Editing Uracil (section)

===In DNA===
Uracil is rarely found in DNA, and this may have been an evolutionary change to increase genetic stability. This is because cytosine can deaminate spontaneously to produce uracil through hydrolytic deamination. Therefore, if there were an organism that used uracil in its DNA, the deamination of cytosine (which undergoes base pairing with guanine) would lead to formation of uracil (which would base pair with adenine) during DNA synthesis.
[[Uracil-DNA glycosylase]] excises uracil bases from double-stranded DNA. This enzyme would therefore recognize and cut out both types of uracil – the one incorporated naturally, and the one formed due to cytosine deamination, which would trigger unnecessary and inappropriate repair processes.<ref>{{Cite journal|vauthors=Békési A, Vértessy BG|date=2011|title=Uracil in DNA: error or signal?|url=https://www.scienceinschool.org/2011/issue18/uracil|journal=Science in School|pages=18|archive-url=https://web.archive.org/web/20160323021752/http://www.scienceinschool.org/2011/issue18/uracil|archive-date=23 March 2016}}</ref>

This problem is believed to have been solved in terms of evolution, that is by "tagging" (methylating) uracil. Methylated uracil is identical to thymine. Hence the hypothesis that, over time, thymine became standard in DNA instead of uracil. So cells continue to use uracil in RNA, and not in DNA, because RNA is shorter-lived than DNA, and any potential uracil-related errors do not lead to lasting damage. Apparently, either there was no evolutionary pressure to replace uracil in RNA with the more complex thymine, or uracil has some chemical property that is useful in RNA, which thymine lacks. Uracil-containing DNA still exists, for example in: 
* DNA of several [[phage]]s<ref>{{cite journal | vauthors = Wang Z, Mosbaugh DW | title = Uracil-DNA glycosylase inhibitor of bacteriophage PBS2: cloning and effects of expression of the inhibitor gene in Escherichia coli | journal = Journal of Bacteriology | volume = 170 | issue = 3 | pages = 1082–1091 | date = March 1988 | pmid = 2963806 | pmc = 210877 | doi = 10.1128/JB.170.3.1082-1091.1988 }}</ref>
* [[Endopterygote]] development
* Hypermutations during the synthesis of vertebrate antibodies.{{citation needed|date=December 2018}}