The number of chromosomes in different organisms is up to 100. Chromosomes. - But still, is it possible to cross a man with a monkey

Heredity and variability in wildlife exist due to chromosomes, genes, (DNA). It is stored and transmitted as a chain of nucleotides in DNA. What is the role of genes in this phenomenon? What is a chromosome in terms of the transmission of hereditary traits? Answers to such questions allow you to understand the principles of coding and genetic diversity on our planet. In many ways, it depends on how many chromosomes are included in the set, on the recombination of these structures.

From the history of the discovery of "particles of heredity"

Studying plant and animal cells under a microscope, many botanists and zoologists in the middle of the 19th century drew attention to the thinnest filaments and the smallest ring-shaped structures in the nucleus. More often than others, the German anatomist Walter Flemming is called the discoverer of chromosomes. It was he who used aniline dyes to process nuclear structures. Flemming called the discovered substance "chromatin" for its ability to stain. The term "chromosomes" was coined by Heinrich Waldeyer in 1888.

Simultaneously with Flemming, the Belgian Edouard van Beneden was looking for an answer to the question of what a chromosome is. A little earlier, German biologists Theodor Boveri and Eduard Strasburger conducted a series of experiments proving the individuality of chromosomes, the constancy of their number in different types living organisms.

Background of the chromosome theory of heredity

American researcher Walter Sutton found out how many chromosomes are contained in the cell nucleus. The scientist considered these structures to be carriers of units of heredity, signs of an organism. Sutton discovered that chromosomes are made up of genes that transmit properties and functions from parents to offspring. The geneticist in his publications gave descriptions of chromosome pairs, their movement in the process of division of the cell nucleus.

Regardless of the American colleague, Theodore Boveri led the work in the same direction. Both researchers in their writings studied the issues of transmission of hereditary traits, formulated the main provisions on the role of chromosomes (1902-1903). Further development Boveri-Sutton theory took place in the laboratory of Nobel laureate Thomas Morgan. The outstanding American biologist and his assistants established a number of regularities in the placement of genes in the chromosome, developed a cytological base that explains the mechanism of the laws of Gregor Mendel, the founding father of genetics.

Chromosomes in a cell

The study of the structure of chromosomes began after their discovery and description in the 19th century. These bodies and threads are found in prokaryotic organisms (non-nuclear) and eukaryotic cells (in nuclei). The study under a microscope made it possible to establish what a chromosome is from a morphological point of view. This is a mobile thread-like body, which is distinguishable in certain phases of the cell cycle. In interphase, the entire volume of the nucleus is occupied by chromatin. In other periods, chromosomes are distinguishable in the form of one or two chromatids.

These formations are better seen during cell divisions - mitosis or meiosis. In more often one can observe large chromosomes of a linear structure. They are smaller in prokaryotes, although there are exceptions. Cells often include more than one type of chromosome, for example, mitochondria and chloroplasts have their own small "heredity particles".

Shapes of chromosomes

Each chromosome has an individual structure, differs from other staining features. When studying morphology, it is important to determine the position of the centromere, the length and placement of the arms relative to the constriction. The set of chromosomes usually includes the following forms:

• metacentric, or equal arms, which are characterized by a median location of the centromere;
• submetacentric, or uneven shoulders (the constriction is shifted towards one of the telomeres);
• acrocentric, or rod-shaped, in them the centromere is located almost at the end of the chromosome;
• point with a hard-to-define shape.

Functions of chromosomes

Chromosomes are made up of genes, the functional units of heredity. Telomeres are the ends of the chromosome arms. These specialized elements serve to protect against damage and prevent fragments from sticking together. The centromere performs its tasks when the chromosomes are duplicated. It has a kinetochore, it is to it that the fission spindle structures are attached. Each pair of chromosomes is individual in the location of the centromere. The spindle fibers of division work in such a way that one chromosome leaves for daughter cells, and not both. Uniform doubling in the process of division is provided by the points of origin of replication. The duplication of each chromosome begins simultaneously at several such points, which noticeably speeds up the entire process of division.

Role of DNA and RNA

It was possible to find out what a chromosome is, what function this nuclear structure performs after studying its biochemical composition and properties. In eukaryotic cells, nuclear chromosomes are formed by a condensed substance called chromatin. According to the analysis, it contains high-molecular organic substances:

Nucleic acids are directly involved in the biosynthesis of amino acids and proteins, ensure the transfer of hereditary traits from generation to generation. DNA is found in the nucleus of eukaryotic cells, RNA is concentrated in the cytoplasm.

