Material of Chromosomes

FIBRE FOLDED MODEL

A chromosome is a strand of DNA that has been coiled as much as possible. DNA coils up by attaching to histone proteins, forming structures called nucleosomes. Chromatin is the lowest level of coiling produced by the association of DNA and histones. The nucleosomes continue coiling through the work of other proteins to form the highly coiled chromosomes visible during mitosis.

The DNA double helix wraps around a central core of eight histone molecules to form a single nucleosome. A second histone (H1 in the illustration) fastens the DNA to the nucleosome core. (Modified from Access Excellence)

How DNA is coiled and coiled again is shown in the series of drawings below. The DNA double helix is about 2 nm in width (and unspecified length). By the time the DNA double helix is fully condensed in the late prophase chromosome it has been thickened by a factor of nearly a 1000 times.

The chromatin material of the eukaryotic chromosomes according to its percentage of DNA, RNA and proteins and consequently due to its, staining property has been classified into following by classical cytologists:

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