Unit – IV
Packaging of DNA
- The length of a DNA double helix molecule in a typical mammalian cell is approximately 2.2 metres. This length can be obtained by multiplying the total number of base pairs present in the DNA double helix, which is 6.6 X 109, with the distance between two consecutive base pairs, which is 0.34 × 10-9 m. Such a long polymer is packaged within a typical nucleus of size 10 -6 m, by condensing it by coiling and supercoiling (coiled coil) to fit in the nucleus. which is different for both prokaryotes and eukaryotes.
- In prokaryotes, the negatively charged DNA is arranged in large loops and is held together by a few positively-charged proteins, called the nucleoid.
- In eukaryotes, a single molecule of negatively charged DNA is packaged around a pool of positively charged proteins called histones.
- Histones are proteins that are rich in basic amino acid residues lysines and arginines. They carry a positive charge on their outer side chain. There are four types of histones H2A, H2B, H3 and H4. Two types of each, thus four molecules of histones form an octamer. The negatively charged DNA is wrapped around the positively charged histone octamer to form a nucleosome. It is held in place by the H1 histone.
- A typical nucleosome has around two hundred (146 + 54) base pairs of DNA helix and it is the nucleosomes that make up the repeating unit in a chromatin.
- Under an electron microscope, nucleus shows chromatin network. In the network, nucleosome can be seen as beads on the string. 1 and 3/4 th turn around histone octamer consist of 146 base pairs and is called core DNA. Adjacent DNA acts as linking strand consists of 54 base pairs and is called linker DNA. H1 histone is present in the linker region and as DNA makes two complete turns it is present where DNA starts wrapping the histone and leaves it.
- The thin and long nucleosome fibre is coiled again to form supercoiled structure to make solenoid fibre with diameter 30nm or 300 A°. Nucleosome and solenoid fibres are characteristic of the nucleus at interphase.
- For further packing of chromatin additional protein Non-Histone Chromosomal (NHC) proteins are responsible. The chromatin fibres are of two types, euchromatin and heterochromatin.
- The euchromatin fibres are of thirty to eighty nanometres in diameter and are loosely packed and stain light. While, heterochromatin fibres are of about three hundred nanometres in diameter and are more densely packed and stain dark.
- The euchromatin is transcriptionally active, while heterochromatin is inactive.
- These chromatin fibres coil further and condense to form short and thick bodies called chromosomes, which are further packaged within the nucleus.