History of the discovery of chromosomes
Drawing from W. Flemming’s book depicting different stages of salamander epithelial cell division (W. Flemming. Zellsubstanz, Kern und Zelltheilung. 1882)
In different articles and books, priority for the discovery of chromosomes is given to different people, but most often the year of discovery of chromosomes is called 1882, and their discoverer is the German anatomist W. Fleming. However, it would be fairer to say that he did not discover chromosomes, but in his fundamental book “Zellsubstanz, Kern und Zelltheilung” (German) he collected and organized information about them, supplementing them with the results of his own research. The term "chromosome" was proposed by the German histologist Heinrich Waldeyer in 1888; "chromosome" literally means "colored body", since the basic dyes are well bound by chromosomes.
Now it is difficult to say who made the first description and drawing of chromosomes. In 1872, the Swiss botanist Carl von Negili published a work in which he depicted certain bodies that appear in place of the nucleus during cell division during the formation of pollen in a lily ( Lilium tigrinum) and Tradescantia ( Tradescantia). However, his drawings do not allow us to unequivocally state that K. Negili saw exactly chromosomes. In the same 1872, the botanist E. Russov presented his images of cell division during the formation of spores in a fern of the genus Zhovnik ( Ophioglossum) and lily pollen ( Lilium bulbiferum). In his illustrations it is easy to recognize individual chromosomes and stages of division. Some researchers believe that the German botanist Wilhelm Hofmeister was the first to see chromosomes long before K. Negili and E. Russow, back in 1848-1849. At the same time, neither K. Negili, nor E. Russov, nor even more so V. Hofmeister realized the significance of what they saw.
After the rediscovery of Mendel's laws in 1900, it took only one or two years for it to become clear that chromosomes behaved exactly as expected from “particles of heredity.” In 1902 T. Boveri and in 1902-1903 W. Setton ( Walter Sutton) independently of each other were the first to put forward a hypothesis about the genetic role of chromosomes. T. Boveri discovered that the sea urchin embryo Paracentrotus lividus can develop normally only if there is at least one, but complete set of chromosomes. He also found that different chromosomes are not identical in composition. W. Setton studied gametogenesis in the locust Brachystola magna and realized that the behavior of chromosomes in meiosis and during fertilization fully explains the patterns of divergence of Mendelian factors and the formation of their new combinations.
Experimental confirmation of these ideas and the final formulation of the chromosome theory was made in the first quarter of the 20th century by the founders of classical genetics, who worked in the USA with the fruit fly ( D. melanogaster): T. Morgan, K. Bridges ( C.B.Bridges), A. Sturtevant ( A.H. Sturtevant) and G. Möller. Based on their data, they formulated the “chromosomal theory of heredity,” according to which the transmission of hereditary information is associated with chromosomes, in which genes are localized linearly, in a certain sequence. These findings were published in 1915 in the book The Mechanisms of Mendelian Heredity.
In 1933, T. Morgan received the Nobel Prize in Physiology or Medicine for his discovery of the role of chromosomes in heredity.
Eukaryotic chromosomes
The basis of the chromosome is a linear (not closed in a ring) macromolecule of deoxyribonucleic acid (DNA) of considerable length (for example, in the DNA molecules of human chromosomes there are from 50 to 245 million pairs of nitrogenous bases). When stretched, the length of a human chromosome can reach 5 cm. In addition to it, the chromosome includes five specialized proteins - H1, H2A, H2B, H3 and H4 (the so-called histones) and a number of non-histone proteins. The amino acid sequence of histones is highly conserved and practically does not differ in the most diverse groups of organisms.
Primary constriction
Chromosome constriction (X. n.), in which the centromere is localized and which divides the chromosome into arms.
Secondary constrictions
A morphological feature that allows the identification of individual chromosomes in a set. They differ from the primary constriction by the absence of a noticeable angle between the chromosome segments. Secondary constrictions are short and long and are localized at different points along the length of the chromosome. In humans, these are chromosomes 9, 13, 14, 15, 21 and 22.
Types of chromosome structure
There are four types of chromosome structure:
- telocentric(rod-shaped chromosomes with a centromere located at the proximal end);
- acrocentric(rod-shaped chromosomes with a very short, almost invisible second arm);
- submetacentric(with shoulders of unequal length, resembling the letter L in shape);
- metacentric(V-shaped chromosomes with arms of equal length).