Genes

X-ray diffraction analysis showed that DNA forms a double helix, the chains of which consist of nucleotides. They are a deoxyribose carbohydrate, a phosphate group, and one of four nitrogenous bases:

Sections of helical deoxyribonucleoprotein filaments are genes that carry encoded information about the sequence of amino acids in proteins or RNA. During reproduction, hereditary traits are passed from parents to offspring in the form of gene alleles. They determine the functioning, growth and development of a particular organism. According to a number of researchers, those sections of DNA that do not encode polypeptides perform regulatory functions. The human genome can contain up to 30,000 genes.

Set of chromosomes

The total number of chromosomes, their features - feature kind. In fruit flies, their number is 8, in primates - 48, in humans - 46. This number is constant for cells of organisms that belong to the same species. For all eukaryotes, there is the concept of "diploid chromosomes". This is full set, or 2n, as opposed to haploid - half the number (n).

Chromosomes in one pair are homologous, identical in shape, structure, location of centromeres and other elements. Homologues have their own characteristic features that distinguish them from other chromosomes in the set. Staining with basic dyes allows you to see, study distinctive features each pair. is present in the somatic - in the sex (the so-called gametes). In mammals and other living organisms with a heterogametic male sex, two types of sex chromosomes are formed: the X chromosome and Y. Males have a set of XY, females - XX.

Human chromosome set

The cells of the human body contain 46 chromosomes. All of them are combined into 23 pairs that make up the set. There are two types of chromosomes: autosomes and sex chromosomes. The first form 22 pairs - common for women and men. The 23rd pair differs from them - the sex chromosomes, which are non-homologous in the cells of the male body.

Genetic traits are related to gender. They are transmitted by a Y and an X chromosome in men, two Xs in women. Autosomes contain the rest of the information about hereditary traits. There are techniques that allow you to individualize all 23 pairs. They are clearly visible in the drawings when painted in specific color. It is noticeable that the 22nd chromosome in the human genome is the smallest. Its stretched DNA is 1.5 cm long and has 48 million base pairs. Special histone proteins from the composition of chromatin perform compression, after which the thread occupies thousands of times less space in the cell nucleus. Under an electron microscope, the histones in the interphase nucleus resemble beads strung on a strand of DNA.

Genetic diseases

There are more than 3 thousand hereditary diseases different type caused by damage and abnormalities in the chromosomes. Down syndrome is one of them. A child with such a genetic disease is characterized by a lag in mental and physical development. With cystic fibrosis, there is a malfunction in the functions of the external secretion glands. Violation leads to problems with sweating, secretion and accumulation of mucus in the body. It makes it difficult for the lungs to work, and can lead to suffocation and death.

Violation of color vision - color blindness - immunity to certain parts of the color spectrum. Hemophilia leads to a weakening of blood clotting. Lactose intolerance prevents the human body from absorbing milk sugar. In family planning offices, you can find out about the predisposition to a particular genetic disease. In large medical centers there is an opportunity to undergo appropriate examination and treatment.

Gene therapy is a direction of modern medicine, elucidation of the genetic cause of hereditary diseases and its elimination. Via latest methods normal genes are introduced into pathological cells instead of disturbed ones. In this case, doctors relieve the patient not of the symptoms, but of the causes that caused the disease. Only correction of somatic cells is carried out, methods of gene therapy are not yet applied massively in relation to germ cells.

containing genes. The name "chromosome" comes from Greek words(chrōma - color, color and sōma - body), and is due to the fact that during cell division they are intensely stained in the presence of basic dyes (for example, aniline).

Many scientists, since the beginning of the 20th century, have thought about the question: “How many chromosomes does a person have?”. So until 1955, all the "minds of mankind" were convinced that the number of chromosomes in a person is 48, i.e. 24 couples. The reason was that Theophilus Painter (a Texas scientist) incorrectly counted them in preparative sections of human testes, by court order (1921). In the future, other scientists, using different methods of counting, also came to this opinion. Even having developed a method for separating chromosomes, the researchers did not challenge Painter's result. The error was discovered by scientists Albert Levan and Jo-Hin Tjo in 1955, who accurately calculated how many pairs of chromosomes a person has, namely 23 (more than modern technology).

