Which polymerases possess a 3' to 5' exonuclease proofreading activity?

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

Answer: (B)

The concept being tested is how DNA polymerases correct mistakes during synthesis using a 3' to 5' exonuclease proofreading activity. This proofreading domain allows the enzyme to remove a mispaired nucleotide from the 3' end and then resynthesize correctly, which greatly improves replication fidelity.

In bacterial replication, the polymerases that carry out this proofreading function are DNA polymerase I and DNA polymerase III. Both have a 3' to 5' exonuclease activity that can excise misincorporated bases as they extend DNA. DNA polymerase III is the main replicative enzyme and relies on this proofreading to reduce errors, while DNA polymerase I also has proofreading capability and contributes during primer processing and repair.

DNA polymerase II, while involved in DNA repair pathways, is not the primary proofreading polymerase used during normal replication, so it is not considered in this context as having the standard 3' to 5' proofreading activity for replication fidelity.

So, the combination that best fits 3' to 5' exonuclease proofreading during replication is polymerases I and III.