The chromosome type is constant for each homologous chromosome and may be constant in all members of the same species or genus.
Satellites
Satellite- this is a round or elongated body, separated from the main part of the chromosome by a thin chromatin thread, with a diameter equal to or slightly smaller than the chromosome. Chromosomes with a satellite are usually referred to as SAT chromosomes. The shape, size of the satellite and the thread connecting it are constant for each chromosome.
Nucleolar zone
Zones of the nucleolus ( nucleolar organizers) - special areas with which the appearance of some secondary constrictions is associated.
Chromonema
Chromonema is a helical structure that can be seen in decompacted chromosomes through an electron microscope. It was first observed by Baranetsky in 1880 in the chromosomes of Tradescantia anther cells, the term was introduced by Veidovsky. A chromonema can consist of two, four or more threads, depending on the object being studied. These threads form two types of spirals:
- paranemic(spiral elements are easy to separate);
- plectonemic(the threads are tightly intertwined).
Chromosomal rearrangements
Violation of the structure of chromosomes occurs as a result of spontaneous or provoked changes (for example, after irradiation).
- Gene (point) mutations (changes at the molecular level);
- Aberrations (microscopic changes visible using a light microscope):
Giant chromosomes
Such chromosomes, which are characterized by their enormous size, can be observed in some cells at certain stages of the cell cycle. For example, they are found in the cells of some tissues of dipteran insect larvae (polytene chromosomes) and in the oocytes of various vertebrates and invertebrates (lampbrush chromosomes). It was on preparations of giant chromosomes that signs of gene activity were revealed.
Polytene chromosomes
Balbiani were first discovered in 2010, but their cytogenetic role was revealed by Kostov, Paynter, Geitz and Bauer. Contained in the cells of the salivary glands, intestines, tracheas, fat body and Malpighian vessels of dipteran larvae.
Lamp brush chromosomes
There is evidence that bacteria have proteins associated with nucleoid DNA, but histones have not been found in them.
Human chromosomes
Each nucleated human somatic cell contains 23 pairs of linear chromosomes, as well as numerous copies of mitochondrial DNA. The table below shows the number of genes and bases in human chromosomes.
| Chromosome | Number of genes | Total bases | Sequenced bases |
|---|---|---|---|
| 4 234 | 247 199 719 | 224 999 719 | |
| 1 491 | 242 751 149 | 237 712 649 | |
| 1 550 | 199 446 827 | 194 704 827 | |
| 446 | 191 263 063 | 187 297 063 | |
| 609 | 180 837 866 | 177 702 766 | |
| 2 281 | 170 896 993 | 167 273 993 | |
Video tutorial 1: Cell division. Mitosis
Video tutorial 2: Meiosis. Phases of meiosis
Lecture: A cell is the genetic unit of a living thing. Chromosomes, their structure (shape and size) and functions
Cell - genetic unit of living things
The basic unit of life is the individual cell. It is at the cellular level that processes occur that distinguish living matter from nonliving matter. In each cell, hereditary information about the chemical structure of proteins that must be synthesized in it is stored and intensively used, and therefore it is called the genetic unit of the living. Even anucleated red blood cells in the initial stages of their existence have mitochondria and a nucleus. Only in a mature state do they not have structures for protein synthesis.
To date, science does not know any cells that do not contain DNA or RNA as a carrier of genomic information. In the absence of genetic material, the cell is not capable of protein synthesis, and therefore life.
DNA is not only found in nuclei; its molecules are contained in chloroplasts and mitochondria; these organelles can multiply inside the cell.
DNA in a cell is found in the form of chromosomes - complex protein-nucleic acid complexes. Eukaryotic chromosomes are localized in the nucleus. Each of them is a complex structure of:
The only long DNA molecule, 2 meters of which is packed into a compact structure measuring (in humans) up to 8 microns;
Special histone proteins, whose role is to pack chromatin (the substance of the chromosome) into the familiar rod-shaped shape;
Chromosomes, their structure (shape and size) and functions
This dense packing of genetic material is produced by the cell before dividing. It is at this moment that the densely packed formed chromosomes can be examined under a microscope. When DNA is folded into compact chromosomes called heterochromatin, messenger RNA cannot be synthesized. During the period of cell mass gain and interphase development, the chromosomes are in a less packed state, which is called interchromatin, in which mRNA is synthesized and DNA replication occurs.
