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turns. This forms a nucleosome of size 110 x 60 
(11 x 6 nm ). DNA present between two adjacent nucleosome is called linker DNA. It is attached to H1 histone protein. Length of linker DNA varies from species to species. Nucleosome chain gives a beads on string appearance under electron microscope. The nucleosomes further coils to form solenoid. It has diameter of 30 nm as found in chromatin. The beads-on-string structure in chromatin is packaged to form chromatin fibres
that are further coiled and condensed at metaphase stage of cell division to form chromosomes. The packaging at higher level requires additional set of proteins that collectively are referred to as non-histone chromosomal (NHC) proteins. In a typical nucleus, some region of chromatin are loosely packed (and stains light) and are referred to as euchromatin. The chromatin that is more densely packed and stains dark is called as
heterochromatin, specifically euchromatin is said to be transcriptionally active and heterochromatin is transcriptionally inactive.
Acidic protein is also called NHC (non histone chromosomal) protein, it is of three types:
(i) Scaffold or structural NHC protein.
(ii) Functional NHC protein e.g., DNA polymerase, RNA polymerase.
(iii) Regulatory NHC protein e.g., HMG (controls gene expression).
Chemical Composition of Chromosome
A chromosome consists of following chemical compositions
a) DNA 40%
b) RNA 1.2%
c)Histone protein 50%
d) Acidic proteins 8.5%
e) Lipid traces
f) Ca2 Mg2, Fe2 traces