Which DNA repair pathway corrects base-base mismatches after replication?

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

Answer: (D)

Mismatch repair fixes base-base mismatches after replication. When a mispaired base slips past the proofreading activity of DNA polymerase, this repair system steps in to scan for the incorrect pairing and determine which strand is the newly synthesized one. It uses strand-specific signals to target the erroneous strand, then nicks the DNA and removes a short stretch containing the mismatch. The gap is filled in by DNA polymerase and sealed by ligase, restoring correct base pairing. This process is distinct from other repair pathways: nucleotide excision repair removes bulky, helix-distorting lesions like thymine dimers; base excision repair fixes small, damaged bases such as oxidized or deaminated bases; and homologous recombination repairs double-strand breaks.