What sets of chromosomes are there? How many chromosomes does the sperm nucleus contain and what features does the chromosome set of sperm have? Who benefits from being wrong?

Chromosomes are the nucleoprotein structures of a eukaryotic cell in which most of the hereditary information is stored. Due to their ability to self-reproduce, it is chromosomes that provide the genetic connection of generations. Chromosomes are formed from a long DNA molecule, which contains a linear group of many genes, and all the genetic information be it about a person, animal, plant or any other living creature.

The morphology of chromosomes is related to the level of their spiralization. So, if during the interphase stage the chromosomes are maximized, then with the onset of division the chromosomes actively spiral and shorten. They reach their maximum shortening and spiralization during the metaphase stage, when new structures are formed. This phase is most convenient for studying the properties of chromosomes and their morphological characteristics.

History of the discovery of chromosomes

Back in the middle of the 19th century before last, many biologists, studying the structure of plant and animal cells, drew attention to thin threads and tiny ring-shaped structures in the nucleus of some cells. And so the German scientist Walter Fleming used aniline dyes to treat the nuclear structures of the cell, which is called “officially” opens the chromosomes. More precisely, he named the discovered substance “chromatid” for its ability to stain, and the term “chromosomes” was introduced into use a little later (in 1888) by another German scientist, Heinrich Wilder. The word "chromosome" comes from the Greek words "chroma" - color and "somo" - body.

Chromosomal theory of heredity

Of course, the history of the study of chromosomes did not end with their discovery; in 1901-1902, American scientists Wilson and Saton, independently of each other, drew attention to the similarity in the behavior of chromosomes and Mendeleev’s factors of heredity - genes. As a result, scientists came to the conclusion that genes are located in chromosomes and it is through them that genetic information is transmitted from generation to generation, from parents to children.

In 1915-1920, the participation of chromosomes in gene transmission was proven in practice in a series of experiments carried out by the American scientist Morgan and his laboratory staff. They managed to localize several hundred hereditary genes in the chromosomes of the Drosophila fly and create genetic maps of the chromosomes. Based on these data, the chromosomal theory of heredity was created.

Chromosome structure

The structure of chromosomes varies depending on the species, so the metaphase chromosome (formed in the metaphase stage during cell division) consists of two longitudinal threads - chromatids, which connect at a point called the centromere. A centromere is a region of a chromosome that is responsible for the separation of sister chromatids into daughter cells. It also divides the chromosome into two parts, called the short and long arms, and is also responsible for the division of the chromosome, since it contains a special substance - the kinetochore, to which the spindle structures are attached.

Here the picture shows the visual structure of a chromosome: 1. chromatids, 2. centromere, 3. short chromatid arm, 4. long chromatid arm. At the ends of the chromatids there are telomeres, special elements that protect the chromosome from damage and prevent fragments from sticking together.

Shapes and types of chromosomes

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 to 10 microns. Depending on the type of chromosome, its staining abilities also differ. Depending on the location of the centromere, the following forms of chromosomes are distinguished:

• Metacentric chromosomes, which are characterized by a central location of the centromere.
• Submetacentric, they are characterized by an uneven arrangement of chromatids, when one arm is longer and the other is shorter.
• Acrocentric or rod-shaped. Their centromere is located almost at the very end of the chromosome.

Functions of chromosomes

The main functions of chromosomes, both for animals and plants and all living beings in general, are the transfer of hereditary, genetic information from parents to children.

Set of chromosomes

The importance of chromosomes is so great that their number in cells, as well as the characteristics of each chromosome, determine the characteristic feature of a particular biological species. So, for example, the Drosophila fly has 8 chromosomes, the y has 48, and the human chromosome set is 46 chromosomes.

In nature, there are two main types of chromosome sets: single or haploid (found in germ cells) and double or diploid. The diploid set of chromosomes has a pair structure, that is, the entire set of chromosomes consists of chromosome pairs.

Human chromosome set

As we wrote above, the cells of the human body contain 46 chromosomes, which are combined into 23 pairs. All together they make up the human chromosome set. The first 22 pairs of human chromosomes (they are called autosomes) are common to both men and women, and only 23 pairs - sex chromosomes - vary between sexes, which also determines a person’s gender. The set of all pairs of chromosomes is also called a karyotype.

