Editing DNA (section)

== Evolution ==
{{further|RNA world hypothesis}}
DNA contains the genetic information that allows all forms of life to function, grow and reproduce. However, it is unclear how long in the 4-billion-year [[Timeline of evolution|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.<ref name="Joyce-2002">{{cite journal | vauthors = Joyce GF | title = The antiquity of RNA-based evolution | journal = Nature | volume = 418 | issue = 6894 | pages = 214–21 | date = July 2002 | pmid = 12110897 | doi = 10.1038/418214a | bibcode = 2002Natur.418..214J | s2cid = 4331004 }}</ref><ref>{{cite journal | vauthors = Orgel LE | title = Prebiotic chemistry and the origin of the RNA world | journal = Critical Reviews in Biochemistry and Molecular Biology | volume = 39 | issue = 2 | pages = 99–123 | year = 2004 | pmid = 15217990 | doi = 10.1080/10409230490460765 | citeseerx = 10.1.1.537.7679 | s2cid = 4939632 }}</ref> 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 [[ribozyme]]s.<ref>{{cite journal | vauthors = Davenport RJ | s2cid = 85976762 | title = Ribozymes. Making copies in the RNA world | journal = Science | volume = 292 | issue = 5520 | pages = 1278a–1278 | date = May 2001 | pmid = 11360970 | doi = 10.1126/science.292.5520.1278a }}</ref> This ancient [[RNA world hypothesis|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.<ref>{{cite journal | vauthors = Szathmáry E | title = What is the optimum size for the genetic alphabet? | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 89 | issue = 7 | pages = 2614–18 | date = April 1992 | pmid = 1372984 | pmc = 48712 | doi = 10.1073/pnas.89.7.2614 | bibcode = 1992PNAS...89.2614S | doi-access = free }}</ref> 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.<ref>{{cite journal | vauthors = Lindahl T | title = Instability and decay of the primary structure of DNA | journal = Nature | volume = 362 | issue = 6422 | pages = 709–15 | date = April 1993 | pmid = 8469282 | doi = 10.1038/362709a0 | bibcode = 1993Natur.362..709L | s2cid = 4283694 }}</ref> Claims for older DNA have been made, most notably a report of the isolation of a viable bacterium from a salt crystal 250 million years old,<ref>{{cite journal | vauthors = Vreeland RH, Rosenzweig WD, Powers DW | title = Isolation of a 250 million-year-old halotolerant bacterium from a primary salt crystal | journal = Nature | volume = 407 | issue = 6806 | pages = 897–900 | date = October 2000 | pmid = 11057666 | doi = 10.1038/35038060 | bibcode = 2000Natur.407..897V | s2cid = 9879073 }}</ref> but these claims are controversial.<ref>{{cite journal | vauthors = Hebsgaard MB, Phillips MJ, Willerslev E | title = Geologically ancient DNA: fact or artefact? | journal = Trends in Microbiology | volume = 13 | issue = 5 | pages = 212–20 | date = May 2005 | pmid = 15866038 | doi = 10.1016/j.tim.2005.03.010 }}</ref><ref>{{cite journal | vauthors = Nickle DC, Learn GH, Rain MW, Mullins JI, Mittler JE | title = Curiously modern DNA for a "250 million-year-old" bacterium | journal = Journal of Molecular Evolution | volume = 54 | issue = 1 | pages = 134–37 | date = January 2002 | pmid = 11734907 | doi = 10.1007/s00239-001-0025-x | bibcode = 2002JMolE..54..134N | s2cid = 24740859 }}</ref>

Building blocks of DNA ([[adenine]], [[guanine]], and related [[organic molecules]]) may have been formed extraterrestrially in [[outer space]].<ref name="Callahan">{{cite journal | vauthors = Callahan MP, Smith KE, Cleaves HJ, Ruzicka J, Stern JC, Glavin DP, House CH, Dworkin JP | title = Carbonaceous meteorites contain a wide range of extraterrestrial nucleobases | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 108 | issue = 34 | pages = 13995–98 | date = August 2011 | pmid = 21836052 | pmc = 3161613 | doi = 10.1073/pnas.1106493108 | bibcode = 2011PNAS..10813995C | doi-access = free }}</ref><ref name="Steigerwald">{{cite web | vauthors = Steigerwald J |title=NASA Researchers: DNA Building Blocks Can Be Made in Space |url=http://www.nasa.gov/topics/solarsystem/features/dna-meteorites.html |publisher=[[NASA]] |date=8 August 2011 |access-date=10 August 2011 |url-status=live |archive-url=https://web.archive.org/web/20150623004556/http://www.nasa.gov/topics/solarsystem/features/dna-meteorites.html |archive-date=23 June 2015 }}</ref><ref name="DNA">{{cite web |author=ScienceDaily Staff |title=DNA Building Blocks Can Be Made in Space, NASA Evidence Suggests |url=https://www.sciencedaily.com/releases/2011/08/110808220659.htm |date=9 August 2011 |website=[[ScienceDaily]] |access-date=9 August 2011 |url-status=live |archive-url=https://web.archive.org/web/20110905105043/https://www.sciencedaily.com/releases/2011/08/110808220659.htm |archive-date=5 September 2011 }}</ref> Complex DNA and [[RNA]] [[organic compound]]s 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 [[meteorite]]s. Pyrimidine, like [[polycyclic aromatic hydrocarbons]] (PAHs), the most carbon-rich chemical found in the [[universe]], may have been formed in [[red giant]]s or in interstellar [[cosmic dust]] and gas clouds.<ref name="NASA-20150303">{{cite web | vauthors = Marlaire R |title=NASA Ames Reproduces the Building Blocks of Life in Laboratory |url=http://www.nasa.gov/content/nasa-ames-reproduces-the-building-blocks-of-life-in-laboratory |date=3 March 2015 |work=[[NASA]] |access-date=5 March 2015 |url-status=live |archive-url=https://web.archive.org/web/20150305083306/http://www.nasa.gov/content/nasa-ames-reproduces-the-building-blocks-of-life-in-laboratory/ |archive-date=5 March 2015 }}</ref>

[[Ancient DNA]] has been recovered from ancient organisms at a timescale where genome evolution can be directly observed, including from extinct organisms up to millions of years old, such as the [[woolly mammoth]].<ref name="CNN-20210217">{{cite news | vauthors = Hunt K |title=World's oldest DNA sequenced from a mammoth that lived more than a million years ago |url=https://www.cnn.com/2021/02/17/world/mammoth-oldest-dna-million-years-ago-scn/index.html |date=17 February 2021 |work=[[CNN|CNN News]] |access-date=17 February 2021 }}</ref><ref name="NAT-20210217">{{cite journal | vauthors = Callaway E | title = Million-year-old mammoth genomes shatter record for oldest ancient DNA – Permafrost-preserved teeth, up to 1.6 million years old, identify a new kind of mammoth in Siberia. |date=17 February 2021 |journal=[[Nature (journal)|Nature]] |volume=590 |issue=7847 |pages=537–538 |doi=10.1038/d41586-021-00436-x |issn=0028-0836 |pmid=33597786 | bibcode = 2021Natur.590..537C |doi-access=free }}</ref>