Molecular Structure of Eukaryotic Chromatin
Chromatin- The negatively charged DNA gets wrapped with basic protein molecules called histones forming stable thread like structure called chromatin because these thread like structure appear as coloured thread like structures when stained with chromosomal stains (like Acetocarmine, Feulgen stain).
During cell division, these chromatin gets condensed and coiled on itself several times that appears as a distinct visible structure in the dividing cells. These structures are called chromosomes.
Structure of Chromosome and Chromatin
Chromatin and chromosomes are made up of:
- Single molecule of double stranded linear DNA, which is the polymer of nucleotide and appear as double stranded right handed helix. The backbone of double stranded DNA helix is negatively charged and which provide the basis for non-covalent interaction of DNA with basic proteins called histone.
- Histone protein- These are the basic proteins. These proteins are rich in basic amino acids like Lysine and Arginine.
- There are five types of histone proteins that differ in their Arginine/Lysine ratio named as H2A, H2B, H3, H4 and H1.
- Sometimes, some derivatives of these histone protein occurs that play different role in maintaining the genomic stability and transcription.
- Derivatives of H2A are H2AX and H2AZ whereas derivative of H3 is CEND-Histone which provides specificity for the assembly of spindle fibre at kinetochore or centromere.
- These histone proteins are highly conserved.
- These histone proteins get associated with the DNA helix at the minor grooves forming stable chromatin.
- Thus, histone play very important role in stabilizing the eukaryotic DNA and its packaging during different stages of cell division. Thus, histone play structural as well as regulatory role in organisation of eukaryotic chromosome.
Levels of Packaging of DNA
In Eukaryotes, the DNA undergoes several levels of compactation and packaging during different stage of cell cycle. The histone and non-histone proteins play very crucial role in packaging of long eukaryotic DNA into discrete chromosomes.
The eukaryotic DNA undergoes three level of packaging:
First level of DNA packaging
Nucleosome- The eukaryotic chromatin appears as “beads on strings”. The beads are present at a regular interval; these beaded structures were called nucleosome by Oudet et al.
- D. Kornberg called nucleosomes as the fundamental unit of eukaryotic chromatin.
- The eukaryotic chromosomes are made up of repeated units of nucleosome containing constant length of DNA of 200 base pairs associated with histone proteins.
- The nucleosomes are disc-shaped particles of 10 Å diameter that constitute the first level of DNA coiling in the form of nucleo-proteinaceous fibre of 10 Å
- Each nucleosome consists of nucleosome core particle and linker DNA associated with H1 Histone core particle is a octamer molecule which consists of dimers of each H2A, H2B, H3 and H4. These core particles get permanently associated with 146 base pairs of double stranded DNA helix resistant to micrococcal enzyme.
- Linker DNA- It may vary from 14 base pairs to 40 base pairs connecting adjacent nucleosome. The H1 histone also called linker histone seal the super helix of DNA when it completes 1 ¾ round around the core particle constituting the chromatosomes that contains DNA length of 200 base pairs and appear as beads on the string on the eukaryotic chromatin.
- The association of DNA super helix with histone forming nucleosome particles reduces the length of DNA seven times ensuring the packaging ratio of 1:7. This 10 nm nucleosome fibre is the fundamental structure of eukaryotic chromatin does not occur as 10 nm fibre in the nucleus at any stage of cell cycle but it undergoes further packaging and compactation.
Second level Packaging of Eukaryotic Chromatin/ DNA packaging
- The 10 nm nucleosome fibre coiled upon themselves forming nucleosome solenoid having diameter of 30 nm.
- Each turn of solenoid contains nucleosome particles.
- The formation of nucleosome solenoid is accomplished by the association of H1 histone of adjacent nucleosome particle.
- H3 and H4 histone of core particles tail also facilitate the coiling of nucleosome.
- The 30 nm chromatin fibre occurs in the interphase stage of the cell cycle because 30 nm fibre is easily accessible for the proteins and enzymes that are responsible for DNA replication and transcription.
- In 30 nm fibre DNA reduces 4 folds of its initial length.
Third level Packaging of Eukaryotic Chromatin
- During the cell division 30 nm chromatin fibre undergoes further packaging reducing the initial length of DNA to 1000 folds. As the result DNA appears as 1 µm thick visible chromosome during the metaphase stage of cell cycle.
- The third level of packaging is accomplished by non-histone protein called topoisomerase type II that forms protein scaffold from which nucleosome super solenoid radiate out compacting the eukaryotic chromatin into visible metaphasic chromosome.
- In eukaryotic chromatin have been evolved in order to pack enormous large DNA of genome into small space inside the nucleus, to stabilize the DNA from the actively of nuclease enzyme present in nucleus.
- The eukaryotic chromatin is a dynamic structure that varies from one stage of cell cycle to the other and histone, a regulatory protein regulate the activity and expression of eukaryotic DNA.
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