The human chromosome set has this type, 22 pairs of double diploid chromosomes contain all our hereditary information, and the last pair differs, in men it consists of a pair of conditional X and Y sex chromosomes, while in women there are two X chromosomes.

All animals have a similar structure of the chromosome set, only the number of non-sex chromosomes in each of them is different.

Genetic diseases associated with chromosomes

A malfunction of chromosomes, or even their incorrect number itself, is the cause of many genetic diseases. For example, Down syndrome appears due to the presence of an extra chromosome in the human chromosome set. And such genetic diseases as color blindness and hemophilia are caused by malfunctions of existing chromosomes.

Chromosomes, video

And finally, an interesting educational video about chromosomes.

This article is available in English - .

Heredity and variability in living nature exist thanks to chromosomes, genes, (DNA). It is stored and transmitted as a chain of nucleotides as part of DNA. What role do genes play in this phenomenon? What is a chromosome from the point of view of transmission of hereditary characteristics? Answers to questions like these provide insight into coding principles and genetic diversity on our planet. It largely depends on how many chromosomes are included in the set and on the recombination of these structures.

From the history of the discovery of “particles of heredity”

Studying plant and animal cells under a microscope, many botanists and zoologists in the middle of the 19th century drew attention to the thinnest threads and the smallest ring-shaped structures in the nucleus. More often than others, the German anatomist Walter Flemming is called the discoverer of chromosomes. It was he who used aniline dyes to treat nuclear structures. Flemming called the discovered substance “chromatin” for its ability to stain. The term “chromosomes” was introduced into scientific use in 1888 by Heinrich Waldeyer.

At the same time as Flemming, the Belgian Eduard van Beneden was looking for an answer to the question of what a chromosome is. A little earlier, German biologists Theodor Boveri and Eduard Strassburger conducted a series of experiments proving the individuality of chromosomes and the constancy of their number in different species of living organisms.

Prerequisites for the chromosomal theory of heredity

American researcher Walter Sutton found out how many chromosomes are contained in the cell nucleus. The scientist considered these structures to be carriers of units of heredity, characteristics of the organism. Sutton discovered that chromosomes consist of genes through which properties and functions are passed on to offspring from their parents. The geneticist in his publications gave descriptions of chromosome pairs and their movement during the division of the cell nucleus.

Regardless of his American colleague, work in the same direction was carried out by Theodore Boveri. Both researchers in their works studied the issues of transmission of hereditary characteristics and formulated the main provisions on the role of chromosomes (1902-1903). Further development of the Boveri-Sutton theory took place in the laboratory of Nobel laureate Thomas Morgan. The outstanding American biologist and his assistants established a number of patterns of gene placement on the chromosome and developed a cytological basis that explains the mechanism of the laws of Gregor Mendel, the founding father of genetics.

Chromosomes in a cell

The study of the structure of chromosomes began after their discovery and description in the 19th century. These bodies and filaments are found in prokaryotic organisms (non-nuclear) and eukaryotic cells (in nuclei). Study under a microscope made it possible to establish what a chromosome is from a morphological point of view. It is a mobile filamentous body that is visible during certain phases of the cell cycle. In interphase, the entire volume of the nucleus is occupied by chromatin. During other periods, chromosomes are distinguishable in the form of one or two chromatids.

These formations are better visible during cell division - mitosis or meiosis. More often, large chromosomes of a linear structure can be observed. In prokaryotes they are smaller, although there are exceptions. Cells often contain more than one type of chromosome, for example mitochondria and chloroplasts have their own small “particles of inheritance”.

Chromosome shapes

Each chromosome has an individual structure and differs from others in its coloring features. When studying morphology, it is important to determine the position of the centromere, the length and placement of the arms relative to the constriction. The set of chromosomes usually includes the following forms:

• metacentric, or equal arms, which are characterized by a median location of the centromere;
• submetacentric, or unequal arms (the constriction is shifted towards one of the telomeres);
• acrocentric, or rod-shaped, in which the centromere is located almost at the end of the chromosome;
• dotted with a difficult-to-define shape.

