Poor ecology, life in constant stress, priority of career over family - all this has a bad effect on a person’s ability to bear healthy offspring. Sadly, about 1% of babies born with serious chromosome abnormalities grow up mentally or physically retarded. In 30% of newborns, deviations in the karyotype lead to the formation of congenital defects. Our article is devoted to the main issues of this topic.

The main carrier of hereditary information

As is known, a chromosome is a certain nucleoprotein (consisting of a stable complex of proteins and nucleic acids) structure inside the nucleus of a eukaryotic cell (that is, those living beings whose cells have a nucleus). Its main function is the storage, transmission and implementation of genetic information. It is visible under a microscope only during processes such as meiosis (division of a double (diploid) set of chromosome genes during the creation of germ cells) and mycosis (cell division during the development of the organism).

As already mentioned, a chromosome consists of deoxyribonucleic acid (DNA) and proteins (about 63% of its mass) on which its thread is wound. Numerous studies in the field of cytogenetics (the science of chromosomes) have proven that DNA is the main carrier of heredity. It contains information that is subsequently implemented in a new organism. This is a complex of genes responsible for hair and eye color, height, number of fingers, etc. Which genes will be passed on to the child are determined at the time of conception.

Formation of the chromosome set of a healthy organism

A normal person has 23 pairs of chromosomes, each of which is responsible for a specific gene. There are 46 in total (23x2) - how many chromosomes a healthy person has. We get one chromosome from our father, the other is passed on from our mother. The exception is 23 pairs. It is responsible for the gender of a person: female is designated as XX, and male as XY. When the chromosomes are in a pair, this is a diploid set. In germ cells they are separated (haploid set) before being subsequently united during fertilization.

The set of characteristics of chromosomes (both quantitative and qualitative) examined within one cell is called a karyotype by scientists. Violations in it, depending on the nature and severity, lead to the occurrence of various diseases.

Deviations in the karyotype

When classified, all karyotype abnormalities are traditionally divided into two classes: genomic and chromosomal.

With genomic mutations, an increase in the number of the entire set of chromosomes, or the number of chromosomes in one of the pairs, is noted. The first case is called polyploidy, the second - aneuploidy.

Chromosomal abnormalities are rearrangements both within and between chromosomes. Without going into scientific jungle, they can be described as follows: some sections of chromosomes may not be present or may be doubled to the detriment of others; The sequence of genes may be disrupted, or their location may be changed. Disturbances in structure can occur in every human chromosome. Currently, the changes in each of them are described in detail.

Let's take a closer look at the most well-known and widespread genomic diseases.

Down syndrome

It was described back in 1866. For every 700 newborns, as a rule, there is one baby with a similar disease. The essence of the deviation is that a third chromosome is added to the 21st pair. This happens when the reproductive cell of one of the parents has 24 chromosomes (with double 21). The sick child ends up with 47 chromosomes – that’s how many chromosomes a Down person has. This pathology is facilitated by viral infections or ionizing radiation suffered by parents, as well as diabetes.

Children with Down syndrome are mentally retarded. Manifestations of the disease are visible even in appearance: an overly large tongue, large, irregularly shaped ears, a skin fold on the eyelid and a wide bridge of the nose, whitish spots in the eyes. Such people live on average forty years, because, among other things, they are susceptible to heart disease, problems with the intestines and stomach, and undeveloped genitals (although women may be capable of childbearing).

The older the parents are, the higher the risk of having a sick child. Currently, there are technologies that make it possible to recognize a chromosomal disorder at an early stage of pregnancy. Older couples need to undergo a similar test. It will not hurt young parents if one of them has had Down syndrome in their family. The mosaic form of the disease (the karyotype of some cells is damaged) is formed already at the embryonic stage and does not depend on the age of the parents.

Patau syndrome

This disorder is trisomy of the thirteenth chromosome. It occurs much less frequently than the previous syndrome we described (1 in 6000). It occurs when an extra chromosome is attached, as well as when the structure of chromosomes is disrupted and their parts are redistributed.

