As the term says heterochromatin and euchromatin, both are different forms of chromatin. Chromatin is DNA wrapped around histone proteins. Heterochromatin and euchromatin are different levels of condensation of chromatin.
After mitosis when two daughter cells are separated, the highly condensed chromosome will return back to chromatin phase which has two outcomes. One is the loose interphase condition which is called euchromatin and around 10% remains condensed throughout the interphase which is called heterochromatin. So when we observe the nucleus we see the darkly stained heterochromatin near the nuclear envelope which is highly condensed and thus transcriptionally inactive. And in the center we observe lightly stained euchromatin which is transcriptionally active.
Euchromatin: ‘Eu’ = well. Euchromatin is less condensed form and thus it is available for transcription. Therefore, all the genes needed to be expressed are present in euchromatin region. It is gene rich.
Heterochromatin: ‘Hetero’ = different. Heterochromatin is highly condensed form and thus it is transcriptionally inactive. It is gene desert. As the term says hetero or different meaning there are two different forms of heterochromatin.
- Constitutive Heterochromatin:
Constitutive = continuously present. That means it is the type of heterochromatin which is always or continuously in condensed form.
It remains in condensed state in all the cells at all the times. It is always transcriptionally inactive. Therefore, it represents the DNA that is permanently silenced.
It has lot of repeated sequences and has low genes (gene desert).
Examples: Centromere and Telomere
- Facultative Heterochromatin:
Facultative = capable of but not restricted to a particular function or mode. That means it is the type of heterochromatin which can also exist in euchromatin phase.
It is specifically inactivated and it can revert back to active euchromatin phase.
It is silenced by histone deacetylation and RNAi.
Example: X-chromosome inactivation (Barr body) where one of the X-chromosome is specifically inactivated (heterochromatin phase). But during meiosis this inactivated X-chromosome need to go back to its active euchromatin stage otherwise half of the daughter cells would get inactivated X-chromosome.
Why heterochromatin is located near the nuclear envelope and euchromatin in the center?
Since the transcription is very low or not present near the nuclear envelope and heterochromatin is transcriptionally inactive we find heterochromatin near the nuclear envelope. Whereas the center of a nucleus has high levels of transcription taking place so in center we will find euchromatin because it is transcriptionally active form of chromatin.
I hope this helps 🙂
Watch a video on this topic here.