How many chromosomes does the sperm nucleus contain and what features does the chromosome set of sperm have? Which pair represents a woman's sex chromosomes?

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.

Chromosome sizes different types are also different. 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).

24. Gender, its predestination (progamous, syngamous, epigamous).

Floor - This is a set of characteristics and properties of an organism that determine its participation in reproduction.

The sex of an individual can be determined:

a) before fertilization of the egg by the sperm (programmatic sex determination);

b) at the moment of fertilization (syngamous sex determination);

c) after fertilization (epigamous sex determination).

Before fertilization, sex is determined in some organisms as a result of the division of eggs into fast and slow growing ones. The first (larger) after fusion with the male gamete give rise to females, and the second (small) give rise to males. In rotifers, which are capable of reproducing in addition to the usual sexual reproduction with fertilization, parthenogenetically, part of the parthenogenetic eggs lose half of their chromosomes during development. From such eggs males develop, and the rest gives rise to females.

In the marine annelid Bonellia, sex determination occurs during the process of ontogenesis: if the larva settles on the bottom, a female develops from it, and if it attaches to the proboscis of an adult female, then a male.

In the vast majority of eukaryotes, sex is determined at the moment of fertilization and is determined genotypically by the chromosome set that the zygote receives from its parents. The cells of male and female animals differ in their pair of chromosomes. This pair is called sex chromosomes (heterosomes) in contrast to the rest - autosomes. Sex chromosomes are usually referred to as X and Y chromosomes. Depending on their combination in organisms, there are 5 types of chromosomal sex determination:

1) XX, XO (O denotes absence of chromosomes) found in Protenor species (insects);

2) XX, XY - it is characteristic, for example, of Drosophila, mammals (including humans);

3) XY, XX - this type of sex determination is typical for butterflies, birds, and reptiles;

4) XO, XX - observed in aphids;

5) haplodiploid type (2n, n) is found, for example, in bees: males develop from unfertilized haploid eggs, females from fertilized diploid eggs.

The specific mechanisms linking the development of male or female sex with a certain combination of sex chromosomes varies from organism to organism. In humans, for example, gender is determined by the presence of the Y chromosome: it contains the TDP gene, it encodes the testicle - the determining factor that determines the development of the male sex.

In Drosophila, the Y chromosome contains the fertility gene, which is responsible for the fertility of the male, and sex is determined by the balance of the number of X chromosomes and the number of sets of autosomes (a typical diploid organism contains, respectively, two sets of autosomes). The X chromosomes contain genes that determine development along the female path, and the autosomes - along the male path.

If the ratio of the number of X chromosomes to the number of sets of autosomes is 0.5, then a male develops, and if it is 1, then a female develops.

In addition to normal males and females, intersex individuals sometimes appear - individuals whose sexual characteristics occupy an intermediate position between male and female (not to be confused with hermaphrodites!). This can be caused by both aneuploidy of sex chromosomes in gametes and various disorders (for example, hormonal) during the process of sex differentiation.

Sex chromosomes, unlike autosomes, are not designated by serial numbers, but by the letters X, Y, W or Z, and the absence of a chromosome is indicated by the number 0. In this case, one of the sexes is determined by the presence of a pair of identical sex chromosomes (homogametic sex, XX or WW), and the other by a combination of two unpaired chromosomes or the presence of only one sex chromosome (heterogametic sex, XY, WZ or X0). In humans, as in most mammals, the homogametic sex is female (XX), the heterogametic sex is male (XY). In birds, on the contrary, the heterogametic sex is female (WZ), and the homogametic sex is male (WW). Amphibians and reptiles have species (for example, all species of snakes) with homogametic males and heterogametic females, and some turtles (the cross-breasted turtle Staurotypus salvinii and the black freshwater turtle Siebenrockiella crassicollis), on the contrary, have heterogametic males and homogametic females. In some cases (in the platypus), sex is determined not by one, but by five pairs of sex chromosomes

