Number of chromosomes in animals. Extra chromosome in humans

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 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).

MOSCOW, July 4— RIA Novosti, Anna Urmantseva. Who has the larger genome? As you know, some creatures have a more complex structure than others, and since everything is written in DNA, then this should also be reflected in its code. It turns out that a person with his developed speech must be more complex than a small round worm. However, if you compare us with a worm in terms of the number of genes, you get about the same thing: 20 thousand genes of Caenorhabditis elegans versus 20-25 thousand of Homo sapiens.

Even more offensive for the “crown of earthly creatures” and the “king of nature” are comparisons with rice and corn - 50 thousand genes in relation to human 25.

However, maybe we think wrong? Genes are “boxes” in which nucleotides are packaged—the “letters” of the genome. Maybe count them? Humans have 3.2 billion nucleotide pairs. But the Japanese crow's eye (Paris japonica) - a beautiful plant with white flowers - has 150 billion base pairs in its genome. It turns out that a person should be 50 times simpler than some flower.

And the lungfish protoptera (lungfish - having both gill and pulmonary respiration) turns out to be 40 times more complex than humans. Maybe all fish are somehow more complex than people? No. The poisonous fugu fish, from which the Japanese prepare a delicacy, has a genome eight times smaller than that of humans and 330 times smaller than that of the lungfish Protoptera.
All that remains is to count the chromosomes - but this confuses the picture even more. How can a person be equal in number of chromosomes to an ash tree, and a chimpanzee to a cockroach?

Evolutionary biologists and geneticists encountered these paradoxes a long time ago. They were forced to admit that the size of the genome, no matter how we try to calculate it, is strikingly unrelated to the complexity of the organization of organisms. This paradox was called the “C-value mystery,” where C is the amount of DNA in the cell (C-value paradox, the exact translation is “genome size paradox”). And yet some correlations between species and kingdoms exist.

© Illustration by RIA Novosti. A. Polyanina

© Illustration by RIA Novosti. A. Polyanina

It is clear, for example, that eukaryotes (living organisms whose cells contain a nucleus) have, on average, larger genomes than prokaryotes (living organisms whose cells do not contain a nucleus). Vertebrates have, on average, larger genomes than invertebrates. However, there are exceptions that no one has yet been able to explain.

There have been suggestions that genome size is related to the length of an organism's life cycle. Using plants as an example, some scientists have argued that perennial species have larger genomes than annuals, usually with a difference of several times. And the smallest genomes belong to ephemeral plants, which go through the full cycle from birth to death within a few weeks. This issue is currently being actively discussed in scientific circles.

Explains the leading researcher at the Institute of General Genetics. N.I. Vavilova of the Russian Academy of Sciences, Professor of the Texas Agromechanical University and the University of Gottingen Konstantin Krutovsky: “The size of the genome is not related to the duration of the life cycle of the organism! For example, there are species within the same genus that have the same genome size, but may differ in life expectancy tens, if not hundreds of times. In general, there is a connection between genome size and evolutionary advancement and complexity of organization, but with many exceptions. Basically, genome size is associated with ploidy (copy number) of the genome (and polyploids are found in both plants and animals) and. amount of highly repetitive DNA (simple and complex repeats, transposons and other mobile elements)."

There are also scientists who have a different point of view on this issue.

Do all living organisms have chromosomes? Do all mammalian cells have these structures? How many chromosomes does this or that organism have? Geneticists study such questions. Many similar questions have already been answered. Data on the number, size and shape of chromosomes are increasingly being used in other biological sciences. Particularly in taxonomy.

Chromosomes are information structures

What is a chromosome? If we examine a eukaryotic cell under high magnification, then in the normal state of this “building block” of the organism, we will not see any chromosome-like structures. They are formed only before cell division, and immediately after the end of reproduction, dense structures disappear, as if dissolving. Chromosomes are necessary for the uniform distribution of information material between daughter cells. They are formed by a DNA molecule and proteins that maintain the dense structure of the chromosome.

What is a karyotype

Each chromosome has its own size and shape. One type of organism is characterized by a certain set of chromosomes. Different individuals of the same species always have the same amount of these information structures; these structures have a size and shape characteristic of a particular species.

