FIG. 4.

Effect of salt concentration on defect-free DNA. Supercoiled 2µm DNA at 1 pN stretching force under 0.01 (blue dashed line), 0.1 (red solid line), and 0.5 M Na⁺ (green dot-dashed line). (a) Extension and (b) Torque shows a more rounded buckling transition for lower salts due to lower stability of plectoneme superhelices. Note that the torque increases in the buckled state for lower salts corresponding to increase of DNA twist due to less twist screening by unstable plectoneme superhelices. (c) Number of plectoneme domains increase in the buckled state for lower salts, whereas, the buckled state is constituted of a single plectoneme domain at higher salts. (d) Average size of a domain increases in the higher salt case. For lower salts, proliferation of multiple domains lead to a very small domain size in the buckled state.