The main elements of chromosome structure are:
Centromere. This is a part of a chromosome with a special nucleotide sequence. It connects two chromatids and participates in conjugation. It is to this that the protein filaments of the cell division spindle tubes are attached.
Telomeres. These are the terminal sections of chromosomes that are not capable of connecting with other chromosomes; they play a protective role. They consist of repeating sections of specialized DNA that form complexes with proteins.
DNA replication initiation points.
Prokaryotic chromosomes are very different from eukaryotic ones, being DNA-containing structures located in the cytoplasm. Geometrically, they are a ring molecule.
The chromosome set of a cell has its own name - karyotype. Each type of living organism has its own characteristic composition, number and shape of chromosomes.
Somatic cells contain a diploid (double) chromosome set, half of which is received from each parent.
Chromosomes responsible for encoding the same functional proteins are called homologous. The ploidy of cells can be different - as a rule, gametes in animals are haploid. In plants, polyploidy is currently a fairly common phenomenon, used in the creation of new varieties as a result of hybridization. Violation of the amount of ploidy in warm-blooded animals and humans causes serious congenital diseases such as Down syndrome (the presence of three copies of chromosome 21). Most often, chromosomal abnormalities lead to the inability of the organism.
In humans, the complete chromosome set consists of 23 pairs. The largest known number of chromosomes, 1600, was found in the simplest planktonic organisms, radiolarians. Australian black bulldog ants have the smallest chromosome set - only 1.
Life cycle of a cell. Phases of mitosis and meiosis
Interphase, in other words, the period of time between two divisions is defined by science as the life cycle of a cell.
During interphase, vital chemical processes occur in the cell, it grows, develops, and accumulates reserve substances. Preparation for reproduction involves doubling the contents - organelles, vacuoles with nutritional contents, and the volume of the cytoplasm. It is thanks to division, as a way to quickly increase the number of cells, that long life, reproduction, an increase in the size of the body, its survival from wounds and tissue regeneration are possible. The following stages are distinguished in the cell cycle:
Interphase. Time between divisions. First, the cell grows, then the number of organelles, the volume of reserve substances increases, and proteins are synthesized. In the last part of interphase, the chromosomes are ready for subsequent division - they consist of a pair of sister chromatids.
Mitosis. This is the name of one of the methods of nuclear division, characteristic of bodily (somatic) cells, during which 2 cells are obtained from one, with an identical set of genetic material.
Gametogenesis is characterized by meiosis. Prokaryotic cells have retained the ancient method of reproduction - direct division.
Mitosis consists of 5 main phases:
- Anaphase. The forces of microtubules tear off chromosome connections in the centromere region and forcefully stretch them towards the poles of the cell. In this case, chromosomes sometimes take a V-shape due to the resistance of the cytoplasm. A ring of protein fibers appears in the area of the metaphase plate.
- Telophase. Its beginning is considered to be the moment when the chromosomes reach the division poles. The process of restoration of the internal membrane structures of the cell begins - the ER, Golgi apparatus, and nucleus. The chromosomes are unpacked. Nucleoli assemble and ribosome synthesis begins. The fission spindle disintegrates.
- Cytokinesis. The last phase in which the protein ring that appears in the central region of the cell begins to shrink, pushing the cytoplasm towards the poles. The cell divides into two and a protein ring of the cell membrane is formed in place.
Prophase. Its beginning is considered to be the moment when the chromosomes become so densely packed that they are visible under a microscope. Also, at this time, the nucleoli are destroyed and a spindle is formed. Microtubules are activated, the duration of their existence decreases to 15 seconds, but the rate of formation also increases significantly. The centrioles diverge to opposite sides of the cell, forming a huge number of constantly synthesized and disintegrated protein microtubules, which extend from them to the centromeres of the chromosomes. This is how the fission spindle is formed. Membrane structures such as the ER and Golgi apparatus break up into separate vesicles and tubes, randomly located in the cytoplasm. Ribosomes are separated from the ER membranes.
Metaphase. A metaphase plate is formed, consisting of chromosomes balanced in the middle of the cell by the efforts of opposite centriole microtubules, each pulling them in their own direction. At the same time, the synthesis and disintegration of microtubules continues, a kind of “bulkhead” of them. This phase is the longest.
Regulators of the mitosis process are specific protein complexes. The result of mitotic division is a pair of cells with identical genetic information. In heterotrophic cells, mitosis occurs faster than in plant cells. In heterotrophs, this process can take from 30 minutes, in plants – 2-3 hours.
