Chromosomes are the main structural elements of the cell nucleus, which are carriers of genes in which hereditary information is encoded. Having the ability to reproduce themselves, chromosomes provide a genetic link between generations.
The morphology of chromosomes is related to the degree of their spiralization. For example, if at the stage of interphase (see Mitosis, Meiosis) the chromosomes are maximally unfolded, i.e., despiralized, then with the beginning of division the chromosomes intensively spiralize and shorten. Maximum spiralization and shortening of chromosomes is achieved at the metaphase stage, when relatively short, dense structures that are intensely stained with basic dyes are formed. This stage is most convenient for studying the morphological characteristics of chromosomes.
The metaphase chromosome consists of two longitudinal subunits - chromatids [reveals elementary threads in the structure of chromosomes (the so-called chromonemas, or chromofibrils) 200 Å thick, each of which consists of two subunits].
The sizes of plant and animal chromosomes vary significantly: from fractions of a micron to tens of microns. The average lengths of human metaphase chromosomes range from 1.5-10 microns.
The chemical basis of the structure of chromosomes are nucleoproteins - complexes (see) with the main proteins - histones and protamines.
Rice. 1. The structure of a normal chromosome.
A - appearance; B - internal structure: 1-primary constriction; 2 - secondary constriction; 3 - satellite; 4 - centromere.
Individual chromosomes (Fig. 1) are distinguished by the localization of the primary constriction, i.e., the location of the centromere (during mitosis and meiosis, spindle threads are attached to this place, pulling it towards the pole). When a centromere is lost, chromosome fragments lose their ability to separate during division. The primary constriction divides the chromosomes into 2 arms. Depending on the location of the primary constriction, chromosomes are divided into metacentric (both arms are equal or almost equal in length), submetacentric (arms of unequal length) and acrocentric (the centromere is shifted to the end of the chromosome). In addition to the primary one, less pronounced secondary constrictions may be found in chromosomes. The small terminal portion of the chromosomes, separated by a secondary constriction, is called a satellite.
Each type of organism is characterized by its own specific (in terms of the number, size and shape of chromosomes) so-called chromosome set. The totality of a double, or diploid, set of chromosomes is designated as a karyotype.
Rice. 2. Normal chromosome set of a woman (two X chromosomes in the lower right corner).
Rice. 3. The normal chromosome set of a man (in the lower right corner - X and Y chromosomes in sequence).
Mature eggs contain a single, or haploid, set of chromosomes (n), which makes up half of the diploid set (2n) inherent in the chromosomes of all other cells of the body. In the diploid set, each chromosome is represented by a pair of homologues, one of which is of maternal and the other of paternal origin. In most cases, the chromosomes of each pair are identical in size, shape and gene composition. The exception is sex chromosomes, the presence of which determines the development of the body in a male or female direction. The normal human chromosome set consists of 22 pairs of autosomes and one pair of sex chromosomes. In humans and other mammals, female is determined by the presence of two X chromosomes, and male by one X and one Y chromosome (Fig. 2 and 3). In female cells, one of the X chromosomes is genetically inactive and is found in the interphase nucleus in the form (see). The study of human chromosomes in health and disease is the subject of medical cytogenetics. It has been established that deviations in the number or structure of chromosomes from the norm that occur in reproductive organs! cells or in the early stages of fragmentation of a fertilized egg, cause disturbances in the normal development of the body, causing in some cases the occurrence of some spontaneous abortions, stillbirths, congenital deformities and developmental abnormalities after birth (chromosomal diseases). Examples of chromosomal diseases include Down's disease (an extra G chromosome), Klinefelter's syndrome (an extra X chromosome in men) and (the absence of a Y or one of the X chromosomes in the karyotype). In medical practice, chromosomal analysis is carried out either directly (on bone marrow cells) or after short-term cultivation of cells outside the body (peripheral blood, skin, embryonic tissue).
Chromosomes (from the Greek chroma - color and soma - body) are thread-like, self-reproducing structural elements of the cell nucleus, containing factors of heredity - genes - in a linear order. Chromosomes are clearly visible in the nucleus during the division of somatic cells (mitosis) and during the division (maturation) of germ cells - meiosis (Fig. 1). In both cases, chromosomes are intensely stained with basic dyes and are also visible on unstained cytological preparations in phase contrast. In the interphase nucleus, the chromosomes are despiralized and are not visible in a light microscope, since their transverse dimensions exceed the resolution limits of the light microscope. At this time, individual sections of chromosomes in the form of thin threads with a diameter of 100-500 Å can be distinguished using an electron microscope. Individual non-despiralized sections of chromosomes in the interphase nucleus are visible through a light microscope as intensely stained (heteropyknotic) areas (chromocenters).
