Image of the Y chromosome under an electron microscope. Photo from visualphotos.com.
The mysteries of the Y chromosome
The Y chromosome is unlike the other 45 chromosomes in the human genome. She does not have a pair, she "collects" all possible mutations and many researchers are sure that soon the male sex chromosome will disappear altogether. Moreover, as it turned out recently, it is not really needed for reproduction.
Scientists predict that the human Y chromosome could potentially completely lose its function and disappear from the genome within the next ten million years. The "male" sex chromosome differs significantly from other chromosomes, and, in particular, from the X chromosome, in that the individual is not able to exchange genetic sites during reproduction. As a result, its hereditary material has become impoverished and the chromosome has accumulated mutations that are passed on from generation to generation. But don't panic: as recent studies have shown, in the future, people will be able to have children without the participation of the Y chromosome.
Male feature
Until recently, it was believed that the X and Y chromosomes appeared about 300 million years ago, but more recently, scientists have found that chromosomal sex determination was absent 166 million years ago.
According to the most common theory, the X and Y chromosomes evolved from a pair of identical chromosomes, when a gene arose in ancient mammals, one of the alleles of which directed the development of an organism in a male pattern. The chromosomes carrying this allele became Y chromosomes, and the second chromosome in this pair became the X chromosome. Thus, the X and Y chromosomes initially differed in only one gene. Over time, genes useful for males and harmful or not important for females began to develop on the Y chromosome.
The Y chromosome does not recombine with the X chromosome during the maturation of germ cells (gametogenesis), so it can only change as a result of mutations. The resulting genetic information is not discarded or "diluted" with new gene variations, therefore, practically unchanged, it is passed from father to son for many generations. Over time, the number of harmful mutations inevitably increases.
In the process of gametogenesis, spermatozoa undergo multiple cell divisions, and each of them provides an opportunity for the accumulation of mutations. In addition, sperm are in the highly oxidative environment of the testes, which contributes to the emergence of new mutations. That is why the Y chromosome “breaks down” much more often than other chromosomes.
Stop the decay of the "male" chromosome
In the course of evolution, the human Y chromosome has lost most of its original genes, and now, according to various estimates, contains 45 to 90 genes, compared with about 1400 genes on the X chromosome. Previously, scientists made a prediction that with an estimated rate of loss of 4.6 genes per million years, the human Y chromosome could potentially completely lose its function within the next 10 million years.
But there is another opinion: the authors of a study conducted at the Whitehead Institute for Biomedical Research that the rapid loss of genes - the genetic "decay" that characterized the early evolution of the male sex chromosome, has come to naught, and the Y chromosome will remain relatively stable for the next tens of millions of years. ...
The researchers sequenced 11 million base pairs of the Y chromosome of rhesus monkeys. Comparing this sequence with a similar region on the male sex chromosome, as well as on the Y chromosome of chimpanzees, scientists have come to the conclusion that the genetic makeup of the male sex chromosome has remained almost unchanged over the past 25 million years.
According to one of the authors of the study, Jennifer Hughes, since “in humans, only one gene has been lost by the Y chromosome compared to rhesus monkeys, we can be sure that in the next millions of years male the chromosome will not disappear. "
Conception withoutY-chromosomes
Hawaiian researchers believe that only two genes from the Y chromosome are enough for male mice to conceive healthy offspring. The authors of the article believe that in the future, a technique may appear that makes it possible to do without the Y chromosome at all in human reproduction. In addition, the result obtained is potentially of great importance in the fight against male infertility.
Scientists used sex cells obtained from male mice, in which only two genes were left from the Y chromosome - SRY (Sex-determining Region of Y) - the most significant gene on the Y chromosome, which is responsible for the development of the male-type organism, the production of male hormones and spermatogenesis, and spermatogonia proliferation factor Eif2s3y. The researchers found that Eif2s3y is the only gene on the Y chromosome that is required for normal sperm formation.
The resulting male germ cells were then in vitro oocytes were fertilized using the intracytoplasmic injection (ROSI) method. The developed embryos were implanted into the uterus of the females. As a result of this procedure, 9 percent of pregnancies ended in the birth of healthy offspring, while in males with a full Y chromosome, this figure was 26 percent. In the future, according to scientists, it is possible to do without the Y chromosome altogether if it is defective. If genes interacting with genes of the Y chromosome are found on other chromosomes, then the activation of such partner genes could theoretically completely replace their functions.