Somatic and germ cells contain a different set of chromosomes in biological species, which cannot be said about the morphological features of chromosomes, which are constant. have a doubled (diploid set), which is divided into pairs of identical (homologous) chromosomes, which are similar in morphology (structure) and size. One part is always paternal, the other maternal. Human germ cells (gametes) are represented by a haploid (single) set of chromosomes. When an egg is fertilized, they unite in one nucleus of the zygote of haploid sets of female and male gametes. This restores the double set. It is possible to say with accuracy how many chromosomes a person has - there are 46 of them, while 22 pairs of them are autosomes and one pair is sex chromosomes (gonosomes). Sexual differences have both morphological and structural (composition of genes). At female body a pair of gonosomes contains two X chromosomes (XX pair), while the male has one X and one Y chromosome each (XY pair).

Morphologically, chromosomes change during cell division, when they double (with the exception of germ cells, in which doubling does not occur). This is repeated many times, but no change in the chromosome set is observed. Chromosomes are most visible at one of the stages of cell division (metaphase). In this phase, the chromosomes are represented by two longitudinally split formations (sister chromatids), which narrow and unite in the region of the so-called primary constriction, or centromere (an obligatory element of the chromosome). Telomeres are the ends of a chromosome. Structurally, human chromosomes are represented by DNA (deoxyribonucleic acid), which encodes the genes that make up them. Genes, in turn, carry information about some a certain sign.

How many chromosomes a person has will depend on his individual development. There are such concepts as: aneuploidy (change in the number of individual chromosomes) and polyploidy (the number of haploid sets is more than diploid). The latter can be of several types: the loss of a homologous chromosome (monosomy), or the appearance (trisomy - one extra, tetrasomy - two extra, etc.). All this is a consequence of genomic and chromosomal mutations that can lead to such pathological conditions like: Klinefelter syndrome, Shereshevsky-Turner syndrome and other diseases.

Thus, only the twentieth century gave answers to all questions, and now every educated inhabitant of the planet Earth knows how many chromosomes a person has. It is on what will be the composition of the 23rd pair of chromosomes (XX or XY) that the sex of the unborn child depends, and this is determined during the fertilization and fusion of the female and male sex cells.

First, let's agree on terminology. Human chromosomes were finally counted a little more than half a century ago - in 1956. Since then we have known that somatic, that is, not germ cells, there are usually 46 of them - 23 pairs.

Chromosomes in a pair (one received from the father, the other from the mother) are called homologous. They contain genes that perform the same functions, but often differ in structure. The exception is the sex chromosomes - X and Y, the gene composition of which does not completely coincide. All other chromosomes except the sex chromosomes are called autosomes.

Number of sets of homologous chromosomes - ploidy- in germ cells it is equal to one, and in somatic cells, as a rule, two.

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.

Share honestly

Most often, the wrong number of chromosomes is the result of unsuccessful cell division. In somatic cells, after DNA duplication, the maternal chromosome and its copy are linked together by cohesin proteins. Then protein complexes of kinetochore sit on their central parts, to which microtubules are later attached. When dividing along microtubules, kinetochores disperse to different poles of the cell and pull chromosomes along with them. If the cross-links between copies of the chromosome are destroyed ahead of time, then microtubules from the same pole can attach to them, and then one of the daughter cells will receive an extra chromosome, and the second will remain deprived.

Meiosis also often passes with errors. The problem is that the construction of linked two pairs of homologous chromosomes can twist in space or separate in the wrong places. The result will again be an uneven distribution of chromosomes. Sometimes the sex cell manages to track this so as not to transmit the defect by inheritance. Extra chromosomes are often misfolded or broken, which triggers the death program. For example, among spermatozoa there is such a selection for quality. But the eggs were less fortunate. All of them are formed in humans even before birth, prepare for division, and then freeze. Chromosomes are already doubled, tetrads are formed, and division is delayed. In this form, they live until the reproductive period. Then the eggs mature in turn, divide for the first time and freeze again. The second division occurs immediately after fertilization. And at this stage, it is already difficult to control the quality of the division. And the risks are greater, because the four chromosomes in the egg remain cross-linked for decades. During this time, breakdowns accumulate in cohesins, and chromosomes can spontaneously separate. Therefore, the older the woman, the greater the likelihood of incorrect chromosome divergence in the egg.

