Which repair pathway removes a single damaged base and fills the gap?

• A. Base excision repair removes bulky lesions.
• B. Nucleotide excision repair removes single damaged bases.
• C. Mismatch repair removes single damaged bases.
• D. Base excision repair removes single damaged bases via glycosylases and fills the gap.

Answer: (D)

Base excision repair targets small, damaged bases. It starts with a DNA glycosylase recognizing the altered base and removing just that base, creating an abasic site. The backbone is then nicked, and a DNA polymerase fills in the resulting single-nucleotide gap before ligase seals the strand. This sequence—removing a single damaged base and filling the small gap—is exactly what base excision repair does. In contrast, bulky, helix-distorting lesions are handled by nucleotide excision repair, and mismatches from replication are fixed by mismatch repair, so the described mechanism specifically matches base excision repair.