ADVERTISEMENTS:
The below mentioned article provides notes on DNA-Histone Complex.
Eukaryotic nuclear DNA remains complexed with basic histone proteins, RNA and acidic proteins in a compact form of organization in which the majority of DNA sequences are functionally inactive. DNA with the histone proteins form a subunit having the same type of design in all eukaryotes.
Each subunit consists of about 200 bp of DNA organized by an octamer of small histone proteins into a beaded structure. DNA lies on the surface of this particle, while the protein components constitute the interior. So the structure can be described as ‘a string on beads’.
ADVERTISEMENTS:
These tightly packed beaded structures are called nucleosomes. They get released when the interphase nuclei are suspended in a solution of low ionic strength. The mass of the protein is always greater than the mass of DNA. The proteins are of two types — the histones and the non-histones. The RNA fraction represents the nascent chain on the DNA template.
The histones are positively charged hydrophilic proteins. They are composed of a single polypeptide chain and are of five types, divided in three major categories: (a) arginine-rich histones, (b) slightly lysine -rich histones, (c) very lysine-rich histones. The arginine-rich histones are called H3 and H4. The slightly lysine-rich histones are H2A and H2B, and the very lysine-rich proteins are the H 1 histones.
The mucleosome contains ~ 200 bp of DNA associated with a histone octamer. The octamer consists of two copies of each H2A, H2B, H3 and H4, which are known as core histones. The DNA- histone association is illustrated diagrammatically in Fig. 3.54.
The nucleosome consists of approximately equal masses of DNA and histones. Histones neutralize a large portion of the phosphate groups of DNA. Histones make the DNA heat stable and protect it from radiation damage. Lysine-rich histones repress DNA from transcription. The histones make the elongated DNA molecule fairly compact and protect the DNA from mechanical stress.
ADVERTISEMENTS:
In general, the prokaryotes lack the histones. However, in 1976, Stewart from Scotland and Haselkorn from Chicago reported the occurrence of a histone-like protein HU which binds DNA non- specifically.
The non-histones include all other proteins of chromatin. They are more variable in different tissues and species. They comprise a relatively smaller proportion of the mass than histones. Probably they are concerned with gene expression.
