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Packaging of DNA Helix

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Packaging of DNA Helix - Lesson Summary

The length of a DNA double helix in a typical mammalian cell is about 2.2 metres.  This length is obtained by multiplying the total number of base pairs present in the DNA double helix, which is 6.6 X 10 9, with the distance between two consecutive base pairs, which is .34 X 10 -9. Such a long polymer is packaged within a typical 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. While in eukaryotes, a single molecule of negatively charged DNA is packaged around a pool of positively charged proteins called histones.

The charge in a protein is decided by the abundance of amino acid residues with charged side chains. The proteins can be positively charged if they are rich in basic amino acid residues like lysines and arginines, which carry positive charges in their side chains.
 
The histones form a unit of eight molecules known as the histone octamer, around which, a negatively charged DNA molecule is wrapped to form a nucleosome. It is held in place by the H1 histone. A typical nucleosome has around two hundred base pairs of DNA helix and it is the nucleosomes that make up the repeating unit in a chromatin.

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.

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