containing genes. The name "chromosome" comes from the Greek words (chrōma - color, color and sōma - body), and is due to the fact that during cell division they are intensely stained in the presence of basic dyes (for example, aniline).

Many scientists, since the beginning of the 20th century, have thought about the question: “How many chromosomes does a person have?”. So until 1955, all the "minds of mankind" were convinced that the number of chromosomes in a person is 48, i.e. 24 couples. The reason was that Theophilus Painter (a Texas scientist) incorrectly counted them in preparative sections of human testes, by court order (1921). In the future, other scientists, using different methods of counting, also came to this opinion. Even having developed a method for separating chromosomes, the researchers did not challenge Painter's result. The error was discovered by scientists Albert Levan and Jo-Hin Tjo in 1955, who accurately calculated how many pairs of chromosomes a person has, namely 23 (a more modern technique was used in their calculation).

Somatic and germ cells contain a different set of chromosomes in biological species, which cannot be said about the morphological features of chromosomes, which are constant. have a doubled (diploid set), which is divided into pairs of identical (homologous) chromosomes, which are similar in morphology (structure) and size. One part is always paternal, the other maternal. Human germ cells (gametes) are represented by a haploid (single) set of chromosomes. When an egg is fertilized, they unite in one nucleus of the zygote of haploid sets of female and male gametes. This restores the double set. It is possible to say with accuracy how many chromosomes a person has - there are 46 of them, while 22 pairs of them are autosomes and one pair is sex chromosomes (gonosomes). Sexual differences have both morphological and structural (composition of genes). In a female organism, a pair of gonosomes contains two X chromosomes (XX pair), and in a male organism, one X and one Y chromosome (XY pair).

Morphologically, chromosomes change during cell division, when they double (with the exception of germ cells, in which doubling does not occur). This is repeated many times, but no change in the chromosome set is observed. Chromosomes are most visible at one of the stages of cell division (metaphase). In this phase, the chromosomes are represented by two longitudinally split formations (sister chromatids), which narrow and unite in the region of the so-called primary constriction, or centromere (an obligatory element of the chromosome). Telomeres are the ends of a chromosome. Structurally, human chromosomes are represented by DNA (deoxyribonucleic acid), which encodes the genes that make up them. Genes, in turn, carry information about a particular trait.

How many chromosomes a person has will depend on his individual development. There are such concepts as: aneuploidy (change in the number of individual chromosomes) and polyploidy (the number of haploid sets is more than diploid). The latter can be of several types: the loss of a homologous chromosome (monosomy), or the appearance (trisomy - one extra, tetrasomy - two extra, etc.). All this is a consequence of genomic and chromosomal mutations that can lead to such pathological conditions as Klinefelter, Shereshevsky-Turner syndromes and other diseases.

Thus, only the twentieth century gave answers to all questions, and now every educated inhabitant of the planet Earth knows how many chromosomes a person has. It is on what will be the composition of the 23rd pair of chromosomes (XX or XY) that the sex of the unborn child depends, and this is determined during the fertilization and fusion of the female and male sex cells.

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

We propose to understand this issue in order to dot the i's once and for all.

When looking at family photos, you might have noticed that members of the same kinship look alike: children look like parents, parents look like grandparents. This similarity is passed down from generation to generation through amazing mechanisms.

All living organisms, from single-celled to African elephants, have 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, its implementation and transmission.

How many chromosomes does a person have

As early as the end of the 19th century, scientists found that the number of chromosomes in different species is not the same.

For example, peas have 14 chromosomes, y - 42, and in humans - 46 (i.e. 23 pairs). Hence, it is tempting to conclude that the more there are, the more complex the creature that possesses them. However, in reality this is not at all the case.

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

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

It is on what will be the composition of the chromosomes of the twenty-third pair (XX or XY) that the sex of the unborn child depends. This is determined during fertilization and the fusion of the female and male reproductive cells.

This fact may seem strange, but in terms of the number of chromosomes, a person is 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 in the presence of an extra chromosome or in the absence of at least one of the 46, a person has a mutation and serious developmental abnormalities (Down's disease, etc.).

