Question

If 20% of the DNA in a relaxed circular-covalently-closed plasmid is induced to change from B-DNA to Z-DNA by transferring to a high salt solution. The plasmid is then a) treated with an enzyme that has type IB topoisomerase activity and b) transferred back to a low salt solution. What is the value of the writhing number when compared to the initial value?

a) Increased
b) Decreased
c) Unchanged
d) Cannot be determined

Answer 1

The value of the writhing number of the plasmid DNA after inducing the conversion from B-DNA to Z-DNA, then treating with type IB topoisomerase, followed by a transfer back to a low salt solution should remain unchanged.

Step-by-step explanation:

If 20% of the DNA in a relaxed circular-covalently-closed plasmid is induced to change from B-DNA to Z-DNA by transferring to a high salt solution and is subsequently treated with an enzyme that has type IB topoisomerase activity before being transferred back to a low salt solution, the writhing number will likely be unchanged.

This is because type IB topoisomerases are enzymes that can introduce single strand breaks to alleviate or introduce supercoiling, but they do so in a reversible manner that does not change the overall linking number of the DNA. Since the writhing number is a component of the linking number, and type IB topoisomerases do not permanently alter linking number, the value of the writhing number compared to the initial value should remain unchanged after these treatments.