Class 12 Biology Molecular Basis of Inheritance Packaging of DNA helix

Packaging of DNA helix

  • Length of DNA is found to be far greater than dimension of a typical nucleus

Total number of base pairs in a typical mammalian cell= 6.6 * 109

Distance between two base pairs= 0.34nm

Therefore, Length of DNA= 0.34 *10-9 *6.6 *109 = 2.2m

Size of a nucleus is of the order of 10-6m

So, Length of DNA is greater than the size of nucleus (It is as if the house is smaller than the person who wants to stay in).


DNA is packaged very strategically to fit inside the nucleus.

  • In Prokaryotes, DNA is organized into loops held by proteins. The region where DNA is present is termed as ‘Nucleoid’.


  • In Eukaryotes, there exist positively charged basic proteins called Histones.
  • DNA wrap around the histone octamer (group of 8 histone proteins) to form a Nucleosome. Each nucleosome contains 200 base pairs of DNA helix. Nucleosomes in chromatin are seen as ‘beads-on-string’ under Electron microscope.


Based on different types of DNA packaging, there are two forms of Chromatin.

  • Euchromatin
    • Less condensed structure with looser DNA packaging
    • Lightly stained when observed under microscope
    • Contains less DNA
    • Transcriptionally active
    • Found in eukaryotes & prokaryotes
  • Heterochromatin
    • Highly condensed structure with tighter DNA packaging
    • Dark stained when observed under microscope
    • Contains more DNA
    • Transcriptionally inactive, as those regions of the genes which need to interact with proteins for transcription is inaccessible.
    • Found in eukaryotes


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