Which DNA replication enzyme has a 5' → 3' exonuclease function to remove the primer?

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

Answer: (A)

During replication, the RNA primer that starts each short DNA segment must be removed and replaced with DNA. The enzyme that does this uses a 5' to 3' exonuclease activity to digest the RNA primer from its 5' end, while its own polymerase activity adds the correct DNA in its place. This combination of primer removal and DNA synthesis is characteristic of DNA polymerase I in bacteria. After the primer is removed and the gap filled, DNA ligase seals the remaining nick to make a continuous strand. Other enzymes listed either lack this 5' to 3' exonuclease function, or their primary roles are proofreading or sealing nicks rather than primer removal, so they don’t carry out this specific primer cleanup.