Male chromosomes. Which chromosome is responsible for the sex of the unborn child? Mysteries of the Y chromosome: a fragile creature that will soon disappear

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Subtitles

Genes, DNA and chromosomes are what make us unique. They are a set of instructions passed down to you from your father and mother. These instructions are found in your cells. And all living organisms are made up of cells. There are many types of cells - nerve cells, hair cells or skin cells. They all differ in shape and size, but each has certain components. The cell has an outer boundary called the membrane, which contains a fluid called the cytoplasm. The cytoplasm contains the nucleus, in which the chromosomes are located. Each human cell usually has 23 pairs of chromosomes, or 46 in total. 22 pairs of these are called autosomes and are the same in men and women. The 23rd pair are sex chromosomes; they are different in men and women. Women have 2 X chromosomes, men have one X and one Y chromosome. Chromosomes are long molecules of DNA - deoxyribonucleic acid. The shape of DNA resembles a twisted ladder. And it's called a double helix. The steps in the ladder are 4 bases: Adenine - A Thymine - T Guanine - G And Cytosine - C A section of DNA is called a gene. The body reads genes as recipes for making proteins. The length and order of bases in the DNA of genes determines the size and shape of the resulting proteins. The size and shape of a protein determine its function in the body. Proteins make up the cells that form the tissues that make up organs, such as our eyes or skin. Thus, genes determine whether you are a cow, an apple or a person and what you look like - the color of your hair, skin, eyes and everything else.

General information

The cells of most mammals contain two sex chromosomes: a Y chromosome and an X chromosome in males, two X chromosomes in females. In some mammals, such as the platypus, sex is determined not by one, but by five pairs of sex chromosomes. At the same time, the sex chromosomes of the platypus are more similar to the Z chromosome of birds, and the SRY gene is probably not involved in its sexual differentiation.

Origin and evolution

Before the appearance of the Y chromosome

Recombination inhibition

Ineffective selection

If genetic recombination is possible, the genome of the offspring will differ from the parent. In particular, a genome with fewer deleterious mutations can be obtained from parental genomes with a large number of deleterious mutations.

If recombination is impossible, then if a certain mutation appears, it can be expected that it will appear in future generations, since the process of reverse mutation is unlikely. For this reason, in the absence of recombination, the number of harmful mutations increases over time. This mechanism is called a Möller ratchet.

Part of the Y chromosome (95% in humans) is incapable of recombination. It is believed that this is one of the reasons why she is susceptible to gene damage.

Y chromosome age

Until recently, it was believed that the X and Y chromosomes appeared about 300 million years ago. However, recent research, particularly sequencing of the platypus genome, suggests that chromosomal sex determination was absent as early as 166 million years ago, with the divergence of monotremes from other mammals. This re-evaluation of the age of the chromosomal sex determination system is based on studies showing that sequences on the X chromosome of marsupials and placental mammals are present in the autosomes of the platypus and birds. More old rating was based on erroneous reports of the presence of these sequences in the X chromosome of the platypus.

Human Y chromosome

In humans, the Y chromosome consists of more than 59 million base pairs, accounting for almost 2% of the human DNA in the cell nucleus. The chromosome contains just over 86 genes, which encode 23 proteins. The most significant gene on the Y chromosome is the SRY gene, which serves as a genetic “switch” for the development of the organism according to male type. Traits inherited through the Y chromosome are called holandric.

The human Y chromosome is unable to recombine with the X chromosome, except for small pseudoautosomal regions at the telomeres (which make up about 5% of the chromosome length). These are relict areas of ancient homology between the X and Y chromosomes. The main part of the Y chromosome that is not subject to recombination is called NRY. non-recombining region of the Y chromosome) . This part of the Y chromosome allows one to determine direct paternal ancestors through the evaluation of single-nucleotide polymorphisms.

