Figure 7.

Structural deformation of DNA leading to a twist of supercoiling change dependent on the method used to determine the position of the origin, the method used here, or that in which the origin lies on a line connecting two carbon atoms on the bases. In structure (a), the same as structure (d) in Fig. 6, both methods lead to an origin for all four base pairs located in the same position. However when base pair 3 is buckled as shown to form structure (b), the origin determined by our method (blue dot) moves, but that of the other method (red dot) does not. One then observes a method-dependent twist of supercoiling for the middle step.