Heterochromatin Vs Euchromatin Over view

Heterochromatin Definition

When stained with nuclear stains, heterochromatin, which contains sequences that are not active during transcription, exhibits strong stains.

It comes in up to four or five different forms, each of which is identified by different combinations of epigenetic markers.
Heteropycnosis, which is the differential staining of chromosomal sections, may occur from the staining of heterochromatin.
The genes on this chromosome are often inactive and not expressed, which distinguishes them from euchromatin.
The periphery of the nucleus contains heterochromatin. Additionally, prokaryotic cells do not have it, indicating that this type evolved later.
However, constitutive heterochromatin and facultative heterochromatin are the two types that are most frequently seen.
In all cells of the same species, constitutive heterochromatin typically stores the same DNA sequences. It may be found in structural forms like centromeres and telomeres and is often repeated.
The genes present close to the densely packed chromosomes may be impacted by the genes in constitutive heterochromatin.
Genes 1, 9, 16, and the Y chromosome in males have higher concentrations of this heterochromatin.
Genes that are often silenced by a variety of methods are packaged by facultative heterochromatin. However, unlike constitutive heterochromatin, facultative chromatin packages diverse genes in various animals within the same species.
Despite not being repeated, the facultative chromosome shares the same structural elements as constitutive heterochromatin.
The process of morphogenesis or differentiation controls the production of facultative heterochromatin.
In humans, one of the two X chromosomes found in females is expressed as euchromatin, while the other is inactivated as facultative heterochromatin.
Heterochromatin serves a variety of purposes. Some of these include maintaining chromosomal integrity and gene control.
The closely packed DNA in heterochromatin protects the chromosomes from numerous protein factors that might cause DNA binding or improper chromosomal destruction by endonucleases.
Additionally, heterochromatin permits epigenetic marker inheritance and gene control.

Euchromatin Definition

With less strong staining and DNA sequences that are transcriptionally active or may become so at some point during growth, euchromatin is a more loosely packed form of DNA.

About 90% of an organism’s genome is made up of euchromatin, which is located in the nucleus’ core.
After staining, it shows as light-colored bands under an optical microscope.
Heteropycnosis is prevented by the homogeneous staining of all euchromatin components.
However, it appears as an elongated 10 nm microfibril under an electron microscope.
Euchromatin’s structure may be seen as a thread of beads that has been unfurled, with the beads representing nucleosomes. Histone proteins found in the nucleosomes cover a certain amount of DNA.
Since the histone proteins’ covering of the euchromatin is loose, the individual DNA sequences may be exposed.
A region of the chromosome known as the histone tail is thought to regulate the conformation of euchromatin.
The bacterial genome’s single confirmation of chromosomes, euchromatin, raises the possibility that it developed before heterochromatin.
In contrast to heterochromatin, euchromatin does not have two forms. Only constitutive euchromatin is present.
Given that it includes genes that are transcribed into RNA and ultimately into proteins, euchromatin is very significant.
Because the DNA in euchromatin has an unfolded form, regulatory proteins and RNA polymerase may attach to the sequences and start the transcription process.
When they are not to be transcribed and are no longer active, certain genes in the euchromatin may be transformed into heterochromatin.
Gene expression and replication are controlled by the conversion of euchromatin to heterochromatin.
Some genes, such as housekeeping genes, are constantly organised in the euchromatin conformation for this function since they must be continuously copied and translated.

Key Differences (Heterochromatin vs Euchromatin)
Basis for Comparison	Heterochromatin	Euchromatin
Definition	Heterochromatin is a tightly packed or condensed DNA that is characterized by intense stains when stained with nuclear stains and transcriptionally inactive sequences.	Euchromatin is a more lightly packed DNA that is characterized by less intense staining and DNA sequences that are transcriptionally active or might become transcriptionally-active at some point during growth.
Staining	Heterochromatin is darkly stained under nuclear stains.	Euchromatin is lightly stained under nuclear stains.
DNA conformation	In heterochromatin, the DNA is tightly bound or condensed.	In euchromatin, the DNA is lightly bound or compressed.
DNA conformation	The DNA in heterochromatin is folded with the histone proteins.	The DNA in euchromatin is unfolded to form a beaded structure.
Genes	The genes present in heterochromatin are usually inactive.	The genes present in euchromatin are either already active or will be active during growth.
Transcription	Heterochromatin is transcriptionally-inactive.	Euchromatin is transcriptionally-active.
DNA content	Heterochromatin has more amount of DNA tightly compressed with the histone proteins.	Euchromatin has less amount of DNA lightly compressed with the histone proteins.
Content in genome	Heterochromatin forms a smaller part of the genome. In humans, it makes about 8-10% of the genome.	Euchromatin forms a more significant part of the genome. In humans, it makes about 90-92% of the genome.
Found in	Heterochromatin is found only in eukaryotes.	Euchromatin is found in both prokaryotes and eukaryotes.
Types	Heterochromatin exists in two forms; constitutive and facultative heterochromatin.	Euchromatin exists in a single form; constitutive euchromatin.
Location within the nucleus	Heterochromatin is present towards the periphery of the nucleus.	Euchromatin is present in the inner body of the nucleus.
Heteropycnosis	Heterochromatin exhibits heteropycnosis.	Euchromatin doesn’t exhibit heteropycnosis.
Replicative	Heterochromatin is a late replicative that replicate later than euchromatin.	Euchromatin is an early replicative that replicate earlier than euchromatin.
Genetic processes	Heterochromatin is not affected by genetic processes where the alleles are not varied.	Euchromatin is affected by various genetic processes that result in variation within the alleles.
Function	Heterochromatin maintains the structural integrity of the genome and allows the regulation of gene expression.	Euchromatin allows the genes to be transcribed and variation to occur within the genes.
Examples	Telomeres and centromeres, Barr bodies, one of the X chromosomes, genes 1, 9, and 16 of humans are some examples of heterochromatin.	All the chromosomes in the genome except the heterochromatin are examples of euchromatin.

References and Sources

Murakami Y. (2013) Heterochromatin and Euchromatin. In: Dubitzky W., Wolkenhauer O., Cho KH., Yokota H. (eds) Encyclopedia of Systems Biology. Springer, New York, NY
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