DNA Replication and DNA Storage Practice Test

Relieving torsional stress during DNA unwinding is done by topoisomerase. As the DNA double helix is opened by helicase, positive supercoils accumulate ahead of the replication fork. Topoisomerase temporarily cuts one (type I) or both (type II) strands of DNA, allowing the helix to unwind or for another segment to pass through the break, and then reseals the DNA. This cutting-and-rejoining directly relieves the torsional strain that would otherwise stall replication.

DNA polymerase adds nucleotides to synthesize new DNA and doesn’t relieve supercoiling. Helicase unwinds the DNA but doesn’t cut and reseal it, which can actually increase torsion if left unchecked. Primase lays down RNA primers for DNA synthesis and is not involved in torsional stress relief. Therefore, the enzyme that best fits the description is topoisomerase.