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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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Homologous and Homozygous – What is the Difference?

Homologous and Homozygous – What is the Difference?

Homologous or homozygous? Let’s understand the difference in this post. We will start with homologous and move to homozygous because that would make it easy to understand.

  • Homologous –

    this term we use many times with reference to chromosome, isn’t it? Homologous chromosomes right! What this means is, in humans; as we are diploid organisms we have total of 46 chromosomes or 23 pairs. Half of these chromosomes come from mother (23 chromosomes) and half of these chromosomes come from father (23 chromosomes). That is how we have total of 46 chromosomes or 23 pairs of chromosomes.

Therefore, for every given pair, one chromosome is maternal and one chromosome is paternal (figure 1). Both these chromosomes in a pair would have the same genes which codes for the same traits. Even the location of all these genes would be same on both the chromosomes (because basically they are the same chromosome but coming from two parents).

Figure 1. Chromosome in pair having the same genes at same locations

Let’s say for example, we are talking about genes which codes for eye color and hair color. Figure 1 shows the position of both these genes on chromosome number 1. Whether it is a maternal chromosome 1 or paternal chromosome 1, these two genes are located on the same place for these chromosomes. This means for every given pair of chromosome, all the genes are same and their location is also same. That is why we call them homologous chromosomes.

Homologous Chromosome = Chromosomes in pair having the same genes (codes for same traits) at same locations (where ever it is present on maternal chromosome, it will be present in the same location on paternal chromosome).

  • Homozygous –

    as just discussed above, we are diploid organisms. This means we have two copies of every chromosome and thus we have two copies of every gene (figure 2). In one of our previous post we had seen that copies of the same gene are called alleles .

In short, alleles are different form of the same gene which might differ slightly in sequence giving us slightly different phenotype such as gene for eye pigmentation can give brown, green, blue etc. pigmentation.

Figure 2. Chromosome pair Homozygous for eye and hair color trait

Figure 2 shows gene for eye color. If both the chromosome 1, maternal as well as paternal codes for the same color eye pigment (blue) that means they have the same allele. In this case they are called homozygous for eye color trait. Let’s say for hair pigmentation both the chromosome 1 has allele for brown hair then they are called homozygous for hair color trait.

Homozygous = when on chromosome pair for a given trait both the alleles are same.

It is also possible that a given chromosome pair has two different alleles for the same gene, right! In that case they are called heterozygous for a given trait.

Remember, we use the term homozygous or heterozygous for individual gene. We cannot use it for the complete chromosome because a chromosome would have many different genes present and for each of them they might be either homozygous or heterozygous. Whereas homologous is the term we use with reference to chromosome in a pair.

I hope this post was helpful 🙂

To understand more details, watch a video on this topic here.