What is the role of DNA polymerase during replication aside from adding nucleotides?

• A. Proofreads each DNA strand
• B. Unwinds the DNA double helix
• C. Seals nicks in the backbone
• D. Adds nucleotides to the strand and proofreads

Answer: (A)

DNA polymerase not only builds the new strand but also checks its work through proofreading. It has a 3' to 5' exonuclease activity that can remove a mispaired nucleotide right after it’s added, allowing the polymerase to reinsert the correct one. This proofreading step drastically increases replication accuracy, keeping mutation rates low. The other activities mentioned—unwinding the helix (done by helicase) and sealing nicks in the backbone (done by ligase)—aren't functions of the polymerase. When you see a option that combines adding nucleotides with proofreading, the part that stands apart is proofreading, which is the role addressed here.