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Heterochromatin and Euchromatin – What is the Difference?

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.

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Homozygous, Heterozygous and Hemizygous – What is the Difference?

Homozygous, heterozygous and hemizygous describes the genotype for a single gene in a diploid organism. What does it mean? In humans, since we are diploid organisms we have two copies of every chromosome and they are called homologous chromosomes – one copy comes from mother and one copy comes from father. So that means we have two copies for every given gene because we have two copies of every chromosome. The locus of these genes is same for homologous chromosomes. For example, locus of gene X would be same on maternal as well as on paternal chromosome. So based on these two copies of every gene or alleles (alleles = different version of the same gene) we have different genotypes.

At this point it is important that we know the difference between gene and alleles to understand this concept. In case you want to know more about it visit our blog on Gene and Alleles.

Let’s understand homozygous and heterozygous first. Let’s say we are talking about eye pigment gene and let’s assume that we have two alleles for this gene – one allele codes for brown color eye (B) and the other allele codes for blue color (b). On the homologous pairs of chromosome we can either have same alleles (both either brown or blue) or different alleles (brown and blue). Only these possible combinations can be there. So based on whether we have got the same alleles or different alleles we describe the terms homozygous and heterozygous.

  • Homozygous: Homo = Same

Homozygous means both alleles present on the homologous chromosomes for a given gene are same. For example, either both alleles code for brown color, so the genotype BB or both alleles code for blue color, so the genotype is bb.

Figure 1. Homozygous

  • Heterozygous: Hetero = Different

Heterozygous means both alleles present on the homologous chromosomes for a given gene are different. For example, one chromosome in homologous pair contains brown color allele (B) and  the other chromosome contains blue color allele (b). So the genotype is Bb.

Figure 2. Heterozygous

  • Hemizygous: Hemi = Half

In simple terms hemizyogous means hemi means half. That means only half is present which means only one allele is present instead of two in a diploid organism. This condition is present in males for sex linked genes on X chromosome. Because males are XY, they have only one copy of X chromosome. So for all the genes present on X chromosomes for male, they have only one allele. So we can say only half of the alleles are present as compare to females with genotype XX.

Figure 3. Hemizygous

Hemizygous condition can also occur in autosomal chromosome under abnormal condition such as deletion or loss of chromosome where one of the autosomal allele is missing. So for that particular gene on autosomal chromosome it is called hemizygous.

Figure 4. Summary

I hope this post helps J

To understand this topic more, watch the video on it 🙂