Which repair pathway fixes oxidative and deaminated bases and abasic sites?

• A. Base excision repair (BER)
• B. Nucleotide excision repair (NER)
• C. Mismatch repair (MMR)
• D. Direct reversal

Answer: (A)

Base excision repair is designed for small, non-helix-distorting base damages, such as oxidative modifications, deaminated bases, and abasic sites. The process begins when a DNA glycosylase recognizes and removes the damaged base, creating an abasic site. An AP endonuclease then cuts the backbone, and a DNA polymerase fills in the single-nucleotide gap before ligase seals the strand. This pathway specifically handles single-base lesions and the intermediate abasic sites that arise from base removal, making it the best fit for fixing oxidative and deaminated bases as well as abasic sites. In contrast, nucleotide excision repair targets bulky, helix-distorting lesions; mismatch repair handles replication errors like mispaired bases; and direct reversal fixes only a narrow set of damages (such as certain methylated bases or pyrimidine dimers) without removing the damaged base.