The set of chromosomes in mammals. How many chromosomes do different animals have. White color of cats

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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 puffer fish, from which the Japanese prepare a delicacy, has a genome eight times smaller than that of a person, 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, genomes larger 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 the length of an organism's life cycle. 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 of the Russian Academy of 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 may differ in lifespan in 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 number) 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.

Bad ecology, life in constant stress, the priority of a career over a family - all this has a bad effect on a person's ability to bring healthy offspring. It is regrettable, but about 1% of babies born with serious disorders in the chromosomal set grow up mentally or physically retarded. In 30% of newborns, deviations in the karyotype lead to the formation of congenital malformations. Our article is devoted to the main issues of this topic.

The main carrier of hereditary information

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

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

Formation of the chromosome set of a healthy organism

A normal person has 23 pairs of chromosomes, each of which is responsible for a specific gene. There are 46 (23x2) in total - how many chromosomes a healthy person has. One chromosome is inherited from our father, the other is inherited from our mother. The exception is 23 pairs. She is responsible for the gender of a person: female is designated as XX, and male as XY. When chromosomes are paired, this is a diploid set. In germ cells, they are separated (haploid set) before the next connection during fertilization.

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

Deviations in the karyotype

All karyotype disorders in the classification are traditionally divided into two classes: genomic and chromosomal.

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

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

Let us dwell in more detail on the most well-known and widespread genomic diseases.

Down syndrome

It was described as early as 1866. For every 700 newborns, as a rule, there is one baby with a similar disease. The essence of the deviation is that the third chromosome joins the 21st pair. This happens when there are 24 chromosomes in the germ cell of one of the parents (with a doubled 21). In a sick child, as a result, there are 47 of them - that's how many chromosomes a Down person has. This pathology is promoted by viral infections or ionizing radiation transferred by parents, as well as diabetes.

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

The risk of having a sick child is higher, the older the parents. Currently, there are technologies that allow to recognize a chromosomal disorder at an early stage of pregnancy. Older couples need to pass a similar test. He will not interfere with young parents, if in the family of one of them there were patients with Down syndrome. The mosaic form of the disease (the karyotype of a part of the cells is damaged) is formed already at the stage of the embryo and does not depend on the age of the parents.

Patau Syndrome

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

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

The infant mortality rate for this disease is about 70%. Most of them do not live up to 3 years. Individuals prone to this syndrome most often have heart and / or brain defects, problems with other internal organs (kidneys, spleen, etc.).

Edwards syndrome

Most babies with 3 eighteenth chromosomes die shortly after birth. They have pronounced malnutrition (digestion problems that prevent the child from gaining weight). The eyes are set wide, the ears are low. Often there is a heart defect.

conclusions

In order to prevent the birth of a sick child, it is desirable to undergo special examinations. Without fail, the test is shown to women in labor after 35 years; parents whose relatives were susceptible to similar diseases; patients with thyroid problems; women who have had miscarriages.

​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 in simple terms what a chromosome is?

- 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 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 breaks and mergers of chromosomes was quite intensive, and now we are observing the results of this process, when often each species has characteristic features of the 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 humans 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). The red wolf in America has long been considered a separate species, 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.

Humans have 23 pairs of chromosomes, while higher apes have 24. It turns out (geneticists are increasingly inclined to this) that the second pair of human chromosomes was formed from the fusion of pairs of other chromosomes of ancestral anthropoids, which is also shown in the figure presented at the beginning of the chapter. That's 48 Pongid chromosomes versus 46 humans! Paris Conference of Geneticists and 1971 and 1975 approved a very illustrative table of the homology of human chromosomes and three great apes. It shows: the chimpanzee is our closest relative with almost the same karyotype as ours (the pygmy chimpanzee is especially close to us in terms of chromosomes).

But one should not think that other monkeys, including the lower ones, are very far from humans in terms of the structure of chromosomes. Many marmosets, some callicebuses, uakari, even the vari lemur have the same number of chromosomes as humans - 46 (double set); in capuchins - 54; in howler monkeys - 44-52 (different species); monkeys - from 48 to 72; in macaques and baboons - 42; langurs have 44; most gibbons have 44 (the siamang has 50). But the relationship of primates is estimated, of course, not only by the number of chromosomes. If you "stretch" all the chromosomes of each species into one line, it turns out to be the same length for all primate species. Only the number of centromeres (that is, in fact, the number of chromosomes) and the distribution of arms change. They have the same total amount of the substance of heredity - DNA.

In the 60s. a great similarity between the karyotypes of humans and many species of lower apes has been established. When studying the phylogeny of chromosomes of 60 species of primates from mouse microcebus to humans, the French geneticist B. Dutrillo (1979) established a complete analogy, approximately 70% of non-repeating colored bands. Proof of close similarity and relationship are also "human" genetic diseases in monkeys: Down's syndrome, alkaptonuria, developmental anomalies. The histocompatibility complex (tissue affinity required for organ transplantation) is localized in the genes on the chromosomes of chimpanzees, gorillas, orangutans and rhesus monkeys in the same way - the coloration of these areas in monkeys is completely identical to the pattern on human chromosome 6. The genes "responsible" for coding five vital enzymes in capuchin are located on chromosomes 2, 9 and 15 - they are encoded in exactly the same way in human chromosomes of the same structure, but with a different numbering.