Functions of chromosomes

Chromosomes consist of genes - functional units of heredity. Telomeres are the ends of chromosome arms. These specialized elements serve to protect against damage and prevent fragments from sticking together. The centromere performs its tasks during chromosome doubling. It has a kinetochore, and it is to this that the spindle structures are attached. Each pair of chromosomes is individual in the location of the centromere. The spindle threads work in such a way that one chromosome at a time goes to the daughter cells, and not both. Uniform doubling during division is provided by the origins of replication. Duplication of each chromosome begins simultaneously at several such points, which significantly speeds up the entire division process.

Role of DNA and RNA

It was possible to find out what a chromosome is and what function this nuclear structure performs after studying its biochemical composition and properties. In eukaryotic cells, nuclear chromosomes are formed by a condensed substance - chromatin. According to the analysis, it contains high-molecular organic substances:

Nucleic acids are directly involved in the biosynthesis of amino acids and proteins and ensure the transmission of hereditary characteristics from generation to generation. DNA is contained in the nucleus of a eukaryotic cell, RNA is concentrated in the cytoplasm.

Genes

X-ray diffraction analysis showed that DNA forms a double helix, the chains of which consist of nucleotides. They represent the carbohydrate deoxyribose, a phosphate group, and one of four nitrogenous bases:

Regions of helical deoxyribonucleoprotein strands are genes that carry encoded information about the sequence of amino acids in proteins or RNA. During reproduction, hereditary characteristics from parents are transmitted to offspring in the form of gene alleles. They determine the functioning, growth and development of a particular organism. According to a number of researchers, those sections of DNA that do not encode polypeptides perform regulatory functions. The human genome can contain up to 30 thousand genes.

Set of chromosomes

The total number of chromosomes and their features are a characteristic feature of the species. In the Drosophila fly their number is 8, in primates - 48, in humans - 46. This number is constant for the cells of organisms that belong to the same species. For all eukaryotes there is the concept of “diploid chromosomes”. This is a complete set, or 2n, as opposed to haploid - half the number (n).

Chromosomes in one pair are homologous, identical in shape, structure, location of centromeres and other elements. Homologues have their own characteristic features that distinguish them from other chromosomes in the set. Staining with basic dyes allows you to examine and study the distinctive features of each pair. is present in the somatic ones - in the reproductive ones (the so-called gametes). In mammals and other living organisms with a heterogametic male sex, two types of sex chromosomes are formed: the X chromosome and the Y. Males have a set of XY, females have a set of XX.

Human chromosome set

The cells of the human body contain 46 chromosomes. All of them are combined into 23 pairs that make up the set. There are two types of chromosomes: autosomes and sex chromosomes. The first form 22 pairs - common for women and men. What differs from them is the 23rd pair - sex chromosomes, which are non-homologous in the cells of the male body.

Genetic traits are associated with gender. They are transmitted by a Y and an X chromosome in men and two X chromosomes in women. Autosomes contain the rest of the information about hereditary traits. There are techniques that allow you to individualize all 23 pairs. They are clearly distinguishable in the drawings when painted in a certain color. It is noticeable that the 22nd chromosome in the human genome is the smallest. Its DNA, when stretched, is 1.5 cm long and has 48 million nitrogen base pairs. Special histone proteins from the composition of chromatin perform compression, after which the thread takes up thousands of times less space in the cell nucleus. Under an electron microscope, the histones in the interphase core resemble beads strung on a strand of DNA.

Genetic diseases

There are more than 3 thousand hereditary diseases of various types caused by damage and abnormalities in chromosomes. These include Down syndrome. A child with such a genetic disease is characterized by delays in mental and physical development. With cystic fibrosis, a malfunction occurs in the functions of the exocrine glands. Violation leads to problems with sweating, secretion and accumulation of mucus in the body. It makes it difficult for the lungs to function and can lead to suffocation and death.

