Chromatin Types

The complex of DNA and specific proteins, known as chromatin, is found inside the eukaryotic cells. There are two chromatin types: euchromatin and heterochromatin. The euchromatin form is less dense, while the heterochromatin form is highly dense. The role of chromatin is to prevent DNA damage, regulate gene expression and DNA replication. 

Observing the chromatin under a microscope, it appears in a string form with beads known as nucleosomes. When a cell divides, and the DNA gets duplicated, one can observe the chromatin under the microscope. The euchromatin is less dense as compared to the heterochromatin type. 

 What is Heterochromatin?

The chromatin type, such as heterochromatin, is a highly dense structure formed due to modifications in the proteins known as the histones. The heterochromatin is made of eukaryotic genomes and has functions such as DNA repair/replication. The heterochromatin present in the nucleus is separate from the euchromatin and is localized around the nucleolus. It also actively takes part in genome organization and is of repressive structure. 

However, the heterochromatin can also fold into higher-order structures, thereby inducing negative supercoiling of the DNA, although it has a stable structure. The heterochromatin type of chromatin is transcriptionally inactive, and so are the genes found in heterochromatin. While the euchromatin type is present both in the eukaryotic and prokaryotic cells, the heterochromatin is present only in the eukaryotic cells. The heterochromatin is present in approximately about 8-10% of living beings.

Heterochromatin functions

In this section, we shall address the types of chromatin functions, starting with the heterochromatin functions. These are as follows:

1. The histones present in the core of heterochromatin are hypoacetylated, thus making the residues of lysine positively charged. However, this increases the histone-DNA interaction. 
2. The H4-K16 type of histone in the heterochromatin is less acetylated, thereby encouraging the folding of high structure orders of chromatin. 
3. The H3-K4 and the K79 type of histone present in heterochromatin are hypomethylated, implying that the heterochromatin is transcriptionally less active. 
4. The heterochromatin DNA is accessible in less amount than the euchromatin because of the chromatin modifications.
5. The SIR2, SIR3, SIR4 proteins also play an important role in heterochromatin formation. 
6. The heterochromatin secures the DNA so that the DNA regions get access to proteins during gene expression as it has a compact nature, thereby preventing endonuclease damage. 

What is Euchromatin?

 As compared to heterochromatin, euchromatin is one of the chromatin types that are less densely packed in DNA and actively participates in transcription. Approximately 90-92% of the entire body genomes are euchromatic in nature. The euchromatin is made of the repeating units of nucleosomes being up to 11 mm in diameter. The core of such nucleosomes consists of pairs of four proteins known as H3, H3, H2A, and H2B. 

Under magnification, the euchromatin looks like a long string with beads and is found both in eukaryotic cells as well as prokaryotic cells. The euchromatin is affected by the genetic process and consists of just a single constitutive type. It replicates early as compared to the heterochromatin and does not show the process of heteropycnosis.

Euchromatin Functions

 Here in this section, we shall address the euchromatin chromatin types as follows: 

1. The euchromatin actively participates in the DNA to mRNA transcription.
2. It is associated with the active transcription of the genes.
3. Euchromatin unfolded structures enable the regulatory proteins present in the genes and the complex polymerase of RNA in order to bind the DNA sequence. 
4. The euchromatin changes to heterochromatin under the process of DNA repair/replication
5. The euchromatin enables the transcription of genes to happen within the genes themselves.
6. It is known that the euchromatic chromatin does not follow the process of heterozygous. 

Conclusion

The two types of chromatin are heterochromatin and euchromatin. Chromatin is a complex form of proteins and DNA that forms the chromosomes in the nucleus. The heterochromatin form is highly dense in nature, while the euchromatin type is less dense in nature. Various functions of heterochromatin and euchromatin are also discussed in the article.