The term chromosomes 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, well-visible bodies in a light microscope, are revealed only shortly before cell division.
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Lecture #6
CHROMOSOMES
Chromosomes are the main functional auto-reproducing 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. Chromosomes proper, as clear, dense, well-visible bodies under a light microscope, are revealed only shortly before cell division. In the interphase itself, chromosomes are not seen as dense bodies, 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 animal or plant species, but varies significantly in different objects. It is not related to the level of organization of living organisms. Primitive organisms may have many chromosomes, while highly organized organisms may have much less. For example, in some radiolarians, the number of chromosomes reaches 1000-1600. The record holder among plants in terms of the number of chromosomes (about 500) is the grass fern, 308 chromosomes in the mulberry tree. Let us give examples of the quantitative content of chromosomes in some organisms: crayfish - 196, humans - 46, chimpanzees - 48, soft wheat - 42, potatoes - 18, Drosophila - 8, house fly - 12. The smallest number of chromosomes (2) is observed in one of roundworm races, the haplopapus composite plant has only 4 chromosomes.
The size of chromosomes in different organisms varies widely. So, the length of chromosomes can vary from 0.2 to 50 microns. The smallest chromosomes are found in some protozoa, fungi, algae, very small chromosomes in flax and sea reeds; they are so small that they are hardly visible in a light microscope. The longest chromosomes are found in some orthopteran insects, in amphibians and in lilies. The length of human chromosomes is in the range of 1.5-10 microns. The thickness of the chromosomes ranges from 0.2 to 2 microns.
The morphology of chromosomes is best studied at the time 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, most of the chromosomes can easily find a zoneprimary constrictionthat divides a chromosome into two shoulder . In the region of the primary constriction is located centromere or kinetochore . It is a plate-like structure shaped like a disk. It is connected by thin fibrils with 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 are involved 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 area - satellite . The dimensions and shape of the satellite are constant for each chromosome. The size and length of the secondary constrictions are also quite constant. Some secondary constrictions are specialized sections of chromosomes associated with the formation of the nucleolus (nucleolar organizers), the rest are not associated with the formation of the nucleolus and their functional role is not fully understood. Arms of chromosomes end in end segments - telomeres. The telomeric ends of chromosomes are not able to connect with other chromosomes or their fragments, in contrast to the ends of chromosomes that lack telomeric regions (as a result of breaks), which can join the same broken ends of other chromosomes.
According to the location of the primary constriction (centromere), the following are distinguished types of chromosomes:
1. metacentric- the centromere is located in the middle, the arms are equal or almost equal in length, in metaphase it acquires V-shaped;
2. submetacentric- the primary constriction is slightly shifted to one of the poles, one shoulder is slightly longer than the other, in metaphase it has L-shaped;
3. acrocentric- the 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. telocentric- the centromere is located at the end of the chromosome, but such chromosomes are not found in nature.
Usually each chromosome has only one centromere (monocentric chromosomes), but chromosomes may occur dicentric (with 2 centromeres) andpolycentric(possessing multiple 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 an 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) - makes up about 90% of the mass of both chromosomes isolated from interphase nuclei and chromosomes of dividing cells. The content of DNP is constant for each chromosome of a given type of organism.
Of the mineral components, the most important are calcium and magnesium ions, which give plasticity to the 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 curled thread of DNP is located, 4-10 nm thick.
Elementary fibrils of DNP are the main component that is included in the structure of mitotic and meiotic chromosomes. Therefore, in order to understand the structure of such chromosomes, it is necessary to know how these units are organized in the compact body of chromosomes. Intensive study of the ultrastructure of chromosomes began in the mid-1950s, which is associated with the introduction of electron microscopy into cytology. There are 2 hypotheses for the organization of chromosomes.
one). Uninemnaya the hypothesis states that there is only one double-stranded DNP molecule in the chromosome. This hypothesis has morphological, autoradiographic, biochemical and genetic confirmations, which makes this point of view the most popular today, since at least for a number of objects (Drosophila, yeast fungi) it is proven.
2). Polynemic the hypothesis is that several double-stranded DNP molecules are combined into a bundle - lameness , and, in turn, 2-4 chromonemes, twisting, form a chromosome. Almost all observations of chromosome polynemy 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 polynemy observed in the cells of higher plants are characteristic only of these objects.
