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  Section: Genetics » Organization of Genetic Material » Packaging of DNA as Nucleosomes
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Solenoid model

Organization of Genetic Material 1.  Packaging of DNA as Nucleosomes in Eukaryotes
Techniques leading to nucleosome discovery
Subunit of chromatin - the nucleosome
Spatial arrangement of histones
Relation between different nucleosomes
Solenoid model
Loops, domains and scaffolds in chromatin
Chromatin replication and nucleosome assembly
Phasing and modification of nucleosomes in active genes

Solenoid Model
It was also shown that 10 nm fibre of nucleosomes gets coiled upon itself to form 30 nm wide helix with five or six nucleosomes per helix. In this helix successive turns came close together, so that their centre to centre distance was about 10 nm. This 30 nm structure was called a solenoid (term solenoid in physics is used for a wire coiled on a central axis). Formation of solenoid from nucleosomes can be compared with winding of a cable (DNA) on a spool (nucleosome) and then folding of wrapped spools. A suggested structure of solenoid is shown in Figure 27.11.

It was also proved that HI protein helped in folding of 100 Å fibre into 300 Å solenoid, because when HI was removed, this ordered folding was found to be absent and only irregular clumping of nucleosomes could be observed. It has been shown that HI molecules aggregate by cross linking to form polymers and may thus control the formation of solenoid. It is also speculated that solenoid has to fold or coil again during condensation of chromatin.

The above account gives patterns of coiling and packing of DNA. Since 60 nm (600 Å) long DNA (166 base pairs) is coiled in a nucleosome, only 5.5 nm (55 Å) long, and then nucleosomes are coiled in 30 nm (300 Å) wide solenoid fibres, it gives DNA a packing ratio of 1 : 50. However, in highly condensed chromosomes, the packing ratio is actually 1 : 5,000, which is 100 times greater than provided by solenoid model. This 100 times packing of the solenoid, should take place by further coiling and folding of solenoid.

Solenoid model, with a helix having about six nucleosomes per turn, where HI molecules of adjacent nucleosomes are in contact (redrawn from Sci. Amer., Vol. 244 1981).
Fig. 27.11. Solenoid model, with a helix having about six nucleosomes per turn, where HI molecules of adjacent nucleosomes are in contact (redrawn from Sci. Amer., Vol. 244 1981).


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