What is Nucleosome?
Nucleosome is one of the sections of DNA, which is wrapped around a core of proteins. Where the DNA forms a complex with protein inside a nucleus which is called as chromatin.
This chromatin allows the DNA to get condensed and thus reduces its volume. When this chromatin is extended and observed under a microscope, it resembles a structure of beads on a string.
Where these tiny beads are called as nucleosomes which has an approximate diameter of about 11nm. The nucleosome is one of the fundamental subunits of chromatin.
Each nucleosome constitutes a set of eight proteins which are wrapped around a DNA. And this is commonly referred to as histone octamer. Thus, each histone octamer has two copies of histone proteins of H2A, H2B, H3 and H4.
This chain of nucleosomes is further compacted and made into a highly organised complex structure of DNA and a protein named chromosome.
Nucleosome is a Unit of Chromatin
Nucleosomes are considered as the basic unit of chromatin as if the cells are the basic unit of life. Thus, the basic structural and functional unit of chromatin is the nucleosome which is made up of eight histone proteins, so it is also called as histone octomer.
While observing chromatin under an electron microscope it looked like a bead present on the string. It gave a clue that nucleosomes exist and the other clue is got by chemical cross-linking of histones in chromatin.
Structure of Nucleosome
When chromatin or whole chromosomes are let to read in air or water interface and examined under an electron microscope, a fibres of length 250 Angstrom are noted.
These fibres are generally known as deoxy-nucleosome proteins fibres which is present in the DNA complex along with the proteins.
This removal of protein can be carried out by several methods which reveals the fibre of 10nm, which acts as a subunit of chromatin.
Pronase is used in the digestion of 10nm fibre which leaves the sensitive DNAs of 2nm with the single DNA double helix.
It is now found that only a small fraction of eukaryotic chromatin is found in an active form and the remain part in a particular nucleus remains inactive.
Actually, there are two kinds of active regions which are recognised as the nuclear regions for coding the rRNA. And non-ribosomal cistrons.
Here the active chromatin shows the molecules of nascent RNA along its axis of DNA. Which is separated by the non-transcribed spacers.
Where as the inactive chromatin has a structure, which looks alike of beads present on the strings, and finally this is called as nucleosomes.
In the year 1974, researcher Olins and Olins experimented chicken embryo with the Millers technique and found the numerous fibres present along with a knob like structure which are released out with the nuclei. Beaded structures were also called as nu bodies.
Where as the other researchers, Oudet, Grosss-Bellard and Chambon seen many similar beads in the chromatin preparation which is collected from many vertebrates, in the year 1975. And they introduced them into nucleosome.
It is generally present in all eukaryotic cells and they are studied in plants and animals in their mitotic and polytene chromosomes.
The bead connecting to the strings constitutes one repair unit and it is structurally made up of about 200 base pairs in the DNA along with all the five histone proteins.
This bead consists of about 140 base pairs of DNAs which is wound in to two complete super-helical turns which is around a protein core and consists of 2 molecules of the histones each. H2A, H2B, H3 and H4.
There are about 60 base pairs that are present in the DNA which connects the strings with the histone H1. Where as the core particles are used for the construction of beads. And the connecting filaments are present in the linker region.
Thus, the DNA in the chromatin is packed with a series of repeated units of two distinct parts such as core particles and a liker region.
Where the chromatin present in the protozoans and in lower eukaryotes shows the similar length of 140 base pairs in the DNA.
But the length of the linker is variable even within the same tissues comparing with different species.
The nucleosome model of chromatin structure is attractive as it explains the packing of long strands of the DNA as the smaller repeat units which are known as nucleosomes.
Chromatin and Nucleosomes Structure
On analysing chromatin biochemically, it supports the concept of nucleosome. When chromatin is digested with the micro-coccal nuclease, it produces an oligomeric fragment of DNA with the specific lengths which corresponds with the repeat units which is being observed in electron microscope.
X-ray diffraction and scattering studies on crystallised DNA also proved the existence of nucleosomes.
In the diffraction of x-rays, it has been shown that it consists of a double helical DNA which is wound around a core of histone proteins with the super helix having a pitch of about 28A with the 1.75 windings around the DNA.
Function of Nucleosome
Nucleosomes are considering as the basic packaging unit in the DNA which is built with histone proteins and it is coiled around the DNA.
It generally serves as a scaffold for the formation of higher order of the structure of chromatin. It also acts as a lyre for controlling the expression of genes by following the concept of gene expression.
This functions smartly in reducing the over all length of the DNA which is present in the nucleus and help to keen the chromatin in an organised manner.
It is also known for its three primary functions first they provide space for packing and stabilising the negative coiling of the genomic DNA, through invivo.
The histones in a nucleosome are post transitionally replaced by the variants of histones for providing an additional epigenetic layer of information to get associated with the genome.
The third thing is that nucleosomes have the capability to regulate the transacting factors directly to function the elements in the chromosomes by virtue.
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- Histone core modifications regulating nucleosome structure and dynamics. Nat Rev Mol Cell Biol . 2014 Nov;15(11):703-8.
- A brief review of nucleosome structure. FEBS Lett . 2015 Oct 7;589(20 Pt A):2914-22.