The concept of "chromosome" is not as new in science as it might seem at first glance. For the first time, this term was proposed to designate the intranuclear structure of a eukaryotic cell more than 130 years ago by the morphologist W. Waldeyer. Embedded in the name is 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 includes special chromosomal proteins, nucleosomes and DNA. Proteins can make up to 65% of the mass of a chromosome. Chromatin is a dynamic molecule and can take on a huge number of configurations.
Chromatin proteins make up a significant part of its mass and are divided into two groups:
- Histone proteins - contain basic 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 (about 1/5 of the total number of histones) - are nuclear proteina matrix that forms a structural network in the interphase nucleus. It is she who is the basis that determines the morphology and metabolism of the nucleus.
Currently, in cytogenetics, chromatin is divided into two varieties: heterochromatin and euchromatin. The division of chromatin into two species occurred due to the ability of each species to stain with specific dyes. This is an efficient DNA imaging technique used by cytologists.
Heterochromatin
Heterochromatin is parts of a chromosome partially condensed in 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.
Structure of heterochromatin
Heterochromatin has a simple structure (see figure).
Heterochromatin is packed into globules called nucleosomes. Nucleosomes form even more dense structures and thus “interfere” with reading information from DNA. Heterochromatin is formed in the process of methylation of H3 histone at lysine 9, and is subsequently associated with protein 1 (HP1 - Heterochromatin Protein 1). Also interacts with other proteins, including H3K9-methyltransferases. Such a large number of protein interactions with each other 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 separate parts, but also whole chromosomes, which remain in a condensed state throughout the entire cell cycle. They are in the S-phase and are subject to replication. Scientists believe that heterochromatin regions do not carry the genes that encode the protein, or the number of such genes is very small. Instead of such genes, the nucleotide sequences of heterochromatin mostly consist of simple repeats.
Types of heterochromatin
Heterochromatin is of two types: facultative and structural.
- Facultative heterochromatin is chromatin that is formed during the formation of a helix of one of the 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 the euchromatic state. The condensed state for facultative heterochromatin is a temporary phenomenon. This is its main difference from the structural one. An example of facultative heterochromatin is the body of chromatin, which determines the female gender. Since such a structure consists of two homologous X-chromosomes of somatic cells, one of them can just form facultative heterochromatin.
- Structural heterochromatin is a structure formed by a highly coiled state. It persists throughout the cycle. As mentioned above, the condensed state for structural heterochromatin is a constant phenomenon, in contrast to an optional one. Structural heterochromatin is also calledconstitutive, it is well detected by C-color. It is located away from the nucleus and occupies the centromeric regions, but is sometimes localized in other regions of the chromosome. Often, during interphase, aggregation of various sections of structural heterochromatin can occur, resulting in the formation of chromocenters. In this type of heterochromatin, there is no transcription property, that is, there are no structural genes. The role of such a segment of the chromosome is not entirely clear until now, so scientists tend to only support the function.
Euchromatin
Euchromatin are parts of chromosomes that are decondensed in interphase. Such a locus is a loose, but at the same time a 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 phase of mitosis, almost all euchromatin condenses and becomes an integral part of the chromosome. Synthetic functions during this period, the chromosomes do not perform. Therefore, cellular chromosomes can be in two functional and structural states:
- Active or working state. At this time, the chromosomes are almost completely or completely decondensed. They are involved in the process of transcription and reduplication. All of these processes occur directly in the cell nucleus.
- Inactive state of metabolic dormancy (non-working). In this state, the chromosomesare 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 weakly colored structures in the form of threads containing transcribed genes are formed.
The structure of each chromosome has its own, unique, variant of the location of chromatin: euchromatin and heterochromatin. This feature of the cells allows cytogeneticists to identify individual chromosomes.
