What is the function of topoisomerase during DNA replication?

• A. Unwinds the DNA ahead of the fork
• B. Seals nicks in the backbone
• C. Relieves supercoiling by creating transient breaks to allow rotation
• D. Synthesizes RNA primers

Answer: (C)

During DNA replication, the DNA ahead of the replication fork becomes overwound as the strands are separated. Topoisomerase relieves this supercoiling by making transient breaks in the DNA backbone, allowing the strands to rotate and then resealing the breaks. This prevents torsional strain from bottlenecking replication and lets the fork advance smoothly. The other functions are handled by different enzymes: helicase unwinds the DNA, DNA ligase seals nicks in the backbone, and primase synthesizes RNA primers needed to start synthesis.