see also

Sources

1. Grützner F, Rens W, Tsend-Ayush E; et al. (2004). “In the platypus a meiotic chain of ten sex chromosomes shares genes with the bird Z and mammal X chromosomes.” Nature. 432 : 913-917. DOI:10.1038/nature03021.
2. Warren WC, Hillier LDW, Graves JAM; et al. (2008). “Genome analysis of the platypus reveals unique signatures of evolution” . Nature. 453 : 175-183. DOI:10.1038/nature06936.
3. Veyrunes F, Waters PD, Miethke P; et al. (2008). “Bird-like sex chromosomes of platypus imply recent origin of mammal sex chromosomes” . Genome Research. 18 : 965-973. DOI:10.1101/gr.7101908.
4. Lahn B, Page D (1999). “Four evolutionary strata on the human X chromosome.” Science. 286 (5441): 964-7. DOI:10.1126/science.286.5441.964. PMID.
5. Graves J.A.M. (2006). “Sex chromosome specialization and degeneration in mammals.” Cell. 124 (5): 901-14. DOI:10.1016/j.cell.2006.02.024. PMID.
6. Graves J. A. M., Koina E., Sankovic N. (2006). “How the gene content of human sex chromosomes evolved.” Curr Opin Genet Dev. 16 (3): 219-24. DOI:10.1016/j.gde.2006.04.007. PMID.
7. Graves J.A. The degenerate Y chromosome--can conversion save it? (English) // Reproduction, fertility, and development. - 2004. - Vol. 16, no. 5 . - P. 527-534. - DOI:10.10371/RD03096. - PMID 15367368.[to correct ]

How does the process of birth of men and women take place? The X and Y chromosomes are responsible for this. And it all begins when 400 million sperm rush to search for an egg. It's not that much difficult task, as it might seem at first glance. IN human body The egg can be compared to a huge star, towards which small sperm star fighters are rushing from all sides.

Now let's talk about chromosomes. They contain all the information necessary for the creation of man. A total of 46 chromosomes are needed. They can be compared to 46 thick volumes of an encyclopedia. Each person receives 23 chromosomes from their mother, and the remaining 23 from their father. But only 2 are responsible for sex, and one must be the X chromosome.

If you get a set of 2 X chromosomes, you will use the women's restroom for the rest of your life. But if the set consists of X and Y, then in this case you are doomed to go to the men's room for the rest of your days. At the same time, you need to know that the man bears full responsibility for gender, since the Y chromosome is contained only in the sperm, and it is absent in the egg. So the birth of boys or girls is entirely dependent on male genetic material.

A remarkable fact is that to recreate the male sex, the Y chromosome is not needed at all. Only an initial push is needed to start the development program of the male body. And it is provided by a special sex determination gene.

X and Y chromosomes are not equal. The first one takes on the main work. And the second only protects the genes associated with it. There are only 100 of them, while the X chromosome carries 1,500 genes.

From each X chromosome, one gene is needed to form the male sex. And for the formation of the female sex, two genes are needed. It's like a pie recipe with one cup of flour. If you take two glasses, then everything will change dramatically.

However, you should know that the female embryo, having two X chromosomes, ignores one of them. This behavior is called inactivation. This is done so that 2 copies of the X chromosomes do not produce twice as many genes as required. This phenomenon is referred to as gene dosage compensation. An inactivated X chromosome will be inactive in all subsequent cells resulting from division.

This shows that the cells of the female embryo form quite complex mosaic, collected from inactive and active paternal and maternal X chromosomes. As for the male embryo, no inactivation of the X chromosome occurs in it. This means that women are genetically more complex than men. This is a rather loud and bold statement, but a fact is a fact.

But as for the genes of the X chromosome, of which there are 1,500, many of them are associated with brain activity and determine human thinking. We all know that the chromosome sequence of the human genome was determined in 2005. It was also found that a high percentage of genes on the X chromosome provide the generation of a protein that is involved in the formation of the medulla.

Some of the genes are involved in the formation of the brain mental activity. These are verbal skills social behavior, intellectual abilities. Therefore, today scientists consider the X chromosome to be one of the main points of knowledge.

The male Y chromosome is not a dead end of evolution, but is changing very actively. Such conclusions were made by geneticists when comparing the set of genes in the chromosome of humans and chimpanzees, which survived 6 million years of separate evolution. The unexpected genetic diversity is explained by the peculiarities of the functioning of genes involved in the formation of germ cells.

