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Heterochromatin is a tightly packed or condensed DNA that is characterized by intense stains when stained with nuclear stains, containing transcriptionally inactive sequences.
- It exists in multiple variations, up to four to five state, each of which is marked with combinations of epigenetic markers.
- The staining of heterochromatin might result in heteropycnosis; heteropycnosis is the differential staining of parts of chromosomes.
- This chromosome is different from euchromatin in that the genes in these chromosomes are usually inactivated and are not expressed.
- Heterochromatin is present in the nucleus towards the periphery. It is also not present in prokaryotic cells, indicating this form appeared later during evolution.
- However, the two most common heterochromatin include; constitutive heterochromatin and facultative heterochromatin.
- Constitutive heterochromatin usually packages the same sequences of DNA in all cells of the same species. It is usually repetitive and is present in structural forms like telomeres and centromeres.
- The genes in constitutive heterochromatin might affect the genes present near the tightly packed chromosomes.
- In humans, genes 1, 9, 16, and the Y chromosomes in men contain larger quantities of this heterochromatin.
- Facultative heterochromatin packages genes that are usually silenced through various mechanisms; however, unlike constitutive heterochromatin, facultative chromatin packages different genes in different organisms within the same species.
- The facultative chromosome is not repetitive but has the same structural components as the constitutive heterochromatin.
- The formation of facultative heterochromatin is regulated by the process of morphogenesis or differentiation.
- In humans, one of the two X chromosomes in women is inactivated as facultative heterochromatin while the other is expressed as euchromatin.
- Heterochromatin has multiple functions. Some of which include gene regulation and chromosomes integrity.
- The tightly packaged DNA in heterochromatin prevents the chromosomes from various protein factors that might lead to the binding of DNA or the inaccurate destruction of chromosomes by endonucleases.
- Besides, heterochromatin also allows gene regulation and the inheritance of epigenetic markers.
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.
- Euchromatin is present towards the center of the nucleus and accounts for about 90% of the genome in an organism.
- Under an optical microscope, it appears as light-colored bands after staining. All parts of euchromatin are uniformly stained, which doesn’t result in heteropycnosis.
- Under an electron microscope, however, it appears as an elongated 10 nm microfibril.
- The structure of euchromatin can be represented as an unfolded set of beads in a string where the beads are the nucleosomes. The nucleosomes contain histone proteins that coat a particular number of DNA around.
- In euchromatin, the wrapping around by histone proteins is loose, and thus the individual DNA sequences might be accessible.
- The conformation of euchromatin is said to be controlled by a methylated part in the chromosome called histone tail.
- Euchromatin is the only confirmation of chromosomes in the case of the prokaryotic genome, which suggests that this form evolved earlier than heterochromatin.
- Unlike heterochromatin, euchromatin doesn’t exist in two forms. It only exists as constitutive euchromatin.
- Euchromatin is extremely important as it contains genes that are transcripted into RNA, which are then translated into proteins.
- The unfolded structure of DNA in euchromatin allows regulatory proteins and RNA polymerase to bind to the sequences so that the process of transcription can initiate.
- It is possible for some genes in the euchromatin to be converted into heterochromatin when they are not to be transcribed and are no longer active.
- The transformation of euchromatin to heterochromatin acts as a method for regulating gene expression and replication.
- For this purpose, some genes like housekeeping genes are always arranged in euchromatin conformation as they have to be continuously replicated and transcribed.
Key Differences (Heterochromatin vs Euchromatin)
Basis for Comparison
|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.|
|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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- <1% – https://en.wikipedia.org/wiki/X_chromosome