Figure 13. Map of the human X chromosome

In dragonflies, it has been shown that the XY form is evolutionarily more recent than the XO form. Another point of view is that sex chromosomes originate from an ordinary pair of autosomes carrying sex-determining genes. Therefore, in some species (more primitive), the Y chromosome is the same size as the X chromosome, conjugates with it completely or partially, and participates in crossing over. And in other species, it is small, connected end to end with the X chromosome, without crossing over. During the process of evolution, the Y chromosome for some reason loses active genes, degrades and disappears, because the XY form precedes XO.

Figure 14. Sex chromosomes (X and Y)

The Y chromosome is the most variable chromosome in the genome. In humans, it is genetically almost empty (the gene for hairy ears and membranes between the toes). In other species it may contain many active genes - in guppies - about 30 Y-genes for male coloration (and only 1 autosomal gene).

Drosophila Y chromosome. Contains 9 genes: 6 determine male fertility, 3 bobbed rRNA gene cluster. The activity of bb genes leads to the formation of the nucleolus. The nucleolus-forming bb gene is also present on the X chromosome - the site for pairing of X and Y chromosomes - the collohaes site. Responsible for conjugation are short sequences of nucleotides (240 bp) located between the rRNA genes in the X and Y chromosomes. Removal of the bb locus - no conjugation of sex chromosomes. Another gene - crystal - affects the behavior of chromosomes in meiosis. Its deletion disrupts the segregation of chromosomes in meiosis.

Drosophila has 6 male fertility factors. Of these, 3 are very large - they occupy 10% of the Y chromosome each, i.e. 4000 kb each

There are 2 types of sequences in the DNA of the Y chromosome:

Y - specific - families of 200-2000 copies, organized into clusters of tandemly repeated units 200-400 bp long. Probably located in loops.

Y-associated (found on other chromosomes).

Human Y chromosome

The Y chromosome is the smallest of the 24 chromosomes in humans and contains about 2-3% of the DNA of the haploid genome, amounting to approximately 51 Mb. Of the total volume of Y-chromosome DNA, 21.8 Mb have been sequenced to date. The short arm of the Y chromosome (Yp) contains approximately 11 Mb, and the long arm (Yq) contains 40 Mb of DNA, of which about 7 Mb is in the euchromatic part of Yq and about 3 Mb of DNA is in the centromeric region of the chromosome. Most (~60%) of the long arm of the Y chromosome is functionally inactive heterochromatin, approximately 24 Mb in size. The Y chromosome has several regions: pseudoautosomal regions (PARs); - euchromatic region of the short arm (Yp11); - euchromatic region of the proximal part of the long arm (Yq11); - heterochromatic region of the distal part of the long arm (Yq12); - region of pericentromeric heterochromatin.

The Y chromosome contains about 100 functional genes. Due to the presence of homologous PAR regions on the X and Y chromosomes (telomeres), sex chromosomes regularly conjugate and recombine with portions of these regions in zygotene and pachytene of prophase I of meiosis. However, most (~95%) of the Y chromosome does not take part in recombination, and is therefore called the non-recombinant region of the Y chromosome (NRY - Non Recombinant Region Y chromosome).

The heterochromatic region of the long arm of the Y chromosome is genetically inert and contains various types of repeats, including highly repetitive sequences of the two families DYZ1 and DYZ2, each of which is represented by approximately 5000 and 2000 copies, respectively.

Based on a comparative analysis of the genes of the X and Y gonosomes, three groups of genes are distinguished in the Y chromosome:

1. PAR genes (PAR - Pseudoautosomal Region; genes of the pseudoautosomal regions PAR1 and PAR2), localized in the telomeric regions of the Y chromosome;

2. X-Y homologous genes localized in the non-recombining regions Yp and Yq;

3. 3. Y-specific genes located in the non-recombining regions of Yp and Yq.

