Chromatin:

• The cells of eukaryotic organisms have constant DNA content (C value) that is characteristic for each species and much higher than in bacteria.
• Most of the DNA is present that are Basic because they have a high content of arginine or lysine.
• There are only five histones. The four fundamental histones, H2A, H2B, H3 and H4 are present twice every 200 base pairs of DNA. The fifth histone, Hl is present only once pre 200 base pairs of DNA, and it varies considerably between species and even within tissues of the same species.
• Chromatin is formed by a series of repeating units called nucleosomes. About 10 nm in diameter.

1. Each nucleosome contains a histone octamer consisting of two of each of the four histones H2A, H2B, H3, and H4 with about 200 pairs or DNA coiled on the outside of the nucleosome.
2. Dispersed chromatin is called euchromatin and folded chromatin is called Heterochromatin.
3. Small scattered areas of heterochromatin are called chromocentres.
4. During cell division all chromatin fibers condense mainly by coiling and supercoiling into visible thick threads chromosomes.