CHEMICAL COMPOSITION OF CHROMATIN

1. Chromatin threads composed of DNA (31 %), RNA (2-5%) and proteins (Histon-36%, Non histon 28%).
2. Histone and protamines are basic proteins. Histone protein composed of mainly Arginine, lysine and Histidine amino acids. Protamine has more arginine but lysine is absent. Histone occupy the major grooves of DNA at an angle of 30° to the helix, and it neutralise negative charges of intemucleotide phosphate in DNA. In procaryotes polyamines have simillar functions.
3. Histone protein discovered by Kossel. (Kossel also discovered purines and pyrimidines). On the basis of relative amount of arginine and lysine there are five types of histone proteins i.e. H₂A, H₂B, H₃, H₄ and H₁.H₃ and H₄ are arginine rich, H₂A and H₂B are slightly lysine rich and H₁ is highly lysine rich.
4. Non Histone protein of chromatin mainly composed of Tryptophan and Tyrosine amino acids. Non histone proteins occur more in euchromatin part where these activate segments of DNA for transcription.
5. Klug worked out ultrastructure of chromatin. Under electron microscope chromatin appears beaded. Beads are called Nucleosome or Nu-body by Wood cock and P. oudet (1975) .Nucleosomes are 11 nm in diameter and 5.7 nm in height. Nucleosome model of chromatin proposed by R. Kornberg. Each nucleosome composed of 140 pairs of nucleotides and 8 histone molecules.
6. Central core of nucleosome composed of histon and nucleotides of DNA form two coils (1% ) around this octamer of histone (two molecules each of H₂A, H₂B, H₃, H₄). Nucleosomes are linked in a string by thin threads called linker. Linker composed of 60 pairs of nucleotides and one histone molecule (H₁),
7. In chromatin, nucleosomes form coils called Solenoid of 30 nm in diameter. Each solenoid composed of six nucleosomes.