Sometimes they give us amazing surprises. For example, do you know what chromosomes are and how they affect?

We propose to look into this issue in order to dot the i’s once and for all.

Looking at family photographs, you may have probably noticed that members of the same family resemble each other: children look like parents, parents look like grandparents. This similarity is passed on from generation to generation through amazing mechanisms.

All living organisms, from single-celled organisms to African elephants, contain chromosomes in the cell nucleus - thin, long threads that can only be seen with an electron microscope.

Chromosomes (ancient Greek χρῶμα - color and σῶμα - body) are nucleoprotein structures in the cell nucleus, in which most of the hereditary information (genes) is concentrated. They are designed to store this information, implement it and transmit it.

How many chromosomes does a person have

At the end of the 19th century, scientists discovered that the number of chromosomes in different species is not the same.

For example, peas have 14 chromosomes, y have 42, and in humans – 46 (that is, 23 pairs). Hence the temptation arises to conclude that the more there are, the more complex the creature that possesses them. However, in reality this is absolutely not the case.

Of the 23 pairs of human chromosomes, 22 pairs are autosomes and one pair are gonosomes (sex chromosomes). The sexes have morphological and structural (gene composition) differences.

In a female organism, a pair of gonosomes contains two X chromosomes (XX-pair), and in a male organism, one X-chromosome and one Y-chromosome (XY-pair).

The sex of the unborn child depends on the composition of the chromosomes of the twenty-third pair (XX or XY). This is determined by fertilization and the fusion of the female and male reproductive cells.

This fact may seem strange, but in terms of the number of chromosomes, humans are inferior to many animals. For example, some unfortunate goat has 60 chromosomes, and a snail has 80.

Chromosomes consist of a protein and a DNA (deoxyribonucleic acid) molecule, similar to a double helix. Each cell contains about 2 meters of DNA, and in total there are about 100 billion km of DNA in the cells of our body.

An interesting fact is that if there is an extra chromosome or if at least one of the 46 is missing, a person experiences a mutation and serious developmental abnormalities (Down's disease, etc.).

The term chromosome was first proposed by V. It is very difficult to identify chromosome bodies in the nuclei of interphase cells using morphological methods. The chromosomes themselves, as clear, dense bodies clearly visible in a light microscope, are revealed only shortly before cell division.

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Lecture No. 6

CHROMOSOMES

Chromosomes are the main functional autoreproducing structure of the nucleus, in which DNA is concentrated and with which the functions of the nucleus are associated. The term “chromosomes” was first proposed by W. Waldeyer in 1888.

It is very difficult to identify chromosome bodies in the nuclei of interphase cells using morphological methods. The chromosomes themselves, as clear, dense bodies that are clearly visible in a light microscope, are revealed only shortly before cell division. In the interphase itself, chromosomes as dense bodies are not visible, since they are in a loosened, decondensed state.

Number and morphology of chromosomes

The number of chromosomes is constant for all cells of a given species of animal or plant, but varies significantly among different objects. It is not related to the level of organization of living organisms. Primitive organisms can have many chromosomes, while highly organized ones have much fewer. For example, in some radiolarians the number of chromosomes reaches 1000-1600. The record holder among plants for the number of chromosomes (about 500) is the grass fern; the mulberry tree has 308 chromosomes. Let us give examples of the quantitative content of chromosomes in some organisms: crayfish 196, humans 46, chimpanzees 48, soft wheat 42, potatoes 18, fruit flies 8, house flies 12. The smallest number of chromosomes (2) is observed in one of Ascaris races, the Asteraceae plant Haplopapus has only 4 chromosomes.

The size of chromosomes varies widely among different organisms. Thus, the length of chromosomes can vary from 0.2 to 50 microns. The smallest chromosomes are found in some protozoa, fungi, and algae; very small chromosomes are found in flax and sea reeds; they are so small that they are difficult to see with a light microscope. The longest chromosomes are found in some orthopteran insects, amphibians and liliaceae. The length of human chromosomes is in the range of 1.5-10 microns. The thickness of chromosomes ranges from 0.2 to 2 microns.

