Compaction of Single DNA molecules Dr Ming Li

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

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

DNA must be packaged • • Human genome (in diploid cells) = 6× 109 bp× 0. 34 nm/bp = 2. 04 × 109 nm = 2 m/cell Diameter of nucleus = 5 -10 mm Very thin (2. 0 nm), extremely fragile (Must be protected)

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! ?

Magnetic bead Magnet

End-to-end distance vs time

Connecting DNA to two beads Biotin ended digoxigenin

The machine

The machine

Magnetic bead Magnet

End-to-end distance vs time

End-to-end distance vs time .

Phenomenological energy landscape

Quantized condensation! 1 turn 1½ turns 2 turns

Kulic and Schiessel, PRL, 92, 228101 (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 p. N

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

Summary Ø 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. Ø 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! http: //sm. iphy. ac. cn