Cancer protection?
Recently in the magazine Nature published data showing that the loss of the Y chromosome in blood cells (leukocytes), often observed in older men, is associated with an increased risk of developing cancer and earlier mortality compared to women.
This phenomenon was first described about 50 years ago and until now its causes and consequences remained largely unclear. Now Swedish scientists have studied blood samples from 1,153 elderly men aged 70 to 84, who were observed in clinics from 40 years old. As it turned out, men, in most of whose blood samples showed the loss of the Y chromosome, lived on average 5.5 years less than those who did not have such a phenomenon. In addition, the increase in the number of these blood cells significantly increased the risk of men dying from cancer.
"Many people think that the Y chromosome contains only genes involved in sex determination and sperm production, but in fact, its genes are also involved in other important functions, for example, they can potentially play a role in preventing the development of tumors," the authors noted in my article. "Our hypothesis is that age-related loss of the Y chromosome disrupts the immune vigilance of blood cells, which allows tumor cells to grow uncontrollably and transform into cancer."
The results obtained suggest that a blood test for the presence of leukocytes that have lost the Y chromosome may become a new approach to identifying an increased risk of developing cancer in men. At the same time, the researchers emphasized that the presence of such cells in small numbers is not very dangerous, but their predominance may indicate a high risk of developing cancer.
The Y chromosome, the most important male sex characteristic, is rarely influenced by external factors. Due to the fact that the chromosome is unpaired, it does not participate in recombination and accumulates all mutations, both harmful and useful. Scientists have predicted an imminent end to this strange gene cluster, but it still holds - as befits a real male chromosome.
Elena Sharifullina
Dear Biological Losers!
here and again my cup of patience has flowed and I want to shine a flashlight in the face of truants and poets. Well, yes, there are bloopers and there are bloopers.
The sacramental knowledge that a llama poops more like "horse apples" than "cow cakes" will not take you to the writer's Olympus. But if your chromosome suddenly got into your DNA, this is already a blunder at the level of disregard for the foundations of life. Do you want to screw something scientifically similar into a work? Take a look at the wiki ... So that it turns out that in the left boot of your hero there are two cities, one of which he travels to work.
Genes are small links in a chain. Two long chains sharpened in a spiral make up the DNA strand. This thread, packed in a special way, is called a chromosome. Each chromosome looks a little differently, they are all located in the nucleus of the cell IN EVERY CELL of the body. and at a certain period of the existence of a cell, they can be specially processed and seen through a special microscope. A person has 46 chromosomes. Two of these 46 are called sexual, because depends on them
Can you imagine the carcass device? These are bones connected to form a skeleton. And between the ends of the bones and in different ways, many, many muscles are stretched around and around. That eat meat. I can't imagine how you can separate the meat from the carcass with several movements... It's not a coconut with a shell.resolutely took away the knife and the shrew and ordered:
Sit down and wait.
He is clever, with a few short strokes, separated the charred meat from the carcass, then sliced the roast in two and held out half to Samantha:
There are many differently attached muscles and in no way can you first separate the meat, and then SLIT it into two parts.
Muscles will not be removed in one piece. The carcass, yes, can be cut into two parts.
But if he first separated the meat, then a bunch of pieces formed, which are already cut apart by the ninada, it is enough to push it in two directions))
According to the plot, there is a possibility that the dog sniffs out drugs, so it is clear that the dog is driven away. But one could motivate that the dog will scare the animals, but the animals in the cages sleep under anesthesia. And then a little problem arises. In animals, sterility means the inability to reproduce, because sterility in the sense of the absence of contamination with bacteria and other "unclean material" does not make sense when we mean living animals, because a horde of all kinds of microorganisms always lives on them and in them.These animals are sterile. Remove the dog. It can be contagious.
That is, in principle, the phrase is wild from the lips of the accompanying staff.
Caldera (Spanish caldera - cauldron) is a vast circus-shaped basin with steep walls and a more or less flat bottom of volcanic origin. The caldera differs from the crater by its formation features and large size.
