Which DNA replication enzymes have a 3' → 5' exonuclease function?

Which DNA replication enzymes have a 3' → 5' exonuclease function?

• A. DNA polymerases I and III
• B. DNA polymerases I and II
• C. DNA polymerases II and III
• D. DNA polymerase II

Answer: (A)

3' to 5' exonuclease proofreading is what keeps DNA replication accurate by removing mispaired nucleotides from the end of the growing strand. In bacteria, DNA polymerase III is the main replicative enzyme and uses a 3'→5' exonuclease activity (via its proofreading subunit) to scan and excise mistakes as replication proceeds. DNA polymerase I also has a 3'→5' exonuclease activity, providing proofreading capability in addition to its well-known 5'→3' exonuclease activity used for primer removal and nick translation. Because of these proofreading activities, both polymerases I and III are classically associated with removing errors during DNA synthesis, making them the best-supported pairing for this function. While DNA polymerase II can also have 3'→5' proofreading in some contexts, the combination of I and III is the clearest and most consistent answer for this question.

3' to 5' exonuclease proofreading is what keeps DNA replication accurate by removing mispaired nucleotides from the end of the growing strand. In bacteria, DNA polymerase III is the main replicative enzyme and uses a 3'→5' exonuclease activity (via its proofreading subunit) to scan and excise mistakes as replication proceeds. DNA polymerase I also has a 3'→5' exonuclease activity, providing proofreading capability in addition to its well-known 5'→3' exonuclease activity used for primer removal and nick translation. Because of these proofreading activities, both polymerases I and III are classically associated with removing errors during DNA synthesis, making them the best-supported pairing for this function. While DNA polymerase II can also have 3'→5' proofreading in some contexts, the combination of I and III is the clearest and most consistent answer for this question.