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DNA Coiling.ppt
- 1. DNA Coiling
- 2. DNA is a big molecule • Ruptured viral particle (bacteriophage) X95 000 • DNA about 1.7µm © 2010 Paul Billiet ODWS
- 3. Supercoiling of topologically constrained DNA • Topologically closed DNA can be circular (covalently closed circles) or loops that are constrained at the base • The coiling (or wrapping) of duplex DNA around its own axis is called supercoiling.
- 4. Different topological forms of DNA
- 5. THE NUCLEOSOME • Uncoiled the DNA of a human would stretch 2m • The average diameter of a nucleus is 10µm • The problem: To pack the DNA into the nucleus and yet have access to the genetic information. © 2010 Paul Billiet ODWS
- 6. THE NUCLEOSOME • Histones are made of a lot of basic amino acids • Combined with acidic DNA they make a stable nucleoprotein called chromatin Histone core = an octomer (8 histone molecules) Histone molecules © 2010 Paul Billiet ODWS
- 7. DNA wraps twice round the core DNA strand © 2010 Paul Billiet ODWS
- 8. DNA held in place by another histone (H1) = the nucleosome H1 © 2010 Paul Billiet ODWS
- 9. Nucleosomes are joined by linker DNA Linker DNA
- 10. DNA super coiling DNA double helix Nucleosome beads on a string 30-nm chromatin fibre
- 11. DNA super coiling 30-nm chromatin fibre Chromosome in extended form Condensed section of chromosome in mitosis © 2010 Paul Billiet ODWS
- 12. DNA super coiling Condensed section of chromosome in mitosis Condensed chromosome © 2010 Paul Billiet ODWS
- 13. Negative and positive supercoils • Negative supercoils twist the DNA about its axis in the opposite direction from the clockwise turns of the right-handed (R-H) double helix. – Underwound (favors unwinding of duplex). – Has right-handed supercoil turns. • Positive supercoils twist the DNA in the same direction as the turns of the R-H double helix. – Overwound (helix is wound more tightly). – Has left-handed supercoil turns.
- 14. Components of DNA Topology : Twist • The clockwise turns of R-H double helix generate a positive Twist (T). • The counterclockwise turns of L-H helix (Z form) generate a negative T. • T = Twisting Number B form DNA: + (# bp/10 bp per twist) A form NA: + (# bp/11 bp per twist) Z DNA: - (# bp/12 bp per twist)
- 15. Components of DNA Topology : Writhe • W = Writhing Number • Refers to the turning of the axis of the DNA duplex in space • Number of times the duplex DNA crosses over itself Relaxed molecule W=0 Negative supercoils, W is negative Positive supercoils, W is positive
- 16. Components of DNA Topology : Linking number • L = Linking Number = total number of times one strand of the double helix (of a closed molecule) encircles (or links) the other. • L = W + T
- 17. L cannot change unless one or both strands are broken and reformed • A change in the linking number, DL, is partitioned between T and W, i.e. • DL=DW+DT • if DL = 0, then DW= -DT
- 18. Relationship between supercoiling and twisting
- 19. DNA in most cells is negatively supercoiled • The superhelical density is simply the number of superhelical (S.H.) turns per turn (or twist) of double helix. • Superhelical density = s = W/T = -0.05 for natural bacterial DNA –i.e., in bacterial DNA, there is 1 negative S.H. turn per 200 bp • (calculated from 1 negative S.H. turn per 20 twists = 1 negative S.H. turn per 200 bp)
- 20. Negatively supercoiled DNA favors unwinding • Negative supercoiled DNA has energy stored that favors unwinding, or a transition from B- form to Z DNA. • For s = -0.05, DG=-9 Kcal/mole favoring unwinding Thus negative supercoiling could favor initiation of transcription and initiation of replication.