Stretching DNA

A statistical mechanical treatment of the wormlike chain model (WLC) is used to analyze experiments in which double-stranded DNA, tethered at one end, is stretched by a force applied directly to the free end, by an electric field, or by hydrodynamic flow. All experiments display a strong-stretching regime where the end-to-end distance approaches the DNA contour length as 1/(force)(1/2), which is a clear signature of WLC elasticity. The elastic properties of DNA become scale dependent in the presence of electrostatic interactions; the effective electric charge and the intrinsic bending elastic constant are determined from experiments at low salt concentration. We also consider the effects of spontaneous bends and the distortion of the double helix by strong forces.