Figure 15. Y chromosome

The first group is represented by genes of pseudoautosomal regions (regions). They are identical on the X and Y chromosomes and are inherited as autosomal genes. The PAR1 region is located at the end of the short arm of the Y chromosome, it is larger in size than the PAR2 region, located at the end of the long arm of the Y chromosome, and its size is approximately 2.6 Mb. Since PAR1 deletions lead to disturbances in gonosome conjugation during meiosis in men and can lead to male infertility, it is assumed that the PAR regions are essential for the normal course of spermatogenesis in men.

The second group of genes contains X-Y homologous, but not identical genes, which are localized in non-recombining regions of the Y chromosome (on Yp and Yq). It includes 10 genes, represented in one copy on the Y chromosome, most of them are expressed in humans in many tissues and organs, including the testes and prostate gland. It is still unknown whether these X-Y homologous genes are functionally interchangeable.

The third group of genes consists of 11 genes that are located in the non-recombining region of the Y-gonosome (NRY). All of these genes, with the exception of the SRY gene (Sex-Determining Region Y Chromosome), represented by one copy, are multicopy, and their copies are located on both arms of the Y chromosome. Some of them are candidate genes for the AZF factor (Azoospermia factor, or azoospermia factor).

Little is known about the exact functions of most of these genes. The products encoded by the genes of the non-recombining region of the Y chromosome have various functions, for example, transcription factors, cytokine receptors, protein kinases and phosphatases, which can influence cell proliferation and/or cell signaling.

The AZF (Azoospermia Factor) locus is located on the long arm of the Y chromosome - it contains genes that control the process of differentiation of germ cells, i.e. spermatogenesis. There are 3 regions in this locus - a (800 kb), b (3.2 million bp), c (3.5 million bp). Microdeletions of sections of this locus are one of the main genetic causes of male infertility. Microdeletions of the long arm of the Y chromosome are found in 11% of men with azoospermia and in 8% of men with severe oligozoospermia. With the deletion of the entire c-region of the AZF locus, a block may occur in mitosis and meiosis during spermatogenesis; On histological preparations in such patients, the majority of seminiferous tubules lack germ cells.

The Y chromosome is characterized by specific features that sharply distinguish it from other human chromosomes: 1) depletion of genes;

2) enrichment in repeating blocks of nucleotides. Presence of significant heterochromatic regions;

3) the presence of a region of homology with the X chromosome - the pseudoautosomal region (PAR) (Chernykh, Kurilo, 2001).

The Y chromosome, as a rule, is not large - 2-3% of the haploid genome. However, the coding capacity of its DNA in Homo sapiens is sufficient for at least several thousand genes. However, in this object, only about 40 so-called CrG islands enriched in GC pairs are detected on the Y chromosome, usually flanking the majority of genes. The actual list of genetic functions associated with this chromosome is half as long. The phenotypic influence of this chromosome in mice is limited by testis weight, testosterone levels, serological HY antigen, organ sensitivity to androgens, and sexual behavior. Most of the genes on this chromosome have X-chromosomal analogues. Most of the Y chromosome sequences are homologous to the DNA of the X chromosome or autosomes, and only some of them are strictly unique.

The presence of pseudoautosomal regions allowing meiotic pairing and recombination is usually considered a necessary condition for fertility. Interestingly, the size of the meiotic pairing region is significantly longer than PAR. In humans, there are two pseudoautosomal regions at the top of the short and long arms of the X chromosome. However, only for the first of them obligate metabolism in meiosis, the presence of chiasmata, and the effect on fertility have been established.

