This article will discuss such structures of eukaryotic cells as chromosomes, the structure and function of which are studied by a branch of biology called cytology.
Discovery history
Being the main components of the cell nucleus, chromosomes were discovered in the 19th century by several scientists at once. The Russian biologist I. D. Chistyakov studied them in the process of mitosis (cell division), the German anatomist Waldeyer discovered them during the preparation of histological preparations and called them chromosomes, that is, staining bodies for the rapid reaction of these structures when interacting with the organic dye fuchsin.
Fleming summarized the scientific facts about the function of chromosomes in cells with a formed nucleus.
External structure of chromosomes
These microscopic formations are located in the nuclei - the most important organelles of the cell, and serve as a place for storing and transmitting the hereditary information of a given organism. Chromosomescontain a special substance - chromatin. It is a conglomerate of thin filaments - fibrils and granules. From a chemical point of view, this is a combination of linear DNA molecules (there are about 40%) with specific histone proteins.
Complexes, which include 8 peptide molecules and DNA strands, twisted on protein globules, like on coils, are called nucleosomes. The deoxyribonucleic acid region forms 1.75 turns around the stem portion and is an ellipsoid approximately 10 nanometers long and 5-6 wide. The presence of these structures (chromosomes) in the nucleus is a systematic feature of the cells of eukaryotic organisms. It is in the form of nucleosomes that chromosomes perform the function of preserving and transmitting all genetic traits.
Dependence of the structure of the chromosome on the phase of the cell cycle
If a cell is in a state of interphase, which is characterized by its growth and intensive metabolism, but the absence of division, then the chromosomes in the nucleus look like thin despiralized threads - chromonemes. Usually they are intertwined, and it is impossible to visually separate them into separate structures. At the moment of cell division, which is called mitosis in somatic cells, and meiosis in sex cells, the chromosomes begin to spiral and thicken, becoming clearly visible under a microscope.
Levels of chromosome organization
The units of heredity are chromosomes, the science of genetics studies in detail. Scientists have found that the nucleosomal filament,containing DNA and histone proteins form a first-order helix. Dense packing of chromatin occurs due to the formation of a higher-order structure - a solenoid. It self-organizes and condenses into an even more complex supercoil. All of the above levels of chromosome organization take place 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 compared to the filamentous chromonemes of the interphase period by about 19 thousand times. In such a compact form, the chromosomes of the nucleus, whose functions are to transmit the hereditary characteristics of the organism, become ready for the division of somatic or germ cells.
Chromosome morphology
The functions of chromosomes can be explained by studying their morphological features, which are best traced in the mitotic cycle. It has been proven that even in the synthetic stage of 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 mother. This is achieved as a result of the process of reduplication - the self-doubling of DNA, which occurs with the participation of the DNA polymerase enzyme.
In cytological preparations prepared at the time of the metaphase of mitosis, in plant or animal cells under a microscope, it is clearly seen that each chromosome consists of two parts, calledchromatids. In the further phases of mitosis - anaphase and, especially, telophase - they are completely separated, as a result of which each chromatid becomes a separate chromosome. It contains a continuously compacted DNA molecule, as well as lipids, acidic proteins and RNA. Of the mineral substances, it contains magnesium and calcium ions.
Auxiliary structural elements of the chromosome
In order for the functions of chromosomes in the cell to be carried out to the full, these units of heredity have a special device - the primary constriction (centromere), which never spiralizes. It is she who divides the chromosome into two parts, called shoulders. Depending on the location of the centromere, geneticists classify chromosomes as equal-armed (metacentric), unequal-armed (submetacentric) and acrocentric. On the primary constrictions, 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 ones are in contact with microfilaments (filament spindle threads), attaching to them.
Due to the reduction of filaments (microfilaments), a strictly ordered distribution of the chromatids that make up the chromosome between daughter cells is carried out. Some chromosomes have one or more secondary constrictions that do not participate in mitosis, since fission spindle threads cannot attach to them, but it is these sections (secondary constrictions) that provide control over the synthesis of nucleoli - organelles that respondfor the formation of ribosomes.
What is a karyotype
Well-known genetic scientists Morgan, N. Koltsov, Setton at the beginning of the 20th century scrupulously studied chromosomes, their structure and functions in somatic and germ cells - gametes. They found that each cell of all biological species is characterized by a certain number of chromosomes that have a specific shape and size. It was proposed to call the entire set of chromosomes in the nucleus of a somatic cell a karyotype.
In popular literature, the karyotype is often identified with the chromosome set. In fact, these are not identical concepts. For example, in humans, the karyotype is 46 chromosomes in the nuclei of somatic cells and is denoted by the general formula 2n. But such cells as, for example, hepatocytes (liver cells) have several nuclei, their chromosome set is designated as 2n2=4n or 2n4=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 that in somatic (in body cells), such a set is called haploid and is denoted as n. All other cells in the body have a set of 2n, which is called diploid.
Morgan's chromosome theory of heredity
American geneticist Morgan discovered the law of linked inheritance of genes, conducting experiments on the hybridization of fruit flies-Drosophila. Thanks to his research, the functions of the chromosomes of germ cells were studied. Morgan proved that genes located in neighboringloci of the same chromosome are inherited predominantly together, that is, linked. If the genes are far apart in the chromosome, then crossing over is possible between the sister chromosomes - the exchange of sections.
Thanks to Morgan's research, genetic maps were created that study the functions of chromosomes and are widely used in genetic consultations to resolve questions about possible pathologies of chromosomes or genes that lead to hereditary diseases in humans. The importance of the conclusions made by the scientist cannot be overestimated.
In this article, we examined the structure and functions of chromosomes that they perform in the cell.
