The distance between two consecutive base pairs is 0.34 nm (0.34 x 10–⁹ m). If the length of DNA double helix in a typical mammalian cell is calculated (simply by multiplying the total number of bp with distance between two consecutive bp, that is, 6.6 x 10⁹ bp X 0.34 x 10–⁹ m/bp), it comes out to be approximately 2.2 metres. A length that is far greater than dimension of a typical nucleus (approx 10^-6m).

Nucleosome

In eukaryotes, this organisation is much more complex. There is a set of positively charged, basic proteins called ‘histones’. A protein acquires charge depending upon the abundance of amino acids residues with charged side chains. Histones are rich in amino acids residues lysines and arginines. Both the amino acids residues carry positive charges in their side chains. Histone organised to form a unit of eight histone molecules called as ‘histone octamer’.

The negatively charged DNA is wrapped around positively charged histone-octamer to form a structure called ‘Nucleosome’.

A typical nucleosome contains 200 bp of DNA helix. Nucleosomes constitute the repeating unit of a structure in nucleus called ‘Chromatin’, thread-like stained (coloured) bodies seen in nucleus. The nucleosomes in chromatin are seen as “beads-on-string” structure when viewed under electron microscope (EM).