During DNA replication, what is the primary function of topoisomerase (e.g., DNA gyrase in bacteria) ahead of the replication fork?

• A. Join Okazaki fragments by sealing nicks in the sugar-phosphate backbone
• B. Synthesize RNA primers required to initiate DNA synthesis
• C. Relieve torsional strain (supercoiling) by transiently cutting and rejoining DNA strands
• D. Unwind the DNA double helix by breaking hydrogen bonds between base pairs

Answer: (C)

Verified step by step guidance

1. Understand the context of DNA replication, where the DNA double helix must be unwound to allow the replication machinery to copy each strand.
2. Recognize that as the replication fork progresses, the unwinding of DNA ahead of the fork causes torsional strain, leading to supercoiling of the DNA molecule.
3. Identify the role of topoisomerase enzymes, such as DNA gyrase in bacteria, which act ahead of the replication fork to manage this supercoiling.
4. Know that topoisomerase relieves torsional strain by transiently cutting one or both strands of the DNA, allowing it to rotate and release the supercoils, and then rejoining the DNA strands to maintain integrity.
5. Distinguish this function from other enzymes involved in replication, such as ligase (which joins Okazaki fragments), primase (which synthesizes RNA primers), and helicase (which unwinds the DNA helix by breaking hydrogen bonds).