chromatosome

A chromatosome is a structural unit of chromatin that consists of a nucleosome core particle with the linker histone H1 bound and a segment of linker DNA. The core particle contains roughly 147 base pairs of DNA wrapped around a histone octamer (two each of H2A, H2B, H3, and H4); the linker DNA connects neighboring nucleosomes and provides a site for H1 binding.

H1 binds at the DNA entry and exit points of the nucleosome, helping to stabilize the nucleosome and promote higher-order folding of the chromatin fiber. The exact length of the associated linker DNA can vary, typically ranging from a few to several tens of base pairs, and the presence of H1 plus this linker DNA helps define the chromatosome structure.

Functionally, chromatosomes are thought to contribute to chromatin compaction and organization, potentially serving as an intermediate between the beads-on-a-string form of nucleosomes and more condensed higher-order fibers such as the 30 nm fiber. The precise arrangement of chromatosomes in living cells, and the extent to which a uniform chromatosome model applies, remain areas of ongoing research.

The concept originated from early electron microscopy observations of chromatin, where a nucleosome core particle appeared with an additional mass attributed to bound H1 and linker DNA. Understanding chromatosomes aids in explaining mechanisms of gene regulation, DNA accessibility, and the impact of linker histone variants on chromatin dynamics.