Color vision impairment - color blindness - insensitivity to certain parts of the color spectrum. Hemophilia leads to weakened blood clotting. Lactose intolerance prevents the human body from digesting milk sugar. In family planning offices you can find out about your predisposition to a particular genetic disease. In large medical centers it is possible to undergo appropriate examination and treatment.

Gene therapy is a direction of modern medicine, identifying the genetic cause of hereditary diseases and eliminating it. Using the latest methods, normal genes are introduced into pathological cells instead of damaged ones. In this case, doctors relieve the patient not from the symptoms, but from the causes that caused the disease. Only correction of somatic cells is carried out; gene therapy methods are not yet applied en masse to germ cells.

Set of chromosomes

Rice. 1. Image of a set of chromosomes (right) and a systematic female karyotype 46 XX (left). Obtained by spectral karyotyping.

Karyotype- a set of characteristics (number, size, shape, etc.) of a complete set of chromosomes inherent in the cells of a given biological species ( species karyotype), this organism ( individual karyotype) or line (clone) of cells. A karyotype is sometimes also called a visual representation of the complete chromosome set (karyogram).

Determination of karyotype

The appearance of chromosomes changes significantly during the cell cycle: during interphase, chromosomes are localized in the nucleus, as a rule, despiralized and difficult to observe, therefore, to determine the karyotype, cells are used in one of the stages of their division - metaphase of mitosis.

Karyotype determination procedure

For the procedure for determining the karyotype, any population of dividing cells can be used; to determine the human karyotype, either mononuclear leukocytes extracted from a blood sample, the division of which is provoked by the addition of mitogens, or cultures of cells that rapidly divide normally (skin fibroblasts, bone marrow cells) are used. The cell culture population is enriched by stopping cell division at the metaphase stage of mitosis by adding colchicine, an alkaloid that blocks the formation of microtubules and the “stretching” of chromosomes to the poles of cell division and thereby preventing the completion of mitosis.

The resulting cells at the metaphase stage are fixed, stained and photographed under a microscope; from the set of resulting photographs, so-called photos are formed. systematic karyotype- a numbered set of pairs of homologous chromosomes (autosomes), images of the chromosomes are oriented vertically with short arms up, they are numbered in descending order of size, a pair of sex chromosomes is placed at the end of the set (see Fig. 1).

Historically, the first non-detailed karyotypes that made it possible to classify according to chromosome morphology were obtained using Romanovsky-Giemsa staining, but further detailing of the chromosome structure in karyotypes became possible with the advent of differential chromosome staining techniques.

Classical and spectral karyotypes

Rice. 2. An example of determining translocation by a complex of transverse marks (stripes, classic karyotype) and by a spectrum of areas (color, spectral karyotype).

To obtain a classic karyotype, chromosomes are stained with various dyes or their mixtures: due to differences in the binding of the dye to different parts of the chromosomes, staining occurs unevenly and a characteristic banded structure is formed (a complex of transverse marks, English. banding), reflecting the linear heterogeneity of the chromosome and specific for homologous pairs of chromosomes and their sections (with the exception of polymorphic regions, various allelic variants of genes are localized). The first chromosome staining method to produce such highly detailed images was developed by the Swedish cytologist Kaspersson (Q-staining). Other dyes are also used, such techniques are collectively called differential chromosome staining:

• Q-staining- Kaspersson staining with quinine mustard with examination under a fluorescent microscope. Most often used for the study of Y chromosomes (rapid determination of genetic sex, detection of translocations between the X and Y chromosomes or between the Y chromosome and autosomes, screening for mosaicism involving Y chromosomes)
• G-staining- modified Romanovsky-Giemsa staining. The sensitivity is higher than that of Q-staining, therefore it is used as a standard method for cytogenetic analysis. Used to identify small aberrations and marker chromosomes (segmented differently than normal homologous chromosomes)
• R-staining- acridine orange and similar dyes are used, and areas of chromosomes that are insensitive to G-staining are stained. Used to identify details of homologous G- or Q-negative regions of sister chromatids or homologous chromosomes.
• C-staining- used to analyze centromeric regions of chromosomes containing constitutive heterochromatin and the variable distal part of the Y chromosome.
• T-staining- used to analyze telomeric regions of chromosomes.

