Question

what is the function of topoisomerase?
o elongating new dna at a replication fork by adding nucleotides to the existing chain
o adding methyl groups to bases of dna
o unzipping of the double helix
o relieving strain in the dna ahead of the replication fork

Explanation:

Topoisomerase is an enzyme that functions in DNA replication. During DNA replication, as the replication fork moves forward, the DNA ahead of it becomes over - wound (positively supercoiled), creating strain. Topoisomerase acts to relieve this strain. Unzipping of the double helix is mainly done by helicase. Adding methyl groups to DNA bases is a process related to DNA methylation (not the function of topoisomerase). Elongating new DNA at a replication fork by adding nucleotides to the existing chain is the function of DNA polymerase.

Answer:

relieving strain in the DNA ahead of the replication fork