31 May 2023

It is a conformation of DNA that is compacted and is essential for cell subsistence. It must be taken into account that it intervenes in the correct separation of the genetic material in the cell fragmentation, and also, its structure is involved in the expression of genes. And although it is of great importance for genetic cell control, the process of constitution of this material is not exactly known.

Definition of heterochromatin

It can be defined as a portion of chromatin, that is, DNA and histone proteins, They are densely packed eukaryotic chromosomes and are generally associated with silent regions of the genome.

However, it is generally located around the nucleolus in the immediate vicinity of the nuclear envelope. Similarly, it is important to note that there is a degree of compaction greater than that of heterochromatin when the cells make up chromosomes in division processes, either in mitosis or meiosis. Then the chromosomes are packed in both euchromatin and heterochromatin.

On the other hand, while Euchromatin enables DNA to transcribe and replicate, Heterochromatin has a structure that does not accept DNA and RNA, which makes DNA transcription and replication impossible.

It must be taken into account that a conversion can be generated between the heterochromatin and euchromatin. This is a mechanism for organizing gene expression and although not all the modalities used are well known, it seems to be the passage of both levels of packaging, which are due to chemical alterations in histones. For this reason, heterochromatin has more magnitudes of methylation and euchromatin has a higher level of acetylation.

In the year 1928, emil heitz, carried out histological investigations where he was able to define heterochromatin as those condensed and dark chromosomal sections in the nucleus of the interface. In addition, it is made up of aggregates that require many additional proteins, such as those contained in HP1.

What is the function of heterochromatin?

It must be taken into account that this chromatic portion belongs to the regions telomeric and centromericwhere it exercises transcendental functions from the perspective of cell division and the protection of chromosomal boundaries.

On the one hand, the centromeres work actively in cell division, where they allow the transfer of double chromosomes to both limits of the dividing cell, while the rest of the genes remain compact and inactive.

Another function of heterochromatin is the repression of recombination, where it protects the integrity of the genome and prevents it from taking place illegitimately between repetitive and dispersed sets of DNA in the genome. This is very important to gain control of the transposable components parasitismwhere it is silenced by heteromatinization.

Until recently, heterochromatic DNA was believed to be Junk DNA, as the scientists could not find a function for the sequences incorporated into the regions. Since more than 80% of human genomic DNA does not encode either cellular proteins or RNA molecules with regulatory powers.

On the other hand, at present, the formation of heterochromatic DNA is important for the regulation of some processes during the growth and development of living beings. In addition, the heterochromatin regions are structurally essential.

On the other hand, heterochromatin was found to have structural functions in eukaryotic chromosomes. Since in the heterochromatic regions of a chromosome separates pieces of it, which has diverse patterns of gene activity. That is, the heterochromatic zones serve as spacers between the different active regions, which is important from the point of view of gene transcription.

This is essential to be able to maintain the dissociation of chromosomes in the cellular division and improve the integrity of the genome. The results have generated a new perspective on the prevention of unwanted transcription through the regulation of chromatin structure.

What are the characteristics of heterochromatin?

Heterochromatin is a region of chromatin usually found in the ends of chromosomes and is characterized by having a high density of DNA packaging and low gene expression. Some of the features are:

  • dense packing: It has a high density of DNA wrapping, that is, the DNA strands are very close together and the histones that form the nucleosome join together. This makes chromatin less accessible to the cellular machinery that needs to access genetic information.
  • Low gene expression: heterochromatin has little gene declaration, thanks to the dense structure making it difficult for the cellular team to access the genetic information that is in it.
  • Location at the ends of chromosomes: it is usually found at the boundaries of chromosomes, where it helps maintain their integrity and prevent their deterioration.
  • Protective function: because it helps to prevent the improper recombination of genetic material and to avoid the activation of genes that should remain inactive in certain cells.
  • Variation of classes: the amount and location of heterochromatin varies between species and can be a source of genetic diversity.

Where is heterochromatin found?

They are housed in the nucleus of eukaryotic cells. It is a region of chromatin that is characterized by a high density of DNA packing and a low gene expression.

Heterochromatin can be found in different core parts, but it is generally found at the ends of chromosomes, called telomeres, and in regions near the centromere. In some cases, it may be present on other extensions of the chromosome.

However, heterochromatin is located depending on the type. On the one hand, the constitutive abounds in genomic regions that have a high density of consecutive sequences, and are compact during the cell cycle. On the other hand, facultative heterochromatin is found at loci that They are regulators in the different stages development. In this way, they represent transiently condensed regions, which change with cell signals and gene activity.

What are the types of heterochromatin?

The structure of heterochromatin can be described taking into account its types. In this case, the two main ones are: constitutive and the optionalwhich are identified based on their characteristics, and although there are other types, they carry mixed qualities of those mentioned above.

Now, constitutive heterochromatin is found at the ends of chromosomes and is essential for the genome stability. It is very compact and remains in a densely packed state throughout the cell cycle.

Similarly, facultative heterochromatin may vary between different cells and tissues. Unlike the constitutive, this is not essential for the stability of the genome. In addition, it can be present in different regions of the chromosome in different cells.

Constitutive heterochromatin

It is the stable structure of heterochromatin, which means that it does not break free to form euchromatin. Also has repeating strings of DNA called satellite DNA and can be found at telomeres and centromeres. Usually involved in structural functions, an example of this type of heterochromatin is the X chromosome in females.

It is in a compact conformation that It is not accessible for the cellular machinery, therefore it is not transcribed. It is kept in a dense configuration where it prevents unwanted recombination and loss of genetic material.

According to studies carried out, it was possible to determine that the constitutive heterochromatin has a lot of richness and breeding value, more than what was thought. In this way, large amounts of genes of great importance for the development of certain phases have been found. Some of these genes are:

  • RpL5
  • Light
  • Nipped-A
  • Nipped-B
  • Rolled
  • Bobbed
  • kl-5

facultative heterochromatin

It is found in localities rich in genomes, where it can be converted to euchromatin, thus allowing gene expression in specific cell samples or in certain developmental studies. In addition, it has those genes that do not communicate, but can do so at any time.

It can be in a loosely packed or densely packed state, depending on cellular signals and environmental factors. Sometimes facultative heterochromatin can be unpacked and transcribed in certain cells or at certain times in the cell cycle.

This type of heterochromatin is reversible, therefore its structure can change according to the cell cycle and it is characterized by other types of duplicate DNA sequences and known as line sequences. An example of this is the barr body or sex chromatin X.

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