It has been suggested that mammalian sex chromosomes originate from the ancestral autosome as a result of independent cycles: addition - recombination - degradation. PAR, in this terminology, is only a relic of this latest addition. Next, degradation and loss of the corresponding Y chromosome parts and inactivation of the X chromosome occur. All genes present on the Y chromosome either have real selective value (for example, SRY) or are on the way to extinction. Each Y-chromosomal gene, which rapidly diverges, amplifies, or is prone to extinction, has its homologue on the X chromosome, which is more conserved and active in both sexes. Thus, Sox3, a putative X-chromosomal homolog of SRY, encodes almost identical products in humans, mice, and marsupials, and is expressed in the nervous systems of both sexes. SRY diverges rapidly and is active only in the gonadal tubercle. This Y-chromosomal gene is amplified in many mice and rats.

Thus, the Y chromosome, the only one in the mammalian genome, does not directly work to realize the phenotype. Its genetic significance is associated with continuity between generations, in particular with the control of gametogenesis and primary sex determination. Strict selection acts only on a few of its genes; the rest of the DNA is more plastic.

The subject of genetic research is the phenomena of heredity and variability. American scientist T-X. Morgan created the chromosomal theory of heredity, which proves that each biological species can be characterized by a specific karyotype, which contains such types of chromosomes as somatic and sex chromosomes. The latter are represented by a separate pair, distinguished by male and female individuals. In this article we will study what structure female and male chromosomes have and how they differ from each other.

What is a karyotype?

Each cell containing a nucleus is characterized by a certain number of chromosomes. It is called a karyotype. In different biological species, the presence of structural units of heredity is strictly specific, for example, the human karyotype is 46 chromosomes, chimpanzees - 48, crayfish - 112. Their structure, size, shape differ in individuals belonging to different systematic taxa.

The number of chromosomes in a body cell is called the diploid set. It is characteristic of somatic organs and tissues. If as a result of mutations the karyotype changes (for example, in patients with Klinefelter syndrome the number of chromosomes is 47, 48), then such individuals have reduced fertility and in most cases are infertile. Another hereditary disease associated with sex chromosomes is Turner-Shereshevsky syndrome. It occurs in women who have 45 rather than 46 chromosomes in their karyotype. This means that in a sexual pair there are not two X chromosomes, but only one. Phenotypically, this manifests itself in underdevelopment of the gonads, weakly expressed secondary sexual characteristics and infertility.

Somatic and sex chromosomes

They differ both in shape and in the set of genes that make up them. The male chromosomes of humans and mammals are included in the heterogametic sexual pair XY, which ensures the development of both primary and secondary male sexual characteristics.

In male birds, the sexual pair contains two identical ZZ male chromosomes and is called homogametic. Unlike chromosomes that determine the sex of an organism, the karyotype contains hereditary structures that are identical in both males and females. They are called autosomes. There are 22 pairs of them in the human karyotype. Sexual male and female chromosomes form 23 pairs, so a man’s karyotype can be represented as a general formula: 22 pairs of autosomes + XY, and women - 22 pairs of autosomes + XX.

Meiosis

The formation of germ cells - gametes, the fusion of which forms a zygote, occurs in the sex glands: testes and ovaries. In their tissues, meiosis occurs - the process of cell division leading to the formation of gametes containing a haploid set of chromosomes.

Oogenesis in the ovaries leads to the maturation of eggs of only one type: 22 autosomes + X, and spermatogenesis ensures the maturation of two types of gomets: 22 autosomes + X or 22 autosomes + Y. In humans, the sex of the unborn child is determined at the moment of fusion of the nuclei of the egg and sperm and depends from the karyotype of the sperm.

Chromosomal mechanism and sex determination

We have already looked at the moment at which sex is determined in a person - at the moment of fertilization, and it depends on the chromosomal set of the sperm. In other animals, representatives of different sexes differ in the number of chromosomes. For example, in marine worms, insects, and grasshoppers, in the diploid set of males there is only one chromosome from the sexual pair, and in females - both. Thus, the haploid set of chromosomes of the male sea worm Acirocanthus can be expressed by the formulas: 5 chromosomes + 0 or 5 chromosomes + x, and females have only one set of 5 chromosomes + x in their eggs.