Recently, the so-called technique has been used. spectral karyotyping , which consists of staining chromosomes with a set of fluorescent dyes that bind to specific regions of chromosomes. As a result of this staining, homologous pairs of chromosomes acquire identical spectral characteristics, which not only greatly facilitates the identification of such pairs, but also facilitates the detection of interchromosomal translocations, that is, movements of sections between chromosomes - translocated sections have a spectrum that differs from the spectrum of the rest of the chromosome.

Karyotype analysis

Comparison of complexes of transverse marks in classical karyotypy or areas with specific spectral characteristics makes it possible to identify both homologous chromosomes and their individual sections, which makes it possible to determine in detail chromosomal aberrations - intra- and interchromosomal rearrangements, accompanied by a violation of the order of chromosome fragments (deletions, duplications, inversions, translocation). Such an analysis is of great importance in medical practice, making it possible to diagnose a number of chromosomal diseases caused by both gross violations of karyotypes (violation of the number of chromosomes), and violation of the chromosomal structure or multiplicity of cellular karyotypes in the body (mosaicism).

Nomenclature

Fig.3. Karyotype 46,XY,t(1;3)(p21;q21),del(9)(q22): translocation (transfer of a fragment) between the 1st and 3rd chromosomes, deletion (loss of a section) of the 9th chromosome are shown. Marking of chromosome regions is given both by complexes of transverse marks (classical karyotyping, stripes) and by fluorescence spectrum (color, spectral karyotyping).

To systematize cytogenetic descriptions, the International System for Cytogenetic Nomenclature (ISCN) was developed, based on differential staining of chromosomes and allowing for a detailed description of individual chromosomes and their regions. The entry has the following format:

[chromosome number] [arm] [region number].[band number]

the long arm of a chromosome is designated by the letter q, short - letter p, chromosomal aberrations are indicated by additional symbols.

Thus, the 2nd band of the 15th section of the short arm of the 5th chromosome is written as 5p15.2.

For the karyotype, an entry in the ISCN 1995 system is used, which has the following format:

[number of chromosomes], [sex chromosomes], [features].

Abnormal karyotypes and chromosomal diseases

Disturbances in the normal karyotype in humans occur in the early stages of development of the organism: if such a disturbance occurs during gametogenesis, in which the parental sex cells are produced, the karyotype of the zygote formed during their fusion is also disturbed. With further division of such a zygote, all cells of the embryo and the organism that develops from it have the same abnormal karyotype.

However, karyotype disturbances can also occur in the early stages of zygote fragmentation; the organism developed from such a zygote contains several cell lines (cell clones) with different karyotypes; such a multiplicity of karyotypes of the whole organism or its individual organs is called mosaicism.

As a rule, karyotype disorders in humans are accompanied by multiple developmental defects; most of these anomalies are incompatible with life and lead to spontaneous abortions in the early stages of pregnancy. However, a fairly large number of fetuses (~2.5%) with abnormal karyotypes are carried to term until the end of pregnancy.

Some human diseases caused by karyotype abnormalities,

Karyotypes	Disease

The set of chromosomes contained in the nucleus is called chromosome set . The number of chromosomes in a cell and their shape are constant for each type of living organism.

Number (diploid set) of chromosomes in some species of plants and animals

Somatic cells are usually diploid (contain a double set of chromosomes - 2n). In these cells, chromosomes are presented in pairs. The diploid set of chromosomes of cells of a particular type of living organism, characterized by the number, size and shape of chromosomes, is called karyotype . Chromosomes belonging to the same pair are called homologous. One of them is inherited from the paternal body, the other from the maternal one. Chromosomes of different pairs are called non-homologous . They differ from each other in size, shape, and locations of primary and secondary constrictions. Chromosomes that are the same in both sexes are called autosomes. The chromosomes on which male and female sexes differ from each other are called sex chromosomes, or heterochromosomes . A human cell contains 46 chromosomes or 23 pairs: 22 pairs of autosomes and 1 pair of sex chromosomes. Sex chromosomes are referred to as X and Y chromosomes. Women have two X chromosomes, and men have one X and one Y chromosome.
Sex cells haploid (contain a single set of chromosomes - n). In these cells, the chromosomes are presented in the singular and do not have a pair in the form of a homologous chromosome.

