The functions of chromosomes and their structure. What is the function of the chromosomes in the cell?

In this article, we will consider such structures of eukaryotic cells as chromosomes whose structure and functions are studied by the branch of biology called cytology.

History of the discovery

The cells, the main components of the nucleus, were discovered by chromosomes in the 19th century by several scientists. Russian biologist I. D. Chistyakov studied their mitosis (cell division), the German anatomist Valdeier discovered them during the preparation of histological preparations and called chromosomes, that is, the staining bodies for the rapid reaction of these structures when interacting with the organic dye fuchsin.

Fleming summarized the scientific facts about the function that chromosomes perform in cells that have a formed nucleus.

External structure of chromosomes

These microscopic formations are found in the nuclei - the most important organelles of the cell, and serve as a place for storing and transmitting the hereditary information of this organism. Chromosomes contain a special substance - chromatin. It is a conglomerate of fine filaments - fibrils and granules. From a chemical point of view, this is a combination of linear DNA molecules (about 40% of them) with specific histone proteins.

Complexes comprising 8 molecules of peptides and DNA strands, twisted on protein globules, as on coils, are called nucleosomes. The deoxyribonucleic acid portion forms 1.75 revolutions about the core portion and has the form of an ellipsoid of about 10 nanometers in length and 5-6 in width. The presence of these structures (chromosomes) in the nucleus serves as a systematic feature of cells of eukaryotic organisms. It is in the form of nucleosomes that the chromosome performs the function of preserving and transmitting all genetic traits.

Dependence of the chromosome structure on the phase of the cell cycle

If the cell is in a state of interphase, which is characterized by its growth and intensive metabolism, but lack of division, then the chromosomes in the nucleus have the form of thin, depolarized filaments, the chromone. Usually they are intertwined, and it is impossible to visually separate them into separate structures. At the time of the onset of cell division, which is called mitosis in somatic cells, and meiosis in sexes, the chromosomes begin to spiral and thicken, becoming well distinguishable in a microscope.

Levels of chromosome organization

Units of heredity are chromosomes, the science of genetics is studying in detail. Scientists have determined that the nucleosome strand containing DNA and histone proteins form a first-order helix. The dense packing of chromatin occurs due to the formation of a structure of a higher order - a solenoid. He self-organizes and compacts into an even more complex super-helix. All of the above levels of chromosome organization pass during the preparation of the cell for division.

It is in the mitotic cycle that the structural units of heredity, consisting of genes containing DNA, are shortened and thickened in comparison with the thread-like chromons of the interphase period by approximately 19 thousand times. In such a compact form of the chromosome of the nucleus, whose functions consist in the transfer of hereditary characteristics of the organism, they are ready to divide somatic or sex cells.

Morphology of chromosomes

The functions of chromosomes can be explained by studying their morphological features, which are best traced in the mitotic cycle. It is proved that even in the synthetic phase of the interphase, the mass of DNA in the cell doubles, since each of the daughter cells formed as a result of division must have the same amount of hereditary information as the original maternal. This is achieved as a result of the process of reduplication - self-duplication of DNA, which occurs with the participation of the DNA polymerase enzyme.

In cytological preparations prepared at the moment of mitosis metaphase, in plant or animal cells under a microscope it is clearly seen that each chromosome consists of two parts called chromatids. In the further phases of mitosis-anaphase and, especially, telophase-their complete separation occurs, as a result of which each chromatid becomes a separate chromosome. It contains a continuously compacted DNA molecule, as well as lipids, acid proteins and RNA. Of the mineral substances in it there are ions of magnesium and calcium.

Auxiliary structural elements of the chromosome

That the chromosome functions in the cell were fully realized, these units of heredity have a special adaptation - the primary constriction (centromere), which never spirals. It is she who divides the chromosome into two parts, called the shoulders. Depending on the location of the centromere, genetics classify chromosomes into equinox (metacentric), non-equable (sub-metacentric), and acrocentric. On the primary neck, special formations are formed-kinetochores, which are disc-shaped protein globules located on both sides of the centromere. The kinetochores themselves consist of two sections: the outer contacts with microfilaments (filament spindle fibers), attaching to them.

Due to the reduction of threads (microfilaments), a strictly ordered distribution of the chromatids that make up the chromosome between the daughter cells is carried out. Some chromosomes have one or more secondary constrictions that do not participate in mitosis, since they can not be joined by filament spindle threads, but these parts (secondary constrictions) provide control over the synthesis of nucleoli - organelles, which are responsible for the formation of ribosomes.

What is karyotype?

Well-known genetic scientists Morgan, N. Koltsov, and Setton at the beginning of the 20th century carefully studied the chromosomes, their structure and their functions in somatic and sex cells-gametes. They found that each cell of all biological species is characterized by a certain number of chromosomes, having a specific shape and size. It was suggested that the whole set of chromosomes in the nucleus of a somatic cell be called a karyotype.

In popular literature, karyotype is often identified with a chromosome set. In fact, these are not identical concepts. For example, in a human, the karyotype is 46 chromosomes in the somatic cell nuclei and is designated by the general formula 2n. But such cells, as for example hepatocytes (liver cells) have several nuclei, their chromosomal set is designated as 2n * 2 = 4n or 2n * 4 = 8n. That is, the number of chromosomes in such cells will be more than 46, although the karyotype of hepatocytes is 2n, that is, 46 chromosomes.

The number of chromosomes in germ cells is always half as much as in somatic cells (in the cells of the body), such a set is called haploid and is designated as n. All other cells of the body have a set of 2n, which is called diploid.

Chromosome theory of Morgan's heredity

American geneticist Morgan discovered the law of linked inheritance of genes, conducting experiments on the hybridization of fruit flies-fruit flies. Thanks to his research, the functions of chromosomes of germ cells were studied. Morgan proved that genes located in adjacent loci of the same chromosome are inherited mainly together, that is, linked. If the genes are far from each other in the chromosome, then crossing-sister-exchange of sites is possible between sister chromosomes.

Thanks to Morgan's research, genetic maps have been created that help to study the functions of chromosomes and widely use them in genetic consultations to address the possible pathologies of chromosomes or genes that lead to hereditary diseases in humans. The importance of the conclusions made by the scientist is difficult to overestimate.

In this article we have examined the structure and functions of chromosomes, which they perform in the cell.