Histone

Histones are a group of proteins, that support the structure of chromatin by binding DNA in the nuclei of eukaryotic cells. They are positively charged, basic, as they contain a large amount of basic amino acids (such as arginine and lysine). There are strong ionic interactions between the positively charged histones and negatively charged DNA strands (phosphate groups). We can distinguish 5 subtypes of histones by electrophoresis (see the table below):

H2A, H2B, H3 and H4 are the core histones, while H1 is the linker which is not a part of the histone octamer. Histones interact with the DNA strands in the form of an octamer (meaning it consists of 8 parts) - 2 molecules of each core histone form the 8-protein complex core (histone octamer) around which DNA is wound. Together with the wound DNA, the histone octamers form a nucleosome with the diameter of 10 nm. Nucleosomes are both a structural and functional complex of DNA (cca 150 base pairs) and a histone octamer - this gives the chromatin a "beads on a string" appearance under an electron miscroscope.

The role of histone octamers is to control gene expression by reversible modifications of histones, which translate into changes in the structure of the chromozomes they are associated with. The histone modifications are reversible, post-translational - examples include mainly acetylation, methylation, but also phosphorylation, ubiquitinylation, etc.

MNEMONIC:

Linker histone H1 is not a part of the octamer and is connected to outer part of the nucleosome. Each nucleosome is associated exactly with one H1 histone, though its precise structure is not yet known. Its function is further stabilisation of the chromatic fibre and aid in formation of a higher complex of nucleosome array - solenoid. Solenoid is formed by 8-10 nucleosomes - essentially, it is a chromatin fibre of a higher caliber (30 nm in diameter). The biggest human chromozome contains around 3000 solenoids.

That said, the primary structure of histones has been evolutionarily well-conserved. To give an example, the difference in the sequence of amino acids in the H4 histone of a broad bean and the analogous histone in cattle (more specifically beef thymus gland) is only 2 conservative amino acid exchanges - Ile instead of Val at the 60 position, and Arg instead of Lys at the 77 position. This serves as a proof that the structural effect of histones is greater than its regulatory role in gene expression (as mentioned above).

V interfázi se aktivnější část chromozomu nachází v některé z popsaných spiralizací (tzv. euchromatin). Heterochromatin a chromozomy v metafázi obsahují struktury vyššího řádu než solenoid.