But, of course, the greatest similarity of chromosomes was found in humans with chimpanzees - it reaches 90-98% (according to different authors). It is curious to remember: two species of monkeys, representatives of the same genus - the Brass monkey (diploid chromosome set 62) and the talapoin monkey (chromosome 54) are homologous only in 10 pairs of chromosomes, i.e., much less related than humans and chimpanzees.

Now, after considering the main, fundamental features of the similarity of humans and monkeys in terms of chromosomes, the relationship of primates and other indicators related to genetic relationship will be clear. As we remember, genes and their receptacle - chromosomes - are sections of nuclear (hence, nucleic) acids present in each cell, more precisely, deoxyribonucleic acid (DNA). Already in the 60s, immediately after the great discoveries of the 50s. In the 20th century, when the role and structure of DNA was established, its intensive study and comparison in different organisms began. So, they learned to hybridize the DNA of different species. If it is heated, it, normally double-stranded, "unwinds" into single strands, on which you can "grow" (overlay) the same strand of DNA from another animal, if it has similar genes. When these strands cool down, they will curl up again into a double common helix, but only as much as the organisms that host these two DNAs are related.

It turned out that human and bird DNA hybridize by 10%, human and mouse - by 19%, human and larger mammals - by 30-40%, but human and rhesus monkey - by 66-74%.

As for the chimpanzee, here, as mentioned, the hybrid with human DNA reaches, according to various authors, up to 90-98%. The temperature at which this spliced ​​DNA "melts" (it is different for hybrids of different proximity and therefore also an indicator of the relationship of their hosts) fully confirms the special closeness of humans to other primates.

When the rapidly evolving DNA of non-nuclear formations of the cell - mitochondria - was discovered, skeptics expressed doubts about the reliability of the data obtained on the basis of comparisons of nuclear DNA (although it is well known that it is the main material of chromosomes localized, as said, in the cell nucleus): after all, DNA mitochondria, according to some authors, changes 5-10 times faster than nuclear and, thus, presents us with genetic changes, as it were, in an enlarged form.

Californian biochemists conducted a study (in which Alan Wilson, already known to us, participated) specifically to study the DNA of mitochondria. The method used by them is extremely accurate. It is based on the determination of DNA sections cleaved by highly specific enzymes - restriction endonucleases. These enzymes recognize strictly defined sequences of DNA nucleotides and cut the molecule only in these places. As a result, even minor changes in the composition or order of nucleotides become available for analysis.

By building maps of sites (or, as scientists say, sites) of the action of various restriction enzymes, one can analyze very closely related DNA molecules, for example, subtypes of the same virus, etc. And all the same result - an extraordinary relationship! And to the same extent, which is established by biochemical and genetic methods already known to the reader, chimpanzees and gorillas are as close as possible to humans. Further away is the orangutan, a little further - the gibbons.

The same conclusion was made when studying the "satellite", satellite DNA of chromosomes, when mapping the interferon gene family, etc.

After such a great similarity in chromosomes (DNA), no one can be surprised by the "striking" similarity of blood proteins and tissues of humans and monkeys - after all, they, proteins, receive a "program" from the parental substances encoding them, which are so close, as we have seen. , i.e. from genes, from DNA! Proteins are now mainly studied along with immunological methods by determining the sequence of amino acids, the order, the alternation of which, as it became known also in the 50s, constitutes the "physiognomy" of each protein.

We have already seen the level of similarity of albumin protein in humans and various animals. In general, it is detected in approximately the same sequence for other proteins, but sometimes it is higher - according to these indicators, African anthropoids are closer to humans. Here is the data on transferrin - immunological similarity is expressed as a percentage as follows: in humans with chimpanzees and gorillas - 100% (complete identity!), With Old World monkeys - from 50 to 75, with other animals - either below 4%, or zero, lack of resemblance. Professor G. A. Annenkov reasonably suggested that "a high degree of identity in structure and function extends to many blood serum proteins of all (or most) primates."

And here are the data on low-density lipoproteins, which play a crucial role in the development of atherosclerosis: their immunological similarity in humans with reptiles and fish is 1-10%, with birds - 10, with pigs - 35-58, with various narrow-nosed monkeys - 80-85 , with chimpanzees - more than 90%. Another related blood component, apolipoprotein, is also, according to immunological studies, homologous in humans and various monkeys, but indistinguishable in the plasma of humans, chimpanzees, and gorillas.

The similarity of man and monkeys in the structure and properties of many hormones is incomparable with any other animals. Growth hormone is very species-specific, but is the same in humans and even macaques. Introduced to a child from monkeys, it will act as effectively as the same hormone from humans (established by Nobel laureate American Lee Cho Hao). Almost complete identity was established recently (Watechem et al., 1982) when studying the nucleotide sequence of DNA encoding the hormone insulin of humans and cynomolgus monkeys, in the hormone itself, in its protein, only two substitutions in the amino acid sequence can be found.

As shown by Sukhumi endocrinologists N. P. Goncharov, G. V. Katsia, V. Yu. Butnev, there are no animals in nature that are as close to humans as monkeys, in particular baboons, by the nature of the exchange of steroid hormones produced by the adrenal glands and playing a colossal role in the reproductive system. Mice, rabbits, rats, which, I note, are constantly used in research on steroidogenesis, produce the hormone corticosterone in the greatest amount, while in humans and monkeys, cortisol is the predominant hormone of this group. The ratio of the two named hormones in both primates is almost the same and differs strikingly from their proportions in rodents.