The morphology of chromosomes is best studied at the moment of their greatest condensation, in metaphase and at the beginning of anaphase. The chromosomes of animals and plants in this state are rod-shaped structures of different lengths with a fairly constant thickness; in most chromosomes it is easy to find the zoneprimary constriction, which divides the chromosome into two shoulder . In the area of ​​the primary constriction there is centromere or kinetochore . It is a plate-like, disc-shaped structure. It is connected by thin fibrils to the body of the chromosome in the region of the constriction. The kinetochore is poorly understood structurally and functionally; Thus, it is known that it is one of the centers of tubulin polymerization; bundles of microtubules of the mitotic spindle grow from it, going towards the centrioles. These bundles of microtubules take part in the movement of chromosomes to the poles of the cell during mitosis. Some chromosomes havesecondary constriction. The latter is usually located near the distal end of the chromosome and separates a small section satellite . The size and shape of the satellite are constant for each chromosome. The size and length of the secondary constrictions are also very constant. Some secondary constrictions are specialized regions of chromosomes associated with the formation of the nucleolus (nucleolar organizers); others are not associated with the formation of the nucleolus and their functional role is not fully understood. Chromosome arms end in terminal sections telomeres. The telomeric ends of chromosomes are not able to join with other chromosomes or their fragments, in contrast to the ends of chromosomes lacking telomeric regions (as a result of breaks), which can join the same broken ends of other chromosomes.

Based on the location of the primary constriction (centromere), the following are distinguished: types of chromosomes:

1. metacentricthe centromere is located in the middle, the arms are equal or almost equal in length, in metaphase it acquires V-shape;

2. submetacentricthe primary constriction is slightly shifted to one of the poles, one arm is slightly longer than the other, in metaphase it has L-shape;

3. acrocentricthe centromere is strongly shifted towards one of the poles, one arm is much longer than the other, does not bend in metaphase and has a rod-shaped shape;

4. telocentricThe centromere is located at the end of the chromosome, but such chromosomes have not been found in nature.

Usually each chromosome has only one centromere (monocentric chromosomes), but chromosomes may occur dicentric (with 2 centromeres) andpolycentric(possessing many centromeres).

There are species (for example, sedges) in which the chromosomes do not contain visible centromeric regions (chromosomes with diffusely located centromeres). They're called acentric and are not able to perform ordered movement during cell division.

Chemical composition of chromosomes

The main components of chromosomes are DNA and basic proteins (histones). DNA complex with histonesdeoxyribonucleoprotein(DNP) constitutes about 90% of the mass of both chromosomes isolated from interphase nuclei and chromosomes of dividing cells. The DNP content is constant for each chromosome of a given species of organism.

Of the mineral components, the most important are calcium and magnesium ions, which give plasticity to chromosomes, and their removal makes the chromosomes very fragile.

Ultrastructure

Each mitotic chromosome is covered on top pellicle . Inside is matrix , in which a spirally twisted DNP thread with a thickness of 4-10 nm is located.

Elementary fibrils of DNP are the main component that is included in the structure of mitotic and meiotic chromosomes. Therefore, to understand the structure of such chromosomes, it is necessary to know how these units are organized as part of the compact chromosome body. Intensive study of chromosome ultrastructure began in the mid-50s of the last century, which is associated with the introduction of electron microscopy into cytology. There are 2 hypotheses for the organization of chromosomes.

1). Unimute the hypothesis states that there is only one double-stranded DNP molecule on the chromosome. This hypothesis has morphological, autoradiographic, biochemical and genetic confirmation, which makes this point of view the most popular today, since at least for a number of objects (drosophila, yeast) it is proven.

2). Polynemic the hypothesis is that several double-stranded DNP molecules are combined into a bundle chromonema , and, in turn, 2-4 chromonemas, twisting, form a chromosome. Almost all observations of chromosome polynemism were made using a light microscope on botanical objects with large chromosomes (lilies, various onions, beans, tradescantia, peony). It is possible that the phenomena of polynemia that were observed in the cells of higher plants are characteristic only of these objects.

Thus, it is possible that there are several different principles for the structural organization of chromosomes in eukaryotic organisms.

In interphase cells, many regions of chromosomes are despiralized, which is associated with their functioning. They're called euchromatin. It is believed that the euchromatic regions of the chromosomes are active and contain the entire main set of genes of the cell or organism. Euchromatin is observed in the form of fine granularity or is not visible at all in the nucleus of an interphase cell.

During the cell transition from mitosis to interphase, certain zones of different chromosomes or even entire chromosomes remain compact, spiralized and well stained. These zones are called heterochromatin . It is present in the cell in the form of coarse grains, lumps, and flakes. Heterochromatic regions are usually located in the telomeric, centromeric, and perinucleolar regions of chromosomes, but can also be part of their internal parts. The loss of even significant sections of heterochromatic regions of chromosomes does not lead to cell death, since they are not active and their genes temporarily or permanently do not function.

Matrix is ​​a component of mitotic chromosomes of plants and animals, released during despiralization of chromosomes and consisting of fibrillar and granular structures of ribonucleoprotein nature. Perhaps the role of the matrix is ​​the transfer of RNA-containing material by chromosomes, which is necessary both for the formation of nucleoli and for the restoration of the karyoplasm itself in daughter cells.

