Q.Explain the formation of a nucleosome.

A cell's DNA is vastly longer than the nucleus that contains it (roughly 2.2 m of double helix in a typical human cell, versus a nucleus about a millionth of a metre across), so eukaryotic cells fold and coil DNA through several hierarchical levels. Du Praw's 1965 'unineme' model treats eukaryotic DNA as a single, long, histone-associated coiled molecule. Prokaryotes, lacking a true nucleus, keep their negatively charged DNA compact by association with positively charged proteins in a region called the nucleoid, organised into large loops; prokaryotic DNA is almost circular and, lacking chromatin organisation, is called a genophore. In eukaryotes, Kornberg's nucleosome model has two copies each of histones H2A, H2B, H3 and H4 assembling into a histone octamer, around which DNA wraps nearly twice to form a nucleosome containing about 200 base pairs; neighbouring nucleosomes are joined by linker DNA sealed by histone H1, giving a 'beads-on-a-string' appearance without H1. H1-H1 interactions between …