Question

A plasmid with 4000 bp of closed circular duplex DNA was isolated from E. coli. It 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. This plasmid is further treated with a type IB topoisomerase. Which of the followings is INCORRECT?

a) The plasmid will experience a change in linking number
b) The Z conformation is more energetically favorable in high salt
c) Type IB topoisomerase introduces single-strand breaks
d) The transition is reversible

Answer 1

The incorrect statement about the effects of a type IB topoisomerase on a plasmid in high salt conditions is not explicitly identified; the answer provides details on the mechanisms of topoisomerases and the B-to-Z DNA transition, possibly suggesting that all the given options might be correct under these circumstances.

Step-by-step explanation:

A student asked about the effects of a plasmid with a segment of alternating C and G residues undergoing a transition from B-DNA to Z-DNA upon treatment with a type IB topoisomerase in high salt conditions. To address the question of which statement is incorrect, one must have a good understanding of DNA topology and the role of topoisomerases.

The transition from B-DNA to Z-DNA can lead to changes in the supercoiling of DNA, which can in turn affect the linking number of the plasmid. This is because the Z-DNA conformation can introduce or relieve supercoiling tension. Moreover, type IB topoisomerases introduce single-strand breaks in DNA to relieve or introduce supercoils. After making the necessary changes, they reseal the DNA.

When it comes to the energetic favorability of the Z conformation, it is indeed more energetically favorable in high salt conditions, hence the transition. And importantly, this transition to Z-DNA is reversible upon returning to normal ionic conditions. An incorrect statement regarding this process implies a misunderstanding of these molecular biology concepts. Type IB topoisomerases can alter the linking number by passing one strand of DNA through a break in another strand, thus changing the number of times the strands are interwound.