Chromosome is a DNA-containing thread-like structure in the cell nucleus that carries genes, the units of heredity, arranged in a linear order. Humans have 22 pairs of normal chromosomes and one pair of sex chromosomes. In addition to genes, chromosomes also contain regulatory elements and nucleotide sequences. They house DNA-binding proteins that control the functions of DNA. Interestingly, the word "chromosome" comes from the Greek word "chrome", meaning "color". Chromosomes got this name due to the fact that they have the peculiarity of being painted in different tones. The structure and nature of chromosomes vary from organism to organism. Human chromosomes have always been the subject of constant interest of researchers working in the field of genetics. The wide range of factors that are determined by human chromosomes, the anomalies they are responsible for, and their complex nature have always attracted the attention of many scientists.

Interesting facts about human chromosomes

Human cells contain 23 pairs of nuclear chromosomes. Chromosomes are made up of DNA molecules that contain genes. The chromosomal DNA molecule contains three nucleotide sequences required for replication. When staining chromosomes, the banded structure of mitotic chromosomes becomes apparent. Each strip contains numerous nucleotide pairs of DNA.

Man is a biological species that reproduces sexually and has diploid somatic cells containing two sets of chromosomes. One set is inherited from the mother, while the other is inherited from the father. Reproductive cells, unlike body cells, have one set of chromosomes. Crossing over (crossover) between chromosomes leads to the creation of new chromosomes. New chromosomes are not inherited from either parent. This is the reason for the fact that not all of us exhibit traits that we receive directly from one of our parents.

Autosomal chromosomes are numbered from 1 to 22 in descending order as their size decreases. Each person has two sets of 22 chromosomes, an X chromosome from the mother and an X or Y chromosome from the father.

An abnormality in the contents of a cell's chromosomes can cause certain genetic disorders in humans. Chromosomal abnormalities in humans are often responsible for the occurrence of genetic diseases in their children. Those who have chromosomal abnormalities are often only carriers of the disease, while their children have the disease.

Chromosomal aberrations (structural changes in chromosomes) are caused by various factors, namely the deletion or duplication of part of the chromosome, inversion, which is a change in the direction of the chromosome to the opposite, or translocation, in which part of the chromosome breaks off and joins it to another chromosome.

An extra copy of chromosome 21 is responsible for a very well known genetic disorder called Down syndrome.

Trisomy 18 leads to Edwards syndrome, which can cause death in infancy.

A deletion of part of the fifth chromosome results in a genetic disorder known as 'cried cat' syndrome. People affected by this disease often have mental retardation, and their crying in childhood resembles a cat's cry.

Sex chromosome abnormalities include Turner syndrome, in which female sex characteristics are present but underdeveloped, and XXX syndrome in girls and XXY syndrome in boys, which cause dyslexia in affected individuals.

Chromosomes were first discovered in plant cells. Van Beneden's monograph on fertilized roundworm eggs led to further research. Later, August Weissman showed that the germline was different from the soma and found that the cell nuclei contained hereditary material. He also suggested that fertilization leads to the formation of a new combination of chromosomes.

These discoveries have become cornerstones in the field of genetics. Researchers have already accumulated a fairly significant amount of knowledge about human chromosomes and genes, but much remains to be discovered.

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32. Mitotic chromosomes. Morphological organization and functions. Karyotype (on the example of a person).

Mitotic chromosomes are formed in a cell during mitosis. These are non-working chromosomes, and the DNA molecules in them are packed extremely tightly. Suffice it to say that the total length of the metaphase chromosomes is approximately 104 times less than the length of the entire DNA contained in the nucleus. Due to such compactness of mitotic chromosomes, a uniform distribution of genetic material between daughter cells during mitosis is ensured. Karyotype- a set of features (number, size, shape, etc.) of a complete set of chromosomes, inherent in cells of a given biological species ( species karyotype ), given organism ( individual karyotype ) or line (clone) of cells. A karyotype is sometimes also called a visual representation of the complete chromosome set (karyograms).

Karyotype definition

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

Procedure for determining the karyotype

For the procedure for determining the karyotype, any population of dividing cells can be used; for determining 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 divide rapidly in the norm (skin fibroblasts, bone marrow cells) are used. The enrichment of the cell culture population is carried out by stopping cell division at the stage of mitosis metaphase by adding colchicine, an alkaloid that blocks the formation of microtubules and the "stretching" of chromosomes to the poles of cell division and thereby prevents the completion of mitosis.

