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 further fate 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 among those remaining on homologous chromosome some genes do not work, then the corresponding proteins in the cell will not appear 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. On this 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 also have one more 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 mental faculties men and women.

Who benefits from being wrong

Despite the fact that medicine has been familiar with chromosomal abnormalities for a long time, in Lately 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 same 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 whole population 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.

Polina Loseva

​What mutations, besides Down's syndrome, threaten us? Is it possible to cross a human with a monkey? And what will happen to our genome in the future? The editor of the portal ANTROPOGENESIS.RU talked about chromosomes with a geneticist, head. lab. Comparative Genomics SB RAS Vladimir Trifonov.

− Can you explain plain language what is a chromosome?

- A chromosome is a fragment of the genome of any organism (DNA) in combination with proteins. If in bacteria the entire genome is usually one chromosome, then in complex organisms with a pronounced nucleus (eukaryotes) the genome is usually fragmented, and complexes of long DNA and protein fragments are clearly visible in a light microscope during cell division. That is why chromosomes as staining structures (“chroma” - color in Greek) were described in late XIX century.

- Is there any connection between the number of chromosomes and the complexity of the organism?

- There is no connection. The Siberian sturgeon has 240 chromosomes, the sterlet has 120, but it is sometimes quite difficult to distinguish these two species from each other by external signs. The female Indian muntjac has 6 chromosomes, the males have 7, and their relative has Siberian roe deer there are more than 70 of them (more precisely, 70 chromosomes of the main set and up to a dozen additional chromosomes). In mammals, the evolution of chromosome breaks and fusions was quite intensive, and now we are seeing the results of this process, when often each species has characteristics karyotype (set of chromosomes). But, undoubtedly, the general increase in the size of the genome was a necessary step in the evolution of eukaryotes. At the same time, how this genome is distributed over individual fragments does not seem to be very important.

− What are the common misconceptions about chromosomes? People often get confused: genes, chromosomes, DNA...

- Since chromosomal rearrangements really often occur, people have concerns about chromosomal abnormalities. It is known that an extra copy of the smallest human chromosome (chromosome 21) leads to a rather serious syndrome (Down's syndrome), which has characteristic external and behavioral features. Extra or missing sex chromosomes are also quite common and can have serious consequences. However, geneticists have also described quite a few relatively neutral mutations associated with the appearance of microchromosomes, or additional X and Y chromosomes. I think the stigmatization of this phenomenon is due to the fact that people perceive the concept of the norm too narrowly.

What chromosomal mutations are found in modern man and what do they lead to?

- The most common chromosomal abnormalities are:

- Klinefelter's syndrome (XXY men) (1 in 500) - characteristic external signs, certain health problems (anemia, osteoporosis, muscle weakness and sexual dysfunction), sterility. There may be behavioral differences. However, many symptoms (except sterility) can be corrected by the administration of testosterone. With the use of modern reproductive technologies, it is possible to obtain healthy children from carriers of this syndrome;

- Down syndrome (1 in 1000) - characteristic external signs, delayed cognitive development, short duration life, can be fertile;

- trisomy X (XXX women) (1 per 1000) - most often there are no manifestations, fertility;

- XYY syndrome (men) (1 in 1000) - almost no manifestations, but there may be behavioral features and reproductive problems are possible;

- Turner's syndrome (women CW) (1 per 1500) - short stature and other developmental features, normal intelligence, sterility;

- balanced translocations (1 per 1000) - depends on the type, in some cases malformations and mental retardation may be observed, may affect fertility;

- small extra chromosomes (1 in 2000) - the manifestation depends on the genetic material on the chromosomes and varies from neutral to severe clinical symptoms;

Pericentric inversion of chromosome 9 occurs in 1% of the human population, but this rearrangement is considered as a variant of the norm.

Is the difference in the number of chromosomes an obstacle to crossing? Is there interesting examples crossing animals with different number chromosomes?

- If the crossing is intraspecific or between closely related species, then the difference in the number of chromosomes may not interfere with crossing, but the offspring may be sterile. A lot of hybrids are known between species with different numbers of chromosomes, for example, in horses: there are all variants of hybrids between horses, zebras and donkeys, and the number of chromosomes in all horses is different and, accordingly, hybrids are often sterile. However, this does not exclude the possibility that balanced gametes may be formed by chance.

- What unusual in the field of chromosomes has been discovered recently?

- Recently, there have been many discoveries regarding the structure, functioning and evolution of chromosomes. I especially like the work that has shown that the sex chromosomes formed in different groups of animals quite independently.

- But still, is it possible to cross a man with a monkey?

- It is theoretically possible to obtain such a hybrid. Recently, hybrids of much more evolutionarily distant mammals have been obtained (white and black rhinoceros, alpaca and camel, and so on). Red wolf in America, long considered separate view, but has recently been shown to be a hybrid between a wolf and a coyote. A huge number of feline hybrids are known.


