Difference between Facultative and Constitutive Heterochromatin

DNA is packaged and is present as chromatin scaffolded with non-histone chromosomal proteins. In some places, chromatin is densely packed to form the darkly stained and transcriptionally inactive heterochromatin. Heterochromatin is of two types namely, facultative heterochromatin and constitutive heterochromatin. We will further be understanding the difference between facultative and constitutive heterochromatin.

Chromatin: Introduction

To begin with, in eukaryotes, this organisation is much more complex. There is a set of positively charged, basic proteins called histones. A protein acquires charge depending upon the abundance of amino acid residues with charged side chains. Histones are particularly rich
in the basic amino acid residues lysine and arginine. Both amino acid residues carry positive charges in their side chains specifically. Another key point is that histones are organised to form a unit of eight molecules called histone octamer. The negatively charged DNA is particularly wrapped around the positively charged histone octamer to form a structure called a nucleosome. However, a typical nucleosome contains 200 bp of DNA helix. Nucleosomes constitute the repeating unit of a structure in the nucleus called chromatin, thread-like stained (coloured) bodies seen in the nucleus. To conclude, the nucleosomes in chromatin are seen as a ‘beads-on-string’ structure when viewed under an electron microscope.

Furthermore, the chromatin that is more densely packed and stains dark is called as Heterochromatin. Moreover, Euchromatin is said to be transcriptionally active chromatin, whereas heterochromatin is inactive.

Difference

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Characteristics of Facultative Heterochromatin

1. It particularly includes the region of chromosomes which become heterochromatic in certain cells and tissues.
2. It is reversible from the stability point of view.
3. It is composed of LINE sequences.
4. It is stained by G bands.
5. It plays a role in the regulation of transcription via gene silencing.
6. It does not contain polymorphism. 
7. Its occurrence can be observed in the inactivation of the X chromosome in female somatic cells.

Characteristics of Constitutive Heterochromatin

1. It includes the region of chromosomes which are invariably heterochromatic.
2. Contrarily, it is irreversible from the stability point of view.
3. It is made up of satellite DNA specifically.
4. It is stained by C bands.
5. Conversely, it does not play a role in the regulation of transcription.
6. Unlike, facultative Heterochromatin, it contains polymorphism. 
7. Its occurrence can be observed in the centromeres and telomeres.

Hope this article was helpful in understanding the difference between these two types of heterochromatin.

NCERT- Molecular basis of inheritance- Molecular Basis of Inhertance.pmd (ncert.nic.in)