1. Lab of Soft Matter Physics Institute of Physics Chinese Academy of Sciences Dr. Ming Li mingli@aphy.iphy.ac.cn Compaction of Single DNA molecules

2. Acknowledgement Wen-Bo Fu Xiao-Ling Wang Xing-Hua Zhang Shi-Yong Ran Institute of Physics, CAS National University of Singapore Dr. Jie Yan China National Natural Science Foundation

3. Human genome (in diploid cells) = 6×10 9 bp 6×10 9 bp×0.34 nm/bp = 2.04 ×10 9 nm = 2 m/cell Diameter of nucleus = 5-10  m Very thin (2.0 nm), extremely fragile (Must be protected) DNA must be packaged

5. Formation of the doughnut (toroid)

6. TEM picture of the toroid

7. DNA is a highly charged chain !

8. Attractive interaction is induced by electrostatic correlation

9. How is DNA compacted into a toroid?

10. Watching the action of a DNA! ?

11. Magnet Magnetic bead

12. End-to-end distance vs time

13. Connecting DNA to two beads Biotin ended digoxigenin

16. The machine

19. Magnet Magnetic bead

20. End-to-end distance vs time

21. .

22. Phenomenological energy landscape

23. 1 turn 1½ turns2 turns Quantized condensation!

24. Kulic and Schiessel, PRL, 92, 228101 (2004)

25. F F

26. Quantized (de)condensation is due to a kinetic barrier!

27. DNA wraps on histones to form nucleosomes

28. Chaperones are expected to aid in forming nucleosomes

30. Purified histones are able to condense DNA

31. Exponential vs. Sigmoidal Low protein concentration High protein concentration

32. DNA wraps on histones in a stepwise manner F<2 pN

33. DNA unwraps off histones also in a stepwise manner. F>20 pN

34. Summary  Multivalent cations compact DNA into toroid. The DNA is added onto (stripped off) the toroid in a stepwise manner, suggesting transitions between two metastable structural states.  Purified histones alone are able to condense DNA. The histones compact DNA independently at low concentrations. But they compact DNA cooperatively at high concentrations.

35. Thank you! http://sm.iphy.ac.cn