To generate cells with half the normal number of chromosomes, the body uses another division mechanism - meiosis.
It is associated with the need to produce germ cells; in multicellular organisms, it avoids the constant doubling of the number of chromosomes in the next generation and makes it possible to obtain new combinations of allelic genes. It differs in the number of phases, being longer. The resulting decrease in the number of chromosomes leads to the formation of 4 haploid cells. Meiosis consists of two divisions following each other without interruption.
The following phases of meiosis are defined:
Prophase I. Homologous chromosomes move closer to each other and unite longitudinally. This combination is called conjugation. Then crossing over occurs - double chromosomes cross their arms and exchange sections.
Metaphase I. Chromosomes separate and occupy positions at the equator of the cell spindle, taking on a V-shape due to the tension of the microtubules.
Anaphase I. Homologous chromosomes are stretched by microtubules towards the cell poles. But unlike mitotic division, they separate as whole chromatids rather than as separate ones.
The result of the first meiotic division is the formation of two cells with half the number of intact chromosomes. Between divisions of meiosis, interphase is practically absent, chromosome doubling does not occur, they are already bichromatid.
Immediately following the first, the repeated meiotic division is completely analogous to mitosis - in it, the chromosomes are divided into separate chromatids, distributed equally between new cells.
oogonia go through the stage of mitotic reproduction at the embryonic stage of development, so that the female body is already born with a constant number of them;
spermatogonia are capable of reproduction at any time during the reproductive period of the male body. A much larger number of them are generated than female gametes.
Gametogenesis of animal organisms occurs in the gonads - gonads.
The process of transformation of spermatogonia into spermatozoa occurs in several stages:
Mitotic division transforms spermatogonia into first-order spermatocytes.
As a result of a single meiosis, they turn into second-order spermatocytes.
The second meiotic division produces 4 haploid spermatids.
The period of formation begins. In the cell, the nucleus becomes compacted, the amount of cytoplasm decreases, and a flagellum forms. Also, proteins are stored and the number of mitochondria increases.
The formation of eggs in an adult female body occurs as follows:
From the 1st order oocyte, of which there is a certain number in the body, as a result of meiosis with a halving of the number of chromosomes, 2nd order oocytes are formed.
As a result of the second meiotic division, a mature egg and three small reduction bodies are formed.
This unbalanced distribution of nutrients between the 4 cells is intended to provide a large resource of nutrients for the new living organism.
Ovules in ferns and mosses are formed in archegonia. In more highly organized plants - in special ovules located in the ovary.
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Chromosomes are the main structural elements of the cell nucleus, which are carriers of genes in which hereditary information is encoded. Having the ability to reproduce themselves, chromosomes provide a genetic link between generations.
The morphology of chromosomes is related to the degree of their spiralization. For example, if at the stage of interphase (see Mitosis, Meiosis) the chromosomes are maximally unfolded, i.e., despiralized, then with the beginning of division the chromosomes intensively spiralize and shorten. Maximum spiralization and shortening of chromosomes is achieved at the metaphase stage, when relatively short, dense structures that are intensely stained with basic dyes are formed. This stage is most convenient for studying the morphological characteristics of chromosomes.
The metaphase chromosome consists of two longitudinal subunits - chromatids [reveals elementary threads in the structure of chromosomes (the so-called chromonemas, or chromofibrils) 200 Å thick, each of which consists of two subunits].
The sizes of plant and animal chromosomes vary significantly: from fractions of a micron to tens of microns. The average lengths of human metaphase chromosomes range from 1.5-10 microns.
The chemical basis of the structure of chromosomes are nucleoproteins - complexes (see) with the main proteins - histones and protamines.
Rice. 1. The structure of a normal chromosome.
A - appearance; B - internal structure: 1-primary constriction; 2 - secondary constriction; 3 - satellite; 4 - centromere.
Individual chromosomes (Fig. 1) are distinguished by the localization of the primary constriction, i.e., the location of the centromere (during mitosis and meiosis, spindle threads are attached to this place, pulling it towards the pole). When a centromere is lost, chromosome fragments lose their ability to separate during division. The primary constriction divides the chromosomes into 2 arms. Depending on the location of the primary constriction, chromosomes are divided into metacentric (both arms are equal or almost equal in length), submetacentric (arms of unequal length) and acrocentric (the centromere is shifted to the end of the chromosome). In addition to the primary one, less pronounced secondary constrictions may be found in chromosomes. A small terminal section of chromosomes, separated by a secondary constriction, is called a satellite.
