In addition to organization of DNA in prokaryotes and lower eukaryotes, in eukaryotes the DNA helix is highly organised into the well-defined DNA-protein complex termed as nucleosomes. Among the proteins the most prominent are the histones. The histones are small and basic proteins rich in amino acids such as lysine and/or arginine.
Almost in all eukaryotic cells there are five types of histones e.g. H1, H2A, H2B, H3 and H4. Eight histone molecules (two each of H2A, H2B, H3 and H4) form an octamer ellipsoidal structure of about 11 nm long and 6.5-7 nm in diameter.
DNA coils around the surface of ellipsoidal structure of histones 166 base pairs (about 7/4 turns) before proceeding onto the next and form a complex structure, the nucleosome (Fig. 5.8A- B). Thus a nucleosome is an octamer of four histone proteins complexed with DNA.
The histones play an important role in determining of eukaryotic chromosomes by determining the conformation known as chromatin. The nucleosomes are the repeating units of DNA organization which are often termed as beads. The DNA isolated from chromatin looks like string or beads.
The 146 base pairs of DNA lie in the helical path and the histone-DNA assembly is known as the nucleosome core particle. The stretch of DNA between the nucleosomes is known as ‘linker’ which varies in length from 14 to over 100 base pairs.
The H1 is associated with the linker region and helps the folding of DNA into complex structure called chromatin fibres which in turn get coiled to form chromatin. As a result of maximum folding of DNA, chromatin becomes visible as chromosomes during cell division.