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. 2012 Jul 18;103(2):323–330. doi: 10.1016/j.bpj.2012.05.050

Figure 1.

Figure 1

Supercoiling of DNA under tension. (a) Sketch of supercoiling experiments using magnetic tweezers. A DNA molecule is tethered between a surface and a magnetic bead under constant force and can be simultaneously twisted. (b) DNA length versus applied turns at different forces and 170 mM Na+. Simulations using WLC bending for stretching forces of 0.25, 0.5, 1.0, 2.0, 3.0, and 4.0 pN (bottom to top in gray, light blue, dark blue, red, light green, and dark green, respectively). The corresponding experimental supercoiling curves (taken from Maffeo et al. (22)) are shown behind (light gray). (c) Torque during DNA supercoiling for the simulations shown in panel b. (d) Snapshots of simulated supercoils at 1.0 pN and nine turns in the presence of 30, 60, and 320 mM monovalent ions. (e) DNA length versus applied turns at different ionic strength and a constant stretching force of 3.0 pN. Results from simulations for 30, 60, and 320 mM monovalent ions (blue, red, and black, respectively). Experimental data (taken from Brutzer et al. (20)) are shown behind (lighter color). (Inset) Enlarged view into the buckling region. (f) Torque during DNA supercoiling for the simulation curves shown in panel e. The experimental curves were slightly shifted in vertical direction for a better overlay with the simulation data.