Mistakes can appear in DNA
while it is being synthesized or afterwards, by chemical or photodegradation
. Of course, this is generally a bad thing. Mismatches in DNA may often be propogated during replication, resulting in a mutation
(UDG) is one protein involved in catching a specific type of mistake in DNA. It, along with other enzymes, recognize the mistake and take steps to repair it.
A uracil base is normally found in RNA, not DNA. Uracil may be incorporated into DNA as a mistake instead of thymidine during replication. Also, deamination of a cytosine will convert the base into a uracil. It is estimated that approximately 10,000 uracil bases are mistakenly incorporated into DNA every time a cell divides and spontaneous cytosine deamniation occurs approximately 200 times a day in the average person. The cytosine deaminiation, in particular results in a premutagenic mismatch, which will result in a GC base pair being converted into an AT base pair. UDG is an enzyme suited to repairing this damage, thus keeping mutagenesis in check.
UDG flips the uracil out of the major groove of the DNA and excises it by hydrolysis of the N-glycosidic bond between the uracil and the deoxyribose sugar it is attached to. An endonuclease can then come in and remove the empty site, which is now free to be filled by a DNA polymerase.
Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is believed to have similar activity to UDG. Purified GAPDH can cut uracil out of DNA much the same way UDG does.