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Lecture 8: Genome Packing and Accessibility

Lecture 8: Genome Packing and Accessibility. Rob Phillips California Institute of Technology. (Beautiful work of David Goodsell). (Bustamante et al .). Experiments on Nucleosome Accessibility. Accessibility vs Burial Depth. Dynamics of Nucleosomal Accessibility. (Widom et al .).

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Lecture 8: Genome Packing and Accessibility

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  1. Lecture 8: Genome Packing and Accessibility Rob Phillips California Institute of Technology (Beautiful work of David Goodsell) (Bustamante et al.)

  2. Experiments on Nucleosome Accessibility

  3. Accessibility vs Burial Depth

  4. Dynamics of Nucleosomal Accessibility (Widom et al.) Acceptor Cy5 Donor Cy3 LexA Binding site 147 1 8 27

  5. The Life Cycle of a Bacteriophage • The life cycle involves the infection of the host cell (i.e. injection of its DNA into that cell), exploitation of the machinery of the host cell to make the relevant proteins and DNA, self-assembly and destruction of the host cell. • Each stage involves fascinating physical processes as evidenced by kinetic verbs – infection, replication, assembly, packing, etc. • Model systems for quantitative analysis. Rate of packing: 100bp/sec Self-assembly

  6. Experiments Reveal Crystalline Order for Viral DNA • Cryo-electron microscopy reveals that the packaged DNA is highly ordered. • Crystalline order characterized by concentric rings of DNA wrapped around a central axis. (Cerritelli et al.)

  7. Forces and Packing Rates: As a Function of Fraction Packed Rate of packing (Bustamante et al.) Force resisting further packaging

  8. Viral Packing: Relevant Scales Capsid size = 40nm Governing dimensionless parameter (Hendrix) Persistence length of DNA, length over which DNA can be thought of as being stiff. There is a negative charge every .17nm of length along DNA – electrostatic energy crucial also. Note: kT = 4.1 pN nm – coexistence of thermal and deterministic forces

  9. Viral Packing: Free Energy of Confinement DNA is charged DNA has elasticity Hoop model of packed DNA The idea: set up a free energy function that reflects the competition between these two effects (Riemer et al.,Odijk, Gelbart et al.)

  10. DNA-DNA Interaction: Osmotic Stress Measurements Rau and Parsegian d where F0=55000pN/nm2 and c=.27nm. The measured pressure can be turned into an interaction energy between pairs of DNA strands. This energy, per strand length is:

  11. (nm) (nm) Percent packed T7 asymptote Comparison With Experiments: Geometry of Packing Spacing of packaged DNA in capsids has been measured by cryoEM and x-rays. (Cerritelli et al.)

  12. T7 HK97 Force (pN) 29 T4 Percent packed Predictions for Packing Forces in Other Phage (Jardine et al.) HK97 Wikoff Consider several other bacteriophage under same conditions as that of the Bustamante experiment. Maximum force in the case of T7 nearly a factor of two larger than that in phi29.(Significance for motors in these other phage?)

  13. How Big is a Genome? Part 2

  14. Polymers as Random Walks Bustamante et al.

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