Figure 2.

Sketch of extension (end-to-end distance as a fraction of B-form contour length) vs. force for dsDNA. A, for forces <0.1 pN, the thermal random-walk fluctuations of Fig. 1 give rise to a weak entropic elasticity. B, larger forces (0.1 pN < 10 pN) supress thermal bending and stretch the molecule to near B-form length. C, for forces between 10 and 60 pN simple linear elasticity of DNA is observed, as the double helix starts to be lengthened. D, at about 70 pN, a force plateau is observed, signaling conversion of B-form to overstretched DNA. E, finally, above 70 pN, DNA is entirely converted to its overstretched form.