We had already read that the length of DNA is approximately 2.2 meters. Now you wondered how this much big DNA is packed into a small nucleus?
+ve Histone & -ve DNA
DNA acquire negative charge due to the negatively charged phosphates present in the sugar-phosphate backbone. Some positive charge is needed to counterbalance this negative charge to keep the stability.
In eukaryotes, 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 like residues lysines and arginines. Both the amino acid residues carry positive charges in their side chains.
Histones are organised to form a unit of eight molecules called as 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.
DNA is further packaged by forming coils of nucleosomes, called chromatin. The nucleosomes in chromatin are seen as ‘beads-on-string’ structure when viewed under the electron microscope.
The beads-on-string structure in chromatin is packaged to form
chromatin fibers. These fibers are further coiled and condensed at metaphase stage of cell division to form chromosomes.
The packaging of chromatin at the higher level requires an additional set of proteins that collectively are referred to as Non-histone Chromosomal (NHC) proteins.