DNA

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.

[28] 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.

[30] In humans, the total female diploid nuclear genome per cell extends for 6.37 Gigabase pairs (Gbp), is 208.23 cm long and weighs 6.51 picograms (pg).

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

[47][48] An alternative analysis was 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.

[49] 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.

[65] 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.

[83] 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.

[85] 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.

[106] Various possible functions have been proposed for eDNA: it may be involved in horizontal gene transfer;[107] it may provide nutrients;[108] and it may act as a buffer to recruit or titrate ions or antibiotics.

[113] 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.

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.

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.

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.

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.

[157][158] 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.

[165] DNA profiling was developed in 1984 by British geneticist Sir Alec Jeffreys,[166] and first used in forensic science to convict Colin Pitchfork in the 1988 Enderby murders case.

[167] 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.

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.

[180] Nanomechanical devices and algorithmic self-assembly have also been demonstrated,[181] and these DNA structures have been used to template the arrangement of other molecules such as gold nanoparticles and streptavidin proteins.

[182] DNA and other nucleic acids are the basis of aptamers, synthetic oligonucleotide ligands for specific target molecules used in a range of biotechnology and biomedical applications.

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

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.

[201] 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).

[205] 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",[206] at high hydration levels of DNA.

[207] 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.

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

[209] The 25 April 1953 issue of the journal Nature published a series of five articles giving the Watson and Crick double-helix structure DNA and evidence supporting it.

[216] 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".

[220] In 1986, DNA analysis was first used for criminal investigative purposes when police in the UK requested Alec Jeffreys of the University of Leicester to verify or disprove a suspect's rape-murder "confession".

The structure of the DNA double helix (type B-DNA ). The atoms in the structure are colour-coded by element and the detailed structures of two base pairs are shown in the bottom right.
Simplified diagram
Chemical structure of DNA; hydrogen bonds shown as dotted lines. Each end of the double helix has an exposed 5' phosphate on one strand and an exposed 3′ hydroxyl group (—OH) on the other.
A section of DNA. The bases lie horizontally between the two spiraling strands [ 15 ] ( animated version ).
DNA major and minor grooves. The latter is a binding site for the Hoechst stain dye 33258.
Schematic karyogram of a human. It shows 22 homologous chromosomes , both the female (XX) and male (XY) versions of the sex chromosome (bottom right), as well as the mitochondrial genome (to scale at bottom left). The blue scale to the left of each chromosome pair (and the mitochondrial genome) shows its length in terms of millions of DNA base pairs .
Further information: Karyotype
From left to right, the structures of A , B and Z-DNA
DNA quadruplex formed by telomere repeats. The looped conformation of the DNA backbone is very different from the typical DNA helix. The green spheres in the center represent potassium ions. [ 62 ]
Impure DNA extracted from an orange
A covalent adduct between a metabolically activated form of benzo[ a ]pyrene , the major mutagen in tobacco smoke , and DNA [ 82 ]
Location of eukaryote nuclear DNA within the chromosomes
T7 RNA polymerase (blue) producing an mRNA (green) from a DNA template (orange) [ 101 ]
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.
Interaction of DNA (in orange) with histones (in blue). These proteins' basic amino acids bind to the acidic phosphate groups on DNA.
The lambda repressor helix-turn-helix transcription factor bound to its DNA target [ 124 ]
The restriction enzyme EcoRV (green) in a complex with its substrate DNA [ 128 ]
A current model of meiotic recombination, initiated by a double-strand break or gap, followed by pairing with an homologous chromosome and strand invasion to initiate the recombinational repair process. Repair of the gap can lead to crossover (CO) or non-crossover (NCO) of the flanking regions. CO recombination is thought to occur by the Double Holliday Junction (DHJ) model, illustrated on the right, above. NCO recombinants are thought to occur primarily by the Synthesis Dependent Strand Annealing (SDSA) model, illustrated on the left, above. Most recombination events appear to be the SDSA type.
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. [ 178 ]
Maclyn McCarty (left) shakes hands with Francis Crick and James Watson , co-originators of the double-helix model based on the X-ray diffraction data and insights of Rosalind Franklin and Raymond Gosling .
A blue plaque outside The Eagle pub in Cambridge, England commemorating Crick and Watson
Photo 51 , showing X-ray diffraction pattern of DNA
Pencil sketch of the DNA double helix by Francis Crick in 1953