Which DNA polymerase carries the primary 3'→5' exonuclease proofreading activity during bacterial DNA replication?

• A. DNA polymerase III
• B. DNA polymerase I
• C. DNA polymerase II
• D. RNA polymerase

Answer: (A)

The key idea is that proofreading during bacterial DNA replication is done by an enzyme that can remove mismatched nucleotides from the 3' end as synthesis proceeds. DNA polymerase III does the bulk of replication and carries a 3'→5' exonuclease proofreading activity in its active site. When a wrong nucleotide is incorporated, the newly added base is moved to the exonuclease site, the incorrect nucleotide is removed, and synthesis resumes from the correct 3' end. This proofreading step dramatically increases fidelity and happens continuously during replication.

DNA polymerase I mainly handles removing RNA primers and filling in gaps later, and it has some exonuclease activity but is not the primary proofreading enzyme during replication. DNA polymerase II is more involved in DNA repair pathways than in the core replication process. RNA polymerase is the transcription enzyme and does not participate in DNA replication proofreading. So the enzyme with the primary 3'→5' exonuclease proofreading during bacterial replication is DNA polymerase III.