Chromosome, Heterochromatin, and DNA
We all know that our body is made up of millions of cells, each cell makes up the structural and functional unit of life. Each cell consists of many cellular organelles such as nucleus, mitochondria, lysosome, etc.
Chromosomes are one of the important components which are present in the nucleus of the cell and contains an organized package of DNA, which is very much important in carrying out the body’s metabolic and enzymatic activities and also has a vital i role in carrying out the hereditary characters.
Chromatin is one of the complexes of the DNA and proteins which forms the chromosomes. Chromatin generally occurs in two forms, namely euchromatin and heterochromatin.
Euchromatin is less condensed and can be transcribed, whereas the heterochromatin is highly condensed and is not typically transcribed.
What is Heterochromatin?
Heterochromatin is a condensed or a tightly packed form of DNA, which comes in various forms. These forms stay in a continuous manner in between the two ends of the constitutive heterochromatin and facultative heterochromatin.
These two play an important role in expressing the genes. As it tightly packed, it was said to be inaccessible to the polymerases and therefore it cannot be transcribed.
If much of this DNA, is transcribed. It is continuously turned over the RNA induced transcriptional silencing, recent studies suggest that electron microscopic staining reveal that the dense packing is not due to the chromatin.
Constitutive heterochromatin affects the genes near itself. It occurs in the repetitive manner and it performs structural functions such as centromeres and telomeres.
It acts as an attractor along with expressing the genes and also in repressing signals.
Facultative heterochromatin is due to the result of genes which are silenced through an activity of the histone deacetylation or Piwi-interacting RNA, which is commonly called as PiRNA through RNAi.
This does not occur in a repetitive manner and it also shares the compact structure of the consecutive heterochromatin.
Under specific developmental or environmental signaling of cues, it can sometimes lose its condensed structure and it becomes transcriptionally active.
Heterochromatin is also associated with the di and tri methylation of H3K9 in some certain portions of the genomes. Here H3K9me3 related methyl transferases appears to have pivotal role in modifying the heterochromatin during the lineage commitment which occurs at the onset of organogenesis and it also maintains the lineage fidelity.
Structure of Heterochromatin
Chromatin usually occurs in two varieties such as euchromatin and heterochromatin. These two forms are distinguished cytologically by assuming how intensely they are getting stained; The euchromatin is stained lightly, whereas the heterochromatin is stained intensely such that it looks dark in color, which indicates that it is a tight packaging.
Heterochromatin is usually located at the end of the nucleus. Inspite of this early dichromia, recent evidences in both animals and plants suggest that there were more than two different states of heterochromatin.
But it has also been said that it exists in four or five states which are marked with different combinations in the epigenetic marks.
Heterochromatin usually has its genetically inactive satellite sequences and many of the genes which represses the various extents.
Even though some of them are not able to express their genes in euchromatin at any of the time. Where as both centromeres and telomeres are heterochromatic, as such as how it is present in the Barr body to the second inactivated X-chromosome n the female.
Function of Heterochromatin
Heterochromatin is usually associated with several functions which forms the regulation of gene to protect the integrity of chromosome.
Some roles of heterochromatin help in attributing the dense packaging of DNA, which helps in making it less accessible to the protein factors to bind DNA or its other associating factors.
Some regions of chromatin are very densely packaged along with the fibers which displays a condition compared to that of the chromosomes at its mitosis stage.
Heterochromatin is usually inherited by cloning, During the process of cell division the two daughter cells are produced which contains heterochromatin, in the same regions of DNA which results in epigenetic inheritance.
Variations in the heterochromatin encroaches on the adjacent genes or recedes from the genes to the extreme of the domains.
Transcribable material can be expressed by positioning at the boundary domains. This gives rise to the levels of expression that are varying in the cells from one cell to the other.
Usually, all cells in many of the species has the same regions of the DNA as their packaging sites, especially in a constitutive heterochromatin and as a result any genes which is present within the constitutive heterochromatin in all the cells are poorly expressed.
In most of the organisms this constitutive heterochromatin occurs around the region of chromosomal centromere and near the telomeres.
The regions of the DNA which are packaged in the facultative heterochromatins are not consistent in all the cell type within the pieces, and hence the sequence on one cell is packed in facultative heterochromatin, which means that the genes are expressed poorly so that it can packed in a euchromatin of another cell.
However, the formation of this facultative heterochromatin is regulated often and it is associated with the morphogenesis or differentiation.
One of the species of yeast, such as Saccharomyces cerevisiae, which is also commonly called as budding yeast is considered as one of the eukaryotes and it also has a definite structure of heterochromatin.
Most of its genome are characterized as euchromatin where the regions of the DNA are transcribed very poorly, this type of loci are called as silent mating type loci, the rDNA, sub telomeric regions.
Where as the fission yeast uses another mechanism for the formation of heterochromatin in its centromeres.