Which enzyme helps relieve supercoiling ahead of the replication fork?

Which enzyme helps relieve supercoiling ahead of the replication fork?

• A. DNA polymerase
• B. Primase
• C. Topoisomerase
• D. Ligase

Answer: (C)

When the replication fork advances, helicase unwinds the DNA, which creates torsional strain ahead of the fork in the form of positive supercoils. To keep replication moving, this supercoiling must be relieved. Topoisomerases accomplish this by making transient breaks in the DNA, allowing strands to pass through or rotate to release the tension, and then resealing the breaks. This action directly reduces the supercoiling that would otherwise stall the fork. In bacteria, a type II topoisomerase known as DNA gyrase not only relieves this tension but can introduce negative supercoils, helping keep the DNA under the right level of superhelicity. The other enzymes listed have different roles: polymerase builds new DNA, primase synthesizes RNA primers, and ligase seals nicks after fragments are formed. So the enzyme that relieves supercoiling ahead of the replication fork is topoisomerase.

When the replication fork advances, helicase unwinds the DNA, which creates torsional strain ahead of the fork in the form of positive supercoils. To keep replication moving, this supercoiling must be relieved. Topoisomerases accomplish this by making transient breaks in the DNA, allowing strands to pass through or rotate to release the tension, and then resealing the breaks. This action directly reduces the supercoiling that would otherwise stall the fork. In bacteria, a type II topoisomerase known as DNA gyrase not only relieves this tension but can introduce negative supercoils, helping keep the DNA under the right level of superhelicity. The other enzymes listed have different roles: polymerase builds new DNA, primase synthesizes RNA primers, and ligase seals nicks after fragments are formed. So the enzyme that relieves supercoiling ahead of the replication fork is topoisomerase.