Histones are basic proteins found in chromosomes. They help in packing and organisation of DNA helix in chromatin fibre in the nucleus. They also regulate gene expression.
In eukaryotes, DNA is highly condensed and is wrapped around histones to form chromatin thread, which further condenses to form chromosomes. Histones provide a structural framework to chromosomes and make it more compact to fit inside the nucleus.
Structure
Histones are highly basic proteins.
- They are rich in lysine and arginine residues (basic amino acids).
- The five main types of histones are H1, H2A, H2B, H3 and H4.
- A unit of eight histones form an octamer. Two H2A-H2B dimers and a tetramer of H3 and H4 histones form an octamer, which forms the core of the nucleosome.
- The four histones that form the core are similar in structure and are highly conserved.
- DNA is negatively charged due to phosphate groups and it is wrapped around the core of a positively charged histone octamer. The structure formed by this association is known as the nucleosome.
- The H1 acts as a linker histone. It binds the nucleosome where DNA enters and exits.
- Nucleosome is the repeating unit of chromatin and contains around 200bp of DNA.
- The basic nature of histones is attributed to their solubility in water.
- Histones form various interactions with DNA such as hydrogen bonds, salt bridges, helix-dipoles, non polar interactions.
- Genes that are more active are less associated with histones and those which are inactive are highly associated with histones.
- The replication-dependent histones are expressed during the S-phase of the cell cycle and the replication-independent histones are expressed during the whole cell cycle.
- Histone proteins show post-translational modifications such as methylation, acetylation, phosphorylation, and ADP-ribosylation, etc.
- Histones are conserved through evolution because any mutation in histones may be deleterious.
Functions
Histone plays an important role in the organisation and packaging of DNA inside the nucleus. They also appear to regulate the expression of genes.
- They help in compacting DNA strands into chromosomes. Histones form the core around which DNA is wrapped around.
- Histones show post-translational modifications. It regulates the interaction between DNA and nuclear proteins. It regulates gene expression, condensation during mitosis, spermatogenesis, DNA repair, etc.
- Histone modifications prevent DNA damage and also protect from the UV radiations of the sun.
- It helps in DNA repair.
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