Question

A plasmid with 5000 bp of closed circular duplex DNA was isolated from E. coli. It has a 600-bp segment of alternating C and G residues. Upon transfer to a high salt solution, this segment undergoes a transition from the B conformation to the Z conformation. This plasmid is further treated with a type IB topoisomerase. Which of the followings is INCORRECT?

a) Its final writhing number is zero
b) Its final linking number is 390
c) Its initial twist is 500
d) Its initial linking number is less than 500

Answer 1

The incorrect statement is (d) Its initial linking number is less than 500.

Step-by-step explanation:

The incorrect statement is (d) Its initial linking number is less than 500.

A plasmid with 5000 bp of closed circular duplex DNA was isolated from E. coli. It has a 600-bp segment of alternating C and G residues. When this plasmid is transferred to a high salt solution, the segment undergoes a transition from the B conformation to the Z conformation. Type IB topoisomerases, such as DNA gyrase, are responsible for introducing negative supercoils. The initial linking number of the plasmid is 500, which can be calculated using the formula:

Linking number = Twist + Writhe

In this case, the initial twist is given as 500, so the initial linking number is also 500. The final linking number can be determined by considering the change in the supercoiling resulting from the transition to the Z conformation and the action of the topoisomerase. However, based on the given information, we cannot determine the exact value of the final linking number.