Why is DNA polymerase III essential for replication in prokaryotes?

• A. It unwinds the DNA helix.
• B. It seals nicks between fragments.
• C. It adds nucleotides to the new strand and has proofreading activity.
• D. It synthesizes RNA primers.

Answer: (C)

DNA polymerase III is the enzyme primarily responsible for elongating the new DNA strands during prokaryotic replication and doing so with high fidelity. Its core activity is adding nucleotides to the 3' end of the growing strand in the 5' to 3' direction, using the parental strand as a template. Importantly, it also has proofreading ability through a 3' to 5' exonuclease activity, meaning it can remove incorrectly added bases and replace them with the correct ones right away. This combination—rapid, directional synthesis plus immediate error correction—makes replication accurate and efficient, which is essential for copying the entire genome.

Other enzymes handle different tasks: helicases unwind the double helix, ligases seal nicks after fragments are joined, and primases lay down RNA primers so synthesis can start. DNA polymerase III cannot initiate synthesis on its own and does not perform primer removal and gap filling—that’s handled by primase and other factors, with DNA polymerase I filling in gaps after the RNA primers are removed. Because DNA polymerase III is the main, high-fidelity DNA synthesizer during replication, it is the best answer.