Chromosomes continuously exist in the cell nucleus, undergoing a cycle of reversible spiralization: mitosis-interphase-mitosis. The basic patterns of the structure and behavior of chromosomes in mitosis, meiosis and during fertilization are the same in all organisms.
Chromosomal theory of heredity. Chromosomes were first described by I. D. Chistyakov in 1874 and E. Strasburger in 1879. In 1901, E. V. Wilson, and in 1902, W. S. Sutton, drew attention to parallelism in the behavior of chromosomes and Mendelian factors of heredity - genes - in meiosis and during fertilization and came to the conclusion that genes are located in chromosomes. In 1915-1920 Morgan (T.N. Morgan) and his collaborators proved this position, localized several hundred genes in Drosophila chromosomes and created genetic maps of the chromosomes. Data on chromosomes obtained in the first quarter of the 20th century formed the basis of the chromosomal theory of heredity, according to which the continuity of the characteristics of cells and organisms in a number of their generations is ensured by the continuity of their chromosomes.
Chemical composition and autoreproduction of chromosomes. As a result of cytochemical and biochemical studies of chromosomes in the 30s and 50s of the 20th century, it was established that they consist of constant components [DNA (see Nucleic acids), basic proteins (histones or protamines), non-histone proteins] and variable components (RNA and acidic protein associated with it). The basis of chromosomes is made up of deoxyribonucleoprotein threads with a diameter of about 200 Å (Fig. 2), which can be connected into bundles with a diameter of 500 Å.
The discovery by Watson and Crick (J. D. Watson, F. N. Crick) in 1953 of the structure of the DNA molecule, the mechanism of its autoreproduction (reduplication) and the nucleic code of DNA and the development of molecular genetics that arose after this led to the idea of genes as sections of the DNA molecule. (see Genetics). The patterns of autoreproduction of chromosomes were revealed [Taylor (J. N. Taylor) et al., 1957], which turned out to be similar to the patterns of autoreproduction of DNA molecules (semi-conservative reduplication).
Chromosome set- the totality of all chromosomes in a cell. Each biological species has a characteristic and constant set of chromosomes, fixed in the evolution of this species. There are two main types of sets of chromosomes: single, or haploid (in animal germ cells), denoted n, and double, or diploid (in somatic cells, containing pairs of similar, homologous chromosomes from the mother and father), denoted 2n.
The sets of chromosomes of individual biological species vary significantly in the number of chromosomes: from 2 (horse roundworm) to hundreds and thousands (some spore plants and protozoa). The diploid chromosome numbers of some organisms are as follows: humans - 46, gorillas - 48, cats - 60, rats - 42, fruit flies - 8.
The sizes of chromosomes also vary between species. The length of chromosomes (in metaphase of mitosis) varies from 0.2 microns in some species to 50 microns in others, and the diameter from 0.2 to 3 microns.
The morphology of chromosomes is well expressed in metaphase of mitosis. It is metaphase chromosomes that are used to identify chromosomes. In such chromosomes, both chromatids are clearly visible, into which each chromosome and the centromere (kinetochore, primary constriction) connecting the chromatids are longitudinally split (Fig. 3). The centromere is visible as a narrowed area that does not contain chromatin (see); the threads of the achromatin spindle are attached to it, due to which the centromere determines the movement of chromosomes to the poles in mitosis and meiosis (Fig. 4).
Loss of a centromere, for example when a chromosome is broken by ionizing radiation or other mutagens, leads to the loss of the ability of the piece of chromosome lacking the centromere (acentric fragment) to participate in mitosis and meiosis and to its loss from the nucleus. This can cause severe cell damage.
The centromere divides the chromosome body into two arms. The location of the centromere is strictly constant for each chromosome and determines three types of chromosomes: 1) acrocentric, or rod-shaped, chromosomes with one long and a second very short arm, resembling a head; 2) submetacentric chromosomes with long arms of unequal length; 3) metacentric chromosomes with arms of the same or almost the same length (Fig. 3, 4, 5 and 7).
