What is the role of topoisomerases during replication?

• A. They relieve superhelical tension and tangling ahead of the fork to prevent DNA breakage and enable fork progression.
• B. They synthesize RNA primers.
• C. They degrade RNA primers.
• D. They ligate Okazaki fragments.

Answer: (A)

As the replication fork moves, helicase unwinds the DNA and this creates torsional strain ahead of the fork. Topoisomerases relieve that strain by making transient cuts in one or both DNA strands, allowing the helix to rotate or one segment to pass through another, and then resealing. This removes the supercoiling and prevents breakage, enabling the fork to continue progressing. In bacteria and eukaryotes, this also helps separate replicated chromosomes (decatenation) after copying. Other tasks mentioned belong to different enzymes: RNA primers are synthesized by primase, primers are removed by nucleases, and Okazaki fragments are joined by DNA ligase.