Lecture 2 Flashcards
twists
the number of complete turns one strand make around the axis of the double helix
B-DNA number of bp/turn
- 1 bp/turn
- T = number twists
T= #bp/10
** number of bases would actually be this times two
positive supercoiling
- twisting in the direction of the helix
- e.g. for a right handed helix, turning counter clockwise
- DNA is overwound
negative supercoiling
- twisting in the direction opposite to the helix
- e.g. for a right handed helix, turning clockwise
- DNA becomes underfund
Negative and positive supercoiling leads to _____
tension because B-DNA prefers to maintain that 10 bp/turn
positive supercoiling increases twists while negative supercoiling
decreases twists ( fewer baes twsisting around the center axis)
why does DNA replication generate supercoiling
- separating the strands generates supercoiling
Type 1 topoisomerases
create transient single-strand breaks
Type 2 toposiomerases
create transient double-strand breaks
topoisomerase deal with
DNA supercoiling created during transcription annd replication, and is used to separate DNA strands during cell division.
Type 1 A topo
- uses cleaver nd pass method
- ## cuts the backbone once and then reconnects the sides
Type 1 B topo
- ## cleave and rotate
Type 2 Topo
- double stranded cleave and pass mechanism (changing two strands releases ore tension
cleave and pass method
basically, the line that was bending in the top is now behind in the bottom
Type 1A characteristics
- single strand break
- no ATP
- relaxes NEGATIVE supercoils