Rice. 4. Scheme of chromosome structure in metaphase of mitosis after longitudinal splitting of the centromere: A and A1 - sister chromatids; 1 - long shoulder; 2 - short shoulder; 3 - secondary constriction; 4- centromere; 5 - spindle fibers.
Characteristic features of the morphology of certain chromosomes are secondary constrictions (which do not have the function of a centromere), as well as satellites - small sections of chromosomes connected to the rest of its body by a thin thread (Fig. 5). Satellite filaments have the ability to form nucleoli. The characteristic structure in the chromosome (chromomeres) is thickening or more tightly coiled sections of the chromosomal thread (chromonemas). The chromomere pattern is specific to each pair of chromosomes.
Rice. 5. Scheme of chromosome morphology in anaphase of mitosis (chromatid extending to the pole). A - appearance of the chromosome; B - internal structure of the same chromosome with its two constituent chromonemas (hemichromatids): 1 - primary constriction with chromomeres constituting the centromere; 2 - secondary constriction; 3 - satellite; 4 - satellite thread.
The number of chromosomes, their size and shape at the metaphase stage are characteristic of each type of organism. The combination of these characteristics of a set of chromosomes is called a karyotype. A karyotype can be represented in a diagram called an idiogram (see human chromosomes below).
Sex chromosomes. Genes that determine sex are localized in a special pair of chromosomes - sex chromosomes (mammals, humans); in other cases, the iol is determined by the ratio of the number of sex chromosomes and all others, called autosomes (Drosophila). In humans, as in other mammals, the female sex is determined by two identical chromosomes, designated as X chromosomes, the male sex is determined by a pair of heteromorphic chromosomes: X and Y. As a result of reduction division (meiosis) during the maturation of oocytes (see Oogenesis) in women all eggs contain one X chromosome. In men, as a result of the reduction division (maturation) of spermatocytes, half of the sperm contains an X chromosome, and the other half a Y chromosome. The sex of a child is determined by the accidental fertilization of an egg by a sperm carrying an X or Y chromosome. The result is a female (XX) or male (XY) embryo. In the interphase nucleus of women, one of the X chromosomes is visible as a clump of compact sex chromatin.
Chromosome functioning and nuclear metabolism. Chromosomal DNA is the template for the synthesis of specific messenger RNA molecules. This synthesis occurs when a given region of the chromosome is despiraled. Examples of local chromosome activation are: the formation of despiralized chromosome loops in the oocytes of birds, amphibians, fish (the so-called X-lamp brushes) and swellings (puffs) of certain chromosome loci in multi-stranded (polytene) chromosomes of the salivary glands and other secretory organs of dipteran insects (Fig. 6). An example of inactivation of an entire chromosome, i.e., its exclusion from the metabolism of a given cell, is the formation of one of the X chromosomes of a compact body of sex chromatin.
Rice. 6. Polytene chromosomes of the dipteran insect Acriscotopus lucidus: A and B - area limited by dotted lines, in a state of intensive functioning (puff); B - the same area in a non-functioning state. The numbers indicate individual chromosome loci (chromomeres).
Rice. 7. Chromosome set in a culture of male peripheral blood leukocytes (2n=46).
Revealing the mechanisms of functioning of lampbrush-type polytene chromosomes and other types of chromosome spiralization and despiralization is crucial for understanding reversible differential gene activation.
Human chromosomes. In 1922, T. S. Painter established the diploid number of human chromosomes (in spermatogonia) to be 48. In 1956, Tio and Levan (N. J. Tjio, A. Levan) used a set of new methods for studying human chromosomes : cell culture; study of chromosomes without histological sections on whole cell preparations; colchicine, which leads to the arrest of mitoses at the metaphase stage and the accumulation of such metaphases; phytohemagglutinin, which stimulates the entry of cells into mitosis; treatment of metaphase cells with hypotonic saline solution. All this made it possible to clarify the diploid number of chromosomes in humans (it turned out to be 46) and provide a description of the human karyotype. In 1960, in Denver (USA), an international commission developed a nomenclature for human chromosomes. According to the commission's proposals, the term "karyotype" should be applied to the systematic set of chromosomes of a single cell (Fig. 7 and 8). The term "idiotram" is retained to represent the set of chromosomes in the form of a diagram constructed from measurements and descriptions of the chromosome morphology of several cells.
Human chromosomes are numbered (somewhat serially) from 1 to 22 in accordance with the morphological features that allow their identification. Sex chromosomes do not have numbers and are designated as X and Y (Fig. 8).
