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Nucleosome
is termed as the building blocks of chromosome; and several nucleosomes
together are called chromatin. Each nucleosome composes of a core particle   ?200bp of DNA associated with a linker
histone and histone octamer that consists of two copies each of H2A, H2B, H3,
and H4. However, the generation of nucleosomes from histones in association
with DNA are described in two different pathways depending on the conditions
that are employed; In the first pathway, histone octamer binds to DNA depicting
chromatin to be remodelled. While the other pathway, a tetramer of H32 .
H42 binds first, and the two H2A and H2B dimers are added. Thus, the
assembly of a nucleosome involved ordered association of the core histones with
DNA where H3, H4 tetramers binds to DNA then the two H2A and H2B dimers join
DNA complex to for the final nucleosome. The association of DNA with the
nucleosome is facilitated by many hydrogen bonds between histone and the DNA.
Majorly these hydrogen bonds are between the proteins and the oxygen atoms in
the phosphodiester backbone closer to DNA channel.

Chemical
modifications are part of epigenetic regulation of eukaryotic genome which
occurs in multiple stages such as nucleosome where histone interacts with DNA.
These modifications include, phosphorylation, acetylation and methylation on
serine and lysine residues. Histones acetylation helps to unwrap the DNA from
histone with the use of histone acetyl transferase which results in loose packing
of nucleosome.

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There are
quite a number of proteins that freely interact with nucleosome and one of
these is Sir BAH (Silence information regulation) domain which binds to the
core histones across a large surface of the nucleosome core disk, including
both histone tail and histone-fold regions. Sir3 BAH engages the H4 N-terminal
tail organising an otherwise unstructured region including the H4 basic patch.
Addition interactions are made with surfaces of the nucleosome disk contributed
by H3 H4 and H2B in the functionally defined LRS (loss of Rdna silencing)
region as well as the H2A/H2B acidic patch.

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