** Calimera (Greek Καλημέρα) - good morning, hello!
The Y chromosome may be a symbol of masculinity, but according to modern scientists, it is not the most stable and not even the most necessary collection of genes in the body of mammals.
Sex identifier
Despite the fact that the Y chromosome carries the "main sex determinant", or the SRY gene, which determines whether the embryo will develop male sexual characteristics or not, except for the SRY gene, there are no longer any vital genes in the Y chromosome that are not there in the X chromosome. Accordingly, the Y chromosome is the only chromosome not necessary for life. Women, after all, do just fine with two X chromosomes.
Degeneration rate
In addition, the Y chromosome weakens rapidly, as if fading over time. Because of this, women have two completely normal, healthy X-chromosomes, while men have one full-fledged X-chromosome and a Y-chromosome "dried up" in the process of evolution.
If this rate of degeneration is maintained at its current level, the Y chromosome has only four and a half million years left. After this time, scientists predict the possible degeneration of this chromosome.
This period may seem very long, but this is not entirely true, especially when you consider that life on Earth has existed for three and a half billion years.
Genetic recombination
The Y chromosome was not always a degenerate and unnecessary part of the DNA code. If you look at the state of affairs 166 million years ago, when the very first mammals evolved, the position of the "male" chromosome was completely different.
The early "proto-y chromosome" was originally the same size as the x chromosome and contained a set of the same genes. However, the Y chromosome has one fundamental flaw. Unlike all other chromosomes, of which we have two copies in each of the cells, the Y chromosomes are present there in a single copy and are passed from fathers to sons.
This means that the genes contained in the Y chromosome do not undergo genetic recombination, a kind of “shuffling” of genes that occurs in every generation and helps to eliminate destructive gene mutations.
Deprived of the benefits of “recombination,” the genes on the Y chromosome deteriorate over time and ultimately are eliminated from the genome.
Defense mechanisms
Despite this, recent research has shown that genes on the Y chromosome have developed powerful defense mechanisms aimed at slowing down genetic degradation.
For example, a recent Danish study published in PLoS Genetics focused on examining in detail the genetic code of the Y chromosomes of 62 different participants. Scientists concluded that the Y chromosome regularly undergoes large-scale structural rearrangements aimed at "gene amplification" - numerous copies of healthy genes responsible for sperm production. This "amplification" mitigates gene loss on the Y chromosome.
Genetic palindromes
The study also showed that the Y chromosome developed unusual genetic structures called palindromes (DNA sequences that read the same at both ends, like the word “stomp,” for example). Genetic palindromes protect the Y chromosome from further degradation. In fact, palindromic DNA sequences are capable of "converting" genes, that is, repairing damaged genes using an intact backup copy as a template.
Examining other varieties of Y chromosomes, for example, in other mammals and some other species, scientists have come to the conclusion that the amplification of genes of the Y chromosome is a general principle for representatives of different species.
Scientific debate
On the question of whether the Y chromosome will disappear over time, or will be able to develop sufficient defense mechanisms, the scientific community is divided into two camps. One group insists that the defense mechanisms do an excellent job of protecting the chromosome, the other argues that these processes can only temporarily postpone the inevitable - the complete disappearance of the Y chromosome from the genetic code of living organisms. The debate on this matter continues and is not going to subside.
Disappearing
A leading proponent of the Y chromosome disappearance argument, Jenny Graves of La Trobe University in Australia, argues that in the long term, Y chromosomes are doomed, even if they manage to last a little longer than expected.
In a 2016 article, she points out that Japanese spiny rats and voles have completely lost their Y chromosomes. She argues that the loss of Y-chromosome genes inevitably leads to problems with fertilization, which, in turn, can stimulate the formation of completely new species.
What's in store for men?
According to scientists, even if the Y chromosome in humans disappears, this does not mean that men will also disappear with it. Even in those animal species that do not have a Y chromosome, there is still a division into males and females and natural fertilization and reproduction occurs.
In these cases, the SRY gene, which determines the male sex, switches to another chromosome, meaning that over time, males may completely lose the need for the Y chromosome. However, the new sex-determining chromosome - the one where the SRY gene has moved - will have to undergo the same slow process of degeneration due to the same lack of recombination that doomed the Y chromosome to degradation.
