FIG. 3.

Theoretical curves for supercoiled 5.4 µm (≈ 16 kb) DNA, stretched under 0.5 (blue dashed lines), 1.42 (red solid lines), 3 (cyan dot-dashed lines), and 3.9 pN (magenta dotted lines) forces at 0.1 M Na⁺. Experimental data for 0.5 (blue circles), 1.42 (red triangles), 3 (cyan diamonds), and 3.9 pN (magenta squares) are reproduced from Ref. [9]. (a) Extension and (b) Torque plotted as a function of linking number show twisting behavior at lower linking numbers and plectoneme buckling at higher linking numbers. (c) Equilibrium number of plectoneme domains show proliferation of multiple plectoneme domains in the coexistence state. At higher forces, long molecules show a non-monotonic increase in the number of plectoneme domains at the buckling transition due to the large entropy associated with plectoneme diffusion. However, in the purely-plectoneme state (i.e., the zero extension state, refer to the 0.5 pN case, at linking numbers ≳ 90), high stability of plectoneme superhelices and absence of diffusion entropy results in favoring a single plectoneme domain. Torque in the purely-plectoneme state increases because the DNA twist increases. (d) The steepness in the increase of the average domain size increases in the purely-plectoneme state due to coalescence of plectoneme domains.