Euchromatin and Heterochromatin

Chromosomes are structures that carry the gene of an organism. Genes are composed of DNA molecules and it is also referred to as a physical, chemical and biological unit of heredity. Chromosomes are embedded within the nucleus of a cell. Chromosomes play a key role in cell division and the transfer of genetic material from one cell to another or from one generation to another. These structures are composed of DNA molecules that are firmly attached to the histone protein. Uncondensed DNA resides in a semi-ordered arrangement in eukaryotes’ nuclear chromosomes, where it is muffled around histones to form chromatin, a composite material. These chromatins vary in different phases of replication in eukaryotes. During interphase cell division the chromatin exists in two forms. They are

1. Euchromatin.

2. Heterochromatin.

Euchromatin

Euchromatin is a less densely packed DNA with less strong staining and is found to possess transcriptionally active DNA sequences or could become transcriptionally active at any time throughout growth. Euchromatin is found towards the nucleus’s centre and makes up around 90% of the DNA in a living organism. After staining, it shows as light-coloured bands under an optical microscope. Heteropycnosis does not occur since all regions of euchromatin are coloured consistently. However, it is found to appear as a 10nm long microfibril under an electron microscope. Euchromatin’s structure can be visualised as a string of unfolded beads, with the beads representing nucleosomes. Histone proteins cover a specific number of DNA molecules in the nucleosomes.

Function

Euchromatin plays a key role in transcription by transcribing DNA to mRNA. It facilitates the variation of genes.

Form

Euchromatin exists only in constitutive form as constitutive euchromatin. It does not exist in two forms like heterochromatin.  Chromosome consisting of a methylation region called histone tail is supposed to govern the conformation of euchromatin.

In the case of the bacterial or other prokaryotic genomes, euchromatin is the only existing chromosome conformation, implying that it originated before heterochromatin.

Heterochromatin

Definition 

Heterochromatin is a compressed form of DNA that is rigid containing transcriptionally dormant sequences that exhibit strong stains when stained with nuclear stains. It comes in a variety of forms, ranging from four to five states, each with its own set of epigenetic markers. Heterochromatin staining could lead to heteropycnosis, which is the differential staining of portions of chromosomes. In contrast to euchromatin, the genes on this chromosome are normally inactive and not expressed. Heterochromatin can be detected in the periphery of the nucleus. It is also absent from prokaryotic cells, implying that this form emerged later in the evolution.

Function

Heterochromatin plays a vital role in maintaining the structural conformation of a genome. It also facilitates gene expression. Heterochromatin helps silence certain types of genes that help in DNA repair and replication.

Form

Heterochromatin exists in two forms. They are

1. Constitutive Heterochromatin: It is made up of homogeneous DNA sequences in every cell that is present in a specific species. They are repetitive and are found in centromeres and telomeres. The amount of heterochromatin is at higher levels in 1,9,16 and Y chromosome (in men) genes. 

2. Facultative Heterochromatin: It is made up of heterogeneous DNA sequences in every cell that is present in a specific species. It is non-repetitive and is widely silenced with certain mechanisms of the cell. The formation of heterochromatin is facilitated by the main process called differentiation. It is widely found in any one of the X chromosomes of females.

Differences between Euchromatin and Heterochromatin

The below table gives a detailed description of the differences between Euchromatin and Heterochromatin

Conclusion

Euchromatin and Chromatin are important forms or elements of a chromosome formed during interphase cell division. They play a vital role in maintaining, enhancing, initiating and constraining the process of cell division in the cells. They serve as a basic platform for the development and evolution of genes. They allow the transfer of genetic information and epigenetic markers from one generation to another. On the whole, they act as key elements that support the functions of chromosomes.