How is replication fidelity maintained and what repair systems correct errors after replication?

How is replication fidelity maintained and what repair systems correct errors after replication?

• A. DNA polymerase III proofreading; mismatch repair fixes remaining errors (MutS/MutL in bacteria); other repair pathways fix more.
• B. Only DNA polymerase I proofreads; no other repair mechanisms exist.
• C. RNA polymerase proofreads the DNA; nucleotide excision repair occurs before replication.
• D. Sloppy enzymes produce errors; all errors tolerated.

Answer: (A)

Replication fidelity comes from two main safeguards: the polymerase’s own proofreading during DNA synthesis, and a dedicated repair system that corrects any errors that escape proofreading after replication. In bacteria, DNA polymerase III has 3' to 5' exonuclease activity that immediately removes mispaired nucleotides as they are inserted, reducing errors at the source. If any mismatches still remain after copying the entire chromosome, the mismatch repair system steps in. The MutS protein recognizes the mispair and, with MutL, coordinates removal of a short stretch of the newly made strand containing the error. The gap is then filled in by DNA polymerase and sealed by ligase. This two-tier approach—mistake-proofing during synthesis plus targeted post-replication repair—explains how cells keep replication errors to a minimum. Other repair pathways handle different types of DNA damage not caused by misincorporation, such as base changes or bulky lesions, but they are separate from the specific replication-error correction described here.

Replication fidelity comes from two main safeguards: the polymerase’s own proofreading during DNA synthesis, and a dedicated repair system that corrects any errors that escape proofreading after replication. In bacteria, DNA polymerase III has 3' to 5' exonuclease activity that immediately removes mispaired nucleotides as they are inserted, reducing errors at the source. If any mismatches still remain after copying the entire chromosome, the mismatch repair system steps in. The MutS protein recognizes the mispair and, with MutL, coordinates removal of a short stretch of the newly made strand containing the error. The gap is then filled in by DNA polymerase and sealed by ligase. This two-tier approach—mistake-proofing during synthesis plus targeted post-replication repair—explains how cells keep replication errors to a minimum. Other repair pathways handle different types of DNA damage not caused by misincorporation, such as base changes or bulky lesions, but they are separate from the specific replication-error correction described here.