Lab of Soft Matter Physics Institute of Physics Chinese Academy of Sciences Dr. Ming Li Compaction of Single DNA molecules.

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Presentation transcript:

Lab of Soft Matter Physics Institute of Physics Chinese Academy of Sciences Dr. Ming Li Compaction of Single DNA molecules

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

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

Formation of the doughnut (toroid)

TEM picture of the toroid

DNA is a highly charged chain !

Attractive interaction is induced by electrostatic correlation

How is DNA compacted into a toroid?

Watching the action of a DNA! ?

Magnet Magnetic bead

End-to-end distance vs time

Connecting DNA to two beads Biotin ended digoxigenin

The machine

Magnet Magnetic bead

End-to-end distance vs time

.

Phenomenological energy landscape

1 turn 1½ turns2 turns Quantized condensation!

Kulic and Schiessel, PRL, 92, (2004)

F F

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

DNA wraps on histones to form nucleosomes

Chaperones are expected to aid in forming nucleosomes

Purified histones are able to condense DNA

Exponential vs. Sigmoidal Low protein concentration High protein concentration

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

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

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.

Thank you!