Organization of chromatin fibers

a. Coiled DNA model:

• According to this model, the single DNA molecule of a chromatin fiber is coiled in a manner similar to the wire in a spring.
• The coils are held together by Histone Bridge produced by binding of histone molecule in a large groove of DNA molecules.
• Such a coiled structure would be stabilized as a single histone molecule would bind to several coils of DNA.
• This coiled structure is further coated with chromosomal proteins to yield the basic structure of chromatin fibers.
• This fibers may undergo supercoiling to produce the other type of fiber.

b. Nucleosome-Solenoid model:

• This model was purposed by Romberg and Thomas in 1974 and is universally accepted.
• According to this model, chromatin is composed of a repeating unit called nucleosome.
• One Complete nucleosome consists of the following:

• Nucleosome core
• Linker DNA
• An average of one molecule of H1 histone
• The other associated chromosomal protein

I. Nucleosome core:

• It consists of a histone octamer composed of two molecules, each of histones H2a, H2b, H3 and H4.
• Histone octamer is surrounded by DNA known as core DNA of lengtg 146 bp ( base pair)

ii. Linker DNA:

• The size of linker DNA varies from 8 bp to 114 bp.
• The beads are due to nucleosome cores.
• However, the region of linker DNA initially available for nuclease action is restricted only to a part of each linker.
• But once linker is cleaved, the rest of linker DNA becomes susceptible to nuclease digestion.
• Therefore, it appears that in the natural state of chromatin, DNA most likely passes directly from one nucleosome to the next, and free DNA in the isolated chromatin is probably generated by a loss of some histone octamers during isolation.
• This DNA forms the string part of the beads on a string chromatin fibers and is nuclease susceptible, and the beads are due to nucleosome cores.
• Thus, it joins two neighboring nucleosomes.

iii. H1 histone:

• Each nucleosome contains, on an average, one molecule of H1 histone.
• Complete removal of H1 histone doesn’t affect the structure of nucleosome core, indicating that it is located outside the nucleosome core.
• H1 is most likely located at the linker DNA and could ‘seal’ the DNA in the nucleosome by binding at the sites where DNA enters and leaves the nucleosome.

iv. other Chromosomal proteins:

• Both linker DNA and nucleosome are associated with other chromosomal proteins. Each beads corresponds to a single nucleosome core.
• The nucleosome stack together without any detectable linker DNA between them.
• This produces the 100 A thick chromatin fiber called nucleosome fiber.