Chromosomes

Main article: Genetics

Chromosomes can be seen in a regular microscope.

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Autosomes

Autosomes are a series of chromosomes transmitted from parents (one from the father, the other from the mother), constituting a diploid (double) set of all chromosomes except for sexual ones.

Y-chromosome

The Y chromosome is one of two sex chromosomes in the XY chromosomal sex determination system, which occurs in many animals, most mammals, including humans. It contains the SRY gene, which determines the male sex of the body, as well as the genes necessary for the normal formation of sperm. Unlike other chromosomes, the Y chromosome is transmitted from father to son almost unchanged. If a mutation occurred in the production of germ cells in the Y chromosome of a man, then all his sons will receive an already changed male sex chromosome.

2024: The Y chromosome began to disappear. This can lead to the extinction of man as a species

Men slowly lose their Y chromosome, which after a few million years can lead to the extinction of humans as a species. This is stated in a study by a team of Australian specialists from La Trobe University, the results of which Zdrav.Expert got acquainted with at the end of August 2024.

In humans, as in other mammals, females have two X chromosomes, and males have one X and one Y. Chromosome X contains about 900 genes that perform a wide variety of non-sex-related functions. In turn, Y includes a small number of genes (about 55) and simple repetitive DNA. At the same time, Y contains the SRY gene, which determines the male sex of the body, as well as the genes necessary for the normal formation of sperm.

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Most mammals have X- and Y-chromosomes: the first with a large number of genes, and the second with SRY and a number of others. This system comes with problems due to the unequal dosage of X genes in males and females. The discovery was that the Australian platypus (a waterfowl mammal of the single-pass order) has completely different sex chromosomes, more similar to those of birds. In this animal, the XY pair is a regular chromosome with two equal members. This suggests that previously in mammals X and Y were a common pair of chromosomes.

Thus, the authors of the study conclude, the Y chromosome lost 845 (900 minus 55) active genes over 166 million years, during which people and platypuses developed separately. This corresponds to the loss of about five genes every million years. At this rate, the last 55 genes may disappear after 11 Ma, leading to human extinction. However, two species of rodents are known that have already lost their Y chromosome and are still surviving: these are mole voles in Eastern Europe and spiny rats in Japan.^[1]

2023: How the Y chromosome affects cancer aggressiveness in men

On June 21, 2023, researchers from the George Institute for Global Health Sydney published an article on the contribution of the Y chromosome to the development of colorectal cancer and bladder cancer.

The study found that losing the entire Y chromosome in some cells, which occurs naturally with age, increases the risk of aggressive bladder cancer and may allow bladder tumors to elude detection by the immune system. According to scientists, a certain Y chromosome gene in mice increases the risk of some colorectal cancers spreading to other parts of the body.

Colon cancer

The work is a step toward understanding why so many cancers are predisposed to men, scientists say. It becomes clear that it's not just about lifestyle. Lifestyle has long been blamed on the fact that many non-reproductive cancers are more common and aggressive in men than in women. For example, men are more likely to smoke and drink alcohol. But even with these factors in mind, some differences in the incidence or severity of cancer between men and women persist.

Meanwhile, the researchers also found that the Y chromosome, which is common in men, can be lost spontaneously during cell division. As men age, the proportion of blood cells without a Y chromosome increases, and the excess of such cells is associated with diseases such as heart disease, neurodegenerative conditions and certain types of cancer.

According to the scientific journal Nature, the researchers found that such cancer cells were more aggressive when transplanted into mice than similar cells that preserved the Y chromosome. The scientists also found that immune cells surrounding tumors without a Y chromosome tend not to function. To learn more about how this process could affect bladder cancer, the scientists studied human bladder cancer cells that either spontaneously lost the Y chromosome or removed it using CRISPR-Cas9 genome editing. The team found that such cancer cells were more aggressive when transplanted into mice than similar cells that retained the Y chromosome. They also found that immune cells surrounding tumors without a Y chromosome tend not to function.

According to George Sue Haupt, a cancer researcher at the Institute for Global Health, in mice, a therapeutic antibody capable of restoring the activity of these immune cells turned out to be more effective against such tumors without Y than against tumors that still had a Y chromosome. The team found a similar trend in human tumors. This finding is the study's "most important message." Similar antibodies, called checkpoint inhibitors, are already used in clinical practice against some tumors.

According to the text of the scientific work, the researchers found that a gene on the Y chromosome called KDM5D could weaken connections between tumor cells, helping them separate and spread to other parts of the body. When this gene was removed, tumor cells became less invasive and more likely to be recognized by immune cells. It also represents a potential target for cancer therapy, because it is a target that can be targeted. This context may vary not only with the affected organ, but even with the location of the tumor in the organ and the presence or absence of other genetic mutations.^[2]

X-chromosome

2020: First decoding of human X chromosome

In mid-July 2020, the journal Nature published news that researchers for the first time completely deciphered the human X chromosome, determining the sequence of more than 3 million base pairs. This is an important step in preparing a model of a full-fledged human genome, which scientists promise to create by the end of 2020.

Human genome sequencing is considered one of the most ambitious scientific endeavors. The first version, obtained in 2000, covered a little more than 92% of the genome with an accuracy of more than 99.99%. The first complete assembly of the human X chromosome "telomer-to-telomer" (that is, from beginning to end) was announced by a group of scientists from the National Institute for Human Genome Research in the United States.

The study authors chose the X chromosome because it is associated with a variety of serious diseases, including hemophilia and Duchenne muscular dystrophy. The team did not sequence the X chromosome from a normal human cell, but studied cystic drift cells that contain two identical DNA sequences. This allowed them to obtain more data than studying a single male X chromosome and avoid inaccuracies that can occur when analyzing two female ones.

Human X chromosome first deciphered

The genome is so long that sequencing machines cannot read it in its entirety. Researchers have to cut it into smaller parts to examine them separately, but some of the information is lost during the assembly process. The possibility of decoding the X chromosome has appeared thanks to new technologies for sequencing using nanopores.

Unlike traditional techniques, they allow reading ultra-long sequences without unduly damaging DNA. As a result, the researchers achieved a reading accuracy of 99.995%. Using this technique, the team was able to fill a huge gap in the study of a centromere involving about 3.1 million base pairs of repeating DNA. These previously unexplored fields may prove particularly valuable to science.^[3]

Haplotype, subclade, haplogroup

Haplotype is a collection of genes on regions of the same chromosome, usually inherited together.

Haplogroup is a group of similar haplotypes that have a common ancestor, which has a mutation inherited by all descendants (usually a single nucleotide polymorphism, or a difference in the DNA sequence of the size of one nucleotide (A, T, G or C)).

Subclade is a term used to describe a subgroup (branch) of a haplogroup. It is widely used in the description of DNA genealogical tests of mitochondrial DNA and Y chromosome.

• Subclade R1b1a2a1a2b (S28/U152)

Notes