Editing DNA (section)

=== Alternative DNA structures ===
{{further|Molecular Structure of Nucleic Acids: A Structure for Deoxyribose Nucleic Acid|Molecular models of DNA|DNA structure}}
[[File:Dnaconformations.png|thumb|right|From left to right, the structures of [[A-DNA|A]], [[B-DNA|B]] and [[Z-DNA]]]]

DNA exists in many possible [[Conformational isomerism|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.<ref name=Ghosh /> 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 [[ion]]s, and the presence of [[polyamine]]s in solution.<ref>{{cite journal | vauthors = Basu HS, Feuerstein BG, Zarling DA, Shafer RH, Marton LJ | title = Recognition of Z-RNA and Z-DNA determinants by polyamines in solution: experimental and theoretical studies | journal = Journal of Biomolecular Structure & Dynamics | volume = 6 | issue = 2 | pages = 299–309 | date = October 1988 | pmid = 2482766 | doi = 10.1080/07391102.1988.10507714 }}</ref>

The first published reports of A-DNA [[X-ray diffraction patterns]]—and also B-DNA—used analyses based on [[Patterson function]]s that provided only a limited amount of structural information for oriented fibers of DNA.<ref>
* {{cite journal |vauthors=Franklin RE, Gosling RG |title=The Structure of Sodium Thymonucleate Fibres I. The Influence of Water Content |journal=Acta Crystallogr |volume=6 |issue=8–9 |pages=673–77 |date=6 March 1953 |doi=10.1107/S0365110X53001939 |bibcode=1953AcCry...6..673F |url=http://journals.iucr.org/q/issues/1953/08-09/00/a00979/a00979.pdf |url-status=live |archive-url=https://web.archive.org/web/20160109043915/http://journals.iucr.org/q/issues/1953/08-09/00/a00979/a00979.pdf |archive-date=9 January 2016 |doi-access=free }}
* {{cite journal |vauthors=Franklin RE, Gosling RG |title=The structure of sodium thymonucleate fibres. II. The cylindrically symmetrical Patterson function |journal=Acta Crystallogr |volume=6 |issue=8–9 |pages=678–85 |year=1953|doi=10.1107/S0365110X53001940|bibcode=1953AcCry...6..678F |url=http://journals.iucr.org/q/issues/1953/08-09/00/a00980/a00980.pdf |archive-url=https://web.archive.org/web/20170629084321/http://journals.iucr.org/q/issues/1953/08-09/00/a00980/a00980.pdf |archive-date=2017-06-29 |url-status=live |doi-access=free }}</ref><ref name=NatFranGos>{{cite journal | vauthors = Franklin RE, Gosling RG | title = Molecular configuration in sodium thymonucleate | journal = Nature | volume = 171 | issue = 4356 | pages = 740–41 | date = April 1953 | pmid = 13054694 | doi = 10.1038/171740a0 | url = http://www.nature.com/nature/dna50/franklingosling.pdf | bibcode = 1953Natur.171..740F | s2cid = 4268222 | url-status=live | archive-url = https://web.archive.org/web/20110103160712/http://www.nature.com/nature/dna50/franklingosling.pdf | archive-date = 3 January 2011 | df = dmy-all }}</ref> 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 function]]s.<ref name=NatWilk>{{cite journal | vauthors = Wilkins MH, Stokes AR, Wilson HR | title = Molecular structure of deoxypentose nucleic acids | journal = Nature | volume = 171 | issue = 4356 | pages = 738–40 | date = April 1953 | pmid = 13054693 | doi = 10.1038/171738a0 | url = http://www.nature.com/nature/dna50/wilkins.pdf | bibcode = 1953Natur.171..738W | s2cid = 4280080 | url-status=live | archive-url = https://web.archive.org/web/20110513234223/http://www.nature.com/nature/dna50/wilkins.pdf | archive-date = 13 May 2011 | df = dmy-all }}</ref> In the same journal, [[James Watson]] and [[Francis Crick]] presented their [[Molecular models of DNA|molecular modeling]] analysis of the DNA X-ray diffraction patterns to suggest that the structure was a double helix.<ref name="Watson-1953" />