Cell division

Chromosome set

Chromosome set - a set of chromosomes contained in the nucleus. Depending on the chromosome set, cells are somatic and sexual.

Somatic and germ cells

Cell cycle

Cell cycle (cell life cycle) - the existence of a cell from the moment of its origin as a result of the division of the mother cell until its own division or death. The duration of the cell cycle depends on the type of cell, its functional state and environmental conditions. The cell cycle includes a mitotic cycle and a resting period.
IN rest period (G 0) the cell performs its inherent functions and chooses its future fate - it dies or returns to the mitotic cycle. In continuously reproducing cells, the cell cycle coincides with the mitotic cycle, and there is no rest period.
Mitotic cycle consists of four periods: presynthetic (postmitotic) - G 1, synthetic - S, postsynthetic (premitotic) - G 2, mitosis - M. The first three periods are the preparation of the cell for division ( interphase), the fourth period is the division itself (mitosis).

Interphase - preparation of a cell for division - consists of three periods.

Interphase periods

Eukaryotic cell division

The basis for reproduction and individual development of organisms is cell division.
Eukaryotic cells have three ways of dividing:

• amitosis (direct division),
• mitosis (indirect division),
• meiosis (reduction division).

Amitosis- a rare method of cell division, characteristic of aging or tumor cells. In amitosis, the nucleus is divided by constriction and uniform distribution of the hereditary material is not ensured. After amitosis, the cell is not able to enter into mitotic division.

Mitosis

Mitosis- a type of cell division in which daughter cells receive genetic material identical to that contained in the mother cell. As a result of mitosis, one diploid cell produces two diploid cells, genetically identical to the mother one.

Mitosis consists of four phases.

Phases of mitosis

Phases	Number of chromosomes and chromatids	Processes
Prophase	2n4c	Chromosomes spiral, centrioles (in animal cells) diverge to the poles of the cell, the nuclear membrane disintegrates, nucleoli disappear, and a spindle begins to form.
Metaphase	2n4c	Chromosomes, consisting of two chromatids, are attached by their centromeres(primary constrictions) to the filaments of the spindle. Moreover, they are all located in the equatorial plane. This structure is called metaphase plate.
Anaphase	2n2c	Centromeres divide, and the filaments of the spindle stretch the chromatids separated from each other to opposite poles. Now separated chromatids are called daughter chromosomes.
Telophase	2n2c	Daughter chromosomes reach the cell poles, despiral, spindle filaments are destroyed, a nuclear membrane is formed around the chromosomes, and the nucleoli are restored. The two nuclei formed are genetically identical. This is followed by cytokinesis(cytoplasmic division), which results in the formation of two daughter cells. Organelles are distributed more or less evenly between them.

Biological significance of mitosis:

• genetic stability is achieved;
• the number of cells in the body increases;
• the body grows;
• Phenomena of regeneration and asexual reproduction are possible in some organisms.

Meiosis

Meiosis- a type of cell division accompanied by a reduction in the number of chromosomes. As a result of meiosis, one diploid cell produces four haploid cells, genetically different from the maternal one. During meiosis, two cell divisions occur (the first and second meiotic divisions), and the doubling of the number of chromosomes occurs only before the first division.

Like mitosis, each meiotic division consists of four phases.

Phases of meiosis

Phases	Number of chromosomes and chromatids	Processes
Prophase I	2n4c	Processes similar to those of prophase of mitosis occur. In addition, homologous chromosomes, represented by two chromatids, come closer and “stick together” with each other. This process is called conjugation. In this case, an exchange of sections of homologous chromosomes occurs - crossing over(crossing of chromosomes), that is, the exchange of hereditary information. After conjugation, homologous chromosomes are separated from each other.
Metaphase I	2n4c	Processes similar to those of metaphase of mitosis occur.
Anaphase I	1n2c	Unlike anaphase of mitosis, centromeres do not divide and not one chromatid from each chromosome moves to the poles of the cell, but one chromosome, consisting of two chromatids and held together by a common centromere.
Telophase I	1n2c	Two cells with a haploid set are formed.
Interphase	1n2c	Short. DNA replication (doubling) does not occur and, therefore, diploidity is not restored.
Prophase II	1n2c
Metaphase II	1n2c	Similar to processes during mitosis.
Anaphase II	1n1c	Similar to processes during mitosis.
Telophase II	1n1c	Similar to processes during mitosis.