- And a completely absurd question: is it possible to cross a hamster with a duck?

- Here, most likely, nothing will work out, because over hundreds of millions of years of evolution there are too many genetic differences for a carrier of such a mixed genome to function.


- Is it possible that in the future a person will have fewer or more chromosomes?

- Yes, it is quite possible. It is possible that a pair of acrocentric chromosomes will merge and such a mutation will spread to the entire population.

- What popular science literature would you recommend on the topic of human genetics? What about popular science films?

− Books by the biologist Alexander Markov, the three-volume book “Human Genetics” by Vogel and Motulsky (although this is not pop-science, but there is good reference data there). From films about human genetics, nothing comes to mind ... But here " domestic fish» Shubin is an excellent film and a book of the same name about the evolution of vertebrates.

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Did Charles Darwin at the end of his life renounce his theory of human evolution? Did ancient people find dinosaurs? Is it true that Russia is the cradle of mankind, and who is the Yeti - is it not one of our ancestors who got lost in the centuries? Although paleoanthropology - the science of human evolution - is experiencing a rapid flowering, the origin of man is still surrounded by many myths. These are both anti-evolutionary theories and legends generated by popular culture, and near-scientific ideas that exist among educated and well-read people. Do you want to know how it was "really"? Alexander Sokolov, Chief Editor portal ANTROPOGENESIS.RU, collected a whole collection of such myths and checked how well-founded they are.

At the level of everyday logic, it is obvious that “a monkey is cooler than a person - it has two whole chromosomes more!”. Thus, “the origin of man from apes is finally refuted” ...

Let us remind our dear readers that chromosomes are the things in which DNA is packaged in our cells. A person has 23 pairs of chromosomes (23 we got from mom and 23 from dad. Total 46). The complete set of chromosomes is called a "karyotype". Each chromosome contains a very large DNA molecule tightly coiled.

It is not the number of chromosomes that matters, but the genes that these chromosomes contain. The same set of genes can be packed into different numbers of chromosomes.

For example, two chromosomes were taken and merged into one. The number of chromosomes has decreased, but the genetic sequence that is contained in them has remained the same. (Imagine that a wall was broken between two neighboring rooms. One large room turned out, but the content - furniture and parquet - is the same ...)

The fusion of chromosomes occurred in our ancestor. That is why we have two fewer chromosomes than chimpanzees, despite the fact that the genes are almost the same.

How do we know about the closeness of human and chimpanzee genes?

In the 1970s, when biologists learned to compare genetic sequences different types, this has been done for humans and chimpanzees. Specialists were in for a shock: “ The difference in the nucleotide sequences of the substance of heredity - DNA - in humans and chimpanzees as a whole was 1.1%,- wrote the famous Soviet primatologist E. P. Fridman in the book "Primates". - ... Frog species or squirrels within the same genus differ from each other 20–30 times more than chimpanzees and humans. It was so surprising that I had to urgently somehow explain the discrepancy between molecular data and what is known at the level of the whole organism.» .

And in 1980 in an authoritative magazine Science University of Minneapolis genetics team published The Striking Resemblance of High-Resolution G-Banded Chromosomes of Man and Chimpanzee.

The researchers used the latest methods of coloring chromosomes at that time (transverse stripes appear on the chromosomes different thickness and brightness; each chromosome is distinguished by its own set of stripes). It turned out that in humans and chimpanzees, the striation of chromosomes is almost identical! But what about the extra chromosome? And it’s very simple: if we put the 12th and 13th chromosomes of a chimpanzee in one line opposite the second human chromosome, connecting them at the ends, we will see that together they make up the second human.

Later, in 1991, researchers looked at the point of the alleged fusion on the second human chromosome and found there what they were looking for - DNA sequences characteristic of telomeres - the terminal sections of chromosomes. Another proof that there were once two in place of this chromosome!


But how does such a merger take place? Suppose one of our ancestors had two chromosomes combined into one. He got an odd number of chromosomes - 47, while the rest of the non-mutated individuals still have 48! And how did such a mutant then multiply? How can individuals with different numbers of chromosomes interbreed?

It would seem that the number of chromosomes clearly distinguishes between species and is an insurmountable obstacle to hybridization. What was the surprise of the researchers when, studying karyotypes various mammals, they began to show variation in the number of chromosomes within some species! So, in different populations of the common shrew, this figure can vary from 20 to 33. And the varieties of the musk shrew, as noted in the article by P. M. Borodin, M. B. Rogacheva and S. I. Oda, “differ from each other more than a person from a chimpanzee: animals living in the south of Hindustan and Sri Lanka , have 15 pairs of chromosomes in the karyotype, and all other shrews from Arabia to the islands of Oceania - 20 pairs ... It turned out that the number of chromosomes decreased because five pairs of chromosomes of a typical species merged with each other: 8th with 16th, 9? I am from the 13th, etc.”

