Which of the following DNA repair pathway(s) are able to repair DNA molecules containing deaminated cytosines? If you're uncertain, explain why you're uncertain (3 points).
The correct answers here are (b), (c) and possibly (d). Since uracil pairs with A while cytosine pairs with G, conversion of cytosine to uracil in a double-stranded DNA molecule will convert a correct C:G base pair to an incorrect U:G one. The incorrect U:G base pair can be recognized and repaired by the enzymes of mismatch repair.
As you learned when we discussed base excision repair, there are many DNA N-glycosylases that recognize U in DNA and remove it. Some of these glycosylases are specialized for U in U:G base pairs. So U arising from cytosine deamination can be removed by base excision repair as well as mismatch repair.
Any incorrect base pairing, including incorrect U:G, will lead to a small distortion of the DNA double helix. This distortion should be recognizable by the nucleotide excision repair (NER) pathway. So, even though I didn't explicitly teach you that incorrect U:G pairs can be corrected by NER, you should have been able to deduce that NER might well be able to repair U:G pairs. Due to this uncertainty, I gave full credit for part (d) whether you did or did not circle (d).
Click here to return to list of questions from previous exams.