A connection has been discovered between a number of diseases and birth defects in human development with changes in the number and structure of its chromosomes. (see Heredity).
See also Cytogenetic studies.
All these achievements have created a solid basis for the development of human cytogenetics.
Rice. 1. Chromosomes: A - at the anaphase stage of mitosis in trefoil microsporocytes; B - at the metaphase stage of the first meiotic division in the pollen mother cells of Tradescantia. In both cases, the spiral structure of the chromosomes is visible.
Rice. 2. Elementary chromosomal threads with a diameter of 100 Å (DNA + histone) from interphase nuclei of the calf thymus gland (electron microscopy): A - threads isolated from nuclei; B - thin section through the film of the same preparation.
Rice. 3. Chromosome set of Vicia faba (faba bean) at the metaphase stage.
Rice. 8. Chromosomes are the same as in Fig. 7, sets, systematized according to the Denver nomenclature into pairs of homologues (karyotype).
MOSCOW, July 4— RIA Novosti, Anna Urmantseva. Who has the larger genome? As you know, some creatures have a more complex structure than others, and since everything is written in DNA, then this should also be reflected in its code. It turns out that a person with his developed speech must be more complex than a small round worm. However, if you compare us with a worm in terms of the number of genes, you get about the same thing: 20 thousand genes of Caenorhabditis elegans versus 20-25 thousand of Homo sapiens.
Even more offensive for the “crown of earthly creatures” and the “king of nature” are comparisons with rice and corn - 50 thousand genes in relation to human 25.
However, maybe we think wrong? Genes are “boxes” in which nucleotides are packaged—the “letters” of the genome. Maybe count them? Humans have 3.2 billion nucleotide pairs. But the Japanese crow's eye (Paris japonica) - a beautiful plant with white flowers - has 150 billion base pairs in its genome. It turns out that a person should be 50 times simpler than some flower.
And the lungfish protoptera (lungfish - having both gill and pulmonary respiration) turns out to be 40 times more complex than humans. Maybe all fish are somehow more complex than people? No. The poisonous fugu fish, from which the Japanese prepare a delicacy, has a genome eight times smaller than that of humans and 330 times smaller than that of the lungfish Protoptera.
All that remains is to count the chromosomes - but this confuses the picture even more. How can a person be equal in number of chromosomes to an ash tree, and a chimpanzee to a cockroach?
Evolutionary biologists and geneticists encountered these paradoxes a long time ago. They were forced to admit that the size of the genome, no matter how we try to calculate it, is strikingly unrelated to the complexity of the organization of organisms. This paradox was called the “C-value mystery,” where C is the amount of DNA in the cell (C-value paradox, the exact translation is “genome size paradox”). And yet some correlations between species and kingdoms exist.
© Illustration by RIA Novosti. A. Polyanina
© Illustration by RIA Novosti. A. Polyanina
It is clear, for example, that eukaryotes (living organisms whose cells contain a nucleus) have, on average, larger genomes than prokaryotes (living organisms whose cells do not contain a nucleus). Vertebrates have, on average, larger genomes than invertebrates. However, there are exceptions that no one has yet been able to explain.
There have been suggestions that genome size is related to the length of an organism's life cycle. Using plants as an example, some scientists have argued that perennial species have larger genomes than annuals, usually with a difference of several times. And the smallest genomes belong to ephemeral plants, which go through the full cycle from birth to death within a few weeks. This issue is currently being actively discussed in scientific circles.
Explains the leading researcher at the Institute of General Genetics. N.I. Vavilova of the Russian Academy of Sciences, Professor of the Texas Agromechanical University and the University of Gottingen Konstantin Krutovsky: “The size of the genome is not related to the duration of the life cycle of the organism! For example, there are species within the same genus that have the same genome size, but may differ in life expectancy tens, if not hundreds of times. In general, there is a connection between genome size and evolutionary advancement and complexity of organization, but with many exceptions. Generally, genome size is associated with ploidy (copy number) of the genome (and polyploids are found in both plants and animals) and amount of highly repetitive DNA (simple and complex repeats, transposons and other mobile elements)."
There are also scientists who have a different point of view on this issue.
Containing genes. The name "chromosome" comes from the Greek words (chrōma - color, color and sōma - body), and is due to the fact that when cells divide, they become intensely colored in the presence of basic dyes (for example, aniline).