Chromosome set. Karyotype

The constancy of such characteristics as size, location of primary and secondary constrictions, the presence and shape of satellites determines the morphological individuality of chromosomes. Thanks to this morphological individuality, in many species of animals and plants it is possible to recognize any chromosome set in any dividing cell.

The totality of the number, size and morphology of chromosomes is called karyotype of this type. A karyotype is like the face of a species. Even in closely related species, chromosome sets differ from each other either in the number of chromosomes, or in the size of at least one or several chromosomes, or in the shape of the chromosomes and their structure. Consequently, the structure of the karyotype can be a taxonomic (systematic) character, which is increasingly used in the taxonomy of animals and plants.

A graphic representation of a karyotype is called idiogram.

The number of chromosomes in mature germ cells is called haploid (denoted n ). Somatic cells contain double the number of chromosomes diploid set (2n ). Cells that have more than two sets of chromosomes are called polyploid (3 n, 4 n, 8 n, etc.).

The diploid set contains paired chromosomes that are identical in shape, structure and size, but have different origins (one is maternal, the other is paternal). They're called homologous.

In many higher dioecious animals in the diploid set there are one or two unpaired chromosomes that differ in males and females, this sexual chromosomes. The remaining chromosomes are called autosomes . Cases have been described where a male has only one sex chromosome, and a female has two.

In many fish, mammals (including humans), some amphibians (frogs of the genus Rana ), insects (beetles, Diptera, Orthoptera), the large chromosome is designated by the letter X, and the small by the letter Y. In these animals, in the karyotype of the female, the last pair is represented by two XX chromosomes, and in the male, by XY chromosomes.

In birds, reptiles, certain species of fish, some amphibians (tailed amphibians), and butterflies, the male sex has the same sex chromosomes ( WW -chromosomes), and the female are different ( WZ chromosomes).

In many animals and humans, in the cells of female individuals, one of the two sex chromosomes does not function and therefore remains entirely in a spiraled state (heterochromatin). It is found in the interphase nucleus in the form of a lumpsex chromatinat the inner nuclear membrane. Both sex chromosomes function throughout life in the male body. If sex chromatin is detected in the nuclei of the cells of a male body, this means that he has an extra X chromosome (XXY Kleinfelter's disease). This may occur as a result of impaired spermato- or oogenesis. The study of the content of sex chromatin in interphase nuclei is widely used in medicine to diagnose human chromosomal diseases caused by an imbalance of sex chromosomes.

Karyotype changes

Changes in the karyotype may be associated with a change in the number of chromosomes or a change in their structure.

Quantitative changes in karyotype: 1) polyploidy; 2) aneuploidy.

Polyploidy this is a multiple increase in the number of chromosomes compared to haploid. As a result, instead of ordinary diploid cells (2 n ) are formed, for example, triploid (3 n ), tetraploid (4 n ), octaploid (8 n ) cells. Thus, in onions, whose diploid cells contain 16 chromosomes, triploid cells contain 24 chromosomes, and tetraploid cells contain 32 chromosomes. Polyploid cells are large in size and have increased viability.

Polyploidy is widespread in nature, especially among plants, many species of which arose as a result of multiple doublings of the number of chromosomes. Most cultivated plants, for example, bread wheat, multi-row barley, potatoes, cotton, and most fruit and ornamental plants, are naturally occurring polyploids.

Experimentally, polyploid cells are most easily obtained by the action of an alkaloid colchicine or other substances that disrupt mitosis. Colchicine destroys the spindle, so that already duplicated chromosomes remain in the equatorial plane and do not diverge to the poles. After the cessation of the action of colchicine, the chromosomes form a common nucleus, but a larger one (polyploid). During subsequent divisions, the chromosomes will again double and move towards the poles, but twice the number of them will remain. Artificially obtained polyploids are widely used in plant breeding. Varieties of triploid sugar beet, tetraploid rye, buckwheat and other crops have been created.

In animals, complete polyploidy is very rare. For example, in the mountains of Tibet there lives one of the species of frogs, the population of which on the plain has a diploid chromosome set, and the high-mountain populations have a triploid, or even tetraploid.

In humans, polyploidy leads to sharply negative consequences. The birth of children with polyploidy is extremely rare. Usually the death of the organism occurs at the embryonic stage of development (about 22.6% of all spontaneous abortions are caused by polyploidy). It should be noted that triploidy occurs 3 times more often than tetraploidy. If children with triploidy syndrome are nevertheless born, they have abnormalities in the development of external and internal organs, are practically non-viable and die in the first days after birth.

Somatic polyploidy is more common. Thus, in human liver cells, with age, dividing cells become less and less, but the number of cells with a large nucleus or two nuclei increases. Determining the amount of DNA in such cells clearly shows that they have become polyploid.

