How many chromosomes does a cat have? Genetics provides data on various genomes. DNA record holders: how the human and worm genomes relate to each other The number of chromosomes in different animal species

MOSCOW, 4 Jul— RIA Novosti, Anna Urmantseva. Who has the larger genome? As you know, some creatures have a more complex structure than others, and since everything is written in DNA, then this should also be reflected in its code. It turns out that a person with his developed speech must be more complicated than a small round worm. However, if we compare us with a worm in terms of the number of genes, it will turn out to be about the same: 20 thousand Caenorhabditis elegans genes versus 20-25 thousand Homo sapiens.

Even more offensive for the "crown of earthly creatures" and the "king of nature" are comparisons with rice and corn - 50 thousand genes in relation to human 25.

However, maybe we don't think so? Genes are "boxes" in which nucleotides are packed - "letters" of the genome. Maybe count them? Humans have 3.2 billion base pairs. But the Japanese raven eye (Paris japonica) - a beautiful plant with white flowers - has 150 billion base pairs in its genome. It turns out that a person should be arranged 50 times simpler than a flower.

And the lung-breathing protopter fish (lung-breathing - having both gill and pulmonary breathing), it turns out, is 40 times more difficult than a person. Maybe all fish are somehow more difficult than people? No. poisonous fish Fugu, from which the Japanese make a delicacy, has a genome eight times smaller than that of a human, and 330 times smaller than that of the lungfish protopter.
It remains to count the chromosomes - but this confuses the picture even more. How can a person be equal in number of chromosomes to an ash tree, and a chimpanzee to a cockroach?

These paradoxes have been faced by evolutionary biologists and geneticists for a long time. They were forced to admit that the size of the genome, no matter how we try to calculate it, is strikingly unrelated to the complexity of organisms. This paradox has been called the "C value puzzle", where C is the amount of DNA in a cell (C-value paradox, the exact translation is "genome size paradox"). And yet, there are some correlations between species and kingdoms.

© RIA Novosti illustration. A.Polyanina

© RIA Novosti illustration. A.Polyanina

It is clear, for example, that eukaryotes (living organisms whose cells contain a nucleus) have on average larger genomes than prokaryotes (living organisms whose cells do not contain a nucleus). Vertebrates have, on average, larger genomes than invertebrates. However, there are exceptions that no one has yet been able to explain.

There have been suggestions that genome size is related to duration life cycle organism. Some scientists have argued for plants that perennial species have larger genomes than annual ones, and usually by several times the difference. And the smallest genomes belong to ephemeral plants, which go through a full cycle from birth to death within a few weeks. This issue is now being actively discussed in scientific circles.

Explains the leading researcher at the Institute of General Genetics. N. I. Vavilova Russian Academy Sciences, Professor of the Texas Agromechanical University and the University of Göttingen Konstantin Krutovsky: "The size of the genome is not related to the duration of the life cycle of the organism! For example, there are species within the same genus that have the same genome size, but can differ in life expectancy by tens, if not hundreds of times In general, there is a relationship between genome size and evolutionary advancement and complexity of organization, but with many exceptions.Generally, genome size is associated with the ploidy (copy) of the genome (moreover, polyploids are found in both plants and animals) and the amount of highly repetitive DNA (simple and complex repeats, transposons and other mobile elements).

There are also scientists who take a different point of view on this issue.

Real hedgehogs. Small and medium sized mammals. Body length 13-27 cm. Tail length 1-5 cm. The dorsal surface of the body is covered with needles that extend to the sides. Between the needles are thin, long, very sparse hair.

The ventral side of the body has no needles and is replaced by long and coarse hair. The head is relatively large, wedge-shaped, with a slightly elongated facial region. The auricles are wide and rounded at the base. Their length never exceeds half the length of the head. Coloring the dorsal side of the body is very variable: chocolate-brown or almost black, sometimes almost white. The ventral surface is usually brownish or grayish. The skull is somewhat flattened in the dorso-ventral direction, with an expanded braincase, widely spaced strong zygomatic arches and a shortened rostral part, which has a rather significant width. Bony auditory drums are small in size, flattened. dental formula: I 3/2 C 1/2 P 3/2 M 3/3 = 36.
At hedgehog diploid number of chromosomes 48.

inhabitants various landscapes. They avoid strongly swampy places and solid massifs of tall forests. They prefer forest edges, clearings, thickets of bushes. They are found in the forest-steppe and in the steppe. Activity is predominantly twilight and nocturnal. For the winter, an ordinary hedgehog arranges a ground nest, collecting dry grass and leaves in a heap. The nest is located under heaps of deadwood, under the roots of trees. In October - November, it hibernates, continuing until warm spring days.

By nature of food omnivorous. They eat various invertebrates and vertebrates (mouse-like rodents, lizards, frogs, various insects, their larvae), as well as some plant objects (fruits). Mating in the common hedgehog in the northern part of the range occurs in the spring, shortly after waking up from hibernation. In the tropics, representatives of the genus have no seasonality in reproduction. The common hedgehog has one litter during the year.

Pregnancy approximately 5-6 weeks. The female brings from 3 to 8 cubs (usually about 4). Newborns of the common hedgehog weigh an average of 12 g and have clearly visible needles in the head area. By 15 days, their spiny cover is already well expressed. The eyes open on the 14-18th day after birth. Maturity occurs in the 2nd year of life. Lifespan approximately 6 years old.

Spreading covers Europe, Central Asia, North and Northeast China, the Korean Peninsula and Africa from Morocco and Libya to Angola. The common hedgehog is acclimatized in New Zealand.

The taxonomy of the genus has not been finally established, usually 5 species are distinguished.

The common hedgehog lives in our country (from the northern shores of Lake Ladoga south to the Crimea and the Caucasus, inclusive, in the western regions of northern Kazakhstan, in Western Siberia, in the southern part of the Amur Region and Primorsky Territory) and

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 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. 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 to best case up to 10 years, and often live less than a year. And only trisomy for 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 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.

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 (“chrome” - color in Greek) were described as early as the end of the 19th 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. Females of the Indian muntjac have 6 chromosomes, males have 7, and their relative, the Siberian roe deer, has more than 70 (or rather, 70 chromosomes of the main set and even 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's syndrome (1 per 1000) - characteristic external signs, delayed cognitive development, short life expectancy, may 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? Are there any interesting examples of crossing animals with different numbers of 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, too many genetic differences have accumulated for the carrier of such a mixed genome to be able 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 Shubin's "Inner Fish" is an excellent film and a book of the same name about the evolution of vertebrates.