Each type of organism is characterized by its own specific (in terms of the number, size and shape of chromosomes) so-called chromosome set. The totality of a double, or diploid, set of chromosomes is designated as a karyotype.
Rice. 2. Normal chromosome set of a woman (two X chromosomes in the lower right corner).
Rice. 3. The normal chromosome set of a man (in the lower right corner - X and Y chromosomes in sequence).
Mature eggs contain a single, or haploid, set of chromosomes (n), which makes up half of the diploid set (2n) inherent in the chromosomes of all other cells of the body. In the diploid set, each chromosome is represented by a pair of homologues, one of which is of maternal and the other of paternal origin. In most cases, the chromosomes of each pair are identical in size, shape and gene composition. The exception is sex chromosomes, the presence of which determines the development of the body in a male or female direction. The normal human chromosome set consists of 22 pairs of autosomes and one pair of sex chromosomes. In humans and other mammals, female is determined by the presence of two X chromosomes, and male by one X and one Y chromosome (Fig. 2 and 3). In female cells, one of the X chromosomes is genetically inactive and is found in the interphase nucleus in the form (see). The study of human chromosomes in health and disease is the subject of medical cytogenetics. It has been established that deviations in the number or structure of chromosomes from the norm that occur in reproductive organs! cells or in the early stages of fragmentation of a fertilized egg, cause disturbances in the normal development of the body, causing in some cases the occurrence of some spontaneous abortions, stillbirths, congenital deformities and developmental abnormalities after birth (chromosomal diseases). Examples of chromosomal diseases include Down's disease (an extra G chromosome), Klinefelter's syndrome (an extra X chromosome in men) and (the absence of a Y or one of the X chromosomes in the karyotype). In medical practice, chromosomal analysis is carried out either directly (on bone marrow cells) or after short-term cultivation of cells outside the body (peripheral blood, skin, embryonic tissue).
Chromosomes (from the Greek chroma - color and soma - body) are thread-like, self-reproducing structural elements of the cell nucleus, containing factors of heredity - genes - in a linear order. Chromosomes are clearly visible in the nucleus during the division of somatic cells (mitosis) and during the division (maturation) of germ cells - meiosis (Fig. 1). In both cases, chromosomes are intensely stained with basic dyes and are also visible on unstained cytological preparations in phase contrast. In the interphase nucleus, the chromosomes are despiralized and are not visible in a light microscope, since their transverse dimensions exceed the resolution limits of the light microscope. At this time, individual sections of chromosomes in the form of thin threads with a diameter of 100-500 Å can be distinguished using an electron microscope. Individual non-despiralized sections of chromosomes in the interphase nucleus are visible through a light microscope as intensely stained (heteropyknotic) areas (chromocenters).
Chromosomes continuously exist in the cell nucleus, undergoing a cycle of reversible spiralization: mitosis-interphase-mitosis. The basic patterns of the structure and behavior of chromosomes in mitosis, meiosis and during fertilization are the same in all organisms.
Chromosomal theory of heredity. Chromosomes were first described by I. D. Chistyakov in 1874 and E. Strasburger in 1879. In 1901, E. V. Wilson, and in 1902, W. S. Sutton, drew attention to parallelism in the behavior of chromosomes and Mendelian factors of heredity - genes - in meiosis and during fertilization and came to the conclusion that genes are located in chromosomes. In 1915-1920 Morgan (T.N. Morgan) and his collaborators proved this position, localized several hundred genes in Drosophila chromosomes and created genetic maps of the chromosomes. Data on chromosomes obtained in the first quarter of the 20th century formed the basis of the chromosomal theory of heredity, according to which the continuity of the characteristics of cells and organisms in a number of their generations is ensured by the continuity of their chromosomes.
Chemical composition and autoreproduction of chromosomes. As a result of cytochemical and biochemical studies of chromosomes in the 30s and 50s of the 20th century, it was established that they consist of constant components [DNA (see Nucleic acids), basic proteins (histones or protamines), non-histone proteins] and variable components (RNA and acidic protein associated with it). The basis of chromosomes is made up of deoxyribonucleoprotein threads with a diameter of about 200 Å (Fig. 2), which can be connected into bundles with a diameter of 500 Å.