The resulting cells in the metaphase stage are fixed, stained and photographed under a microscope; from a set of resulting photographs, so-called. systematized karyotype - a numbered set of pairs of homologous chromosomes (autosomes), while images of chromosomes are oriented vertically with short arms up, their numbering is done 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 allowed classification by chromosome morphology were obtained by Romanovsky-Giemsa staining, however, further detailing of the structure of chromosomes in karyotypes became possible with the advent of differential staining techniques for chromosomes.

Classical and spectral karyotypes.

33. Reproduction of chromosomes of pro- and eukaryotes, relationship with the cell cycle.

Typically, the cell cycle in eukaryotes consists of four time periods: mitosis(M),presynthetic(G1),synthetic(S) And postsynthetic(G2) phases (periods). It is known that the total duration of both the entire cell cycle and its individual phases varies significantly not only in different organisms, but also in cells of different tissues and organs of the same organism.

The universal theory of the cell cycle assumes that the cell as a whole passes through a series of states during the cell cycle ( Hartwell L., 1995). In every state critical regulatory proteins undergo phosphorylation or dephosphorylation, which determine the transition of these proteins into an active or inactive state, their relationships and/or cellular localization.

Changes in cell states at certain points in the cycle are organized by a special class of protein kinases - cyclin-dependent kinases(Cyclin-dependent kinases - cdk).CDK form complexes with specific short-lived proteins - cyclins that cause their activation, as well as with other auxiliary proteins.

It is assumed that simplest cell cycle can only consist of two phases - S and M, regulated by the respective cdk. Such a hypothetical cell cycle occurs during early embryogenesis in organisms with large oocytes, such as Xenopus and Drosophila. In these eggs, all the components necessary for numerous divisions are presynthesized during oogenesis and stored in the cytoplasm. Therefore, after fertilization, division occurs extremely quickly, and periods G1 And G2 missing.

Cell proliferation is controlled by a complex network of extracellular and intracellular events leading either to the initiation and maintenance of the cell cycle or to the exit of cells into resting phase.

DNA replication is the central event of the cell cycle.

DNA replication requires the presence of a sufficiently large set of enzymes and protein factors; the packaging of newly synthesized DNA into chromatin also requires de novo histone synthesis. Expression genes, encoding the listed proteins, is specific for the S-phase.

After the completion of replication, when the genetic material is doubled, the cell enters the postsynthetic phase G2, during which preparation for mitosis occurs. As a result of mitosis ( M-phase) the cell is divided into two daughter cells. Usually there are two critical transitions between phases - G1/S And G2/M 0.

Based on the scheme of the cell cycle, it can be concluded that cells would stop at restriction point R V phase G1, if the G1 step were a biosynthetic reaction much more sensitive to inhibition of total protein synthesis than any other reactions specific to individual phases of the cycle.

It was suggested that in order to pass the restriction point R, the concentration of some trigger proteins must exceed a certain threshold level.

According to this model, any conditions that reduce the overall intensity protein synthesis, should delay the accumulation of the threshold concentration of the trigger protein, lengthen the G1 phase and slow down the rate of cell division. Indeed, when cells grow in vitro in the presence of various concentrations of inhibitors of protein synthesis, the cell cycle is greatly extended, while the time required for the passage of the S, G2, and M phases does not change significantly. The observed lengthening of the G1 phase is consistent with this model, assuming that each trigger protein molecule remains active in the cell for only a few hours. This model also makes it possible to explain the inhibition of cell growth with an increase in their density or during starvation; both of these factors are known to reduce protein synthesis and stop the cell cycle at the most sensitive point of the G1 phase - point R.

Apparently, the mechanisms that control cell growth in tissue directly affect the overall intensity of protein synthesis in cells; according to this hypothesis, in the absence of specific stimulating factors (and/or in the presence of inhibitory factors), cells will synthesize proteins only at some basal level that maintains the status quo. Cm RB protein: role in cell cycle regulation. At the same time, the number of proteins with an average renewal rate will be maintained at the same level as in growing cells, and the concentration of unstable proteins (including the trigger protein) will decrease in proportion to the decrease in the rate of their synthesis. Under conditions conducive to accelerating the overall protein synthesis , the amount of the trigger protein will exceed the threshold level, which will allow the cells to pass the restriction point R and start dividing.