Aneuploidy this is an increase or decrease in the number of chromosomes that is not a multiple of the haploid number. Aneuploid organisms, that is, organisms in which all cells contain aneuploid sets of chromosomes, are usually sterile or poorly viable. As an example of aneuploidy, consider some human chromosomal diseases. Kleinfelter's syndrome: the cells of the male body have an extra X chromosome, which leads to general physical underdevelopment of the body, in particular its reproductive system, and mental abnormalities. Down syndrome: an extra chromosome is contained in the 21st pair, which leads to mental retardation, abnormalities of internal organs; the disease is accompanied by some external signs of dementia and occurs in men and women. Turner syndrome is caused by a lack of one X chromosome in the cells of the female body; manifests itself in underdevelopment of the reproductive system, infertility, and external signs of dementia. If one X chromosome is missing in the cells of the male body, death occurs at the embryonic stage.

Aneuploid cells constantly arise in a multicellular organism as a result of disruption of the normal course of cell division. As a rule, such cells quickly die, but in some pathological conditions of the body they reproduce successfully. A high percentage of aneuploid cells is characteristic, for example, of many malignant tumors of humans and animals.

Structural changes in the karyotype.Chromosomal rearrangements, or chromosomal aberrations, occur as a result of single or multiple breaks of chromosomes or chromatids. Chromosome fragments at break sites are able to connect with each other or with fragments of other chromosomes in the set. Chromosomal aberrations are of the following types. Deletion this is the loss of the middle section of the chromosome. Difference this is the detachment of the end section of a chromosome. Inversion tearing off a section of a chromosome, rotating it 180 0 and joining to the same chromosome; this disrupts the order of nucleotides. Duplication breaking off a section of a chromosome and attaching it to a homologous chromosome. Translocation detachment of a section of a chromosome and its attachment to a non-homologous chromosome.

As a result of such rearrangements, dicentric and acentric chromosomes can be formed. Large deletions, differentiations and translocations dramatically change the morphology of chromosomes and are clearly visible under a microscope. Small deletions and translocations, as well as inversions, are detected by changes in the inheritance of genes localized in regions of chromosomes affected by the rearrangement, and by changes in the behavior of chromosomes during the formation of gametes.

Structural changes in the karyotype always lead to negative consequences. For example, “cry of the cat” syndrome is caused by a chromosomal mutation (division) in the 5th pair of chromosomes in humans; manifests itself in abnormal development of the larynx, which leads to “meowing” instead of a normal cry in early childhood, and retardation in physical and mental development.

Chromosome reduplication

The basis of the doubling (reduplication) of chromosomes is the process of DNA reduplication, i.e. the process of self-reproduction of nucleic acid macromolecules, ensuring accurate copying of genetic information and its transmission from generation to generation. DNA synthesis begins with the divergence of strands, each of which serves as a template for the synthesis of a daughter strand. The products of reduplication are two daughter DNA molecules, each of which consists of one parent and one daughter strand. An important place among reduplication enzymes is occupied by DNA polymerase, which carries out synthesis at a rate of about 1000 nucleotides per second (in bacteria). DNA reduplication is semi-conservative, i.e. during the synthesis of two daughter DNA molecules, each of them contains one “old” and one “new” chain (this method of reduplication was proven by Watson and Crick in 1953). Fragments synthesized during reduplication on one strand are “crosslinked” by the enzyme DNA ligase.

Reduplication involves proteins that unwind the DNA double helix, stabilize the untwisted sections, and prevent the molecules from becoming entangled.

DNA reduplication in eukaryotes occurs more slowly (about 100 nucleotides per second), but simultaneously at many points in one DNA molecule.

Since protein synthesis also occurs simultaneously with DNA reduplication, we can talk about chromosome reduplication. Studies conducted back in the 50s of the twentieth century showed that no matter how many longitudinally arranged DNA strands the chromosomes of organisms of different species contain, during cell division the chromosomes behave as if they consist of two simultaneously reduplicating subunits. After reduplication, which occurs in interphase, each chromosome turns out to be double, and even before division begins in the cell, everything is ready for an even distribution of chromosomes between daughter cells. If division does not occur after reduplication, the cell becomes polyploid. During the formation of polytene chromosomes, chromonemas are reduplicated, but do not diverge, due to which giant chromosomes with a huge number of chromonemas are obtained.

Approximately 1 in 150 children are born with chromosomal abnormality. These disorders are caused by errors in the number or structure of chromosomes. Many children with chromosomal problems have mental and/or physical birth defects. Some chromosomal problems ultimately lead to miscarriage or stillbirth.

Chromosomes are thread-like structures found in the cells of our body and containing a set of genes. Humans have about 20–25 thousand genes that determine characteristics such as eye and hair color, and are also responsible for the growth and development of every part of the body. Each person normally has 46 chromosomes, assembled into 23 chromosome pairs, in which one chromosome is inherited from the mother, and the second is inherited from the father.

