Nucleosome and DNA defect geometry.

<p><b>(a)</b> Schematic of the experimental nucleosome structure 1kx5, with DNA in grey; in orange, the 20 bp added to the experimental structure on each end. Histone pairs H3, H4, H2A, H2B color-coded as shown below. Positions 1 to 4 indicate insertion sites of double-strand breaks (DSB). <b>(b)</b> Schematic structure of a DSB; arrows indicate the cut on each backbone; the green-red central base pair is an A⋯T for all models. <b>(c)</b> The circular bent-tube DNA geometry in the nucleosome. At any point along the neutral axis (centerline) a local reference frame is defined by three unit vectors: normal <b>n</b> (blue), tangent <b><i>τ</i></b> (red), and the binormal <b>b</b> (green) directed to the center of curvature. The white slice represents a tube averaging zone for stress calculations.