Nucleosomeis termed as the building blocks of chromosome; and several nucleosomestogether are called chromatin. Each nucleosome composes of a core particle ?200bp of DNA associated with a linkerhistone and histone octamer that consists of two copies each of H2A, H2B, H3,and H4.
However, the generation of nucleosomes from histones in associationwith DNA are described in two different pathways depending on the conditionsthat are employed; In the first pathway, histone octamer binds to DNA depictingchromatin to be remodelled. While the other pathway, a tetramer of H32 .H42 binds first, and the two H2A and H2B dimers are added. Thus, theassembly of a nucleosome involved ordered association of the core histones withDNA where H3, H4 tetramers binds to DNA then the two H2A and H2B dimers joinDNA complex to for the final nucleosome. The association of DNA with thenucleosome is facilitated by many hydrogen bonds between histone and the DNA.
Majorly these hydrogen bonds are between the proteins and the oxygen atoms inthe phosphodiester backbone closer to DNA channel. Chemicalmodifications are part of epigenetic regulation of eukaryotic genome whichoccurs in multiple stages such as nucleosome where histone interacts with DNA.These modifications include, phosphorylation, acetylation and methylation onserine and lysine residues. Histones acetylation helps to unwrap the DNA fromhistone with the use of histone acetyl transferase which results in loose packingof nucleosome.
There arequite a number of proteins that freely interact with nucleosome and one ofthese is Sir BAH (Silence information regulation) domain which binds to thecore histones across a large surface of the nucleosome core disk, includingboth histone tail and histone-fold regions. Sir3 BAH engages the H4 N-terminaltail organising an otherwise unstructured region including the H4 basic patch.Addition interactions are made with surfaces of the nucleosome disk contributedby H3 H4 and H2B in the functionally defined LRS (loss of Rdna silencing)region as well as the H2A/H2B acidic patch.