Patau syndrome is diagnosed by three symptoms: microphthalmos (reduced eye size), polydactyly (more fingers), cleft lip and palate.

The infant mortality rate for this disease is about 70%. Most of them do not live to be 3 years old. In individuals susceptible to this syndrome, heart and/or brain defects and problems with other internal organs (kidneys, spleen, etc.) are most often observed.

Edwards syndrome

Most babies with 3 eighteenth chromosomes die soon after birth. They have pronounced malnutrition (digestive problems that prevent the child from gaining weight). The eyes are set wide and the ears are low. Heart defects are often observed.

conclusions

To prevent the birth of a sick child, it is advisable to undergo special examinations. The test is mandatory for women giving birth after 35 years of age; parents whose relatives were exposed to similar diseases; patients with thyroid problems; women who have had miscarriages.

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From school biology textbooks, everyone has become familiar with the term chromosome. The concept was proposed by Waldeyer in 1888. It literally translates as painted body. The first object of research was the fruit fly.

General information about animal chromosomes

A chromosome is a structure in the cell nucleus that stores hereditary information. They are formed from a DNA molecule that contains many genes. In other words, a chromosome is a DNA molecule. Its amount varies among different animals. So, for example, a cat has 38, and a cow has 120. Interestingly, earthworms and ants have the smallest numbers. Their number is two chromosomes, and the male of the latter has one.

In higher animals, as well as in humans, the last pair is represented by XY sex chromosomes in males and XX in females. It should be noted that the number of these molecules is constant for all animals, but their number differs in each species. For example, we can consider the content of chromosomes in some organisms: chimpanzees - 48, crayfish - 196, wolves - 78, hare - 48. This is due to the different level of organization of a particular animal.

On a note! Chromosomes are always arranged in pairs. Geneticists claim that these molecules are the elusive and invisible carriers of heredity. Each chromosome contains many genes. Some believe that the more of these molecules, the more developed the animal, and the more complex its body is. In this case, a person should have not 46 chromosomes, but more than any other animal.

How many chromosomes do different animals have?

You need to pay attention! In monkeys, the number of chromosomes is close to that of humans. But the results are different for each species. So, different monkeys have the following number of chromosomes:

• Lemurs have 44-46 DNA molecules in their arsenal;
• Chimpanzees – 48;
• Baboons – 42,
• Monkeys – 54;
• Gibbons – 44;
• Gorillas – 48;
• Orangutan – 48;
• Macaques - 42.

The canine family (carnivorous mammals) has more chromosomes than monkeys.

• So, the wolf has 78,
• the coyote has 78,
• the small fox has 76,
• but the ordinary one has 34.
• The predatory animals lion and tiger have 38 chromosomes.
• The cat's pet has 38, while his dog opponent has almost twice as many - 78.

In mammals that are of economic importance, the number of these molecules is as follows:

• rabbit – 44,
• cow – 60,
• horse – 64,
• pig – 38.

Informative! Hamsters have the largest chromosome sets among animals. They have 92 in their arsenal. Also in this row are hedgehogs. They have 88-90 chromosomes. And kangaroos have the smallest amount of these molecules. Their number is 12. A very interesting fact is that the mammoth has 58 chromosomes. Samples were taken from frozen tissue.

For greater clarity and convenience, data from other animals will be presented in the summary.

Name of animal and number of chromosomes:

Number of chromosomes in different animal species

As you can see, each animal has a different number of chromosomes. Even among representatives of the same family, indicators differ. We can look at the example of primates:

• the gorilla has 48,
• the macaque has 42, and the marmoset has 54 chromosomes.

Why this is so remains a mystery.

How many chromosomes do plants have?

Plant name and number of chromosomes:

Video

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.).

The term chromosome was first proposed by V. It is very difficult to identify chromosome bodies in the nuclei of interphase cells using morphological methods. The chromosomes themselves, as clear, dense bodies clearly visible in a light microscope, are revealed only shortly before cell division.