Mystery! Let me remind you that during meiosis - cell division, as a result of which sex cells are formed - each chromosome in the cell must connect with its homologue pair. And here, when merged, an unpaired chromosome appears! Where should she go?

It turns out the problem is solved! PM Borodin describes this process, which he personally registered in 29 chromosome punares. Punare are bristly rats native to Brazil. Individuals with 29 chromosomes were obtained by crossing between 30 and 28 chromosome punare belonging to different populations of this rodent.

During meiosis in such hybrids, paired chromosomes successfully found each other. “And the remaining three chromosomes formed a triple: on the one hand, a long chromosome received from a 28 chromosome parent, and on the other, two shorter ones that came from a 30 chromosome parent. In this case, each chromosome stood in its place"

Genetics is a science that studies the laws of heredity and variability of all living beings. It is this science that gives us knowledge about the number of chromosomes in different types of organisms, the size of chromosomes, the location of genes on them, and how genes are inherited. Genetics also studies mutations that occur during the formation of new cells.

Chromosomal set

Every living organism (the only exceptions are bacteria) has chromosomes. They are located in every cell of the body in a certain amount. In all somatic cells, chromosomes are repeated twice, thrice or large quantity times, depending on the type of animal or variety of plant organism. In germ cells, the chromosome set is haploid, that is, single. This is necessary so that when two germ cells merge, the correct set of genes for the body is restored. However, even in the haploid set of chromosomes, the genes responsible for the organization of the whole organism are concentrated. Some of them may not appear in the offspring if the second germ cell contains stronger features.

How many chromosomes does a cat have?

You will find the answer to this question in this section. Each type of organism, plant or animal, contains a certain set of chromosomes. The chromosomes of one species of creatures have a certain length of the DNA molecule, a certain set of genes. Each such structure has its own size.

And dogs are our pets? A dog has 78 chromosomes. Knowing this number, is it possible to guess how many chromosomes a cat has? It's impossible to guess. Because there is no relationship between the number of chromosomes and the complexity of the organization of the animal. How many chromosomes does a cat have? There are 38 of them.

Chromosome size differences

The DNA molecule, with the same number of genes located on it, can have different lengths in different species.

Moreover, the chromosomes themselves different size. One information structure can contain a long or very short DNA molecule. However, chromosomes are not too small. This is due to the fact that when the daughter structures diverge, a certain weight of the substance is necessary, otherwise the divergence itself will not occur.

The number of chromosomes in different animals

As mentioned above, there is no relationship between the number of chromosomes and the complexity of the organization of the animal, because these structures have a different size.

How many chromosomes a cat has, the same number of other cats: a tiger, a jaguar, a leopard, a cougar and other representatives of this family. Many canids have 78 chromosomes. The same amount domestic chicken. The domestic horse has 64, and the Przewalski's horse has 76.

Humans have 46 chromosomes. The gorilla and chimpanzee have 48, while the macaque has 42.

A frog has 26 chromosomes. In the somatic cell of a pigeon there are only 16 of them. And in a hedgehog - 96. In a cow - 120. In a lamprey - 174.

Next, we present data on the number of chromosomes in the cells of some invertebrates. The ant, like the roundworm, has only 2 chromosomes in each somatic cell. A bee has 16 of them. A butterfly has 380 such structures per cell, and radiolarians have about 1600.

Animal data show different amount chromosomes. It must be added that Drosophila, which geneticists use in the course of genetic experiments, has 8 chromosomes in somatic cells.

The number of chromosomes in different plants

Vegetable world is also extremely diverse in the number of these structures. So, peas and clover each have 14 chromosomes. Onion - 16. Birch - 84. Horsetail - 216, and fern about 1200.

Differences between males and females

Males and females on genetic level differ in only one chromosome. In females, this structure looks like the Russian letter "X", and in males it looks like "Y". In some animal species, females have a "Y" chromosome, and males have an "X".

Traits found on such non-homologous chromosomes are inherited from father to son and from mother to daughter. The information that is fixed on the “Y” chromosome cannot be transferred to a girl, because a person who has this structure is necessarily male.

The same applies to animals: if we see a tricolor cat, we can say for sure that we have a female in front of us.

Because only the X chromosome, which belongs to females, has the corresponding gene. This structure is the 19th in the haploid set, that is, in germ cells, where the number of chromosomes is always two times less than in somatic ones.

The work of breeders

Knowing the structure of the apparatus that stores information about the body, as well as the laws of inheritance of genes and the features of their manifestation, breeders breed new plant varieties.

Wild wheat often has a diploid set of chromosomes. There are not many wild representatives that have a tetraploid set. Cultivated varieties often contain tetraploid and even hexaploid sets of structures in their somatic cells. This improves yield, weather resistance, and grain quality.

Genetics is an interesting science. The device of the apparatus containing information about the structure of the whole organism is similar in all living beings. However, each type of creature has its own genetic characteristics. One of the characteristics of a species is the number of chromosomes. In organisms of the same species, there is always a certain constant amount of them.