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

• A. It relieves positive supercoiling (torsional strain) by introducing negative supercoils using ATP-dependent strand passage.
• B. It seals nicks in the sugar-phosphate backbone by forming phosphodiester bonds between Okazaki fragments.
• C. It unwinds the DNA double helix at the replication fork by breaking hydrogen bonds between base pairs.
• D. It synthesizes short RNA primers required to initiate DNA synthesis on the lagging strand.

Answer: (A)

Verified step by step guidance

1. Understand the context of bacterial DNA replication, where the DNA double helix must be unwound to allow replication machinery to copy the strands.
2. Recognize that as the replication fork progresses, the DNA ahead of the fork becomes overwound, creating positive supercoiling or torsional strain.
3. Identify the role of DNA gyrase, a type II topoisomerase, which acts ahead of the replication fork to manage this torsional strain.
4. Recall that DNA gyrase uses energy from ATP hydrolysis to introduce negative supercoils by cutting both strands of the DNA, passing another segment through the break, and resealing the strands, thereby relieving positive supercoiling.
5. Distinguish this function from other enzymes: DNA ligase seals nicks, helicase unwinds DNA by breaking hydrogen bonds, and primase synthesizes RNA primers.