Supercoiling Flashcards
what is a twist?
the number of complete turns one strand makes around the axis of the double helix
i.e. coiling of single strands in the helix
T = # bp / 10
righthanded turns are – while left-handed turns are –
positive
negative
B-DNA has 10 bp/turn because
it is the most energetically favourable
if you turn B-DNA clockwise from the top
twist would decrease and unwing - unstable
introduce tension; stored mechanical energy
— relieves tension in DNA
supercoiling
if you turn B-DNA counterclockwise from the top
twist would increase and over wind - unstable
introduce tension; stored mechanical energy
underwound RH DNA gives rise to
negative supercoiling
overwound RH DNA gives rise to
positive supercoiling
what happens to the cell when too much tension in DNA?
mRNA replication stops because DNA is too supercoiled -> cell mitigates this with topoisomerase
supercoiling exists outside of
chromatin
what are the 2 types of topoisomerase?
- type I: creates transient ss breaks
- type II: cerates transient ds breaks
what is the type IA topoisomerase mechanism?
what type of supercoils does it work on?
“cleave and pass”
- change which strand in front
relaxes negative (underwound) supercoils
what is the type IB topoisomerase mechanism?
what type of supercoils does it work on?
“cleave and rotate”
- cleave and turn strand around
relaxes any supercoils
what is type II topoisomerase mechanism?
what type of supercoils does it work on?
relaxes any supercoils
which topoisomerase uses ATP?
type II