The discovery by Watson and Crick (J. D. Watson, F. N. Crick) in 1953 of the structure of the DNA molecule, the mechanism of its autoreproduction (reduplication) and the nucleic code of DNA and the development of molecular genetics that arose after this led to the idea of genes as sections of the DNA molecule. (see Genetics). The patterns of autoreproduction of chromosomes were revealed [Taylor (J. N. Taylor) et al., 1957], which turned out to be similar to the patterns of autoreproduction of DNA molecules (semi-conservative reduplication).
Chromosome set- the totality of all chromosomes in a cell. Each biological species has a characteristic and constant set of chromosomes, fixed in the evolution of this species. There are two main types of sets of chromosomes: single, or haploid (in animal germ cells), denoted n, and double, or diploid (in somatic cells, containing pairs of similar, homologous chromosomes from the mother and father), denoted 2n.
The sets of chromosomes of individual biological species vary significantly in the number of chromosomes: from 2 (horse roundworm) to hundreds and thousands (some spore plants and protozoa). The diploid chromosome numbers of some organisms are as follows: humans - 46, gorillas - 48, cats - 60, rats - 42, fruit flies - 8.
The sizes of chromosomes also vary between species. The length of chromosomes (in metaphase of mitosis) varies from 0.2 microns in some species to 50 microns in others, and the diameter from 0.2 to 3 microns.
The morphology of chromosomes is well expressed in metaphase of mitosis. It is metaphase chromosomes that are used to identify chromosomes. In such chromosomes, both chromatids are clearly visible, into which each chromosome and the centromere (kinetochore, primary constriction) connecting the chromatids are longitudinally split (Fig. 3). The centromere is visible as a narrowed area that does not contain chromatin (see); the threads of the achromatin spindle are attached to it, due to which the centromere determines the movement of chromosomes to the poles in mitosis and meiosis (Fig. 4).
Loss of a centromere, for example when a chromosome is broken by ionizing radiation or other mutagens, leads to the loss of the ability of the piece of chromosome lacking the centromere (acentric fragment) to participate in mitosis and meiosis and to its loss from the nucleus. This can cause severe cell damage.
The centromere divides the chromosome body into two arms. The location of the centromere is strictly constant for each chromosome and determines three types of chromosomes: 1) acrocentric, or rod-shaped, chromosomes with one long and a second very short arm, resembling a head; 2) submetacentric chromosomes with long arms of unequal length; 3) metacentric chromosomes with arms of the same or almost the same length (Fig. 3, 4, 5 and 7).
Rice. 4. Scheme of chromosome structure in metaphase of mitosis after longitudinal splitting of the centromere: A and A1 - sister chromatids; 1 - long shoulder; 2 - short shoulder; 3 - secondary constriction; 4- centromere; 5 - spindle fibers.
Characteristic features of the morphology of certain chromosomes are secondary constrictions (which do not have the function of a centromere), as well as satellites - small sections of chromosomes connected to the rest of its body by a thin thread (Fig. 5). Satellite filaments have the ability to form nucleoli. The characteristic structure in the chromosome (chromomeres) is thickening or more tightly coiled sections of the chromosomal thread (chromonemas). The chromomere pattern is specific to each pair of chromosomes.
Rice. 5. Scheme of chromosome morphology in anaphase of mitosis (chromatid extending to the pole). A - appearance of the chromosome; B - internal structure of the same chromosome with its two constituent chromonemas (hemichromatids): 1 - primary constriction with chromomeres constituting the centromere; 2 - secondary constriction; 3 - satellite; 4 - satellite thread.
The number of chromosomes, their size and shape at the metaphase stage are characteristic of each type of organism. The combination of these characteristics of a set of chromosomes is called a karyotype. A karyotype can be represented in a diagram called an idiogram (see human chromosomes below).
Sex chromosomes. Genes that determine sex are localized in a special pair of chromosomes - sex chromosomes (mammals, humans); in other cases, the iol is determined by the ratio of the number of sex chromosomes and all others, called autosomes (Drosophila). In humans, as in other mammals, the female sex is determined by two identical chromosomes, designated as X chromosomes, the male sex is determined by a pair of heteromorphic chromosomes: X and Y. As a result of reduction division (meiosis) during the maturation of oocytes (see Oogenesis) in women all eggs contain one X chromosome. In men, as a result of the reduction division (maturation) of spermatocytes, half of the sperm contains an X chromosome, and the other half a Y chromosome. The sex of a child is determined by the accidental fertilization of an egg by a sperm carrying an X or Y chromosome. The result is a female (XX) or male (XY) embryo. In the interphase nucleus of women, one of the X chromosomes is visible as a clump of compact sex chromatin.