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Lecture No. 6

CHROMOSOMES

Chromosomes are the main functional autoreproducing structure of the nucleus, in which DNA is concentrated and with which the functions of the nucleus are associated. The term “chromosomes” was first proposed by W. Waldeyer in 1888.

It is very difficult to identify chromosome bodies in the nuclei of interphase cells using morphological methods. The chromosomes themselves, as clear, dense bodies clearly visible in a light microscope, are revealed only shortly before cell division. In the interphase itself, chromosomes as dense bodies are not visible, since they are in a loosened, decondensed state.

Number and morphology of chromosomes

The number of chromosomes is constant for all cells of a given species of animal or plant, but varies significantly among different objects. It is not related to the level of organization of living organisms. Primitive organisms can have many chromosomes, while highly organized ones have much fewer. For example, in some radiolarians the number of chromosomes reaches 1000-1600. The record holder among plants for the number of chromosomes (about 500) is the grass fern; the mulberry tree has 308 chromosomes. Let us give examples of the quantitative content of chromosomes in some organisms: crayfish 196, humans 46, chimpanzees 48, soft wheat 42, potatoes 18, fruit flies 8, house flies 12. The smallest number of chromosomes (2) is observed in one of Ascaris races, the Asteraceae plant Haplopapus has only 4 chromosomes.

The size of chromosomes varies widely among different organisms. Thus, the length of chromosomes can vary from 0.2 to 50 microns. The smallest chromosomes are found in some protozoa, fungi, and algae; very small chromosomes are found in flax and sea reeds; they are so small that they are difficult to see with a light microscope. The longest chromosomes are found in some orthopteran insects, amphibians and liliaceae. The length of human chromosomes is in the range of 1.5-10 microns. The thickness of chromosomes ranges from 0.2 to 2 microns.

The morphology of chromosomes is best studied at the moment of their greatest condensation, in metaphase and at the beginning of anaphase. The chromosomes of animals and plants in this state are rod-shaped structures of different lengths with a fairly constant thickness; in most chromosomes it is easy to find the zoneprimary constriction, which divides the chromosome into two shoulder . In the area of ​​the primary constriction there is centromere or kinetochore . It is a plate-like, disk-shaped structure. It is connected by thin fibrils to the body of the chromosome in the region of the constriction. The kinetochore is poorly understood structurally and functionally; Thus, it is known that it is one of the centers of tubulin polymerization; bundles of microtubules of the mitotic spindle grow from it, going towards the centrioles. These bundles of microtubules take part in the movement of chromosomes to the poles of the cell during mitosis. Some chromosomes havesecondary constriction. The latter is usually located near the distal end of the chromosome and separates a small section satellite . The size and shape of the satellite are constant for each chromosome. The size and length of secondary constrictions are also very constant. Some secondary constrictions are specialized regions of chromosomes associated with the formation of the nucleolus (nucleolar organizers); others are not associated with the formation of the nucleolus and their functional role is not fully understood. Chromosome arms end in terminal sections telomeres. The telomeric ends of chromosomes are not able to join with other chromosomes or their fragments, in contrast to the ends of chromosomes lacking telomeric regions (as a result of breaks), which can join the same broken ends of other chromosomes.

Based on the location of the primary constriction (centromere), the following are distinguished: types of chromosomes:

1. metacentricthe centromere is located in the middle, the arms are equal or almost equal in length, in metaphase it acquires V-shape;

2. submetacentricthe primary constriction is slightly shifted to one of the poles, one arm is slightly longer than the other, in metaphase it has L-shape;

3. acrocentricthe centromere is strongly shifted to one of the poles, one arm is much longer than the other, does not bend in metaphase and has a rod-shaped shape;

4. telocentricThe centromere is located at the end of the chromosome, but such chromosomes have not been found in nature.

Usually each chromosome has only one centromere (monocentric chromosomes), but chromosomes may occur dicentric (with 2 centromeres) andpolycentric(possessing many centromeres).

There are species (for example, sedges) in which the chromosomes do not contain visible centromeric regions (chromosomes with diffusely located centromeres). They're called acentric and are not able to perform ordered movement during cell division.

