The number of chromosomes in different organisms. DNA record holders: how the human and worm genomes relate to each other. General about animal chromosomes

​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. 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? Is there interesting examples 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 was unusual in the field of chromosomes was discovered in recent times?

- 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 here " domestic fish» Shubin is an excellent film and a book of the same name about the evolution of vertebrates.

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 of different thickness and brightness appear on the chromosomes; at the same time, each chromosome differs in its own special 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 the karyotypes of various mammals, they began to find a scatter in the number of chromosomes within some species! So, in different populations common shrew this figure can walk 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-chromosomal punare belonging to different populations 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"

Bad ecology, life in constant stress, the priority of a career over a family - all this reflects badly 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 birth defects. 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) a structure inside the nucleus of a eukaryotic cell (that is, those living beings whose cells have a nucleus). Its main function is storage, transmission and implementation 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 chromosome set healthy body

At normal person 23 pairs of chromosomes, each of which is responsible for a specific gene. There are 46 (23x2) in total - how many chromosomes do healthy person. 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 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 older 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 ( large quantity 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. AT without fail the test is shown to women in labor after 35 years; parents whose relatives were susceptible to similar diseases; patients with problems with thyroid gland; women who have had miscarriages.

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) - 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, 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 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.

The presenter explains Researcher 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.