Histone

Histones are basic proteins found in chromosomes that help in packing and organization of DNA helix in chromatin fibre in the nucleus. Additionally, they regulate gene expression.

In eukaryotes, DNA is condensed and wrapped around histones to form chromatin threads, which further condense to form chromosomes. Histones provide a structural support to chromosomes and make them more compact, allowing them to fit inside the nucleus.

Structure

Histones are highly basic proteins.

They are abundant in lysine and arginine residues (basic amino acids).

The five main types of histones are:

Two H2A-H2B dimers and a tetramer of H3 and H4 histones form an octamer, which is a unit of eight histones and forms the core of the nucleosome.

The four histones that form the core are structurally similar and highly conserved.

The structure formed by the association of negatively charged DNA, due to phosphate groups, wrapped around the core of a positively charged histone octamer is known as the nucleosome.

The H1 histone acts as a linker, binding the nucleosome where DNA enters and exits.

The Nucleosome is the repeating unit of chromatin and contains approximately 200 base pairs of DNA.

The solubility of histones in water is attributed to their basic nature.

Histones form various interactions with DNA, including hydrogen bonds, salt bridges, helix-dipoles, and non polar interactions.

Genes that are more active are less associated with histones, whereas those which are inactive are highly associated with histones.

The replication-dependent histones are expressed during the S-phase of the cell cycle, whereas the replication-independent histones are expressed throughout the entire cell cycle.

Histone proteins undergo post-translational modifications, including methylation, acetylation, phosphorylation, and ADP-ribosylation, among others.

Histones are conserved through evolution because any mutation in histones could be detrimental.

Functions

Histones play an important role in the organization and packaging of DNA inside the nucleus, as well as in the regulation of gene expression.

Histones form the core around which DNA is wrapped, thus helping to compact DNA strands into chromosomes.

Histones are known to show post-translational modifications, which regulate the interaction between DNA and nuclear proteins. This modification is essential for gene expression, condensation during mitosis, spermatogenesis, DNA repair, and more.

Histone modifications help to protect DNA from damage and UV radiations from the sun.

It helps in DNA repair.

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