Which enzyme relieves supercoiling ahead of the replication fork?

Which enzyme relieves supercoiling ahead of the replication fork?

• A. Helicase
• B. Primase
• C. Topoisomerase
• D. Ligase

Answer: (C)

Relieving torsional stress from unwinding DNA at the replication fork is done by topoisomerase. As helicase opens the double helix, the ahead-of-fork region becomes overwound, generating positive supercoils that can stall replication. Topoisomerase makes transient breaks in the DNA backbone, allowing the strands to pass through and release the excess twist, then reseals the breaks. This keeps the DNA from becoming too tightly wound and lets replication proceed smoothly. Primase builds RNA primers for DNA synthesis, helicase is the unwinding motor, and ligase seals Okazaki fragments—roles that don’t directly address the need to relieve supercoiling.

Relieving torsional stress from unwinding DNA at the replication fork is done by topoisomerase. As helicase opens the double helix, the ahead-of-fork region becomes overwound, generating positive supercoils that can stall replication. Topoisomerase makes transient breaks in the DNA backbone, allowing the strands to pass through and release the excess twist, then reseals the breaks. This keeps the DNA from becoming too tightly wound and lets replication proceed smoothly. Primase builds RNA primers for DNA synthesis, helicase is the unwinding motor, and ligase seals Okazaki fragments—roles that don’t directly address the need to relieve supercoiling.