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CBSE Class 12-science Answered

EXPLAIN  THE  SOLENOID  MODEL  OF  DNA   PACKAGING  WITH  HISTONES  WITH  THE  HELP  OF  DIAGRAM FOR  5  MARKS

Asked by pardeepkumar2281 | 03 Oct, 2018, 08:09: PM
Expert Answer
The solenoid model of DNA is the DNA packing in eukaryotes. 
  • It is carried out with the help of lysine and arginine-rich basic proteins called histones.
  • The compacted mass is called nucleosome.
  • Four of the five histone proteins (H2A, H2B, H3 and H4) occur in pairs to produce histone octamer called nu body.
  • About 166 bp of DNA is wrapped over the nu body for turns to form a nucleosome of size 11×6 nm.
  • DNA connecting two adjacent nucleosomes is called interbead or linker DNA.
  • Nucleosome and linker DNA together constitute chromatosome.
  • Nucleosome chain gives a ‘beads on string’ appearance under an electron microscope.
  • The nucleosomal organisation in about 10 nm in thickness, which gets further condensed and coiled to produce a solenoid of 30 nm diameter. This solenoid structure undergoes further coiling to produce a chromatin fibre of 30-80 nm and then, a chromatid of 700 nm.
  • Chromatin is held over a scaffold of non-histone chromosomal or NHC proteins.
  • At some places, chromatin is densely packed to form darkly staining heterochromatin. At other places, chromatin is loosely packed to form euchromatin which is lightly stained.
  • Euchromatin is transcriptionally active chromatin whereas heterochromatin is transcriptionally inactive and late replicating or heteropycnotic.
 
 
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