新浪博客
D. DNA must be constrained: plasmid DNA or by proteins
E. Unraveling the DNA at one position changes the superhelicity-
F. Topology only defined for continuous deformation -no strand breakage
Supercoilingof DNA
2. Numerical expression for degree of supercoiling
A. Equation Lk=Tw+Wr
B. L:linkingnumber, # of times that one DNA strand winds about the others strands, is always an integer
C. T: twist,# of revolutions about the duplex helix
D. W: writhe, # of turns of the duplex axis about the superhelicalaxis
by definition the measure of the degree of supercoiling
E. specific linking difference or superhelicaldensity=ΔLk/Lk0
Supercoilingof DNA
1.Topology
A. Right handed supercoiling= negative supercoiling(underwinding)
B. Left handed supercoiling= positive supercoiling
C. Relaxed state is with no bends
D. DNA must be constrained: plasmid DNA or by proteins
E. Unraveling the DNA at one position changes the superhelicity-
F. Topology only defined for continuous deformation -no strand breakage
Supercoilingof DNA
3. DNA compaction requires special form of supercoiling
A. Interwound: supercoilingof DNA in solution
B. Toroidal-tight left handed turns, packing of DNA
both forms are interconvertible
Supercoilingof DNA
4. Methods for measuring supercoiling-based on how compact the DNA is
A. Gel electrophoresis
i. 1 dimensional
ii. 2 dimensional
B. Density sedimentation
Supercoilingof DNA
4. Topoisomerasesare required to relieve torsionalstrain
A. TopoisomerasesI :
breaks only one strand
B. TopoisomeraseII :
breaks both strands