Which enzyme is primarily responsible for removing RNA primers on the lagging strand in bacteria?

• A. DNA polymerase II
• B. DNA polymerase III
• C. RNAse H
• D. DNA polymerase I (via its 5'→3' exonuclease activity) removes RNA primers and fills in DNA.

Answer: (D)

In bacterial replication, the pieces of the lagging strand are started by RNA primers. Those primers must be removed and replaced with DNA before the strand is sealed. The enzyme that handles this primary job is DNA polymerase I. It has a 5' to 3' exonuclease activity that chews away the RNA primers and, using its polymerase activity, fills the resulting gaps with DNA. After this replacement, DNA ligase seals the nicks to finish the fragment.

DNA polymerase III is the main workhorse for extending DNA during synthesis, but it doesn’t remove RNA primers. RNAse H can remove RNA from RNA–DNA hybrids, yet it isn’t the principal primer remover in this context, and it doesn’t fill the gaps with DNA. DNA polymerase II is mainly involved in repair, not primer removal. Therefore, the best answer is DNA polymerase I with its 5' to 3' exonuclease activity to remove primers and fill in the DNA.