Biological significance of meiosis:

• basis of sexual reproduction;
• the basis of combinative variability.

Prokaryotic cell division

Prokaryotes do not have mitosis or meiosis. Bacteria reproduce asexually - cell division using constrictions or partitions, less often budding. These processes are preceded by the doubling of the circular DNA molecule.
In addition, bacteria are characterized by a sexual process - conjugation. During conjugation through a special channel formed between two cells, a DNA fragment of one cell is transferred to another cell, that is, the hereditary information contained in the DNA of both cells changes. Since the number of bacteria does not increase, for correctness the concept of “sexual process” is used, but not “sexual reproduction”.

Living space and other personal property are inherited from parents to children. But it is not only material values ​​that can be inherited: every child has the genes of their parents, the younger generation inherits intangible values ​​from the older generation (the shape of the face, hands, features of the head, hair color, etc.). Deoxyribonucleic acid (DNA) is responsible for transmitting characteristic characteristics from parents to children in the body. This substance contains biological information about variability and is written in the form of a special code. The chromosome stores this code.

So how many chromosomes does a person have? There are only 46 chromosomes, and this is how they are counted: in total, a human cell contains 23 pairs of chromosomes, each pair contains 2 absolutely identical chromosomes, but the pairs differ from each other. So, 45 and 46 are sexual, and this pair is the same only for women; for men they are different. All chromosomes except sex chromosomes are called autosomes. More than half of them consist of proteins. The chromosomes differ in appearance: some are thinner, others are shorter, but each has a twin.

Human chromosome set(or karyotype) is a genetic structure responsible for the transmission of heredity. They can be seen under a microscope only during cell division at the metaphase stage. It is at this moment that chromosomes are formed from chromatin, acquiring ploidy: each living organism has its own ploidy, a human cell has 23 pairs.

Haploid and diploid set of chromosomes

Ploidy– the number of chromosome sets in cell nuclei. In living organisms they can be paired or unpaired. It has already been determined that a diploid set of chromosomes is formed in human cells. Diploid (a complete, double set of chromosomes) is inherent in all somatic cells; in humans it is represented by 44 autosomes and 2 sex chromosomes.

Haploid set of chromosomes– is a single set of unpaired chromosomes of germ cells. With this set, the nuclei contain 22 autosomes and 1 sex. Haploid and diploid sets of chromosomes can be present simultaneously (during the sexual process). At this time, the alternation of the haploid and diploid phases occurs: from the complete set, a single set is formed through division, then two single ones merge, forming a complete set, and so on.

Chromosome disorder. During development, disruptions and disturbances may occur at the cellular level. Changes in a person's karyotype (chromosome set) lead to chromosomal diseases. The most famous of these is Down syndrome. With this disease, a malfunction occurs in 21 pairs, when exactly the same chromosome is added to two identical chromosomes, but an extra third one (triosomy is formed).

Often, when the 21st pair of chromosomes is disrupted, the fetus does not have time to develop and dies, but a child born with Down syndrome is doomed to a shortened life and retarded mental development. This disease is incurable. Violations are known not only in the 21st pair; there are also violations in the 18th (Edwards syndrome), 13th (Patau syndrome) and 23rd (Shereshevsky-Turner syndrome) pair of chromosomes.

Developmental changes at the chromosomal level lead to incurable diseases. The result is reduced vitality, especially of newborn children, and deviations in intellectual development. Children suffering from chromosomal diseases are stunted in growth, and the genitals do not develop according to age. To date, there are no methods to protect cells from the appearance of an incorrect chromosome set.