What is the role of DNA polymerase during DNA replication?

• A. It unwinds the DNA double helix
• B. It synthesizes new DNA strands by adding nucleotides to a growing chain
• C. It seals nicks in the sugar-phosphate backbone
• D. It primes RNA synthesis

Answer: (B)

DNA polymerase’s job is to synthesize new DNA strands by adding nucleotides to a growing chain. It reads the existing template strand and forms the new strand in the 5' to 3' direction, linking nucleotides through phosphodiester bonds starting from a preexisting 3' end provided by a primer. Because initiation requires a primer, the enzyme cannot start from scratch—primer synthesis by another enzyme provides the starting point. On the leading strand, it can work continuously toward the replication fork, while on the lagging strand it produces short segments that are later joined. It also proofreads as it goes, using 3' to 5' exonuclease activity to correct mistakes and maintain high fidelity. Other replication enzymes have different roles: unwinding the helix is done by helicase, creating RNA primers is done by primase, and sealing the final nicks between fragments is done by ligase.