Figure 1.
Sketch of the experimental setup: a dsDNA molecule is fixed by one end to a glass pane while the other end is attached to a mechanical system, symbolized by the shaded disk, which allows one to exert a pulling force Fext and impose a rotation 2πn. For large numbers of turns the molecule coils around itself in a helical way and forms plectonemes. The configuration of the molecule is made of two phases: the tails and the plectonemes. The plectonemic phase is characterized by superhelical radius R and angle α between the tangent t(s) and the helices axis. The dashed parts represent the regions we neglect: the matching region between the tails and the plectonemes and the end loop.