Describe mismatch repair mechanism and its key proteins in bacteria or eukaryotes.

• A. Mismatch repair recognizes mispaired bases post-replication; bacterial MutS/MutL/MutH or eukaryotic MSH/MLH proteins recruit exonucleases and resynthesize the corrected segment.
• B. Mismatch repair uses RNA-guided targeting to correct mismatches.
• C. Mismatch repair occurs before replication.
• D. Mismatch repair is not present in bacteria.

Answer: (A)

Mismatch repair detects and corrects mispaired bases after DNA has been replicated, boosting fidelity by fixing replication mistakes that slip past the polymerase. In bacteria, the MutS protein recognizes the mispair, then recruits MutL, and MutH performs a strand-specific nick on the newly synthesized strand (identified by hemimethylation at GATC sites). This nick allows exonucleases to remove the error-containing segment, after which DNA polymerase fills in the gap and DNA ligase seals the patch. In eukaryotes, similar steps occur with MutS homologs like MSH2-MSH6 or MSH2-MSH3 recognizing the mismatch, and MutL homologs such as MLH1-PMS2 coordinating the subsequent incision, excision, and resynthesis; strand discrimination is achieved through nicks generated during replication and interactions with PCNA, rather than a MutH-like endonuclease. The essential idea is that a mismatch is detected after replication, the correct strand is identified and processed by exonucleases, the missing DNA is resynthesized, and the backbone is sealed, thereby preventing a mismatch from becoming a permanent mutation.