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).

Class_12_Molecular_Basis_Of_Inheritance_Structure_Of_DNA_1

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’.

Class_12_Molecular_Basis_Of_Inheritance_DNA_In_Prokaryotes

  • 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.

Class_12_Molecular_Basis_Of_Inheritance_DNA_In_Eukaryotes

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

Class_12_Molecular_Basis_Of_Inheritance_DNA

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