Artificial fertilization methods
While the Y chromosome is essential for normal human reproduction, there are no more genes that are useful or necessary for the existence of this chromosome. It turns out that if you use modern artificial methods of fertilization, then the Y-chromosome is completely unnecessary.
This means that genetic engineering could soon replace the function of the Y chromosome gene, allowing same-sex female couples or infertile men to reproduce. However, even if it were possible for everyone to become pregnant in this way, it is very unlikely that most healthy people would simply stop giving birth to children in the traditional way, switching to artificial insemination.
While the fate of the Y chromosome is an interesting and hotly debated area of genetic research, there is no need to worry just yet. We don't even know if the Y chromosome will disappear altogether. It is possible that her genes will be able to find a way to protect themselves from degeneration and everything will remain as it was.
You may remember that we said that the chromosomes of men and women are different. Women have 24 perfect pairs of chromosomes, including two X chromosomes. Men have 23 perfect couples and one imperfect couple. An imperfect pair consists of an X chromosome and a stunted Ygrek chromosome. The Y chromosome carries few (if any) genes, and this is a source of great trouble for males.
The gene, located on the 24th pair of chromosomes, is responsible for the human eye's ability to distinguish between red and green. An imperfect allele belonging to the same gene is unable to control this type of color vision. When this deficient gene is the only gene a person possesses, the individual carrying it cannot distinguish red from green. It is the gene for color blindness (color blindness). Let's label the normal gene with the letter N and the gene for color blindness with the letter C.
A man who cannot tell red from green has the C gene on one chromosome in his 24th pair of chromosomes. If he had a normal gene in addition to it, everything would be fine; but he doesn't have it. Another chromosome of the 24th pair is the Y-chromosome, which does not have a single allele of this gene, and possibly genes in general. Let's just call the Y chromosome Y for short.
The gene combination of the male with color blindness is thus CY.
A color-blind man will produce two kinds of sperm cells. One group of cells will receive the normal chromosome 24, which carries the gene for color blindness. This will be a Tina C sperm cell. Another group will receive a Y chromosome without any gene for color vision at all. This will be the Y sperm cell. Both sperm cells will, of course, be formed in equal numbers.
We then assume that this person marries a woman with normal color vision. Both chromosomes of her 24th pair have a normal gene; so she is NN. All her eggs are the same in this respect. All have a normal N.
How will fertilized eggs be formed? Either sperm cell C fertilizes an N egg, or sperm cell Y fertilizes an N egg. Fertilized eggs will all be either NC or NY.
You may remember from the previous chapter that a fertilized egg containing a Y chromosome always develops into a male. All fertilized NY eggs develop into boys. We see that when a color blind man has sons by a normal wife, they are all normal. None of them have the gene for color blindness at all; and the gene for color blindness, accordingly, cannot manifest among their offspring.
When a fertilized egg does not have a Y chromosome, it always develops into a woman. All NC children are therefore girls and all girls are heterozygous. Fortunately, the normal gene is dominant over the gene for color blindness. For this reason, the daughters of a color-blind man and a normal woman can see colors quite normally. However, unlike sons from this marriage, daughters have the gene for color blindness and can pass it on to their children.
Suppose, for example, that one of these heterozygous girls (NC) eventually marries a normal man with one good color vision gene and of course one Y chromosome (NY). What happens next? The girl produces two types of eggs, one type - N and the other - C, of equal quantity. A man produces two types of sperm cells, one type - N and the other - Y, of equal quantity.
Potential fertilized eggs would then be NN, NC, NY and CY. The daughters of this marriage will be without a Y chromosome. They will be either NN or NC. NN girls will be perfectly normal. NC girls see color in exactly the same way, but they are heterozygous. They have the gene for color blindness.
Boys from this marriage will have a Y-chromosome. They will be either NY or CY. NY boys will be perfectly normal. CY boys will be color blind.
This process can continue in future generations. It will always be the boy who will suffer from color blindness, and the girl will almost never have it. However, it is the girl, not the boy, who will pass on the gene for color blindness to her children.