Thus, it is possible that there are several different principles of the structural organization of chromosomes in eukaryotic organisms.
In interphase cells, many sections of chromosomes are despiralized, which is associated with their functioning. They're called euchromatin. It is believed that the euchromatic regions of chromosomes are active and contain the entire main complex of genes of a cell or organism. Euchromatin is observed in the form of fine granularity or is not distinguishable at all in the nucleus of the interphase cell.
During the transition of a cell from mitosis to interphase, certain zones of various chromosomes or even entire chromosomes remain compact, spiralized and stain well. These zones are called heterochromatin . It is present in the cell in the form of large grains, lumps, 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.
The matrix is a component of the mitotic chromosomes of plants and animals, released during despiralization of chromosomes and consisting of fibrillar and granular structures of a ribonucleoprotein nature. It is possible that the role of the matrix consists in the transfer of RNA-containing material by chromosomes, which is necessary both for the formation of nucleoli and for the restoration of the karyoplasm proper in daughter cells.
chromosome set. Karyotype
The constancy of such features as the size, location of the primary and secondary constrictions, the presence and shape of satellites, determines the morphological individuality of chromosomes. Due to this morphological individuality, in many species of animals and plants it is possible to recognize any chromosome of the 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 more chromosomes, or in the shape of the chromosomes and in their structure. Therefore, the structure of the karyotype can be a taxonomic (systematic) feature that is increasingly used in the taxonomy of animals and plants.
The graphic representation of a karyotype is called idiogram.
The number of chromosomes in mature germ cells is called haploid (denoted by n ). Somatic cells contain a double number of chromosomes - diploid set (2 n ). Cells with more than two sets of chromosomes are called polyploid (3n, 4n, 8n, etc.).
The diploid set contains paired chromosomes, identical in shape, structure and size, but having a different origin (one maternal, the other paternal). They're called homologous.
In many higher diploid animals, there are one or two unpaired chromosomes in the diploid set, which differ in males and females - this genital chromosomes. The rest of the chromosomes are called autosomes . Cases are described when the male has only one sex chromosome, and the female has two.
In many fish, mammals (including humans), some amphibians (frogs of the genus Rana ), insects (beetles, Diptera, Orthoptera), the large chromosome is denoted by the letter X, and the small one 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), butterflies, the male sex has the same sex chromosomes ( WW -chromosomes), and female - 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 spiralized state (heterochromatin). It is found in the interphase nucleus in the form of a lumpsex chromatinat the inner nuclear membrane. The sex chromosomes in the male body function both for life. If sex chromatin is found in the nuclei of the cells of the male body, this means that he has an extra X chromosome (XXY - Kleinfelter's disease). This may occur as a result of impaired spermatogenesis or oogenesis. The study of the content of sex chromatin in interphase nuclei is widely used in medicine for diagnosing human chromosomal diseases caused by imbalance of sex chromosomes.
Karyotype changes
Changes in the karyotype may be associated with a change in the number of chromosomes or with 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 the 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. So, in an onion, the diploid cells of which contain 16 chromosomes, the triploid cells contain 24 chromosomes, and the tetraploid cells contain 32 chromosomes. Polyploid cells are characterized by large size and increased viability.
Polyploidy is widespread in nature, especially among plants, many species of which have arisen as a result of multiple doublings in the number of chromosomes. Most cultivated plants, such as soft wheat, multi-row barley, potatoes, cotton, 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 of division, due to which already doubled chromosomes remain in the plane of the equator and do not diverge towards the poles. After the termination of the action of colchicine, the chromosomes form a common nucleus, but already larger (polyploid). During subsequent divisions, the chromosomes will again double and diverge towards the poles, but a double 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, one of the species of frogs lives in the mountains of Tibet, the population of which in the plains 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 due to polyploidy). It should be noted that triploidy occurs 3 times more often than tetraploidy. If children with triploidy syndrome are still born, then they have anomalies in the development of external and internal organs, are practically unviable and die in the first days after birth.