In most mammals, sex is determined by them: the male body is the carrier of the X- and Y-chromosomes, and women “make do” with two X-chromosomes. Once this division did not exist, but as a result of evolution about 300 million years ago, chromosomes differentiated. There are variations whereby some men's cells contain two X chromosomes and one Y chromosome, or one X chromosome and two Y chromosomes; Some women's cells contain three or one X chromosome. Occasionally, female XY organisms or male XX organisms are observed, but the vast majority of people still have a standard configuration of sex chromosomes. For example, the phenomenon of hemophilia is associated with this feature. The defective gene that impairs blood clotting is linked to the X chromosome and is recessive. For this reason, women only endure the disease without suffering from it themselves due to the presence of a duplicate gene due to the second X chromosome, but men in a similar situation carry only a defective gene and get sick.

One way or another, the Y chromosome has traditionally been considered a weak point in male organisms, reducing genetic diversity and preventing evolution. However, recent studies have shown that fears about the extinction of the male race are greatly exaggerated: the Y chromosome does not even think of stagnating. On the contrary, its evolution is very active; it changes much faster than other parts of the human genetic code.

A study published in Nature (Jennifer F. Hughes et al., Chimpanzee and human Y chromosomes are remarkably divergent in structure and gene content) showed that a specific part of the Y chromosome of humans and one of its closest relatives, chimpanzees, is very different . Over the 6 million years of separate evolution of monkeys and humans, the fragment of the chromosome responsible for the production of germ cells has changed by a third or even half. The rest of the chromosome is actually quite constant.

Human evolution. Source "Eternal Youth"

Scientists' assumptions about the conservatism of the Y chromosome were based on objective factors: being transmitted from father to son without changes (for the X chromosome there are as many as three options - two from the mother and one from the father, all of them can exchange genes), it cannot gain genetic diversity from the outside, changing only due to the loss of genes. According to this theory, in 125 thousand years the Y chromosome will finally die out, which could be the end of all humanity.

However, for 6 million years of separate evolution of humans and chimpanzees, the Y chromosome has been successfully changing and progressing. IN new job, conducted at the Massachusetts Institute of Technology, talks about the Y chromosome of chimpanzees. The human Y chromosome was deciphered in 2003 by the same group led by Professor David Page.

The results of the new study surprised geneticists: they expected that the sequence of genes on the two chromosomes would be very similar. For comparison: in the total DNA of humans and chimpanzees, only 2% of genes are different, and the Y chromosome differs by more than 30%!

Professor Page compared the process of evolution of the male chromosome to a change in the appearance of a house, the owners of which remain the same. “Despite the fact that the same people live in the house, almost constantly one of the rooms is completely updated and renovated. As a result, after a certain period of time, as a result of “room-by-room” renovation, the entire house changes. However, this trend is not normal for the entire genome,” he noted.

The reason for this unexpected instability of the Y chromosome is not yet precisely clear. Scientists suggest that genetic diversity in it is ensured by instability to mutations. The usual mechanism for “repairing” genes fails on the Y chromosome, opening the way for new mutations. Statistically, a larger number of them become fixed and change the genome.

In addition, these mutations are subject to significantly greater selection pressure. This is determined by their function - the production of germ cells. Any beneficial mutations will be fixed with to a greater extent probabilities, since they act directly - increasing the ability of an individual to reproduce. At the same time, ordinary mutations have an indirect effect - increasing the body's resistance to disease or harsh conditions environment, For example. Thus, the benefit of a mutation in a nonspecific DNA section will only be revealed if the organism finds itself in corresponding unfavorable conditions. In other cases, mutant and non-mutant organisms will perform similarly. Fertility appears very quickly - already in the second generation. An individual either reproduces as a result of mutation more successfully and leaves numerous offspring, or reproduces noticeably worse and cannot increase the share of its genes in the general population. This mechanism functions more efficiently in chimpanzees, whose females constantly mate with a large number of males. As a result, the germ cells enter into direct competition, and “selection” occurs as efficiently as possible. In humans, due to more conservative models of reproduction, the Y chromosome has not evolved so rapidly, geneticists say. This hypothesis is supported by the fact that the parts of the chromosome involved in sperm production are most different between humans and chimpanzees.

