During DNA replication, what is the primary role of DNA gyrase (a bacterial type II topoisomerase) ahead of the replication fork?

• A. It synthesizes short RNA primers required for DNA polymerase to begin DNA synthesis.
• B. It relieves positive supercoiling (torsional strain) by introducing negative supercoils via transient double-strand breaks and resealing DNA.
• C. It unwinds the parental DNA duplex by breaking hydrogen bonds between base pairs at the replication fork.
• D. It joins Okazaki fragments on the lagging strand by sealing nicks in the sugar-phosphate backbone.

Answer: (B)

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 unwinding the DNA helix ahead of the replication fork creates torsional strain, leading to positive supercoiling that can hinder the progress of the replication fork.
3. Identify the role of DNA gyrase, a bacterial type II topoisomerase, which acts ahead of the replication fork to manage this torsional strain.
4. Know that DNA gyrase introduces transient double-strand breaks in the DNA, passes another segment of the double helix through the break, and then reseals the break, effectively introducing negative supercoils.
5. Conclude that by introducing negative supercoils, DNA gyrase relieves the positive supercoiling and allows the replication fork to proceed smoothly.