Figure 3. The many structural states of the nucleosome.
a | Structural states of the nucleosome that are likely to be interchangeable. These include the tetrasome, which is formed by the wrapping of ~80 bp DNA around an (H3–H4)2 tetramer. Hexasomes (nucleosomes lacking one H2A–H2B heterodimer) are intermediate states during nucleosome assembly or disassembly, as well as during transcription of nucleosome templates by RNA polymerase II. Both hexasomes and fully formed nucleosomes may undergo spontaneous structural transitions that are characterized either by the transient release of the DNA ends (DNA breathing) or by a transient opening of the interface between histone subcomplexes (open state). Some of the specific states may be favoured by DNA sequence, histone variant incorporation or post-translational modifications (PTMs). Histone variants are likely to be incorporated by similar pathways. b | Nucleosomes may also exist in alternative states that vary in the direction of the handedness of the DNA superhelix (left-handed versus right-handed) or in the stoichiometry (hemisome) and structural states (lexosome) of histones. These alternative structures, if they do indeed exist in vivo, would affect DNA accessibility and the interaction of nuclear factors with chromatin.