The staining of non-dividing cells in the nucleus under the light microscope exhibits two distinct regions depending on the intensity of the staining. Lightly stained areas are considered as euchromatin, whereas the darkly stained areas are considered as heterochromatin. Heterochromatin organization is more compact in such a way that their DNA is inaccessible to the proteins which are involved in the gene expression.
Genetic events like chromosomal crossing over are avoided by the compact nature of heterochromatin. Hence, heterochromatin is considered as transcriptionally and genetically inactive.
Two heterochromatin types can be identified in the nucleus: constitutive heterochromatin and facultative heterochromatin. Constitutive heterochromatin contains no genes in the genome, hence it can be retained in its compact structure also during the interphase of the cell.
DNA in the telomeric and centromeric regions belong to the constitutive heterochromatin. Some regions in the chromosomes belong to the constitutive heterochromatin; for example, most of the regions of Y chromosome is constitutionally heterochromatic. These inactive genes may be inactive either in some cells or during some periods. When those genes are inactive, they form facultative heterochromatin. Chromatin structures, beads on a string, 30 nm fiber, active chromosomes in the interphase are shown in figure 2.
Figure 2: Chromatin Structures. Heterochromatin is mainly involved in maintaining the integrity of the genome. The higher packaging of heterocromatin allows the gene expression to be regulated by keeping the DNA regions inaccessible to proteins in gene expression.
The formation of heterochromatin prevents the DNA end damage by endonucleases due to its compact nature. Euchromatin: Euchromatin is the uncoiled form of chromatin. Heterochromatin: Heterochromatin is a part of chromosome. It is tightly packed. Euchromatin: Euchromatin consists of chromatin fibers, and the DNA is wrapped around histone protein chores.
Hence, it is loosely packaged. Heterochromatin: Heterochromatin is a tightly packed form of DNA in the chromosome. Euchromatin: Euchromatin is lightly stained. But, it is stained dark during the mitosis. Heterochromatin: Heterochromatin is stained dark during the interphase. Euchromatin: Euchromatin contains a low DNA density compared to heterochromatin.
Heterochromatin: Heterochromatin contains a high density of DNA. Euchromatin: Euchromatin does not exhibit heteropycnosis. Heterochromatin: Heterochromatin exhibits heteropycnosis. Euchromatin: Euchromatin is found in both prokaryotes and eukaryotes. Heterochromatin can epigenetically administer the expression of nearby genes resulting in varied phenotypes in genetically identical cells.
Biochemical and genetic approaches show that the RNAi machinery plays an important role in the formation of heterochromatin. The structure of Heterochromatin is tightly packed and condensed. The changes in heterochromatin occurs due to the modifications to histones and spreading of silencing complexes cause the changes in structure of chromatin. Due to its repressive structure, heterochromatin does not completely express the genes within it.
Heterochromatin usually folds into higher order structures and this induces an increase in negative supercoiling of DNA. The structure of Heterochromatin is stable and is also dynamic that changes with cell cycle. The formation of chromatin is promoted due to the DNA elements called barriers which promote the formation of active chromatin and remove the nucleosomes. This allows the heterochromatin to spread. The structural functions are regulated by this form of heterochromatin and are found in centromeres and telomeres.
Facultative Heterochromatin is known to change its structure according to the cell cycle. This can be seen to change its structure in the inactivated X-chromosome of females. The structure of heterochromatin can also be determined by the density gradient data in which the heterochromatin appears as regular structure and euchromatin has an irregular structure.
The functional aspects of heterochromatin are determined by the modifications of chromatin. The heterochromatin core histones present in yeast are hypoacetylated which makes the lysine residues to become more positively charged, allowing an increase in the interaction between the histone and DNA, making the nucleosome more closed in structure.
The closed chromatin structure of heterochromatin is due to the low acetylation of Histone H4-K16 in heterochromatin, further promoting the folding of chromatin to high structure orders.
The active transcriptional activity is due to the hypomethylation of heterochromatin at H3-K4 and K The major difference between heterochromatin and euchromatin is that heterochromatin is such part of the chromosomes, which is a firmly packed form and are genetically inactive , while euchromatin is an uncoiled loosely packed form of chromatin and are genetically active. When the non-dividing cells of the nucleus were observed under the light microscope, it exhibited the two regions, on the ground of concentration or intensity of staining.
The dark stained areas are said as heterochromatin and light stained areas are said as euchromatin. They are the parts of chromatin and participate in the protection of DNA in the genome present inside the nucleus.
Emil Heitz in the year , coined the term Heterochromatin and Euchromatin. By focussing on the few more points, we will be able to understand the difference between both types of chromatin. Given below is the comparison chart along with the brief description of them.
Basis for comparison Heterochromatin Euchromatin Meaning The tightly packed form of DNA in the chromosome is called as heterochromatin. The loosely packed form of DNA in the chromosome is called as euchromatin. Low DNA density. Kind of stain Stained dark. Lightly stained. Where they are present These are found at the periphery of the nucleus in eukaryotic cells only. These are found in the inner body of the nucleus of prokaryotic as well as in eukaryotic cells. Transcriptional activity They show little or no transcriptional activity.
They actively participate in the process of transcription. Other features They are compactly coiled. They are loosely coiled. They are late replicative. They are early replicative. Regions of heterochromatin are sticky. Regions of euchromatin are non-sticky.
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