Image from unc.edu

Every woman is not just a mystery, but a mosaic consisting of cells with different sets of active chromosomes. Humans have 23 pairs of chromosomes, and the chromosomes of each pair carry the same sets of genes. The exception is a pair of sex chromosomes. In men, one is called X and the other is called Y, and they differ significantly in their sets of genes. The X chromosome is much larger than the Y chromosome and contains more genes. Both female sex chromosomes are X, and they differ from each other just like the chromosomes within the other 22 pairs. Every woman has two X chromosomes, and every man has only one, and so that they are equally active in women and men, the body regulates their work. To do this, in all cells of a woman’s body, one of the X chromosomes is inactivated. Which of the two sex chromosomes will be disabled is determined by chance for each cell, so that in some of the cells of a woman’s body one X chromosome works, and in the remaining cells the other works.

As a result of this mosaic pattern, women rarely develop diseases associated with damage to the X chromosome. Even if a woman has an X chromosome with a defect in some gene, the other chromosome of the pair, working in half of the cells, saves the situation and prevents the disease from manifesting itself. For a disease associated with damage to the X chromosome to “play out” to its full potential, a woman must receive as many as two copies of this chromosome with a defect in the same gene. This is an unlikely event. At the same time, if a man receives a defective X chromosome (it comes from the mother), she will not have a mate to compensate for the damage, and the disease will show itself.

The X chromosome, unfortunately for men, carries many vital genes, so its breakdown is fraught with dire consequences. Color blindness, hemophilia, Duchenne myopathy, fragile X syndrome, X-linked immunodeficiency are just the most well-known genetic diseases that affect almost exclusively men.

Color blindness

It is a common misconception that only men can be colorblind. This is not true, however, colorblind women are much less common. Only 0.4 percent of women and about 5 percent of men have difficulty distinguishing certain colors. Color blindness is the loss or impairment of one of the pigments associated with recognizing light of a certain color. There are three such pigments in total, and they are sensitive to waves of red, green and of blue color. Any complex color can be thought of as a combination of these three. Each cone cell, which is found in the retina and is responsible for color recognition, contains only one type of pigment. For reasons still unknown, problems with the functioning of the pigments with which we distinguish between red and green colors, are more common than defects in the pigment needed to correctly recognize the color blue.

Genes located on the X chromosome are responsible for the synthesis of pigments. If a man received a chromosome with a defective gene that determines the recognition of, for example, the color red, then only this defective X chromosome will be active in all the cones of his retina - he simply does not have another. Therefore, such a man will not have cones that can correctly recognize the color red. A woman’s retina has a mosaic structure, and even if one of the X chromosomes carries a damaged gene, this chromosome will be active only in part of the cones responsible for recognizing the corresponding color. In other cones, the second chromosome will be active, which carries the normal gene. Such a woman's color perception will be slightly altered, but she will still be able to distinguish all the colors that people usually distinguish.

Hemophilia

Another known disease associated with defects in X chromosome genes is hemophilia, a blood clotting disorder. After an injury in the blood healthy person a complex system of reactions is launched, leading to the formation of fibrin protein filaments. Due to the accumulation of these threads, the blood at the site of injury becomes thicker and clogs the wound. If any stage of the process is disrupted, the blood does not clot at all or does so too slowly, so that the patient may die from blood loss even after the tooth is removed. In addition, patients with hemophilia suffer from spontaneous internal hemorrhages due to the vulnerability of the vessel walls.

The cascade of reactions that ultimately leads to the formation of fibrin threads and blood thickening is very complex, and what more complex system, those more places where it might break. There are three known types of hemophilia associated with defects in three genes encoding proteins that participate in the cascade. Two of these genes are located on the X chromosome, so one man in 5,000 suffers from hemophilia, and only 60 cases of the disease have been recorded in women throughout history.

