In the presence of a eukaryotic condensin and a bacterial type II topoisomerase, the linking number (Lk) of a relaxed closed-circular DNA molecule does not change. However, the DNA becomes highly knotted as shown.
condensin
type il topoisomerase
Formation of the knots requires breakage of the DNA, passage of a segment of DNA through the break, and religation by the topoisomerase.
Given that every reaction of the topoisomerase would be expected to result in a change in linking number, how can Lk remain the same?
O The topoisomerase transiently breaks only one of the two DNA strands.
O The topoisomerase winds and unwinds the DNA without breaking either strand.
O The topoisomerase introduces the same number of positive and negative supercoils in the DNA..
OThe topoisomerase only introduces positive supercoils in the DNA.