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Packaging of DNA Helix
Taken the distance between two consecutive base pairs
as 0.34 nm (0.34×10–9 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 × 109 bp ×
0.34 × 10-9m/bp), it comes out to be approximately
2.2 metres. A length that is far greater than the
dimension of a typical nucleus (approximately 10–6 m).
In prokaryotes, such as, E. coli, though they do
not have a defined nucleus, the DNA is not scattered
throughout the cell. DNA (being negatively charged)
is held with some proteins (that have positive
charges) in a region termed as ‘nucleoid’. The DNA
in nucleoid is organised in large loops held by
proteins.
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 the basic amino acid residues lysine and arginine.
Both the amino acid residues carry positive charges
in their side chains. Histones are organised to form
a unit of eight molecules called histone octamer.
The negatively charged DNA is wrapped around the 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, threadlike stained (coloured) bodies seen in nucleus. The nucleosomes in
chromatin are seen as ‘beads-on-string’ structure when viewed under electron microscope (EM).The beads-on-string structure in chromatin is packaged to form chromatin fibers that are further coiled and condensed at metaphase stage of cell division to form chromosomes. The packaging of chromatin at higher level requires additional set of proteins that collectively are referred to asNon-histone Chromosomal (NHC) proteins. In a typical nucleus, some
region of chromatin are loosely packed (and stains light) and are referred to as euchromatin. The chromatin that is more densely packed and stains dark are called as Heterochromatin. Euchromatin is said to be transcriptionally active chromatin, whereas heterochromatin is inactive