The Nucleosome Core Particle.
Eukaryotic chromatin is organized in a structural hierarchy. The smallest unit is the nucleosome, a complex between ~150 bp of DNA and two copies each of four histones. The DNA strands are colored green and brown. The histone coloring is: H3, blue; H4, light green; H2A, yellow; and H2B, red. Note the "histone-fold" in all four proteins. This is a structural motif comprised of three a-helices connected by two loops (a1-L1-a2-L2-a3). See the close-ups below for how they combine in the heterodimers to form crescent-shaped DNA-binding units.
(cf. Fig. 1a of Luger, et al.)
A description of its use at this site can be found at the Chime Features page.
The Header information from the 1AOI PDB file.