What are the two functions of DNA polymerase during replication?

• A. Adds new nucleotides to the strand
• B. Proofreads each DNA strand
• C. Unwinds the DNA double helix
• D. Adds new nucleotides to the strand and proofs each DNA strand

Answer: (D)

DNA polymerase carries out two essential tasks in replication: it builds the new strand by adding nucleotides to the growing chain, and it proofreads the newly synthesized DNA to correct mistakes. During synthesis, it adds nucleotides to the 3' end of the primer, extending the strand in the 5' to 3' direction and using the template to guide correct base pairing. At the same time, it has a proofreading activity, a 3' to 5' exonuclease function, that can remove mispaired nucleotides right after they are added and then resynthesize the correct sequence. This combination of synthesis and proofreading ensures high-fidelity replication. Other roles, like unwinding the helix, are done by other enzymes, so the most complete description of what DNA polymerase does during replication is that it both adds new nucleotides and proofreads.