Nucleosome Model of Chromosome

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Chrosomes are self-replicating nucleic acid molecules that have particular organisational and functional characteristics and are essential for inheritance, mutation, variety and evolutionary advancement.
Every chromosome’s structure is supported by many layers of DNA wrapped around proteins.
Histones and non-histone chromosomal proteins are the traditional classifications for proteins which attach to DNA to construct eukaryotic chromosomes.
Chromatin is the name given to the complex of both classes of proteins with the nuclear DNA of eukaryotic cells. Nuclear components, chromosomes are the building blocks of a specific organisation.
Chromatin is a highly compacted structure made up of packed DNA that is required for DNA to fit into the nucleus.
A succession of steps is required to assemble DNA into chromatin, starting with the synthesis of the nucleosome, and ending with a complex organisation of domains within the nucleus.
DNA is condensed into an 11 nm fibre in the first step of this process, which indicates a 6–fold amount of compaction. Nucleosome assembly is used to do this.
The nucleosome is chromatin’s smallest structural component, formed by interactions between DNA and histone proteins.⦁
It consists of an octamer core of histones H2A, B, 3 and 4 (or other histone variants in rare instances) and a DNA segment which surrounds the core of the nucleosome. Connecting nucleosome pairs is done with the help of “linker DNA”.

When DNA and related proteins are arranged in the nucleus, the nucleosome model helps explain how DNA and related proteins are arranged in chromosomes.
Roger Kornberg proposed the model in 1974, and it is widely recognised model of chromatin organisation.
It was confirmed and christened by P. Oudet et al., and it is the most widely accepted model of chromatin organisation (1975).

DNA is securely bonded to an equivalent mass of histones in eukaryotes, forming nucleosomes, a repeating array of DNA-protein particles.
Every human chromosome’s DNA double-helix will stretch thousands of times across the cell nucleus.
Histones serve a critical funstion in packing this very long DNA molecule (i.e., nucleosome) into a nucleus with a diameter of only a few micrometres.
Thus, nucleosomes are the chromatin’s primary packing unit particles, giving it a “beads-on-a-string” look in electron micrographs following treatments which unravel higher-order packing.
In addition, every nucleosome comprises two copies of every one of the four nucleosome histones (H2A, H2B, H3, and H4), with a diameter of 11 nm and a height of 5.7 nm.
Hexamer histones (H3, H4, H2A, and H2B) form the nucleus of the DNA helix, that is twisted 146 times around the histone octamer’s protein core.
Every nucleosome is separated from the next by a 54-base pair stretch of linker DNA consisting of a single H1 histone molecule in chromatin.
H1 molecules seal off two complete rotations around the histone octamers, which are 200 bp long.
Nucleosomes repeat at 200 nucleotide or base pair intervals on average. For example, every human cell with 6 × 109 DNA nucleotide pairs includes
3 x 107 nucleosomes, and a eukaryotic gene of 10,000 nucleotide pairs will be connected with 50 nucleosomes.

The DNA is initially assembled with a freshly synthesised tetramer (H3-H4), which is then selectively changed (e.g., H4 is acetylated at Lys5 and Lysl2 (H3-H4)) to produce a sub-nucleosomal particle, that is then followed by the addition of two H2A-H2B dimers.

A nucleosomal core particle with 146 base pairs of DNA binds around the histone octamer as a result of this process. The nucleosome is made up of this core particle and its linker DNA.
The maturation process requires ATP to ensure that the nucleosome cores are spaced evenly to create the nucleo-filament.
The newly integrated histones are de-acetylated during this stage.
Following the insertion of linker histones, the nucleo-filament is folded into the 30 nm fibre, the structure of which is still unknown.
The solenoid model and the zig-zag model are the two main models.
Finally, the nucleus develops a high level of organisation and particular domains as a result of further folding events.

Nucleosome Model of Chromosome Citations

Verma, P. S., & Agrawal, V. K. (2006). Cell Biology, Genetics, Molecular Biology, Evolution & Ecology (1 ed.). S .Chand and company Ltd.
Alberts, B., Johnson, A., Lewis, J., Raff, M., Roberts, K., & Walter, P. (2002). Molecular biology of the cell. New York: Garland Science.
https://www.easybiologyclass.com/nucleosome-model-of-chromosomes-in-eukaryotes-short-notes/
http://www.biologydiscussion.com/cell-biology/nucleosome-model/nucleosome-model-of-chromatin-assembly-cell-nucleus-biology/78886
https://www.mechanobio.info/genome-regulation/what-are-nucleosomes/
https://www.ncbi.nlm.nih.gov/pubmed/958895

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