Thus, a karyotype is the external characteristics of chromosomes and their number in individuals of the same species. Unlike the genome, a karyotype does not include specific characteristics of individuals, but only the appearance of chromosomal structures. Karyotype features help taxonomists correctly distribute living organisms into taxonomic groups.

How many chromosomes do dogs have

Each type of organism has a certain number of chromosomes. This applies to all eukaryotes. Prokaryotes have a circular DNA molecule, which also doubles during cell division and is distributed among daughter cells without the formation of chromosomal structures.

The number of chromosomes varies enormously among different representatives of the animal and plant kingdoms. For example, a person has 46 chromosomes in somatic cells. This is a diploid set. There are 23 structures in human germ cells. How many chromosomes do dogs have? Their number cannot simply be guessed for each organism. The karyotype of a dog consists of 78 chromosomes. How many chromosomes does a wolf have in this case? Here there is a similarity in karyotype. Because all wolves are relatives to each other and to the domestic dog. Almost all wolves also have 78 chromosomes in their somatic cells. The exceptions are the red wolf and

How many chromosomes do dogs have in their reproductive cells? Germ cells always have two times fewer chromosomes than somatic cells. Because they are distributed equally between daughter cells during meiosis.

In addition to dogs and wolves, the canine family also includes foxes. There are 78 chromosomes in a dog's karyotype. How many chromosomes do foxes have? Taxonomic genera of foxes are very heterogeneous in the number of chromosomes. The common fox has 38. The sand fox has 40. The Bengal fox has 60.

How many chromosomes are there in a dog's red blood cells?

Red blood cells are red blood cells that serve as oxygen carriers. How are they structured? Mature red blood cells must contain a large amount of hemoglobin. That is why they do not have many organelles, including chromosomes, since there is no nucleus at all.

However, in the blood of dogs, as in the blood of humans, there are reticulocytes - immature red blood cells. They make up only 1-2 percent of the total number of red blood cells. Reticulocytes contain ribosomal RNA, mitochondria, ribosomes, and the Golgi complex. But after just a day or a day and a half, reticulocytes are transformed into mature red blood cells, which do not contain DNA, and, consequently, chromosomal structures.

How many chromosomes are in the karyotype of other animals

Animal species are very diverse in karyotype. Moreover, the number of chromosomes in the cell nuclei of various animals does not depend on the complexity of the organization of a living being. For example, in a somatic cell of a frog there are 26 chromosomes. Chimpanzees have 48, which is slightly more than humans. Domestic chicken has 78 structures. This is the same number of chromosomes in dogs. The carp has 104, and the lamprey, a jawless vertebrate, has 174.

Chromosome set of plants

The karyotype of plant forms is also extremely diverse. Bread wheat with a hexaploid set of chromosomes has 42 information structures, rye has 14, and corn has 20. Tomatoes have 24 chromosomes in each cell, and rice has the same number. Jerusalem artichoke has 102.

There are absolute record holders in the plant kingdom for the number of chromosomes. These are ferns.

There are about 1200 chromosomes in the cell of this ancient plant. Horsetail has many such structures: 216.

Thus, all eukaryotic cells, except erythrocytes, have chromosomes. Depending on the type of animal or plant, the quantitative composition of chromosomes changes, as well as their size and shape. It is precisely because chromosomes have different sizes that the number of these structures is so different. The smaller the structures, the more likely their number will be greater.

Genetic research of the human body is one of the most necessary for the population of the entire planet. It is genetics that is of great importance for studying the causes of hereditary diseases or predisposition to them. We'll tell you how many chromosomes does a person have, and what this information may be useful for.

How many pairs of chromosomes does a person have?

The cell of the body is designed to store, implement and transmit hereditary information. It is created from a DNA molecule and is called a chromosome. Many people are interested in the question of how many pairs of chromosomes a person has.

Humans have 23 pairs of chromosomes. Until 1955, scientists erroneously calculated the number of chromosomes to be 48, i.e. 24 pairs. The error was discovered by scientists using more precise techniques.

The set of chromosomes is different in somatic and germ cells. The doubled (diploid) set is present only in the cells that determine the structure (somatics) of the human body. One part is of maternal origin, the other part is of paternal origin.