Somatic polyploidy is more common. So, 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. Determination of 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, not a multiple of the haploid. Aneuploid organisms, that is, organisms in which all cells contain aneuploid sets of chromosomes, are usually sterile or non-viable. As an example of aneuploidy, consider some human chromosomal diseases. Kleinfelter syndrome: in the cells of the male body there is an extra X chromosome, which leads to a general physical underdevelopment of the body, in particular its reproductive system, and mental abnormalities. Down syndrome: an extra chromosome is contained in 21 pairs, which leads to mental retardation, anomalies of internal organs; the disease is accompanied by some external signs of dementia, occurs in men and women. Turner syndrome is caused by a lack of one X chromosome in the cells of the female body; manifested in the underdevelopment of the reproductive system, infertility, external signs of dementia. With a lack of one X chromosome in the cells of the male body, a lethal outcome is observed at the embryonic stage.
Aneuploid cells constantly arise in a multicellular organism as a result of a violation of the normal course of cell division. As a rule, such cells die quickly, however, under certain pathological conditions of the body, they successfully multiply. A high percentage of aneuploid cells is characteristic, for example, of many malignant tumors in humans and animals.
Structural changes in the karyotype.Chromosomal rearrangements, or chromosomal aberrations, result from single or multiple breaks in chromosomes or chromatids. Fragments of chromosomes at break points are able to connect with each other or with fragments of other chromosomes of the set. Chromosomal aberrations are of the following types. deletion is the loss of the middle portion of a chromosome. Difishencia is the detachment of the end portion of a chromosome. Inversion - detachment of a chromosome segment, turning it 180 0 and attachment to the same chromosome; this disrupts the order of the nucleotides. duplication detachment of a segment of a chromosome and its attachment to a homologous chromosome. Translocation detachment of a segment 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, divisions 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 a change in the inheritance of genes localized in the regions of chromosomes affected by the rearrangement, and by a change in the behavior of chromosomes during the formation of gametes.
Structural changes in the karyotype always lead to negative consequences. For example, the "cat's cry" syndrome is caused by a chromosomal mutation (deficiency) in the 5th pair of chromosomes in humans; manifests itself in the abnormal development of the larynx, which entails "meow" instead of a normal cry in early childhood, a lag in physical and mental development.
Chromosome reduplication
Doubling (reduplication) of chromosomes is based on the process of DNA reduplication, i.e. the process of self-reproduction of macromolecules of nucleic acids, which ensures the exact copying of genetic information and its transmission from generation to generation. DNA synthesis begins with the separation 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 child strand. An important place among the reduplication enzymes is occupied by DNA polymerase, leading the 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" strand (this method of reduplication was proved by Watson and Crick in 1953). Fragments synthesized during reduplication on the same strand are “crosslinked” by the enzyme DNA ligase.
Reduplication involves proteins that unwind the double helix of DNA, stabilize the untwisted sections, and prevent molecular entanglement.
DNA reduplication in eukaryotes occurs more slowly (about 100 nucleotides per second), but simultaneously at many points in one DNA molecule.
Since protein synthesis occurs simultaneously with DNA replication, we can speak of chromosome reduplication. Studies carried out back in the 1950s showed that no matter how many longitudinal strands of DNA the chromosomes of organisms of different species contain, during cell division, the chromosomes behave as consisting of two simultaneously replicating subunits. After the reduplication that takes place in the interphase, each chromosome is double, and even before the start of division in the cell, everything is ready for an even distribution of chromosomes between the daughter cells. If division does not occur after reduplication, the cell becomes polyploid. During the formation of polytene chromosomes, chromonemes are replicated, but do not diverge, which results in giant chromosomes with a huge number of chromonemes.
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From school textbooks on biology, everyone had a chance to get acquainted with the term chromosome. The concept was proposed by Waldeyer in 1888. It literally translates as a painted body. The first object of research was the fruit fly.
General about animal chromosomes
The chromosome is the structure of the cell nucleus that stores hereditary information. They are formed from a DNA molecule, which contains many genes. In other words, a chromosome is a DNA molecule. Its quantity in different animals is not the same. So, for example, a cat has 38, and a cow has -120. Interestingly, earthworms and ants have the smallest number. 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 for all animals is constant, but for each species their number is different. For example, we can consider the content of chromosomes in some organisms: chimpanzee - 48, crayfish - 196, wolf - 78, hare - 48. This is due to the different level of organization of an 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 its body is more complex. In this case, a person should not have 46 chromosomes, but more than any other animal.
