Taken the distance between two consecutive base pairs- 0.34nm,

if the length of DNA double helixin a typical mammalian cell,  
         =The total no. of base pairs x the distance between two consecutive base pairs

         = approximately 2.2 metres.

Wonderful and efficient packaging system of such a long polymer  in a cell..............

   

     PACKAGING OF DNA

PACKAGING OF DNA HELIX  IN PROKARYOTES

      In prokaryotes with no well defined nucleus the DNA ( Negatively charged) is organized in large loop held by certain positively charged proteins in a region called nucleoid.      

PACKAGING OF DNA IN EUKARYOTES

In Eukaryotes, the negatively charged DNA is wrapped around positively charged histone octamer into a structure called nucleosome.

1. A typical nucleosome consists of the negatively charged DNA wrapped around the positively charged histone octamer.
2. Histone monomer is a basic protein rich in basic amino acids – Lysines and Arginines. Both these amino acids carry positive charges in their side chains. 8 molecules of  Histone proteins(monomers H-2A,H-2B,H-3 and H-4 )  are organized to form Histone Octamer.
3. A negatively DNA helix consisting of 200 bp is wrapped around the positively charged Histone octamer  to form a structure called Nucleosome.
4. The wrapping of DNA around Histone  is further stabilized by the help of H1 histone.  
5. Nucleosomes constitute the repeating unit of the repeating unit of the structure called chromatin. The nucleosomes in chromatin are seen as ‘beads – on – string’
6. The beads on string structure in chromatin is packaged to form chromatin fibres that are further coiled and condensed at metaphase stage of cell division to form chromosomes.
7. The packaging of chromatin at higher level requires additional set of proteins that collectively referred to as Non- histone Chromosomal (NHC) Proteins.

         DIAGRAMS SHOWING ORGANISATION OF HISTONE MONOMERS AND DNA TO FORM NUCLEOSOME,     CHROMATIN AND CHROMOSOME 

        

Sl. No.	EUCHROMATIN	HETEROCHROMATIN
1	In a typical nucleus , some region of chromatin are loosely packed and stains lightly , are referred to as Euchromatin.	The chromatin that is more densely packed and stains dark are called as Heterochromatin.
2	Euchromatin is said to be transcriptionally active chromatin.	Heterochromatin is transcriptionally inactive chromatin.