Which enzyme relieves torsional strain ahead of the replication fork?

Which enzyme relieves torsional strain ahead of the replication fork?

• A. Topoisomerase (gyrase)
• B. Ligase
• C. Primase
• D. DNA polymerase I

Answer: (A)

When DNA is unwound at the replication fork, the strands ahead become overwound, creating torsional strain that can slow or stall replication. Topoisomerases are the enzymes that relieve this tension. A type II topoisomerase, such as gyrase in bacteria, makes transient cuts in the DNA backbone, passes another part of the DNA through the break, and reseals it. This action relieves positive supercoiling ahead of the fork and can even introduce negative supercoils to help keep the DNA relaxed enough for replication to proceed. Ligase simply seals nicks in the sugar-phosphate backbone, primarily joining Okazaki fragments. Primase synthesizes short RNA primers to start DNA synthesis. DNA polymerase I removes RNA primers and fills in gaps after primer removal, but neither primarily relieves torsional strain.

When DNA is unwound at the replication fork, the strands ahead become overwound, creating torsional strain that can slow or stall replication. Topoisomerases are the enzymes that relieve this tension. A type II topoisomerase, such as gyrase in bacteria, makes transient cuts in the DNA backbone, passes another part of the DNA through the break, and reseals it. This action relieves positive supercoiling ahead of the fork and can even introduce negative supercoils to help keep the DNA relaxed enough for replication to proceed.

Ligase simply seals nicks in the sugar-phosphate backbone, primarily joining Okazaki fragments. Primase synthesizes short RNA primers to start DNA synthesis. DNA polymerase I removes RNA primers and fills in gaps after primer removal, but neither primarily relieves torsional strain.