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

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.