Figure 1. Architecture of the nucleosome core particle (PDB ID 1KX5; [2]).

The four histone dimers H3, H4, H2A and H2B are colored in blue, green, red and yellow, respectively. The four histone dimers are arranged about a twofold dyad symmetry axis, which also intersects the middle of the DNA fragment. DNA positions around the nucleosome are described by super helical locations, or SHLs, numbered here. The middle of the DNA fragment at the dyad position is referred to as SHL0. Starting from the dyad along the outer wrap of DNA on the nucleosome (shown by an arrow in the figure), each minor groove facing the histone core is denoted by SHL0.5, SHL1.5, etc. (positive in one direction, negative in the other).