A model for chromatin based upon two symmetrically paired half-nucleosomes.

We propose that the basic unit of chromatin is constructed of two isologously paired heterotypic protein tetramers each containing one molecule of H2A, H2B, H3, and H4 histone. These proteins form a core that holds 140 base pairs (bp) of DNA in a single left-handed, non-interwound DNA super-coil approximately 95 bp in circumference, creating a nucleosome particle (DNA and protein) organized about a dyad axis of symmetry. Such a nucleosome can open up into its separate half-nucleosomes to allow genetic readout without requiring histone displacement.