in 1984 by British geneticist Sir [|Alec Jeffreys], [|[126]] and first used in forensic science to convict Colin Pitchfork in the 1988 [|Enderby murders] case. [|[127]] 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. [|[128]] 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.

Bioinformatics
Further information: [|Bioinformatics] [|Bioinformatics] involves the manipulation, searching, and [|data mining] of biological data, and this includes DNA sequence data. The development of techniques to store and search DNA sequences have led to widely applied advances in [|computer science], especially [|string searching algorithms] , [|machine learning] and [|database theory]. [|[129]] 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. [|[130]] The DNA sequenced may be [|aligned] with other DNA sequences to identify [|homologous] sequences and locate the specific [|mutations] that make them distinct. These techniques, especially [|multiple sequence alignment], are used in studying [|phylogenetic] relationships and protein function. [|[131]] 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. 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. [|[132]] 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 nanotechnology
The DNA structure at left (schematic shown) will self-assemble into the structure visualized by [|atomic force microscopy] at right. [|DNA nanotechnology] is the field that seeks to design nanoscale structures using the [|molecular recognition] properties of DNA molecules. Image from [|Strong, 2004]. Further information: [|DNA nanotechnology] DNA nanotechnology uses the unique [|molecular recognition] properties of DNA and other nucleic acids to create self-assembling branched DNA complexes with useful properties. [|[133]] 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 " [|DNA origami] " method) as well as three-dimensional structures in the shapes of [|polyhedra]. [|[134]] [|Nanomechanical devices] and [|algorithmic self-assembly] have also been demonstrated, [|[135]] and these DNA structures have been used to template the arrangement of other molecules such as [|gold nanoparticles] and [|streptavidin] proteins. [|[136]]

History and anthropology
Further information: [|Phylogenetics] and [|Genetic genealogy] 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]. [|[137]] This field of phylogenetics is a powerful tool in [|evolutionary biology]. If DNA sequences within a species are compared, [|population geneticists] can learn the history of particular populations. This can be used in studies ranging from [|ecological genetics] to [|anthropology] ; For example, DNA evidence is being used to try to identify the [|Ten Lost Tribes of Israel]. [|[138]] [|[139]] DNA has also been used to look at modern family relationships, such as establishing family relationships between the descendants of [|Sally Hemings] and [|Thomas Jefferson]. This usage is closely related to the use of DNA in criminal investigations detailed above. Indeed, some criminal investigations have been solved when DNA from crime scenes has matched relatives of the guilty individual. [|[140]]

History of DNA research
Further information: [|History of molecular biology] [|James D. Watson] and [|Francis Crick] (right), co-originators of the double-helix model, with Maclyn McCarty (left). DNA was first isolated by the [|Swiss] physician [|Friedrich Miescher] who, in 1869, discovered a microscopic substance in the [|pus] of discarded surgical bandages. As it resided in the nuclei of cells, he called it "nuclein". [|[141]] In 1878, [|Albrecht Kossel] isolated the non-protein component of "nuclein", [|nucleic acid], and later isolated its five primary [|nucleobases]. [|[142]] In 1919, [|Phoebus Levene] identified the base, sugar and phosphate nucleotide unit. [|[143]] 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 [|William Astbury] produced the first [|X-ray diffraction] patterns that showed that DNA had a regular structure. [|[144]] [|Rosalind Franklin] used [|X-ray crystallography] to help visualize the structure of DNA. In 1927 [|Nikolai Koltsov] proposed that inherited traits would be inherited via a "giant hereditary molecule" which would be made up of "two mirror strands that would replicate in a semi-conservative fashion using each strand as a template". [|[145]] In 1928, [|Frederick Griffith] discovered that [|traits] 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. [|[146]] This system provided the first clear suggestion that DNA carries genetic information—the [|Avery–MacLeod–McCarty experiment] —when [|Oswald Avery], along with coworkers [|Colin MacLeod] and [|Maclyn McCarty] , identified DNA as the [|transforming principle] in 1943. [|[147]] 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 [|T2 phage]. [|[148]] In 1953, [|James D. Watson] and [|Francis Crick] suggested what is now accepted as the first correct double-helix model of [|DNA structure] in the journal [|//Nature//]. [|[5]] Their double-helix, molecular model of DNA was then based on a single [|X-ray diffraction] image (labeled as " [|Photo 51] ") [|[149]] taken by [|Rosalind Franklin] and [|Raymond Gosling] in May 1952, as well as the information that the DNA bases are paired — also obtained through private communications from [|Erwin Chargaff] in the previous years. [|Chargaff's rules] played a very important role in establishing double-helix configurations for B-DNA as well as A-DNA. Experimental evidence supporting the [|Watson and Crick] model were published in a series of five articles in the same issue of //Nature//. [|[150]] Of these, Franklin and Gosling's paper was the first publication of their own X-ray diffraction data and original analysis method that partially supported the Watson and Crick model; [|[31]] [|[151]] this issue also contained an article on DNA structure by [|Maurice Wilkins] and two of his colleagues, whose analysis and //in vivo// B-DNA X-ray patterns also supported the presence //in vivo// of the double-helical DNA configurations as proposed by Crick and Watson for their double-helix molecular model of DNA in the previous two pages of //Nature//. [|[32]] In 1962, after Franklin's death, Watson, Crick, and Wilkins jointly received the [|Nobel Prize] in [|Physiology or Medicine]. [|[152]] However, Nobel rules of the time allowed only living recipients, but a vigorous debate continues on who should receive credit for the discovery. [|[153]] 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". [|[154]] Final confirmation of the replication mechanism that was implied by the double-helical structure followed in 1958 through the [|Meselson–Stahl experiment]. [|[155]] Further work by Crick and coworkers 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. [|[156]] These findings represent the birth of [|molecular biology].