Dynamic force spectroscopy of a single condensed DNA

The dynamic force vs. the extension relationship of single DNA molecules in the presence of trivalent cation, spermidine, is studied using optical tweezers. A hysteresis between the stretching and relaxing cycles increases with increasing operating speed. The rate- dependent hysteresis is manifested due to the dissipative stretching and the delay of DNA condensation in the relaxing process. Effective friction inside a DNA condensate is estimated quantitatively, and its physical origin is discussed based on the Kramers- type kinetic theory.