Chemical composition of chromosomes

The main components of chromosomes are DNA and basic proteins (histones). DNA complex with histonesdeoxyribonucleoprotein(DNP) constitutes about 90% of the mass of both chromosomes isolated from interphase nuclei and chromosomes of dividing cells. The DNP content is constant for each chromosome of a given species of organism.

Of the mineral components, the most important are calcium and magnesium ions, which give plasticity to chromosomes, and their removal makes the chromosomes very fragile.

Ultrastructure

Each mitotic chromosome is covered on top pellicle . Inside is matrix , in which a spirally twisted DNP thread with a thickness of 4-10 nm is located.

Elementary fibrils of DNP are the main component that is included in the structure of mitotic and meiotic chromosomes. Therefore, to understand the structure of such chromosomes, it is necessary to know how these units are organized as part of the compact chromosome body. Intensive study of chromosome ultrastructure began in the mid-50s of the last century, which is associated with the introduction of electron microscopy into cytology. There are 2 hypotheses for the organization of chromosomes.

1). Unimute the hypothesis states that there is only one double-stranded DNP molecule on the chromosome. This hypothesis has morphological, autoradiographic, biochemical and genetic confirmation, which makes this point of view the most popular today, since at least for a number of objects (drosophila, yeast) it is proven.

2). Polynemic the hypothesis is that several double-stranded DNP molecules are combined into a bundle chromonema , and, in turn, 2-4 chromonemas, twisting, form a chromosome. Almost all observations of chromosome polynemism were made using a light microscope on botanical objects with large chromosomes (lilies, various onions, beans, tradescantia, peony). It is possible that the phenomena of polynemia that were observed in the cells of higher plants are characteristic only of these objects.

Thus, it is possible that there are several different principles for the structural organization of chromosomes in eukaryotic organisms.

In interphase cells, many regions of chromosomes are despiralized, which is associated with their functioning. They're called euchromatin. It is believed that the euchromatic regions of the chromosomes are active and contain the entire main set of genes of the cell or organism. Euchromatin is observed in the form of fine granularity or is not visible at all in the nucleus of an interphase cell.

During the cell transition from mitosis to interphase, certain zones of different chromosomes or even entire chromosomes remain compact, spiralized and well stained. These zones are called heterochromatin . It is present in the cell in the form of coarse grains, lumps, and flakes. Heterochromatic regions are usually located in the telomeric, centromeric, and perinucleolar regions of chromosomes, but can also be part of their internal parts. The loss of even significant sections of heterochromatic regions of chromosomes does not lead to cell death, since they are not active and their genes temporarily or permanently do not function.

Matrix is ​​a component of mitotic chromosomes of plants and animals, released during despiralization of chromosomes and consisting of fibrillar and granular structures of ribonucleoprotein nature. Perhaps the role of the matrix is ​​the transfer of RNA-containing material by chromosomes, which is necessary both for the formation of nucleoli and for the restoration of the karyoplasm itself in daughter cells.

Chromosome set. Karyotype

The constancy of such characteristics as size, location of primary and secondary constrictions, the presence and shape of satellites determines the morphological individuality of chromosomes. Thanks to this morphological individuality, in many species of animals and plants it is possible to recognize any chromosome set in any dividing cell.

The totality of the number, size and morphology of chromosomes is called karyotype of this type. A karyotype is like the face of a species. Even in closely related species, chromosome sets differ from each other either in the number of chromosomes, or in the size of at least one or several chromosomes, or in the shape of the chromosomes and their structure. Consequently, the structure of the karyotype can be a taxonomic (systematic) character, which is increasingly used in the taxonomy of animals and plants.

A graphic representation of a karyotype is called idiogram.

The number of chromosomes in mature germ cells is called haploid (denoted n ). Somatic cells contain double the number of chromosomes diploid set (2 n ). Cells that have more than two sets of chromosomes are called polyploid (3 n, 4 n, 8 n, etc.).

The diploid set contains paired chromosomes that are identical in shape, structure and size, but have different origins (one is maternal, the other is paternal). They're called homologous.

