In DNA repair, what is base excision repair?

• A. It repairs double-strand breaks via homologous recombination.
• B. It removes damaged bases and replaces them with correct nucleotides.
• C. It synthesizes new DNA using RNA templates.
• D. It repairs mismatches during replication.

Answer: (B)

Base excision repair focuses on fixing small, non-helix-distorting base damage by removing the damaged base and replacing it with the correct nucleotide. The process starts when a DNA glycosylase recognizes the damaged base and removes it, creating an abasic site. An AP endonuclease then cuts the backbone, a DNA polymerase fills in the appropriate nucleotide, and a ligase seals the strand. This targeted removal and replacement of individual bases is distinct from other repair pathways: double-strand breaks are repaired by homologous recombination or end joining, mismatches during replication are fixed by mismatch repair, and RNA-templated synthesis is not a repair mechanism. Thus, removing damaged bases and inserting the correct ones best describes base excision repair.