Which enzyme proofreads and joins nucleotides to form DNA?

• A. DNA Ligase
• B. RNA Primase
• C. RNA Polymerase
• D. DNA Polymerase

Answer: (D)

DNA polymerase is the enzyme that builds the new DNA strand by adding complementary nucleotides and also proofreads each incorporated base using its exonuclease activity, correcting mistakes as replication proceeds. This proofreading function is what keeps the DNA sequence accurate. DNA ligase, on the other hand, seals nicks between fragments after synthesis, but does not add nucleotides or proofread. Primase lays down RNA primers to start synthesis, and RNA polymerase makes RNA rather than DNA. So the enzyme that both synthesizes DNA and checks for errors is DNA polymerase.