Question

In a negatively supercoiled circular plasmid (covalently closed), the transition of a small region of B-DNA to Z-DNA can be induced. If this region involves a 12 base pair alternating d(CG) sequence in the closed plasmid, determine changes to W, T, and L after the induced B-to-Z transition. (Remember: ~10 base pairs per right-handed turn for B-DNA and -12 base pairs per left-handed turn for Z-DNA, i.e. Z-DNA twists in the opposite direction).

Answer 1

Covalently closed ,circular DNA molecules can be supercoiled .This corressponds to a teritiary structure of DNA .i.e higher the order folding of elements of secondary structure. Negetive supercoiling is underwining.Identical molecules that differ only in their linking number are called topoisomers or topological isomers .The formation pf negetively supercoiled molecule then,supercoiling is characterized by linking number(L) which is sum of a twiating number(T),and writhing number(W)

L=T+W

Enzymes lcalled topoisomers can catalyse changes in the linking number of covalently closed circular DNA molecules.change in linking number can be detected by gel electrophoresis.

BTOZ DNA transition in which the FRET responds with it showing a distinct change in FRET efficiency for each donor or acceptor configuration allowing reliable sytuctural probing.This transition involves 4 hypothesis explained in form of different models i.e wang model,harvey model etc.