Chromosome functioning and nuclear metabolism. Chromosomal DNA is the template for the synthesis of specific messenger RNA molecules. This synthesis occurs when a given region of the chromosome is despiraled. Examples of local chromosome activation are: the formation of despiralized chromosome loops in the oocytes of birds, amphibians, fish (the so-called X-lamp brushes) and swellings (puffs) of certain chromosome loci in multi-stranded (polytene) chromosomes of the salivary glands and other secretory organs of dipteran insects (Fig. 6). An example of inactivation of an entire chromosome, i.e., its exclusion from the metabolism of a given cell, is the formation of one of the X chromosomes of a compact body of sex chromatin.
Rice. 6. Polytene chromosomes of the dipteran insect Acriscotopus lucidus: A and B - area limited by dotted lines, in a state of intensive functioning (puff); B - the same area in a non-functioning state. The numbers indicate individual chromosome loci (chromomeres).
Rice. 7. Chromosome set in a culture of male peripheral blood leukocytes (2n=46).
Revealing the mechanisms of functioning of lampbrush-type polytene chromosomes and other types of chromosome spiralization and despiralization is crucial for understanding reversible differential gene activation.
Human chromosomes. In 1922, T. S. Painter established the diploid number of human chromosomes (in spermatogonia) to be 48. In 1956, Tio and Levan (N. J. Tjio, A. Levan) used a set of new methods for studying human chromosomes : cell culture; study of chromosomes without histological sections on whole cell preparations; colchicine, which leads to the arrest of mitoses at the metaphase stage and the accumulation of such metaphases; phytohemagglutinin, which stimulates the entry of cells into mitosis; treatment of metaphase cells with hypotonic saline solution. All this made it possible to clarify the diploid number of chromosomes in humans (it turned out to be 46) and provide a description of the human karyotype. In 1960, in Denver (USA), an international commission developed a nomenclature for human chromosomes. According to the commission's proposals, the term "karyotype" should be applied to the systematic set of chromosomes of a single cell (Fig. 7 and 8). The term "idiotram" is retained to represent the set of chromosomes in the form of a diagram constructed from measurements and descriptions of the chromosome morphology of several cells.
Human chromosomes are numbered (somewhat serially) from 1 to 22 in accordance with the morphological features that allow their identification. Sex chromosomes do not have numbers and are designated as X and Y (Fig. 8).
A connection has been discovered between a number of diseases and birth defects in human development with changes in the number and structure of its chromosomes. (see Heredity).
See also Cytogenetic studies.
All these achievements have created a solid basis for the development of human cytogenetics.
Rice. 1. Chromosomes: A - at the anaphase stage of mitosis in trefoil microsporocytes; B - at the metaphase stage of the first meiotic division in the pollen mother cells of Tradescantia. In both cases, the spiral structure of the chromosomes is visible.
Rice. 2. Elementary chromosomal threads with a diameter of 100 Å (DNA + histone) from interphase nuclei of the calf thymus gland (electron microscopy): A - threads isolated from nuclei; B - thin section through the film of the same preparation.
Rice. 3. Chromosome set of Vicia faba (faba bean) at the metaphase stage.
Rice. 8. Chromosomes are the same as in Fig. 7, sets, systematized according to the Denver nomenclature into pairs of homologues (karyotype).
2. Chromosome set of a cell
Chromosomes play an important role in the cell cycle. Chromosomes- carriers of hereditary information of the cell and organism contained in the nucleus. They not only regulate all metabolic processes in the cell, but also ensure the transfer of hereditary information from one generation of cells and organisms to another. The number of chromosomes corresponds to the number of DNA molecules in a cell. The increase in the number of many organelles does not require precise control. During division, the entire contents of the cell are distributed more or less evenly between the two daughter cells. The exception is chromosomes and DNA molecules: they must double and be precisely distributed between newly formed cells.
Chromosome structure
The study of the chromosomes of eukaryotic cells has shown that they consist of DNA and protein molecules. The complex of DNA and protein is called chromatin. A prokaryotic cell contains only one circular DNA molecule, not associated with proteins. Therefore, strictly speaking, it cannot be called a chromosome. This is a nucleoid.
