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’.
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.
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.