Causes of chromosomal abnormalities

Chromosomal abnormalities are usually the result of an error that occurs during the maturation of a sperm or egg. Why these errors occur is not yet known.

Eggs and sperm normally contain 23 chromosomes. When they come together, they form a fertilized egg with 46 chromosomes. But sometimes something goes wrong during (or before) fertilization. For example, an egg or sperm may develop incorrectly, as a result of which they may have extra chromosomes, or, conversely, they may lack chromosomes.

In this case, cells with the wrong number of chromosomes are attached to a normal egg or sperm, as a result of which the resulting embryo has chromosomal abnormalities.

Most common type chromosomal abnormality called trisomy. This means that instead of having two copies of a particular chromosome, a person has three copies. For example, they have three copies of chromosome 21.

In most cases, an embryo with the wrong number of chromosomes does not survive. In such cases, the woman has a miscarriage, usually in the early stages. This often occurs very early in pregnancy, before the woman may even realize she is pregnant. More than 50% of miscarriages in the first trimester are caused by chromosomal abnormalities in the embryo.

Other errors may occur before fertilization. They can lead to changes in the structure of one or more chromosomes. People with structural chromosomal abnormalities usually have a normal number of chromosomes. However, small pieces of a chromosome (or an entire chromosome) may be deleted, copied, reversed, misplaced, or exchanged with part of another chromosome. These structural rearrangements may not have any effect on a person if he has all the chromosomes, but they are simply rearranged. In other cases, such rearrangements can lead to pregnancy loss or birth defects.

Errors in cell division can occur soon after fertilization. This can lead to mosaicism, a condition in which a person has cells with different genetic makeups. For example, people with one form of mosaicism, Turner syndrome, lack an X chromosome in some, but not all, cells.

Diagnosis of chromosomal abnormalities

Chromosomal abnormalities can be diagnosed before the baby is born through prenatal testing, such as amniocentesis or chorionic villus sampling, or after birth using a blood test.

The cells obtained from these tests are grown in the laboratory and then their chromosomes are examined under a microscope. The laboratory makes an image (karyotype) of all of a person's chromosomes, arranged in order from largest to smallest. A karyotype shows the number, size and shape of chromosomes and helps doctors identify any abnormalities.

The first prenatal screening consists of taking a maternal blood test in the first trimester of pregnancy (between 10 and 13 weeks of pregnancy), as well as a special ultrasound examination of the back of the baby's neck (the so-called nuchal translucency).

The second prenatal screening is carried out in the second trimester of pregnancy and consists of a maternal blood test between 16 and 18 weeks. This screening identifies pregnancies that are at higher risk for having genetic disorders.

However, screening tests cannot accurately diagnose Down syndrome or others. Doctors suggest that women who have abnormal screening test results undergo additional tests - chorionic villus sampling and amniocentesis - to definitively diagnose or rule out these disorders.

The most common chromosomal abnormalities

The first 22 pairs of chromosomes are called autosomes or somatic (non-sex) chromosomes. The most common abnormalities of these chromosomes include:

1. Down syndrome (trisomy 21) is one of the most common chromosomal abnormalities, diagnosed in approximately 1 in 800 babies. People with Down syndrome have varying degrees of mental development, characteristic facial features and, often, congenital abnormalities in the development of the heart and other problems.

Modern prospects for the development of children with Down syndrome are much brighter than they were before. Most of them have mild to moderate intellectual disabilities. With early intervention and special education, many of these children learn to read and write and participate in a variety of activities from childhood on.

The risk of Down syndrome and other trisomies increases with maternal age. The risk of having a child with Down syndrome is approximately:

• 1 in 1300 – if the mother is 25 years old;
• 1 in 1000 – if the mother is 30 years old;
• 1 in 400 – if the mother is 35 years old;
• 1 in 100 – if the mother is 40 years old;
• 1 in 35 – if the mother is 45 years old.

2. Trisomy 13 and 18 chromosomes – these trisomies are usually more serious than Down syndrome, but fortunately are quite rare. About 1 in 16,000 babies are born with trisomy 13 (Patau syndrome), and 1 in 5,000 babies are born with trisomy 18 (Edwards syndrome). Children with trisomy 13 and 18 typically suffer from severe mental retardation and many birth defects. Most of these children die before the age of one year.

The last, 23rd pair of chromosomes are the sex chromosomes, called the X chromosomes and the Y chromosomes. Typically, women have two X chromosomes, while men have one X chromosome and one Y chromosome. Sex chromosome abnormalities can cause infertility, growth problems, and learning and behavior problems.