It is possible, of course, that a girl can also suffer from color blindness if it happens that she has two genes for color blindness at once (CO. This can happen if a man with color blindness marries a woman whose father suffered from color blindness and who, is thus a carrier of the gene for color blindness. Then there is an equal chance that a girl born of this marriage will suffer from color blindness. Such cases are known, but they are very rare. (All sons of a woman suffering from color blindness will suffer from color blindness, no matter who she marries. Can you figure it out yourself?)
Whenever a characteristic similar to the gene for color blindness appears in only one and not the other, it is said to be sex-related.
Another gender-related characteristic you may have heard of is hemophilia. This is a condition when the blood, for some reason, is unable to clot. Even a small cut can be fatal for a person suffering from this disease, because without special treatment, the bleeding will not stop.
Hemophilia is inherited in the same way as color blindness. With very rare exceptions, only men get it, but only women pass it on to their children.
By the way, about hemophilia. Queen Victoria may have been heterozygous for the hemophilia gene. Since the normal blood clotting gene is dominant over the hemophilia gene, we cannot be sure. Her blood clotted normally. However, hemophilia suddenly developed in her male offspring. The son of the Russian Tsar Nicholas II (whose wife was one of Victoria's granddaughters) suffered from hemophilia. The eldest son of King of Spain Alfonso XIII (whose wife was one of Victoria's granddaughters) also had hemophilia. We will, however, have an opportunity to mention Queen Victoria again in the next chapter.
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The biological role of the Y chromosome turned out to be much more important than previously thought. The genes contained in it also work outside the reproductive system.
For a while, scientists were seriously worried that the smallest human chromosome might eventually disappear. But, as it turned out, despite a rather significant reduction of genes, she provided herself with "eternal life", due to the fact that it contains genes responsible for very important processes in the body, and not only necessary for the normal functioning of the male reproductive system.
Chromosomal sex determination in different groups of organisms occurs in different ways. XY sex determination works in the vast majority of mammals: females have two identical sex chromosomes XX, and males have two different sex chromosomes X and Y. During evolution, the Y chromosome has lost a huge number of genes, and only 3% of its former greatness remained ... The X chromosome has retained 98% of the original number of genes. Based on these data, the hypothesis of the disappearing Y-chromosome arose. Not in the near future, of course, but millions in five years ...
Scientists have studied the Y chromosome not only in men of different nationalities, but also in representatives of different groups of mammals, including marsupials. As a result of the study, 36 genes were found that are on both sex chromosomes. Representatives of all studied groups have 18 of them.
“The Y chromosome does more than just say that this is a man and that he is capable of reproduction. She says that this is a man who can leave offspring and wants to survive on his own, "- commented on the results of the study, which were recently published in the journal" Nature ", one of the authors, Daniel Winston Bellot from the Whitehead Institute (USA).
The sex chromosomes of mammals evolved over millions of years from two common and identical ancestors. For a long time it was believed that the Y chromosome is responsible only for the male reproductive system, but it turned out that it contains genes that work in other tissues and organs. There are genes whose work depends, among other things, on how many copies of them exist in the genome. Most of the genes found on the sex chromosomes are singular. In the female body, those that need a second copy are on the other X chromosome. It turned out that the Y chromosome also contains second copies of some genes, so that whatever gender the individual is, he will be provided with all the necessary genetic material. We can say that this is a kind of "biological insurance" for the Y-chromosome, because these vital genes are preserved in all groups of mammals.
Another study, the results of which have just been published in Nature by Swiss scientists, suggests that the Y chromosome appeared after mammals appeared. After all, there are other ways of sex determination, even exotic ones, such as dependence on temperature in some reptiles. There is a hypothesis that the situation was approximately the same for dinosaurs, and this temperature dependence ultimately led to their inevitable death. One of the important results was the determination of the age of the SRY gene, which is responsible for the development of the male-type organism in placental mammals. It appeared about 180 million years ago, after the first beasts separated from the rest, but before the marsupials appeared.
The presence of important regulatory genes on the Y chromosome posed a different challenge for scientists. These genes can influence why the clinical picture in men and women is different for a number of diseases. It is possible that when prescribing treatment, it is worth paying attention not only to age, concomitant diseases, but also to the gender of the patient. “Knowing that the Y chromosome can affect the entire genome, and not just be a trigger responsible for the masculinity, it is necessary to take gender into account when studying certain diseases,” said non-participating geneticist Andrew Clark of Cornell University.