Duchenne myopathy

Another important gene located on the X chromosome is the dystrophin protein gene, which is necessary to maintain the integrity of muscle cell membranes. In Duchenne myopathy, the function of this gene is disrupted and dystrophin is not produced. Men who have inherited an X chromosome with such a damaged gene develop progressive muscle weakness, as a result of which boys with this disease cannot walk independently by the age of 12. As a rule, patients die at the age of about 20 years due to respiratory disorders associated with muscle weakness. In girls who received an X chromosome with a faulty dystrophin gene, due to mosaicism, the protein is missing in only half of the body cells. Therefore, women who carry the defective dystrophin gene suffer only from mild muscle weakness, and even then not always.

X-linked severe immunodeficiency

Patients with severe immunodeficiencies are forced to live in completely sterile environments because they are extremely vulnerable to infectious diseases. X-linked severe immunodeficiency occurs due to a mutation in a gene that encodes a common component of several receptors necessary for the interaction of cells of the immune system. As is obvious from the name of the disease, this gene is also located on the X chromosome. Due to dysfunctional receptors, the immune system develops incorrectly from the very beginning, its cells are few in number, function poorly and cannot coordinate their actions. Fortunately this serious disease It is rare: it affects one boy in 100,000. In girls, the occurrence of this disease can be considered almost impossible.

Fragile syndromeX chromosomes

Another important gene located on the X chromosome is the FMR1 gene, which is necessary for normal development nervous system. The functioning of this gene can be disrupted due to a pathological process in which the number of repeating DNA fragments in the gene increases. The point is that exactly copying a repeating number of units is always difficult. Let's imagine that we need to carefully rewrite a long number that contains many identical numbers in a row - it’s easy to make a mistake and write a few numbers more or less. It's exactly the same in DNA. During cell division, when DNA is doubled, the number of repeats can randomly change. It is precisely because of the increase in the number of repeats in a short fragment of DNA on the X chromosome that a “fragile” region can appear that easily breaks during cell division. The FMR1 gene is located next to the “fragile” area, and its work is disrupted. As a result of this pathology, mental retardation occurs, which manifests itself more clearly in men with a “fragile” X chromosome than in women.

Is it always better to have two?X chromosomes than one?

It seems that having two X chromosomes is more beneficial than one: there is less risk of diseases due to bad genes. What about males who have the following sex chromosome composition: XXY? Can we expect them to have an advantage over males with regular composition XY sex chromosomes? It turns out that the composition of XXY chromosomes is not a blessing, but quite the opposite. Men with this set of chromosomes suffer from Klinefelter syndrome, in which many pathologies are observed, but there are no benefits.

Moreover, there are known diseases that are also characterized by large quantities X chromosomes, up to five per genotype. Such pathologies occur in both women and men. If there are excess X chromosomes, all but one are inactivated. However, even if the extra X chromosomes do not work, the more there are, the more severe the disease. Interestingly, intelligence especially suffers from the presence of excess X chromosomes - each extra chromosome of this type leads to a decrease in IQ by an average of about 15 points. It turns out that having a spare X chromosome is good, but not always (an additional X chromosome does not make men any better). Having many spare variants of this sex chromosome is not beneficial for either women or men.

Why are additional inactive X chromosomes harmful, and why does each extra chromosome aggravate the severity of the disease? Firstly, the extra X chromosomes are not turned off immediately, but only after the first 16 days of embryo development. And the earlier during development a disorder occurs, the more diverse and numerous its manifestations will be. Therefore, extra chromosomes can have time to “damage” quite fundamentally, so that pathologies will manifest themselves in completely different areas.

Second, some genes on inactivated X chromosomes somehow escape being turned off. Although the X and Y chromosomes are very different, they still form a pair and have a small number of identical genes. If there are too many sex chromosomes, and these genes remain active on all of them, the gene balance in the cells is disrupted. Therefore, the more extra chromosomes, the more severe the disease.

The X chromosome carries many vital genes, and it is not surprising that its defects have extremely unpleasant manifestations. Women are naturally given the opportunity to “insure themselves” by having an extra copy of the chromosome, which can reduce the severity of the disease. However, such a “reserve” is only good for singular, and all additional X chromosomes lead to the development of severe pathologies. Well, men who do not have a second X chromosome are at greater risk from the very beginning of their conception. Alas.

Yulia Kondratenko