The most common sex chromosome abnormalities include:

1. Turner syndrome – This disorder affects approximately 1 in 2,500 female fetuses. A girl with Turner syndrome has one normal X chromosome and is completely or partially missing a second X chromosome. Typically, these girls are infertile and will not undergo the changes of normal puberty unless they take synthetic sex hormones.

Girls affected by Turner syndrome are very short, although treatment with growth hormone may help increase height. In addition, they have a whole range of health problems, especially with the heart and kidneys. Most girls with Turner syndrome have normal intelligence, although they experience some learning difficulties, especially in mathematics and spatial reasoning.

2. Trisomy X chromosome – About 1 in 1000 women have an extra X chromosome. Such women are very tall. They typically have no physical birth defects, experience normal puberty, and are fertile. Such women have normal intelligence, but may also have serious problems with learning.

Since such girls are healthy and have a normal appearance, their parents often do not know that their daughter has it. Some parents find out that their child has a similar disorder if the mother underwent one of the invasive prenatal diagnostic methods (amniocentesis or choriocentesis) during pregnancy.

3. Klinefelter syndrome – This disorder affects approximately 1 in 500 to 1000 boys. Boys with Klinefelter syndrome have two (and sometimes more) X chromosomes along with one normal Y chromosome. Such boys usually have normal intelligence, although many have problems with learning. When such boys grow up, they have decreased testosterone secretion and are infertile.

4. Disomy on the Y chromosome (XYY) – About 1 in 1,000 men are born with one or more extra Y chromosomes. These men experience normal puberty and are not infertile. Most have normal intelligence, although there may be some learning difficulties, behavioral difficulties and problems with speech and language acquisition. As with trisomy X in women, many men and their parents do not know they have the disorder until prenatal diagnosis.

Less common chromosomal abnormalities

New methods of chromosome analysis can detect tiny chromosomal abnormalities that cannot be seen even under a powerful microscope. As a result, more and more parents are learning that their child has a genetic abnormality.

Some of these unusual and rare anomalies include:

• Deletion – absence of a small section of a chromosome;
• Microdeletion - the absence of a very small number of chromosomes, perhaps only one gene is missing;
• Translocation - part of one chromosome joins another chromosome;
• Inversion - part of the chromosome is skipped, and the order of the genes is reversed;
• Duplication (duplication) - part of the chromosome is duplicated, which leads to the formation of additional genetic material;
• Ring Chromosome – When genetic material is removed from both ends of the chromosome and the new ends join together to form a ring.

Some chromosomal pathologies are so rare that only one or a few cases are known to science. Some abnormalities (for example, some translocations and inversions) may have no effect on a person's health if non-genetic material is missing.

Some unusual disorders may be caused by small chromosomal deletions. Examples are:

• Cry Cat Syndrome (deletion on chromosome 5) - sick children in infancy are distinguished by a high-pitched cry, as if a cat is screaming. They have significant problems in physical and intellectual development. Approximately 1 in 20–50 thousand babies are born with this disease;
• Prader-Will syndromeAnd (deletion on chromosome 15) - sick children have deviations in mental development and learning, short stature and behavioral problems. Most of these children develop extreme obesity. Approximately 1 in 10–25 thousand babies are born with this disease;
• DiGeorge syndrome (chromosome 22 deletion or 22q11 deletion) – About 1 in 4,000 babies are born with a deletion in a specific part of chromosome 22. This deletion causes a variety of problems that may include heart defects, cleft lip/palate (cleft palate and cleft lip), immune system disorders, abnormal facial features and learning problems;
• Wolf-Hirschhorn syndrome (deletion on chromosome 4) – this disorder is characterized by mental retardation, heart defects, poor muscle tone, seizures and other problems. This condition affects approximately 1 in 50,000 babies.

With the exception of people with DiGeorge syndrome, people with the above syndromes are infertile. As for people with DiGeorge syndrome, this pathology is inherited by 50% with each pregnancy.

New methods of chromosome analysis can sometimes pinpoint where genetic material is missing, or where an extra gene is present. If the doctor knows exactly where the culprit is chromosomal abnormality, he can assess the full extent of its influence on the child and give an approximate forecast for the development of this child in the future. Often this helps parents decide to continue the pregnancy and prepare in advance for the birth of a baby that is a little different from everyone else.

Heredity and variability in living nature exist thanks to chromosomes, genes, (DNA). It is stored and transmitted in the form of a chain of nucleotides as part of DNA. What role do genes play in this phenomenon? What is a chromosome from the point of view of transmission of hereditary characteristics? Answers to questions like these provide insight into coding principles and genetic diversity on our planet. It largely depends on how many chromosomes are included in the set and 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 threads 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 treat nuclear structures. Flemming called the discovered substance “chromatin” for its ability to stain. The term “chromosomes” was introduced into scientific use in 1888 by Heinrich Waldeyer.

