Abstract:
Chromatin is a complex of DNA and proteins that was named so by Walther
Flemming in 1882 as a substance that could be visualized by staining. It is
a highly ordered structure made up of arrays of nucleosomes that forms the
genetic material within a eukaryotic cell [1]. The details of the composition of
the nucleosome came with solving of the crystal structure at 2.8 Ao
resolution
by Luger et al [2]. Nucleosome is the repeating unit of chromatin that consists
of 146 bp of DNA wrapped in about 1.67 turns around an octamer of core
histones [3, 4]. X-ray crystallographic studies also revealed that the histone
octamer has a disc-like shape with a thickness of 6 nm and diameter of 6.5
nm [5]. The core histones that form the octamer are H2A, H2B, H3 and H4
each present as two copies within the nucleosome. Core histones are basic
proteins rich in lysine and arginine residues and it is this basic nature of
histones that allows them to be wrapped by the negatively charged DNA.
