The distance between two consecutive base pair is 0. 34 nm or 0.34 into 10 to the power -9 m . The total number of base pair in a mammalian cell is about 6.6 into 10 to the power 9 bp . There fore the length of the DNA is calculated , that is 6.6 into 10 to the power 9 bp into 0.34 into 10 to the power -9 m , it comes to be approximately 2.2 meters.
In prokaryotic cells DNA remains in nucleoid of the cells . As DNA is negatively charged , it is surrounded by some positively charged proteins .
In eukaryotic cells DNA lies inside the nucleus . Hence nucleus is called the cage of DNA . The DNA remains in association with positive charged proteins called Histones .Histones are rich in basic aminoacid residues lysines and arginines .
Histones are organised to form a unit of eight molecules called a histone octamer .
The negatively charged DNA is wrapped around the positively charged histone octomer to form a structure called nucleosome.
A typical nucleosome contains about 200bp of DNA helix .
Nucleosomes constitute the repeating unit of a structure in the nucleus called chromatin .
The packaging of chromatin at higher level requires additional set of proteins that collectively referred as Non-histone chromosomal proteins .
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