Figure 4.

Gradual unwrapping of nucleosomes facilitates fiber stretching. (A) Typical conformations of nucleosomes in extended fibers. For unwrapping U = 0, all nucleosomal DNA is attached to histones (shown in yellow). For U > 0, the unwrapped DNA is represented by the red fragments located between the ends of the yellow fragments and the white markers (NCP positions 1 and 145). An increase in U brings about an increase in the angle between the DNA end vectors (arrows starting at the white markers) so that for U = 25–32 bp, this angle is ∼180°, with the end vectors perfectly aligned along the external force. The alternative asymmetric conformations on the right, with U^L and U^R equal to 10 or 54 bp, have the same average degree of unwrapping as the U = 32 bp case and generate the same level of extension (Z_ext). However, the form of the adhesion energy profile and the stochastic dynamic of unwrapping-rewrapping dictates that these asymmetric conformations are preferable (Figs. 5 and S8). (B) The nucleosome end-to-end distance (between the white markers in (A)) as a function of unwrapping, U. The largest increase in this distance occurs between U = 10 and 25 bp, which roughly corresponds to the plateau limits (Fig. 2). To see this figure in color, go online.