FIG. 9.

Displacement of the defect site. Twist response of 2 µm DNA under 2 pN force at 0.5 M Na⁺ with an intermediate defect (ε = 0.2) located L* = 0.15 (blue), 0.25 (red), 0.5 (cyan), 0.75 (green), and 1 µm (magenta) from one of the DNA ends. The location of the defect site controls the critical size of the pinned plectoneme domain (2L*) nucleated at the buckling point. This results in an increase of the critical linking number for the rebuckling transition for defects located farther away from the end, seen as a shift in extension and torque bumps. For a centrally-located defect (L* = 1 µm, magenta lines) the rebuckling transition does not occur because the critical size of the defect-pinned domain is equal to the total size of the DNA. Note that unpinning of the defect-pinned domain occurs at the rebuckling transition, as expected for intermediate defects.