How many chromosomes do different animals have
Need to pay attention! In monkeys, the number of chromosomes is close to that of humans. But each type has different results. 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 family of canids (carnivorous mammals) has more chromosomes than monkeys.
- So, the wolf has 78,
- coyote - 78,
- in a small fox - 76,
- but the ordinary one has 34.
- The predatory animals of the lion and tiger each have 38 chromosomes.
- The cat's pet has 38, and its dog opponent has nearly 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 the smallest number of these molecules are endowed with kangaroos. Their number is 12. A very interesting fact is that the mammoth has 58 chromosomes. Samples are taken from frozen tissue.
For greater clarity and convenience, the data of other animals will be presented in the summary.
The name of the animal and the number of chromosomes:
| Spotted martens | 12 |
| Kangaroo | 12 |
| yellow marsupial mouse | 14 |
| marsupial anteater | 14 |
| common opossum | 22 |
| Opossum | 22 |
| Mink | 30 |
| American badger | 32 |
| Korsak (steppe fox) | 36 |
| Tibetan fox | 36 |
| small panda | 36 |
| Cat | 38 |
| a lion | 38 |
| Tiger | 38 |
| Raccoon | 38 |
| Canadian beaver | 40 |
| Hyenas | 40 |
| House mouse | 40 |
| Baboons | 42 |
| Rats | 42 |
| Dolphin | 44 |
| rabbits | 44 |
| Human | 46 |
| Hare | 48 |
| Gorilla | 48 |
| American fox | 50 |
| striped skunk | 50 |
| Sheep | 54 |
| Elephant (Asian, Savannah) | 56 |
| Cow | 60 |
| Domestic goat | 60 |
| woolly monkey | 62 |
| Donkey | 62 |
| Giraffe | 62 |
| Mule (a hybrid of a donkey and a mare) | 63 |
| Chinchilla | 64 |
| Horse | 64 |
| Fox gray | 66 |
| white tailed deer | 70 |
| Paraguayan fox | 74 |
| fox small | 76 |
| Wolf (red, red, maned) | 78 |
| Dingo | 78 |
| Coyote | 78 |
| Dog | 78 |
| common jackal | 78 |
| Chicken | 78 |
| Pigeon | 80 |
| Turkey | 82 |
| Ecuadorian hamster | 92 |
| common lemur | 44-60 |
| arctic fox | 48-50 |
| Echidna | 63-64 |
| hedgehogs | 88-90 |
The number of chromosomes in different animal species
As you can see, each animal has a different number of chromosomes. Even among members of the same family, the indicators differ. Consider the example of primates:
- gorilla has 48,
- the macaque has 42, and the monkey has 54 chromosomes.
Why this is so remains a mystery.
How many chromosomes do plants have?
Plant name and number of chromosomes:
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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.
So far, B chromosomes have not been found in humans. But sometimes an additional set of chromosomes appears in cells - then they talk about polyploidy, and if their number is not a multiple of 23 - about aneuploidy. Polyploidy occurs in certain types of cells and contributes to their increased work, while aneuploidy usually indicates violations in the work of the cell and often leads to its death.
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Most often, the wrong number of chromosomes is the result of unsuccessful cell division. In somatic cells, after DNA duplication, the maternal chromosome and its copy are linked together by cohesin proteins. Then protein complexes of kinetochore sit on their central parts, to which microtubules are later attached. When dividing along microtubules, kinetochores disperse to different poles of the cell and pull chromosomes along with them. If the cross-links between copies of the chromosome are destroyed ahead of time, then microtubules from the same pole can attach to them, and then one of the daughter cells will receive an extra chromosome, and the second will remain deprived.
Meiosis also often passes with errors. The problem is that the construction of linked two pairs of homologous chromosomes can twist in space or separate in the wrong places. The result will again be an uneven distribution of chromosomes. Sometimes the sex cell manages to track this so as not to transmit the defect by inheritance. Extra chromosomes are often misfolded or broken, which triggers the death program. For example, among spermatozoa there is such a selection for quality. But the eggs were less fortunate. All of them are formed in humans even before birth, prepare for division, and then freeze. Chromosomes are already doubled, tetrads are formed, and division is delayed. In this form, they live until the reproductive period. Then the eggs mature in turn, divide for the first time and freeze again. The second division occurs immediately after fertilization. And at this stage, it is already difficult to control the quality of the division. And the risks are greater, because the four chromosomes in the egg remain cross-linked for decades. During this time, breakdowns accumulate in cohesins, and chromosomes can spontaneously separate. Therefore, the older the woman, the greater the likelihood of incorrect chromosome divergence in the egg.