If it were possible to stretch the DNA strand of each chromosome, its length would significantly exceed the size of the nucleus. Nuclear proteins - histones - play an important role in the packaging of giant DNA molecules. Recent studies of the structure of chromosomes have shown that each DNA molecule combines with groups of nuclear proteins, forming many repeating structures - nucleosomes(Fig. 2). Nucleosomes are the structural units of chromatin; they are tightly packed together and form a single structure in the form of a helix 36 nm thick.
Rice. 2. Structure of the interphase chromosome: A - electron photograph of chromatin threads; B - nucleosome, consisting of proteins - histones, around which a spirally twisted DNA molecule is located
Most chromosomes in interphase are stretched in the form of threads and contain a large number of despiralized regions, which makes them practically invisible in a conventional light microscope. As mentioned above, before cell division, DNA molecules double and each chromosome consists of two DNA molecules that spiral, connect with proteins and take on distinct shapes. The two daughter DNA molecules are packaged separately to form sister chromatids. Sister chromatids are held together by the centromere and form one chromosome. Centromere is a site of cohesion between two sister chromatids that controls the movement of chromosomes to the poles of the cell during division. The spindle strands are attached to this part of the chromosomes.
Individual chromosomes differ only during cell division, when they are packed as tightly as possible, stain well and are visible under a light microscope. At this time, you can determine their number in the cell and study the general appearance. Each chromosome contains chromosome arms and centromere. Depending on the position of the centromere, three types of chromosomes are distinguished - equal-armed, unequal-armed And single-armed(Fig. 3).
Rice. 3. Chromosome structure. A - diagram of the chromosome structure: 1 - centromere; 2 - chromosome arms; 3 - sister chromatids; 4 - DNA molecules; 5 - protein components; B - types of chromosomes: 1 - equal-armed; 2 - different arms; 3 - single-arm
Chromosome set of cells
The cells of each organism contain a specific set of chromosomes called karyotype. Each type of organism has its own karyotype. The chromosomes of each karyotype differ in shape, size and set of genetic information.
The human karyotype, for example, consists of 46 chromosomes, the fruit fly Drosophila - 8 chromosomes, one of the cultivated species of wheat - 28. The chromosome set is strictly specific for each species.
Studies of the karyotype of various organisms have shown that cells can contain a single and double set of chromosomes. Double, or diploid(from Greek diploos- double and eidos- species), a set of chromosomes is characterized by the presence of paired chromosomes that are identical in size, shape and nature of hereditary information. Paired chromosomes are called homologous(from Greek homois - identical, similar). For example, all human somatic cells contain 23 pairs of chromosomes, i.e. 46 chromosomes are presented in the form of 23 pairs. In Drosophila, 8 chromosomes form 4 pairs. Paired homologous chromosomes are very similar in appearance. Their centromeres are in the same places, and their genes are located in the same sequence.
Rice. 4. Sets of chromosomes of cells: A - skerda plants, B - mosquito, C - fruit flies, D - humans. The set of chromosomes in the Drosophila reproductive cell is haploid
In some cells or organisms there may be a single set of chromosomes called haploid(from Greek haploos- single, simple and eidos- view). In this case, there are no paired chromosomes, i.e. there are no homologous chromosomes in the cell. For example, in the cells of lower plants - algae, the set of chromosomes is haploid, while in higher plants and animals the set of chromosomes is diploid. However, the germ cells of all organisms always contain only a haploid set of chromosomes.
The chromosome set of cells of each organism and species as a whole is strictly specific and is its main characteristic. The chromosome set is usually denoted by a Latin letter n. The diploid set is denoted accordingly 2n, and haploid - n. The number of DNA molecules is indicated by the letter c. At the beginning of interphase, the number of DNA molecules corresponds to the number of chromosomes and in a diploid cell is equal to 2c. Before division begins, the amount of DNA doubles and is equal to 4c.
Questions for self-control
1. What is the structure of the interphase chromosome?
2. Why is it impossible to see chromosomes under a microscope during interphase?
3. How is the number and appearance of chromosomes determined?
4. Name the main parts of a chromosome.
5. How many DNA molecules does a chromosome consist of during the presynthetic period of interphase and just before cell division?
6. Due to what process does the number of DNA molecules in a cell change?
7. Which chromosomes are called homologous?
8. Based on the set of Drosophila chromosomes, identify equal-armed, different-armed and single-armed chromosomes.
9. What are diploid and haploid sets of chromosomes? How are they designated?
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