Which repair pathway detects and corrects small insertions, deletions, and base mispairs?

• A. Base excision repair.
• B. Double-strand break repair.
• C. Nucleotide excision repair.
• D. Mismatch repair (MMR).

Answer: (D)

Mismatch repair fixes errors that slip past the DNA polymerase proofreading during replication. It recognizes distortions in the DNA helix caused by a mispaired base or a small insertion/deletion loop, usually just 1–2 nucleotides. The repair system marks the newly synthesized strand, removes the segment containing the error, synthesizes the correct sequence, and seals the gap. This targeted cleanup prevents mutations from small-scale errors that would otherwise persist after replication. Other pathways handle different problems: base excision repair fixes damaged bases, nucleotide excision repair removes bulky lesions that distort the helix, and double-strand break repair deals with breaks in both strands. So, the pathway that detects and corrects small insertions, deletions, and base mispairs is mismatch repair.