At the same time as Flemming, the Belgian Eduard van Beneden was looking for an answer to the question of what a chromosome is. A little earlier, German biologists Theodor Boveri and Eduard Strassburger conducted a series of experiments proving the individuality of chromosomes and the constancy of their number in different species of living organisms.

Prerequisites for the chromosomal 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, characteristics of the organism. Sutton discovered that chromosomes consist of genes through which properties and functions are passed on to offspring from their parents. The geneticist in his publications gave descriptions of chromosome pairs and their movement during the division of the cell nucleus.

Regardless of his American colleague, work in the same direction was carried out by Theodore Boveri. Both researchers in their works studied the issues of transmission of hereditary characteristics and formulated the main provisions on the role of chromosomes (1902-1903). Further development of the Boveri-Sutton theory took place in the laboratory of Nobel laureate Thomas Morgan. The outstanding American biologist and his assistants established a number of patterns of gene placement on the chromosome and developed a cytological basis 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 filaments are found in prokaryotic organisms (non-nuclear) and eukaryotic cells (in nuclei). Study under a microscope made it possible to establish what a chromosome is from a morphological point of view. It is a mobile filamentous body that is visible during certain phases of the cell cycle. In interphase, the entire volume of the nucleus is occupied by chromatin. During other periods, chromosomes are distinguishable in the form of one or two chromatids.

These formations are better visible during cell division - mitosis or meiosis. More often, large chromosomes of a linear structure can be observed. In prokaryotes they are smaller, although there are exceptions. Cells often contain more than one type of chromosome, for example mitochondria and chloroplasts have their own small “particles of inheritance”.

Chromosome shapes

Each chromosome has an individual structure and differs from others in its coloring 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 unequal arms (the constriction is shifted towards one of the telomeres);
• acrocentric, or rod-shaped, in which the centromere is located almost at the end of the chromosome;
• dotted with a difficult-to-define shape.

Functions of chromosomes

Chromosomes consist of genes - functional units of heredity. Telomeres are the ends of chromosome arms. These specialized elements serve to protect against damage and prevent fragments from sticking together. The centromere performs its tasks during chromosome doubling. It has a kinetochore, and it is to this that the spindle structures are attached. Each pair of chromosomes is individual in the location of the centromere. The spindle threads work in such a way that one chromosome at a time goes to the daughter cells, and not both. Uniform doubling during division is provided by the origins of replication. Duplication of each chromosome begins simultaneously at several such points, which significantly speeds up the entire division process.

Role of DNA and RNA

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

Nucleic acids are directly involved in the biosynthesis of amino acids and proteins and ensure the transmission of hereditary characteristics from generation to generation. DNA is contained in the nucleus of a eukaryotic cell, 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 represent the carbohydrate deoxyribose, a phosphate group, and one of four nitrogenous bases:

Regions of helical deoxyribonucleoprotein strands are genes that carry encoded information about the sequence of amino acids in proteins or RNA. During reproduction, hereditary characteristics from parents are transmitted 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 thousand genes.

Set of chromosomes

The total number of chromosomes and their features are a characteristic feature of the species. In the Drosophila fly their number is 8, in primates - 48, in humans - 46. This number is constant for the cells of organisms that belong to the same species. For all eukaryotes there is the concept of “diploid chromosomes”. This is a complete 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 examine and study the distinctive features of each pair. is present in the somatic ones - in the reproductive ones (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 the Y. Males have a set of XY, females have a set of 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. What differs from them is the 23rd pair - sex chromosomes, which are non-homologous in the cells of the male body.

Genetic traits are associated with gender. They are transmitted by a Y and an X chromosome in men and two X chromosomes 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 distinguishable in the drawings when painted in a certain color. It is noticeable that the 22nd chromosome in the human genome is the smallest. Its DNA, when stretched, is 1.5 cm long and has 48 million nitrogen base pairs. Special histone proteins from the composition of chromatin perform compression, after which the thread takes up thousands of times less space in the cell nucleus. Under an electron microscope, the histones in the interphase core resemble beads strung on a strand of DNA.

Genetic diseases

There are more than 3 thousand hereditary diseases of various types caused by damage and abnormalities in chromosomes. These include Down syndrome. A child with such a genetic disease is characterized by delays in mental and physical development. With cystic fibrosis, a malfunction occurs in the functions of the exocrine glands. Violation leads to problems with sweating, secretion and accumulation of mucus in the body. It makes it difficult for the lungs to function and can lead to suffocation and death.

Color vision impairment - color blindness - insensitivity to certain parts of the color spectrum. Hemophilia leads to weakened blood clotting. Lactose intolerance prevents the human body from digesting milk sugar. In family planning offices you can find out about your predisposition to a particular genetic disease. In large medical centers it is possible to undergo appropriate examination and treatment.

