Question

If 10% of the dna in a relaxed circular-covalently-closed plasmid is induced to change from b-dna (10 bp/turn) to z-dna (12 bp/-ve turn) by transferring to a high salt solution. the plasmid is then a) treated with an enzyme that has type ia 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) remains the same
b) none of the rest
c) decreases
d) cannot be calculated without knowing the size of the plasmid
e) increases

Answer 1

The writhing number of the plasmid remains the same after treatment with type IA topoisomerase and return to low salt solution, as the topoisomerase negates the temporary topology changes.

Step-by-step explanation:

When 10% of the DNA in a relaxed circular-covalently-closed plasmid is induced to change from B-DNA to Z-DNA by a high salt solution, the DNA topology is altered due to the difference in the number of base pairs per turn between the two forms. If the plasmid is then treated with a type IA topoisomerase enzyme, which relieves the pressure resulting from supercoiling, and subsequently returned to a low salt solution, the writhing number of the plasmid compared to its initial value would remain the same. This is because the topoisomerase will break and reform the DNA's phosphate backbone, allowing the plasmid to return to its original supercoiling state, negating the temporary changes induced by the high salt solution.