What are the types of histones?

There are five major types of histone families: H1/H5, H2A, H3, H2B, and H4. Octomers of two of each type of histones form nucleosomes, which are wrapped together in a spiral structure known as a solenoid. Histones H1/H5 are known as linker histones, and H2A, H2B, H3, and H4 are core histones. The H1/H5 family of histones are highly variable, and play a key role in the condensation of chromatin into a 30 nm fiber. It binds to the nucleosome particle near the DNA entry and exit sites. H1 can stabilize nucleosome structure, as well as chromatin structure. The H1 molecule consists of a central globular domain with flexible tail regions at its N- and C- terminal ends. The H2 histone family has the highest sequence divergence, and thus has the highest number of variants known. It is used during the packaging of DNA into the chromatin. H2A has an important role in determining the overall structure of chromatin, as well as gene expression and DNA modification. H2A and H2B are both involved in epigenetic regulation of transcription. Histone H3 has a vital role in activating the spindle assembly in response to a defect in mitosis. Humans have 3 copies of genes encoding H3.2, and 10 copies of genes encoding H3.1 Histone H4 plays a role in the structure of the nucleosome and is highly post-translationally modified. It has been suggested histone H4 is important for the long term regulation of genes.