Chromatin is the chromosomal material extracted from nuclei of eukaryotic cells.
It consists of long double stranded DNA and nearly equal mass of small basic proteins called Histones, and also some non histone protein.
Histones condense the DNA by forming Nucleosomes (DNA wound around histones).H1 is least tightly bound to chromatin hence easily removed.H2A and H2B are lysine rich and are significantly conserved.
H3 and H4 are arginine rich and are highly conserved. H2A, 2B, 3 & 4 are tightly bound to DNA.
- Nucleosomes are primary structural units of chromatin.
- It consists of DNA bound to protein histones, i.e. core of protein histone wrapped with DNA.
- Histones are basic proteins rich in lysine and arginine. These positively charged amino acids interact with negatively charged phosphate group of DNA.
- There are five types of histones H1, H2A, H2B, H3 and H4. Two molecules of each H2A, H2B, H3 and H4 associate one another to form histone octomer.
- Around histone octomer a segment of the DNA helix is wound nearly twice forming nucleosome core.
- A nucleosome associated with histone H1 is called chromatosome.
- This winding of DNA around histones produce net shortening of DNA so that it can be accommodated in the nucleus.
- Further shortening is brought about by further coiling of chromatin fiber to produce solenoid structure with six nucleosome per turn.
- The solenoids are further folded and condensed to form chromosomes.