Chromosomes, eukaryotic histone octamer

The genetic information of eukaryotic cells is propagated in the form of chromosomal DNA. Besides the nucleic acid component, chromosomes contain architectural proteins as stoichiometric components, which are involved in the protective compaction of the fragile DNA double strands. Together, the DNA and proteins form a nucleoprotein structure called chromatin. The fundamental repeating unit of chromatin is the nucleosome core particle. It consists of about 147 base pairs of DNA wrapped around a histone octamer of a (H3/H4)2 tetramer and two (H2A-H2B) heterodimers. One molecule of the linker histone HI (or H5) binds the linker DNA region between two nucleosome core particles (Bates and Thomas 1981). 

In the chromatin of eukaryotic cells DNA forms a coiled-coil structure with an approximately equal weight of a mixture of five basic proteins known as histones. Four of these histones in pairs form an octa-mer around which the DNA duplex occurs in a left-handed helix. The DNA octamer complex is called a nucleosome. Each nucleosome contains about 140 base pairs of DNA in a nuclease-resistant nucleosome core and approximately 60 base pairs of spacer between core particles. Histone HI binds to the chromatin independently of the octamer and is the first histone to dissociate from the chromatin when the ionic strength is raised. Beyond the nucleosome the higher order structure of the chromosome involves coiled-coil structures with varying degrees of regularity. 

---