Gene therapy is a direction of modern medicine, identifying the genetic cause of hereditary diseases and eliminating it. Using the latest methods, normal genes are introduced into pathological cells instead of damaged ones. In this case, doctors relieve the patient not from the symptoms, but from the causes that caused the disease. Only correction of somatic cells is carried out; gene therapy methods are not yet applied en masse to germ cells.

​What mutations, besides Down syndrome, threaten us? Is it possible to cross a man with a monkey? And what will happen to our genome in the future? The editor of the portal ANTHROPOGENES.RU talked about chromosomes with a geneticist, head. lab. comparative genomics SB RAS Vladimir Trifonov.

− Can you explain in simple language what a chromosome is?

− A chromosome is a fragment of the genome of any organism (DNA) in complex 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 fragments of DNA and protein are clearly visible in a light microscope during cell division. That is why chromosomes as colorable structures (“chroma” - color in Greek) were described at the end of the 19th century.

− Is there any relationship between the number of chromosomes and the complexity of an 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 based on external characteristics. Female 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 up to a dozen additional chromosomes). In mammals, the evolution of chromosome breaks and fusions proceeded quite intensively, and now we are seeing the results of this process, when each species often has characteristic features of its karyotype (set of chromosomes). But, undoubtedly, the general increase in genome size was a necessary step in the evolution of eukaryotes. At the same time, how this genome is distributed into individual fragments does not seem to be very important.

− What are some common misconceptions about chromosomes? People often get confused: genes, chromosomes, DNA...

− Since chromosomal rearrangements do occur frequently, 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 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 normal too narrowly.

− What chromosomal mutations occur in modern humans and what do they lead to?

− The most common chromosomal abnormalities are:

− Klinefelter syndrome (XXY men) (1 in 500) – characteristic external signs, certain health problems (anemia, osteoporosis, muscle weakness and sexual dysfunction), sterility. There may be behavioral features. However, many symptoms (except sterility) can be corrected by administering testosterone. Using modern reproductive technologies, it is possible to obtain healthy children from carriers of this syndrome;

− Down syndrome (1 in 1000) – characteristic external signs, delayed cognitive development, short life expectancy, may be fertile;

− trisomy X (XXX women) (1 in 1000) – most often there are no manifestations, fertility;

− XYY syndrome (men) (1 in 1000) – almost no manifestations, but there may be behavioral characteristics and possible reproductive problems;

− Turner syndrome (women with CP) (1 in 1500) – short stature and other developmental features, normal intelligence, sterility;

− balanced translocations (1 in 1000) – depends on the type, in some cases developmental defects and mental retardation may be observed and may affect fertility;

− small additional chromosomes (1 in 2000) – the manifestation depends on the genetic material on the chromosomes and varies from neutral to serious clinical symptoms;

Pericentric inversion of chromosome 9 occurs in 1% of the human population, but this rearrangement is considered a normal variant.

Is the difference in the number of chromosomes an obstacle to crossing?

− 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 descendants may turn out to be sterile. There are a lot of hybrids known between species with different numbers of chromosomes, for example, equines: there are all kinds of hybrids between horses, zebras and donkeys, and the number of chromosomes in all equines is different and, accordingly, the hybrids are often sterile. However, this does not exclude the possibility that balanced gametes may be produced by chance.

- What unusual things have been discovered recently in the field of chromosomes?

− Recently, there have been many discoveries regarding the structure, function and evolution of chromosomes. I especially like the work that showed that sex chromosomes were formed completely independently in different groups of animals.

- Still, is it possible to cross a man with a monkey?

- Theoretically, it is possible to obtain such a hybrid. Recently, hybrids of much more evolutionarily distant mammals (white and black rhinoceros, alpaca and camel, and so on) have been obtained. The red wolf in America has long been considered a separate species, but has recently been proven to be a hybrid between a wolf and a coyote. There are a huge number of feline hybrids known.

- And a completely absurd question: is it possible to cross a hamster with a duck?

- Here, most likely, nothing will work out, because too many genetic differences have accumulated over hundreds of millions of years of evolution for the carrier of such a mixed genome to function.

- Is it possible that in the future a person will have fewer or more chromosomes?

- Yes, this is quite possible. It is possible that a pair of acrocentric chromosomes will merge and such a mutation will spread throughout the population.

− What popular science literature do you recommend on the topic of human genetics? What about popular science films?

− Books by biologist Alexander Markov, the three-volume “Human Genetics” by Vogel and Motulsky (though this is not science-pop, but there is good reference data there). Nothing comes to mind from films about human genetics... But Shubin’s “Inner Fish” is an excellent film and book of the same name about the evolution of vertebrates.