Explain the solenoid model of DNA packaging with histones with the help of diagram for 5 marks
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 interbreed or linker DNA.
- Nucleosome and linker DNA together constitute chromatosome.
- Nucleosome chain gives a ‘beads on string’ appearance under an electron microscope.
- The nucleosomal organization 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 fiber 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.