Gonosomes (sex chromosomes) have only one pair. They differ in gene composition. Therefore, depending on gender, a person has a different composition of the pair of gonosomes. From the fact how many chromosomes do women have, The gender of the unborn child does not depend. A woman has a set of XX chromosomes. Its reproductive cells do not influence the development of sexual characteristics during fertilization of the egg. Belonging to a particular gender depends on the information code about how many chromosomes does a man have. It is the difference between the XX and XY chromosomes that determines the sex of the unborn child. The remaining 22 pairs of chromosomes are called autosomal, i.e. the same for both sexes.

• A woman has 22 pairs of autosomal chromosomes and one pair XX;
• A man has 22 pairs of autosomal chromosomes and one XY pair.

The structure of chromosomes changes during division in the process of doubling somatic cells. These cells are constantly dividing, but the set of 23 pairs has a constant value. The structure of chromosomes is influenced by DNA. The genes that make up the chromosomes form a specific code under the influence of DNA. Thus, the information obtained during the DNA coding process determines the individual characteristics of a person.

Changes in the quantitative structure of chromosomes

A person's karyotype determines the totality of chromosomes. Sometimes it can be modified due to chemical or physical reasons. The normal number of 23 chromosomes in somatic cells can vary. This process is called aneuploidy.

1. The number may be less, then this is monosomy.
2. If there is no pair of autotenous cells, then this structure is called nullisomy.
3. If a third chromosome is added to a pair of cells that make up a chromosome, then this is trisomy.

Various changes in the quantitative set lead to a person receiving congenital diseases. Abnormalities in the structure of chromosomes cause Down syndrome, Edwards syndrome and other conditions.

There is also a variation called polyploidy. With this deviation, a multiple increase in chromosomes occurs, that is, a doubling of a pair of cells that is part of one chromosome. A diploid or sex cell can be present three times (triploidy). If it is present 4 or 5 times, then this increase is called tetraploidy and pentaploidy, respectively. If a person has such a deviation, then he dies within the first days of life. The plant world is quite widely represented by polyploidy. A multiple increase in chromosomes is present in animals: invertebrates, fish. Birds with this anomaly die.

Sometimes they give us amazing surprises. For example, do you know what chromosomes are and how they affect?

We propose to look into this issue in order to dot the i’s once and for all.

Looking at family photographs, you may have probably noticed that members of the same family resemble each other: children look like parents, parents look like grandparents. This similarity is passed on from generation to generation through amazing mechanisms.

All living organisms, from single-celled organisms to African elephants, contain chromosomes in the cell nucleus - thin, long threads that can only be seen with an electron microscope.

Chromosomes (ancient Greek χρῶμα - color and σῶμα - body) are nucleoprotein structures in the cell nucleus, in which most of the hereditary information (genes) is concentrated. They are designed to store this information, implement it and transmit it.

How many chromosomes does a person have

At the end of the 19th century, scientists discovered that the number of chromosomes in different species is not the same.

For example, peas have 14 chromosomes, y have 42, and in humans – 46 (that is, 23 pairs). Hence the temptation arises to conclude that the more there are, the more complex the creature that possesses them. However, in reality this is absolutely not the case.

Of the 23 pairs of human chromosomes, 22 pairs are autosomes and one pair are gonosomes (sex chromosomes). The sexes have morphological and structural (gene composition) differences.

In a female organism, a pair of gonosomes contains two X chromosomes (XX-pair), and in a male organism, one X-chromosome and one Y-chromosome (XY-pair).

The sex of the unborn child depends on the composition of the chromosomes of the twenty-third pair (XX or XY). This is determined by fertilization and the fusion of the female and male reproductive cells.

This fact may seem strange, but in terms of the number of chromosomes, humans are inferior to many animals. For example, some unfortunate goat has 60 chromosomes, and a snail has 80.

Chromosomes consist of a protein and a DNA (deoxyribonucleic acid) molecule, similar to a double helix. Each cell contains about 2 meters of DNA, and in total there are about 100 billion km of DNA in the cells of our body.

An interesting fact is that if there is an extra chromosome or if at least one of the 46 is missing, a person experiences a mutation and serious developmental abnormalities (Down's disease, etc.).