Question

A plasmid with 4000 bp of a relaxed closed circular duplex DNA has a 300-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. Which of the followings is CORRECT AFTER this plasmid is further treated with a type IB topoisomerase that is active at the high salt solution AND THEN transferred back to a low salt solution?

a) The DNA remains in the Z conformation
b) The DNA returns to the B conformation
c) The DNA undergoes supercoiling
d) The DNA undergoes unwinding

Answer 1

When a plasmid undergoes a transition from the B conformation to the Z conformation in a high salt solution, it can be brought back to the B conformation by treating it with a type IB topoisomerase and transferring it back to a low salt solution.

Step-by-step explanation:

When the plasmid with the 300-bp segment of alternating C and G residues undergoes a transition from the B conformation to the Z conformation in a high salt solution, it means that the DNA changes its shape or topology.

To return the DNA back to its original conformation, it needs to be treated with a type IB topoisomerase, which is active at high salt solutions.

This topoisomerase enzyme relieves the pressure resulting from supercoiling by breaking and reforming DNA's phosphate backbone ahead of the replication fork. Therefore, after being treated with the type IB topoisomerase and transferred back to a low salt solution, the DNA will return to the B conformation.