Question

A completely relaxed 500 bp circular DNA molecule has linking number (L)= 50. DNA gyrase, a type II topoisomerase, hydrolyzes two molecules of ATP while adding left-handed supercoils to this molecule. What is the linking number after this occurs?

A) 46

B) 48

C) 52

D) 54

Please explain

Answer 1

The answer is- D) 54

Left handed supercoil means- positive supercoiling. That means DNA gyrase type II topoisomerase will introduce supercoil to the relaxed DNA.

In relaxed state it has 50 turns. So after the action of DNA gyrase type II topoisomerase the linking number would become = 50 + 4 = 54.

Because DNA gyrase type II topoisomerase can add 2 turns at a time. And here 2 molecules of ATP have been utilized. So 4 turns will be added and the linking number will become 54.