What is the function of DNA polymerase during DNA replication?

• A. Unwinds the double helix.
• B. Joins Okazaki fragments.
• C. Synthesizes the new DNA strand by adding nucleotides in a 5' to 3' direction using the old strand as a template; requires primers.
• D. Seals nicks in backbone.

Answer: (C)

DNA polymerase is the enzyme that builds the new DNA strand during replication. It uses the existing strand as a template and adds nucleotides one by one in a 5′ to 3′ direction, so it always extends the strand at its 3′ end. It can only start from a primer, which provides the needed 3′ hydroxyl group to begin synthesis. Because of this directionality and need for a primer, the polymerase copies the template strand to create a complementary new strand. On the leading strand this happens continuously, while on the lagging strand it occurs in short segments called Okazaki fragments, each starting from a primer and then extended by polymerase. Afterward, other enzymes seal the gaps and nicks. The other options describe helicase unwinding the DNA, and ligase sealing or joining fragments, which are actions of different enzymes, not of DNA polymerase.