• This model was proposed by Kornberg and Thomas in 1974 to explain the structure of chromatin fibres. 
  • This has been widely accepted all over the world. 
  • According to this model, chromatin is composed of a repeating unit called nucleosome. 

Important points of this model are as follows:

  • Chromatin fibres of a chromosome are made up of DNA and histone proteins.
  • The repeating unit of chromatin is called nucleosome. 
  • It is a disc-like structure 11nm in diameter and 6 nm in height. 
  • The core of a nucleosome is made up of an octamer of proteins having two molecules each of H2A, H2B, H3 and H4 histones.
  • Around this octamer, a DNA segment having the length of 200 base pairs is wound round making one 3/4 turns. 
  • This segment of DNA in chromatin fibre is nuclease resistant. 
  • The structure of the nucleosome is invariable in all the eukaryotes.
  • P. Oudet et al. (1975) worked extensively on the structure of nucleosomes and proposed that the length of the DNA segment in the core of nucleosome is 146 base pairs. 
  • Two nucleosome units are joined with a segment of DNA, which is called linker.
  •  It consists of 50-70 base pairs. 
  • H1 histone is associated with this linker DNA which makes a connection between two adjacent nucleosomes.
  • The nucleosome model explains the ‘string of beads’ concept of chromatin. 
  • This is just opposite to the concept of ‘beads on string’ explaining the interrelationship of genes and chromosomes. 
  • Aron Clug (1977-80) made further electron microscopic studies of chromosomes and chromatin and proposed the ‘Solenoid model of nucleosome’. 
  • This model describes the dense compaction of DNA in chromosomal chromatids. 
  • It further illustrates that chromatin fibers tightly coil in a chromosome and form lump-like structures. 
  • The average diameter of this chromatin lump is 300 Å in which several nucleosomes of 100 Å diameter are found. 
  • As has been mentioned earlier each nucleosome is made up of protein octamer around which a DNA segment of 200 base pairs was found forming one3/4 turn. 
  • Through the process of supercoiling, such nucleosomes with the help of linker DNA easily form the solenoid-like structure.


  • It was also shown that an 11 nm wide fiber of nucleosomes gets coiled upon itself to form – 30 nm wide helix with five or six nucleosomes per helix. 
  • In this helix successive nucleosome units came close together, so that their center to center distance was about 10 nm. 
  • This 30 nm structure was called a solenoid. 
  • Formation of solenoid from nucleosomes can be compared with winding of a cable on a spool and then folding of wrapped spools.
  • It was also proved that H1 protein helped in folding of 110 A wide fiber into 300 armstrong wide solenoid, 
  • It has been shown that H1 molecules aggregate by cross linking to form polymers and may thus control the formation of solenoids. 
  • The above account gives patterns of coiling and packing of DNA. Since 60 nm along 
  • DNA is coiled in a nucleosome, only 6nm long, and then nucleosomes are coiled in 30 nm wide solenoid fibers, giving DNA a packing ratio of 1:50. 
  • However, in highly condensed chromosomes, the packing ratio is actually 1:5000, which is 100 times greater than provided by solenoid, which would take place by further coiling and folding of solenoid.

Ubiquitination, acetylation, methylation and phosphorylation of histones in the nucleosome.

  • The histone proteins, which are integral parts of nucleosomes undergo a variety of modifications to bring about decondensation of chromatin, to allow access of DNA replication or transcription machinery to naked DNA.
  • These modifications include ubiquitination, acetylation, methylation and phosphorylation of some specific amino acid residues of histones.
  • Acetylation and methylation occur on the free amino groups of lysines residues. Methylation also occurs on arginine and histidine. 
  • Similarly, phosphorylation occurs on the hydroxyl group of serine and histidine. 
  • Methylation and acetylation remove the positive charge on NH3+, while phosphorylation introduces a negative charge in the form of  phosphate group.
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