In many higher dioecious animals in the diploid set there are one or two unpaired chromosomes that differ in males and females, this sexual chromosomes. The remaining chromosomes are called autosomes . Cases have been described when a male has only one sex chromosome, and a female has two.

In many fish, mammals (including humans), some amphibians (frogs of the genus Rana ), insects (beetles, Diptera, Orthoptera), the large chromosome is designated by the letter X, and the small chromosome by the letter Y. In these animals, in the karyotype of the female, the last pair is represented by two XX chromosomes, and in the male, by XY chromosomes.

In birds, reptiles, certain species of fish, some amphibians (tailed amphibians), and butterflies, the male sex has the same sex chromosomes ( WW -chromosomes), and the female are different ( WZ chromosomes).

In many animals and humans, in the cells of female individuals, one of the two sex chromosomes does not function and therefore remains entirely in a spiraled state (heterochromatin). It is found in the interphase nucleus in the form of a lumpsex chromatinat the inner nuclear membrane. Both sex chromosomes function throughout life in the male body. If sex chromatin is detected in the nuclei of the cells of a male body, this means that he has an extra X chromosome (XXY Kleinfelter's disease). This may occur as a result of impaired spermato- or oogenesis. The study of the content of sex chromatin in interphase nuclei is widely used in medicine to diagnose human chromosomal diseases caused by an imbalance of sex chromosomes.

Karyotype changes

Changes in the karyotype may be associated with a change in the number of chromosomes or a change in their structure.

Quantitative changes in karyotype: 1) polyploidy; 2) aneuploidy.

Polyploidy this is a multiple increase in the number of chromosomes compared to haploid. As a result, instead of ordinary diploid cells (2 n ) are formed, for example, triploid (3 n ), tetraploid (4 n ), octaploid (8 n ) cells. Thus, in onions, whose diploid cells contain 16 chromosomes, triploid cells contain 24 chromosomes, and tetraploid cells contain 32 chromosomes. Polyploid cells are large in size and have increased viability.

Polyploidy is widespread in nature, especially among plants, many species of which arose as a result of multiple doublings of the number of chromosomes. Most cultivated plants, for example, bread wheat, multi-row barley, potatoes, cotton, and most fruit and ornamental plants, are naturally occurring polyploids.

Experimentally, polyploid cells are most easily obtained by the action of an alkaloid colchicine or other substances that disrupt mitosis. Colchicine destroys the spindle, so that already doubled chromosomes remain in the equatorial plane and do not diverge to the poles. After the cessation of the action of colchicine, the chromosomes form a common nucleus, but a larger one (polyploid). During subsequent divisions, the chromosomes will again double and move towards the poles, but twice the number of them will remain. Artificially obtained polyploids are widely used in plant breeding. Varieties of triploid sugar beet, tetraploid rye, buckwheat and other crops have been created.

In animals, complete polyploidy is very rare. For example, in the mountains of Tibet there lives one of the species of frogs, the population of which on the plain has a diploid chromosome set, and the high-mountain populations have a triploid, or even tetraploid.

In humans, polyploidy leads to sharply negative consequences. The birth of children with polyploidy is extremely rare. Usually the death of the organism occurs at the embryonic stage of development (about 22.6% of all spontaneous abortions are caused by polyploidy). It should be noted that triploidy occurs 3 times more often than tetraploidy. If children with triploidy syndrome are nevertheless born, they have abnormalities in the development of external and internal organs, are practically non-viable and die in the first days after birth.

Somatic polyploidy is more often observed. Thus, in human liver cells, with age, dividing cells become less and less, but the number of cells with a large nucleus or two nuclei increases. Determining the amount of DNA in such cells clearly shows that they have become polyploid.