What influences sexual dimorphism?

In addition to chromosomal, there are other ways to determine sex. In some invertebrates - rotifers - sex is determined even before the fusion of gametes - fertilization, as a result of which male and female chromosomes form homologous pairs. Females of the marine polychaete Dinophyllus produce two types of eggs during oogenesis. The first ones are small, depleted in yolk, and males develop from them. Others - large, with a huge supply of nutrients - serve for the development of females. In honey bees - insects of the Hymenoptera series - females produce two types of eggs: diploid and haploid. From unfertilized eggs, males develop - drones, and from fertilized eggs - females, who are worker bees.

Hormones and their effect on gender formation

In humans, male glands - the testes - produce sex hormones such as testosterone. They influence both development (anatomical structure of the external and internal genital organs) and physiological features. Under the influence of testosterone, secondary sexual characteristics are formed - skeletal structure, figure features, body hair, timbre of voice. In a woman’s body, the ovaries produce not only sex cells, but also hormones, being Sex hormones, such as estradiol, progesterone, estrogen, contribute to the development of external and internal genital organs, female-type body hair, regulate menstrual cycle and the course of pregnancy.

In some vertebrates, fish, and amphibians, biologically active substances produced by the gonads strongly influence the development of primary and secondary sexual characteristics, but the types of chromosomes do not have such a great impact on the formation of sex. For example, the larvae of marine polychaetes - Bonellias - under the influence of female sex hormones stop their growth (size 1-3 mm) and become dwarf males. They live in the genital tract of females, which have a body length of up to 1 meter. In cleaner fish, males maintain harems of several females. Females, in addition to the ovaries, have the rudiments of the testes. As soon as the male dies, one of the harem females takes over his function (male gonads that produce sex hormones begin to actively develop in her body).

Sex regulation

It is carried out by two rules: the first determines the dependence of the development of the rudimentary gonads on the secretion of testosterone and the hormone MIS. The second rule indicates the exceptional role played by the Y chromosome. The male sex and all the anatomical and physiological characteristics corresponding to it develop under the influence of genes located on the Y chromosome. The interrelation and dependence of both rules in human genetics is called the principle of growth: in an embryo that is bisexual (that is, having the rudiments of the female glands - the Müllerian duct and the male gonads - the Wolffian canal), the differentiation of the embryonic gonad depends on the presence or absence of the Y chromosome in the karyotype.

Genetic information on the Y chromosome

Research by geneticists, in particular T-H. Morgan, it was found that in humans and mammals the gene composition of the X and Y chromosomes is not the same. Human male chromosomes lack some of the alleles present on the X chromosome. However, their gene pool contains the SRY gene, which controls spermatogenesis, leading to the formation of the male sex. Hereditary disturbances of this gene in the embryo lead to the development of a genetic disease - Swire's syndrome. As a result, the female individual developing from such an embryo contains in the XY karyotype a sexual pair or only a section of the Y chromosome containing the gene locus. It activates the development of gonads. In sick women, secondary sexual characteristics are not differentiated and they are infertile.

Y chromosome and hereditary diseases

As noted earlier, the male chromosome differs from the X chromosome both in size (it is smaller) and in shape (it looks like a hook). The set of genes is also specific to it. Thus, a mutation in one of the genes on the Y chromosome is phenotypically manifested by the appearance of a tuft of coarse hair on the earlobe. This sign is typical only for men. There is a known hereditary disease called Klinefelter syndrome. A sick man has extra female or male chromosomes in his karyotype: XXY or XXYY.

The main diagnostic signs are pathological growth of the mammary glands, osteoporosis, and infertility. The disease is quite common: for every 500 newborn boys, there is 1 patient.

To summarize, we note that in humans, as in other mammals, the sex of the future organism is determined at the moment of fertilization, due to a certain combination of sex X and Y chromosomes in the zygote.