Aneuploidy in germ cells inevitably leads to aneuploidy of the embryo. When a healthy egg with 23 chromosomes is fertilized by a sperm with an extra or missing chromosome (or vice versa), the number of chromosomes in the zygote will obviously be different from 46. But even if the germ cells are healthy, this does not guarantee healthy development. In the first days after fertilization, the cells of the embryo actively divide in order to quickly gain cell mass. Apparently, in the course of rapid divisions, there is no time to check the correctness of chromosome segregation, so aneuploid cells can arise. And if an error occurs, then the further fate of the embryo depends on the division in which it happened. If the balance is disturbed already in the first division of the zygote, then the whole organism will grow aneuploid. If the problem arose later, then the outcome is determined by the ratio of healthy and abnormal cells.
Some of the latter may die further, and we will never know about their existence. Or he can take part in the development of the body, and then he will succeed mosaic- different cells will carry different genetic material. Mosaicism causes a lot of trouble for prenatal diagnosticians. For example, at the risk of having a child with Down syndrome, sometimes one or more embryonic cells are removed (at the stage when this should not be dangerous) and the chromosomes are counted in them. But if the embryo is mosaic, then this method becomes not particularly effective.
Third wheel
All cases of aneuploidy are logically divided into two groups: deficiency and excess of chromosomes. The problems that arise with a deficiency are quite expected: minus one chromosome means minus hundreds of genes.
If the homologous chromosome is working normally, then the cell can get away with only an insufficient amount of proteins encoded there. But if some of the genes remaining on the homologous chromosome do not work, then the corresponding proteins will not appear in the cell at all.
In the case of an excess of chromosomes, everything is not so obvious. There are more genes, but here - alas - more does not mean better.
First, extra genetic material increases the load on the nucleus: an additional strand of DNA must be placed in the nucleus and served by information reading systems.
Scientists have found that in people with Down syndrome, whose cells carry an extra 21st chromosome, the work of genes located on other chromosomes is mainly disrupted. Apparently, an excess of DNA in the nucleus leads to the fact that there are not enough proteins that support the work of chromosomes for everyone.
Secondly, the balance in the amount of cellular proteins is disturbed. For example, if activator proteins and inhibitor proteins are responsible for some process in the cell, and their ratio usually depends on external signals, then an additional dose of one or the other will cause the cell to stop responding adequately to the external signal. Finally, an aneuploid cell has an increased chance of dying. When duplicating DNA before division, errors inevitably occur, and the cellular proteins of the repair system recognize them, repair them, and start doubling again. If there are too many chromosomes, then there are not enough proteins, errors accumulate and apoptosis is triggered - programmed cell death. But even if the cell does not die and divides, then the result of such division is also likely to be aneuploids.
You will live
If even within a single cell, aneuploidy is fraught with disruption and death, then it is not surprising that it is not easy for an entire aneuploid organism to survive. At the moment, only three autosomes are known - 13, 18 and 21, trisomy for which (that is, an extra, third chromosome in cells) is somehow compatible with life. This is probably due to the fact that they are the smallest and carry the fewest genes. At the same time, children with trisomy on the 13th (Patau syndrome) and 18th (Edwards syndrome) chromosomes live at best up to 10 years, and more often live less than a year. And only trisomy on the smallest in the genome, the 21st chromosome, known as Down syndrome, allows you to live up to 60 years.
It is very rare to meet people with general polyploidy. Normally, polyploid cells (carrying not two, but four to 128 sets of chromosomes) can be found in the human body, for example, in the liver or red bone marrow. These are usually large cells with enhanced protein synthesis, which do not require active division.