Aneuploidy this is an increase or decrease in the number of chromosomes that is not a multiple of the haploid number. Aneuploid organisms, that is, organisms in which all cells contain aneuploid sets of chromosomes, are usually sterile or poorly viable. As an example of aneuploidy, consider some human chromosomal diseases. Kleinfelter's syndrome: the cells of the male body have an extra X chromosome, which leads to general physical underdevelopment of the body, in particular its reproductive system, and mental abnormalities. Down syndrome: an extra chromosome is contained in the 21st pair, which leads to mental retardation, abnormalities of internal organs; the disease is accompanied by some external signs of dementia and occurs in men and women. Turner syndrome is caused by the lack of one X chromosome in the cells of the female body; manifests itself in underdevelopment of the reproductive system, infertility, and external signs of dementia. If one X chromosome is missing in the cells of the male body, death occurs at the embryonic stage.

Aneuploid cells constantly arise in a multicellular organism as a result of disruption of the normal course of cell division. As a rule, such cells quickly die, but in some pathological conditions of the body they reproduce successfully. A high percentage of aneuploid cells is characteristic, for example, of many malignant tumors of humans and animals.

Structural changes in the karyotype.Chromosomal rearrangements, or chromosomal aberrations, occur as a result of single or multiple breaks of chromosomes or chromatids. Chromosome fragments at break sites are able to connect with each other or with fragments of other chromosomes in the set. Chromosomal aberrations are of the following types. Deletion this is the loss of the middle section of the chromosome. Difference this is the detachment of the end section of a chromosome. Inversion tearing off a section of a chromosome, rotating it 180 0 and joining to the same chromosome; this disrupts the order of nucleotides. Duplication breaking off a section of a chromosome and attaching it to a homologous chromosome. Translocation detachment of a section of a chromosome and its attachment to a non-homologous chromosome.

As a result of such rearrangements, dicentric and acentric chromosomes can be formed. Large deletions, differentiations and translocations dramatically change the morphology of chromosomes and are clearly visible under a microscope. Small deletions and translocations, as well as inversions, are detected by changes in the inheritance of genes localized in regions of chromosomes affected by the rearrangement, and by changes in the behavior of chromosomes during the formation of gametes.

Structural changes in the karyotype always lead to negative consequences. For example, “cry of the cat” syndrome is caused by a chromosomal mutation (division) in the 5th pair of chromosomes in humans; manifests itself in abnormal development of the larynx, which leads to “meowing” instead of a normal cry in early childhood, and retardation in physical and mental development.

Chromosome reduplication

The basis of the doubling (reduplication) of chromosomes is the process of DNA replication, i.e. the process of self-reproduction of nucleic acid macromolecules, ensuring accurate copying of genetic information and its transmission from generation to generation. DNA synthesis begins with the divergence of strands, each of which serves as a template for the synthesis of a daughter strand. The products of reduplication are two daughter DNA molecules, each of which consists of one parent and one daughter strand. An important place among reduplication enzymes is occupied by DNA polymerase, which carries out synthesis at a rate of about 1000 nucleotides per second (in bacteria). DNA reduplication is semi-conservative, i.e. during the synthesis of two daughter DNA molecules, each of them contains one “old” and one “new” chain (this method of reduplication was proven by Watson and Crick in 1953). Fragments synthesized during reduplication on one strand are “crosslinked” by the enzyme DNA ligase.

Reduplication involves proteins that unwind the DNA double helix, stabilize the untwisted sections, and prevent the molecules from becoming entangled.

DNA reduplication in eukaryotes occurs more slowly (about 100 nucleotides per second), but simultaneously at many points in one DNA molecule.

Since protein synthesis also occurs simultaneously with DNA reduplication, we can talk about chromosome reduplication. Studies conducted back in the 50s of the twentieth century showed that no matter how many longitudinally arranged DNA strands the chromosomes of organisms of different species contain, during cell division the chromosomes behave as if they consist of two simultaneously reduplicating subunits. After reduplication, which occurs in interphase, each chromosome turns out to be double, and even before division begins in the cell, everything is ready for an even distribution of chromosomes between daughter cells. If division does not occur after reduplication, the cell becomes polyploid. During the formation of polytene chromosomes, chromonemas are reduplicated, but do not diverge, due to which giant chromosomes with a huge number of chromonemas are obtained.