A model for the sub-structure of the core particle of the nucleosome in chromatin, based on results from neutron and x-ray scattering and other physico-chemical observations, is presented. Two disc-shaped heterotypic tetramers of histones, opposed face-to-face, form the core protein. Each tetramer is enclasped by an annulus of DNA and the two annuli are joined by a linker piece. The mode of self assembly of core particles in solution is derived as are the changes in conformation of the core particle which occur on transcription and chromosome replication.