An additional set of chromosomes complicates the task of their distribution among daughter cells, so polyploid embryos, as a rule, do not survive. Nevertheless, about 10 cases have been described when children with 92 chromosomes (tetraploids) were born and lived from several hours to several years. However, as in the case of other chromosomal anomalies, they lagged behind in development, including mental development. However, for many people with genetic abnormalities, mosaicism comes to the rescue. If the anomaly has developed already during the fragmentation of the embryo, then a certain number of cells may remain healthy. In such cases, the severity of symptoms decreases and life expectancy increases.
Gender injustices
However, there are also such chromosomes, the increase in the number of which is compatible with human life or even goes unnoticed. And this, surprisingly, the sex chromosomes. The reason for this is gender injustice: about half of the people in our population (girls) have twice as many X chromosomes as others (boys). At the same time, the X chromosomes serve not only to determine sex, but also carry more than 800 genes (that is, twice as many as the extra 21st chromosome, which causes a lot of trouble for the body). But girls come to the aid of a natural mechanism to eliminate inequality: one of the X chromosomes is inactivated, twisted and turns into a Barr body. In most cases, the selection occurs randomly, and in some cells the maternal X chromosome is active, while in others the paternal X chromosome is active. Thus, all girls are mosaic, because different copies of genes work in different cells. Tortoiseshell cats are a classic example of such mosaicity: on their X chromosome there is a gene responsible for melanin (a pigment that determines, among other things, coat color). Different copies work in different cells, so the color is spotty and is not inherited, since inactivation occurs randomly.
As a result of inactivation, only one X chromosome always works in human cells. This mechanism allows you to avoid serious trouble with X-trisomy (XXX girls) and Shereshevsky-Turner syndromes (XO girls) or Klinefelter (XXY boys). About one in 400 children is born this way, but vital functions in these cases are usually not significantly impaired, and even infertility does not always occur. It is more difficult for those who have more than three chromosomes. This usually means that the chromosomes did not separate twice during the formation of germ cells. Cases of tetrasomy (XXXXX, XXYY, XXXY, XYYY) and pentasomy (XXXXX, XXXXY, XXXYY, XXYYY, XYYYY) are rare, some of which have been described only a few times in the history of medicine. All of these variants are compatible with life, and people often live to advanced years, with abnormalities manifesting themselves in abnormal skeletal development, genital defects, and mental decline. Tellingly, the extra Y-chromosome itself has little effect on the functioning of the body. Many men with the XYY genotype do not even know about their features. This is due to the fact that the Y chromosome is much smaller than the X and carries almost no genes that affect viability.
The sex chromosomes have another interesting feature. Many mutations in genes located on autosomes lead to abnormalities in the functioning of many tissues and organs. At the same time, most gene mutations on the sex chromosomes manifest themselves only in mental impairment. It turns out that, to a significant extent, the sex chromosomes control the development of the brain. Based on this, some scientists hypothesize that it is they who are responsible for the differences (however, not fully confirmed) between the mental abilities of men and women.
Who benefits from being wrong
Despite the fact that medicine has been familiar with chromosomal abnormalities for a long time, recently aneuploidy continues to attract the attention of scientists. It turned out that more than 80% of tumor cells contain an unusual number of chromosomes. On the one hand, the reason for this may be the fact that proteins that control the quality of division are able to slow it down. In tumor cells, these very control proteins often mutate, so division restrictions are removed and chromosome checking does not work. On the other hand, scientists believe that this may serve as a factor in the selection of tumors for survival. According to this model, tumor cells first become polyploid, and then, as a result of division errors, they lose different chromosomes or their parts. It turns out a whole population of cells with a wide variety of chromosomal abnormalities. Most of them are not viable, but some may accidentally succeed, for example, if they accidentally get extra copies of genes that start division, or lose genes that suppress it. However, if the accumulation of errors during division is additionally stimulated, then the cells will not survive. The action of taxol, a common cancer drug, is based on this principle: it causes systemic nondisjunction of chromosomes in tumor cells, which should trigger their programmed death.
It turns out that each of us can be a carrier of extra chromosomes, at least in individual cells. However, modern science continues to develop strategies to deal with these unwanted passengers. One of them proposes to use the proteins responsible for the X chromosome and incite, for example, the extra 21st chromosome of people with Down syndrome. It is reported that in cell cultures this mechanism was able to be brought into action. So, perhaps in the foreseeable future, dangerous extra chromosomes will be tamed and rendered harmless.
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.).
