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.
Read Also: DNA- Structure, Properties, Types and Functions
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.
Read Also: Different forms of DNA- A form, B form, Z form
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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