The concept of “chromosome” is not as new in science as it might seem at first glance. For the first time to designate the intranuclear structure of a eukaryotic cell by such a term was proposed more than 130 years ago by the morphologist W. Valdeyer. The name implies the ability of the intracellular structure to stain with basic dyes.
First of all ... What is chromatin?
Chromatin is a nucleoprotein complex. Namely, chromatin is a polymer that incorporates into special chromosomal proteins, nucleosomes and DNA. Proteins can make up to 65% of the mass of the chromosome. Chromatin is a dynamic molecule and can take a huge number of configurations.
Chromatin proteins make up a significant part of its mass and are divided into two groups:
- Histone proteins - contain the main amino acids in their composition (for example, arginine and lysine). The arrangement of histones is chaotic in the form of blocks along the entire length of the DNA molecule.
- Non-histone proteins (approximately 1/5 of the total number of histones) are a nuclear protein matrix that forms a structural network in the interphase nucleus. It is she who is the basis determining the morphology and metabolism of the nucleus.
In cytogenetics, chromatin is currently divided into two varieties: heterochromatin and euchromatin. The separation of chromatin into two species was due to the ability of each species to stain with specific dyes. This is an effective DNA visualization technique used by cytologists.
Heterochromatin
Heterochromatin is part of the chromosome partially condensed at interphase. Functionally, heterochromatin is of no value, since it is not active, specifically in relation to transcription. But its ability to stain well is widely used in histological studies.
The structure of heterochromatin
Heterochromatin has a simple structure (see. Figure).
Heterochromatin is packaged in globules called nucleosomes. Nucleosomes form even denser structures and thus “interfere” with reading information from DNA. Heterochromatin is formed during the methylation of histone NS by lysine 9, subsequently associated with protein 1 (HP1-Heterochromatin Protein 1). Also interacts with other proteins, including NZK9-methyltransferases. Such a large number of protein interactions among themselves is a condition for maintaining heterochromatin and its distribution. The primary structure of DNA does not affect the formation of heterochromatin.
Heterochromatin is not only individual parts, but also whole chromosomes that remain in a condensed state throughout the entire cell cycle. It is they in the S phase that undergo replication. Scientists believe that heterochromatin regions do not carry genes that encode a protein, or the number of such genes is very small. Instead of such genes, the nucleotide sequences of heterochromatin for the most part consist of simple repeats.
Types of heterochromatin
Heterochromatin can be of two types: optional and structural.
- Optional heterochromatin is chromatin that is formed during the formation of a helix of one of two chromosomes of the same species; it is not always heterochromatic, but at times. It contains genes with hereditary information. It is read when it enters an euchromatic state. The condensed state for facultative heterochromatin is a temporary phenomenon. This is its main difference from structural. An example of optional heterochromatin is the body of chromatin, which determines female gender. Since such a structure consists of two homologous X chromosomes of somatic cells, one of them can just form optional heterochromatin.
- Structural heterochromatin is a structure formed by a highly helical state. It persists throughout the cycle. As mentioned above, the condensed state for structural heterochromatin is a constant phenomenon, in contrast to the optional one. Structural heterochromatin is also called constitutive; it is well detected by C-staining. It is located far from the nucleus and occupies pericentromeric regions, but is sometimes localized in other regions of the chromosome. Often during the interphase process, aggregation of various regions of structural heterochromatin can occur, resulting in the formation of chromocenters. In this form of heterochromatin, the property of transcription is absent, that is, there are no structural genes. The role of such a part of the chromasome is not entirely clear until now, therefore, scientists are inclined to only support functions.
Euchromatin
Euchromatin is a region of chromosomes that is decondensed at interphase. Such a locus is a loosened, but at the same time small, compact structure.
Functional Features of Euchromatin
This type of chromatin is working and functionally active. It does not have the property of staining and is not determined by histological studies. In the mitosis phase, euchromatin almost completely condenses and becomes an integral part of the chromosome. Synthetic functions in this period of the chromosome do not perform. Therefore, cell chromosomes can be in two functional structural states:
- Active or operational status. At this time, the chromosomes are almost completely or completely decondensed. They are involved in the process of transcription and reduction. All of these processes occur directly in the cell nucleus.
- Inactive state of metabolic dormancy (non-working). In this state, the chromosomes are condensed to the maximum and serve as a transport for the transfer of genetic material to daughter cells. In this state, the genetic material is also distributed.
In the final phase of mitosis, despiralization occurs and faintly colored structures are formed in the form of filaments containing transcribed genes.
The structure of each chromosome has its own, unique, variant of the arrangement of chromatin: euchromatin and heterochromatin. This cell feature allows cytogenetics to identify individual chromosomes.