Aneuploidy in germ cells inevitably leads to aneuploidy of the embryo. When a healthy egg with 23 chromosomes is fertilized by a sperm with an extra or missing chromosome (or vice versa), the number of chromosomes in the zygote will obviously be different from 46. But even if the germ cells are healthy, this does not guarantee healthy development. In the first days after fertilization, the cells of the embryo actively divide in order to quickly gain cell mass. Apparently, in the course of rapid divisions, there is no time to check the correctness of chromosome segregation, so aneuploid cells can arise. And if an error occurs, then the further fate of the embryo depends on the division in which it happened. If the balance is disturbed already in the first division of the zygote, then the whole organism will grow aneuploid. If the problem arose later, then the outcome is determined by the ratio of healthy and abnormal cells.

Some of the latter may die further, and we will never know about their existence. Or he can take part in the development of the body, and then he will succeed mosaic Different cells will carry different genetic material. Mosaicism causes a lot of trouble for prenatal diagnosticians. For example, at the risk of having a child with Down syndrome, sometimes one or more embryonic cells are removed (at the stage when this should not be dangerous) and the chromosomes are counted in them. But if the embryo is mosaic, then this method becomes not particularly effective.

Third wheel

All cases of aneuploidy are logically divided into two groups: deficiency and excess of chromosomes. The problems that arise with a deficiency are quite expected: minus one chromosome means minus hundreds of genes.

If the homologous chromosome is working normally, then the cell can get away with only an insufficient amount of proteins encoded there. But if some of the genes remaining on the homologous chromosome do not work, then the corresponding proteins will not appear in the cell at all.

In the case of an excess of chromosomes, everything is not so obvious. There are more genes, but here - alas - more does not mean better.

First, extra genetic material increases the load on the nucleus: an additional strand of DNA must be placed in the nucleus and served by information reading systems.

Scientists have found that in people with Down syndrome, whose cells carry an extra 21st chromosome, the work of genes located on other chromosomes is mainly disrupted. Apparently, an excess of DNA in the nucleus leads to the fact that there are not enough proteins that support the work of chromosomes for everyone.

Secondly, the balance in the amount of cellular proteins is disturbed. For example, if activator proteins and inhibitor proteins are responsible for some process in the cell, and their ratio usually depends on external signals, then an additional dose of one or the other will cause the cell to stop responding adequately to the external signal. Finally, an aneuploid cell has an increased chance of dying. When duplicating DNA before division, errors inevitably occur, and the cellular proteins of the repair system recognize them, repair them, and start doubling again. If there are too many chromosomes, then there are not enough proteins, errors accumulate and apoptosis is triggered - programmed cell death. But even if the cell does not die and divides, then the result of such division is also likely to be aneuploids.

You will live

If even within a single cell, aneuploidy is fraught with disruption and death, then it is not surprising that it is not easy for an entire aneuploid organism to survive. At the moment, only three autosomes are known - 13, 18 and 21, trisomy for which (that is, an extra, third chromosome in cells) is somehow compatible with life. This is probably due to the fact that they are the smallest and carry the fewest genes. At the same time, children with trisomy on the 13th (Patau syndrome) and 18th (Edwards syndrome) chromosomes live at best up to 10 years, and more often live less than a year. And only trisomy 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 additional Y-chromosome itself does not affect the functioning of the body much. 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 the mental abilities of men and women.

Who benefits from being wrong

Despite the fact that medicine has been familiar with chromosomal abnormalities for a long time, recently aneuploidy continues to attract the attention of scientists. It turned out that more than 80% of tumor cells contain an unusual number of chromosomes. On the one hand, the reason for this may be the fact that proteins that control the quality of division are able to slow it down. In tumor cells, these very control proteins often mutate, so division restrictions are removed and chromosome checking does not work. On the other hand, scientists believe that this may serve as a factor in the selection of tumors for survival. According to this model, tumor cells first become polyploid, and then, as a result of division errors, they lose different chromosomes or their parts. It turns out a whole population of cells with a wide variety of chromosomal abnormalities. Most of them are not viable, but some may accidentally succeed, for example, if they accidentally get extra copies of genes that start division, or lose genes that suppress it. However, if the accumulation of errors during division is additionally stimulated, then the cells will not survive. 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.

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 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 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 on early stage 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.

findings

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 problems with thyroid gland; 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 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 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). 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.