Which enzyme removes RNA primers and fills the resulting gaps with DNA during replication?

• A. DNA polymerase III
• B. DNA polymerase I
• C. RNA primase
• D. DNA ligase

Answer: (B)

When copying DNA, RNA primers laid down on the template must be removed and the gaps filled with DNA. DNA polymerase I is the enzyme that handles this cleanup in bacteria because it has both 5' to 3' exonuclease activity to remove the RNA primer and 5' to 3' polymerase activity to fill in the space with DNA. After it finishes, DNA ligase seals the remaining nick to join the pieces. The other enzymes have different roles: RNA primase makes the primers, DNA polymerase III synthesizes most of the new DNA but doesn’t remove primers, and DNA ligase only seals nicks without filling in DNA. So the enzyme that removes RNA primers and fills the gaps with DNA is DNA polymerase I.