Heterochromatin VS Euchromatin | Definition, Types, Function

Hello, guys. In today’s article, we will study heterochromatin vs euchromatin. Like – what is euchromatin and heterochromatin? What are the functions of euchromatin and heterochromatin? We will know the answers to many such questions today, so let’s start.

What is the definition of heterochromatin vs euchromatin?

Definition of euchromatin

During interphase, chromatin thread remains uncondensed and lightly stained. It is called euchromatin.

Euchromatin is transcriptionally active chromatin. Its nucleosome packing is less condensed and has the following specialities.

1. Histon H1 is less tightly bound.
2. The four nucleosomal histones are present in normal amounts, but they are highly acetylated on lysines near their amino terminus. The acetyl groups are constantly added to these histones by the enzyme histone acetylase and removed by histone deacetylase. (Each acetyl group persists on an average for about 10 minutes only.)
3. Nucleosomes in active chromatin selectively bind two closely related small chromosomal proteins -HMG 14 (high-mobility group.)
4. Active chromatin is highly enriched in a minor variant of histone H2A.

What is the definition of Heterochromatin?

Heterochromatin forms those regions of chromatin that remain condensed and darkly stained during interphase and early prophase.

Heterochromatin is transcriptionally inactive and is resistant to nuclease digestion. These heterochromatin regions fail to replicate during early cycles of DNA synthesis and replicate after the replication of euchromatic regions.

What is euchromatin and heterochromatin?

The cell chromatin can be differentiated into two types. The fine thread-like linen of the chromatin, which stains lightly with basic dyes, is called euchromatin. At certain regions, the chromatin is condensed and darkly stained. This is known as heterochromatin.

The heterochromatin regions can be seen in the interphase and prophase. It does not unravel in telophase like the remaining euchromatin.

The euchromatin also exhibits different affinities to the dyes. The linen of the chromatin, in the form of lightly stained threads, is composed of chromatin (Stains with acidic dyes), and the darkly stained granules (chromomeres) present on the linen are formed of basic rotation or chromatin proper.

IIn some cases, the most significant regions of the nucleus take dark stains with basic fuchsin. These are known as chromocentres of karyosomes. 

Unique sex chromatin bodies or bare bodies are found in the periphery of the nucleus. These are more common in mammalian cells, especially females. Their number usually depends upon the number of sets of X -chromosomes. Usually, there is one sex chromatin body for a diploid set of chromosomes.

What is heterochromatin?

According to Heitz (1929), heterochromatin is found in those areas of chromatin that remain condensed during interphase and early prophase and do not uncoil during telophase, along with a test of the euchromatin.

During interphase, these heterochromatic regions appear darkly stained. Heterochromatin replicates during the S-substage of interphase after the replication of euchromatin.

Types of Heterochromatin – 

It is of the following types.

Constitutive heterochromatin –

It is permanently inactive chromatin. It occupies a fixed position in the interphase and the chromosomes. It occurs at the exact location in both the chromosomes of a pair, such as in the centromeric region, at telomeres, and adjacent to the nucleolar organizer region. Furthermore, it is also called organizer heterochromatin.

Facultative heterochromatin –

, Chromatin, which gets one of the two X-chromosomes in mammalian females, becomes heterochromatic. In mammalian males, where only one X chromosome is present, it remains euchromatic.

Structure of heterochromatin –

Though initially heterochromatin was considered inactive, it is now known that heterochromatin contains polygenes or repetitive DNA for ribosomal RNA, 5S RNA, and tRNA in the nucleolar organizer region. It is formed of small repetitive polynucleotide sequences. Each block contains about 300 nucleotides, and the number of blocks varies from 100 to 10 lacs.

What are the functions of heterochromatin?

1. It protects the vital areas of the genome against external disruptive forces. Around the nucleolar organizer protects the 18S and 28S ribosomal RNA cistrons from mutation and crossing over.
2. Centromeric heterochromatin participates in the separation of chromosomes during cell division.
3. It attracts Homologous chromosomes for pairing during meiosis.
4. It establishes proximity between chromosomal regions that are functionally related and form the chromocentre.
5. Furthermore, it plays an essential role in the replication and transcription of DNA.
6. It creates a fertility barrier that provides means for evolutionary diversity and speciation. 
7. Within the euchromatin, the heterochromatin compartment divides the genome into several functional parts and serves as a full transcription stop.

What is the difference between heterochromatin and euchromatin?

Heterochromatin	Euchromatin
In this, DNA is tightly packed.	In this, DNA is loosely packed.
It has high density of DNA.	In this, the density of DNA is less.
It exhibits heteropycnosis.	Heteropycnosis has not been shown in these.
It appears in inactive state.	It appears in active state.
This occurs only in eukaryotes.	It is present in prokaryotes and eukaryotes.
Replication takes place late in this.	It replicates fast.
It is present on the periphery of the nucleus.	It is present in the interior of the nucleus.
Furthermore, it participates in transcriptional activity.	There is less transcriptional activity in this.

Frequently Asked Questions –

Conclusion –

So, friends, in today’s article, we studied heterochromatin vs. euchromatin. Friends, if you did not like this article, please comment and explain your suggestions and our mistakes.

Thank you so much