The crystal structure of a tetranucleosome. (a) The four nucleosomes are arranged in two zigzag pairs that stack on top of each other. Shown here are the DNA and histone octamers (top) and the DNA only (bottom), in orientations that illustrate the geometry of the two stacks. (b and c) A model of the 30 nm filament made by repeating the tetranucleosome in a continuous zigzag configuration.