Editing Deamination (section)

===Cytosine===
{{Plain image with caption|DesaminierungCtoU.png|Deamination of [[cytosine]] to [[uracil]].|275px|right|bottom|triangle|grey}}
Spontaneous deamination is the [[hydrolysis]] reaction of [[cytosine]] into [[uracil]], releasing [[ammonia]] in the process. This can occur in vitro through the use of [[bisulfite]], which deaminates cytosine, but not [[5-methylcytosine]]. This property has allowed researchers to [[DNA sequencing|sequence]] [[DNA methylation|methylated]] DNA to distinguish non-methylated cytosine (shown up as [[uracil]]) and methylated cytosine (unaltered).

In [[DNA]], this spontaneous deamination is corrected for by the removal of uracil (product of cytosine deamination and ''not'' part of DNA) by [[uracil-DNA glycosylase]], generating an abasic (AP) site. The resulting [[abasic site]] is then recognised by enzymes ([[AP endonuclease]]s) that break a phosphodiester bond in the DNA, permitting the repair of the resulting lesion by replacement with another cytosine. A [[DNA polymerase]] may perform this replacement via [[nick translation]], a terminal excision reaction by its 5'⟶3' exonuclease activity, followed by a fill-in reaction by its polymerase activity. DNA ligase then forms a phosphodiester bond to seal the resulting nicked duplex product, which now includes a new, correct cytosine ([[Base excision repair]]).