Although the ''B-DNA form'' is most common under the conditions found in cells,<ref>{{cite journal | vauthors = Leslie AG, Arnott S, Chandrasekaran R, Ratliff RL | title = Polymorphism of DNA double helices | journal = Journal of Molecular Biology | volume = 143 | issue = 1 | pages = 49–72 | date = October 1980 | pmid = 7441761 | doi = 10.1016/0022-2836(80)90124-2 }}</ref> it is not a well-defined conformation but a family of related DNA conformations<ref>{{cite journal|vauthors=Baianu IC|s2cid=189888972|year=1980|title=Structural Order and Partial Disorder in Biological systems|url=http://cogprints.org/3822/|journal=Bull. Math. Biol.|volume=42|issue=4|pages=137–41|doi=10.1007/BF02462372}}</ref> that occur at the high hydration levels present in cells. Their corresponding X-ray diffraction and scattering patterns are characteristic of molecular [[Paracrystalline|paracrystals]] with a significant degree of disorder.<ref>{{cite book | vauthors = Hosemann R, Bagchi RN | title = Direct analysis of diffraction by matter | publisher = North-Holland Publishers | location = Amsterdam&nbsp;– New York | year = 1962 }}</ref><ref>{{cite journal|vauthors=Baianu IC|title=X-ray scattering by partially disordered membrane systems|journal=Acta Crystallogr A|volume=34|issue=5|pages=751–53|year=1978|doi=10.1107/S0567739478001540|bibcode=1978AcCrA..34..751B|url=http://journals.iucr.org/a/issues/1978/05/00/a15615/a15615.pdf|access-date=29 August 2019|archive-date=14 March 2020|archive-url=https://web.archive.org/web/20200314050140/http://journals.iucr.org/a/issues/1978/05/00/a15615/a15615.pdf|url-status=dead}}</ref>

Compared to B-DNA, the A-DNA form is a wider [[Helix#Properties and types|right-handed]] spiral, with a shallow, wide minor groove and a narrower, deeper major groove. 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.<ref>{{cite journal | vauthors = Wahl MC, Sundaralingam M | title = Crystal structures of A-DNA duplexes | journal = Biopolymers | volume = 44 | issue = 1 | pages = 45–63 | year = 1997 | pmid = 9097733 | doi = 10.1002/(SICI)1097-0282(1997)44:1<45::AID-BIP4>3.0.CO;2-# }}</ref><ref>{{cite journal | vauthors = Lu XJ, Shakked Z, Olson WK | title = A-form conformational motifs in ligand-bound DNA structures | journal = Journal of Molecular Biology | volume = 300 | issue = 4 | pages = 819–40 | date = July 2000 | pmid = 10891271 | doi = 10.1006/jmbi.2000.3690 }}</ref> Segments of DNA where the bases have been chemically modified by [[methylation]] may undergo a larger change in conformation and adopt the [[Z-DNA|Z form]]. Here, the strands turn about the helical axis in a left-handed spiral, the opposite of the more common B form.<ref>{{cite journal | vauthors = Rothenburg S, Koch-Nolte F, Haag F | title = DNA methylation and Z-DNA formation as mediators of quantitative differences in the expression of alleles | journal = Immunological Reviews | volume = 184 | pages = 286–98 | date = December 2001 | pmid = 12086319 | doi = 10.1034/j.1600-065x.2001.1840125.x | s2cid = 20589136 }}</ref> These unusual structures can be recognized by specific Z-DNA binding proteins and may be involved in the regulation of transcription.<ref>{{cite journal | vauthors = Oh DB, Kim YG, Rich A | title = Z-DNA-binding proteins can act as potent effectors of gene expression in vivo | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 99 | issue = 26 | pages = 16666–71 | date = December 2002 | pmid = 12486233 | pmc = 139201 | doi = 10.1073/pnas.262672699 | bibcode = 2002PNAS...9916666O | doi-access = free }}</ref>