Describe base excision repair as it corrects a cytosine deaminated to uracil, including the roles of the key enzymes.

• A. Uracil-DNA glycosylase removes uracil, creating an abasic site; AP endonuclease cuts backbone; DNA polymerase fills in with cytosine; ligase seals.
• B. Uracil-DNA glycosylase removes the sugar backbone from the site; AP endonuclease seals the nick; DNA polymerase seals the gap.
• C. DNA ligase removes uracil and ligates the backbone without removal of uracil.
• D. DNA polymerase fills in the gap without removing uracil.

Answer: (A)

Base excision repair fixes a single damaged base by removing just that base and then restoring the correct nucleotide. When cytosine deaminates to uracil, the repair starts with uracil-DNA glycosylase recognizing and removing the uracil base, leaving an abasic (AP) site. Next, AP endonuclease cleaves the DNA backbone at that abasic site, creating a nick. A DNA polymerase then inserts the correct base, cytosine, opposite the opposing guanine, and finally DNA ligase seals the backbone to restore the intact DNA strand. This sequence ensures uracil is removed and the original C–G pairing is reestablished.