Figure 6.

Plausible chromatin conformations that accommodate positive helical tension of DNA. The topology of DNA in the canon nucleosome and in models a, b, and c is explained in the text. To calculate approximate σ values supported in each model, the ratio of the change in writhe to the change in twist was fixed at ∼2.6:1 in histone-free DNA regions (Boles et al, 1990). For σ over +0.03, the twist regime tends to saturate and deformation occurs mainly by the writhe regime (Koster et al, 2005). We then took ΔTw values no higher than 1-U/200 bp and ΔWr values by averaging (−) and (+) crossings in several planar projections of a given conformation. For simplicity, we depicted histone octamers as spheres, although partial unfolding of octamers may occur in models b and c.