Many scientists, since the beginning of the 20th century, have thought about the question: “How many chromosomes does a person have?” So, until 1955, all the “minds of humanity” were convinced that the number of chromosomes in humans is 48, i.e. 24 pairs. The reason was that Theophilus Painter (Texas scientist) incorrectly counted them in preparative sections of human testes, according to a court decision (1921). Subsequently, other scientists, using different calculation methods, also came to this opinion. Even after developing a method for separating chromosomes, the researchers did not challenge Painter’s result. The error was discovered by scientists Albert Levan and Jo-Hin Thio in 1955, who accurately calculated how many pairs of chromosomes a person has, namely 23 (more modern technology was used to count them).
Somatic and germ cells contain a different chromosome set in biological species, which cannot be said about the morphological characteristics of chromosomes, which are constant. have a doubled (diploid set), which is divided into pairs of identical (homologous) chromosomes, which are similar in morphology (structure) and size. One part is always of paternal origin, the other of maternal origin. Human sex cells (gametes) are represented by a haploid (single) set of chromosomes. When an egg is fertilized, haploid sets of female and male gametes are united in one zygote nucleus. In this case, the double dialing is restored. It is possible to say with accuracy how many chromosomes a person has - there are 46 of them, with 22 pairs of them being autosomes and one pair being sex chromosomes (gonosomes). Sexes have differences - both morphological and structural (gene composition). In a female organism, a pair of gonosomes contains two X chromosomes (XX-pair), and in a male organism, one X- and a Y-chromosome (XY-pair).
Morphologically, chromosomes change during cell division, when they double (with the exception of germ cells, in which duplication does not occur). This is repeated many times, but no change in the chromosome set is observed. Chromosomes are most noticeable at one of the stages of cell division (metaphase). During this phase, the chromosomes are represented by two longitudinally split formations (sister chromatids), which narrow and unite in the area of the so-called primary constriction, or centromere (an obligatory element of the chromosome). Telomeres are the ends of a chromosome. Structurally, human chromosomes are represented by DNA (deoxyribonucleic acid), which encodes the genes that make up them. Genes, in turn, carry information about a specific trait.
Individual development will depend on how many chromosomes a person has. There are such concepts as: aneuploidy (change in the number of individual chromosomes) and polyploidy (the number of haploid sets is greater than the diploid one). The latter can be of several types: loss of a homologous chromosome (monosomy), or appearance (trisomy - one extra, tetrasomy - two extra, etc.). All this is a consequence of genomic and chromosomal mutations, which can lead to pathological conditions such as Klinefelter syndrome, Shereshevsky-Turner syndrome and other diseases.
Thus, only the twentieth century gave answers to all questions, and now every educated inhabitant of planet Earth knows how many chromosomes a person has. The sex of the unborn child depends on the composition of the 23 pairs of chromosomes (XX or XY), and this is determined during fertilization and the fusion of the female and male reproductive cells.
Considering our body at the cellular level, you will definitely come across its structural unit - the chromosome. It is where the genes are contained. From Greek, this concept can be literally translated as “body coloring.” Why such a strange name? The fact is that during cell division, structural units can become colored when interacting with natural dyes. The chromosome is a valuable carrier of information. Therefore, when a person develops the wrong number of chromosomes, this indicates a pathological process.
In contact with
Normal for a healthy person
According to the latest statistics, 1% of newborns today are born with abnormalities at the physiological level, when an insufficient number of chromosomes appears. This problem is already becoming global, causing great concern among doctors. A healthy person (male or female) has 46 chromosomes, that is, 23 pairs. An interesting fact is that until 1996, scientists had no doubt that there were not 23, but 24 pairs of structural units. The mistake was made by Theophilus Painter, a well-known scientist in his circle. It was found and corrected by two other luminaries - Albert Levan and Jo-Hin Tyo.
All chromosomes have the same morphological characteristics, but germ and somatic cells have a different set of structural units. What is this difference?
When cell division occurs (that is, their number begins to double), changes in chromosomes are observed at the morphological level. But, despite the fact that such complex processes occur in our body, the number of chromosomes in a person still remains the same - 46. His intellectual development and general health depend on how many pairs of chromosomes a person should have. That is why it is very important for doctors to pay attention to this issue during the pregnancy planning process. Often, the gynecologist recommends that young couples contact a geneticist who will conduct some important clinical studies.
At conception, a person receives one of the units in a pair from the biological mother, and the second from the biological father. But the sex of the unborn baby depends on the 23rd pair. When studying the human karyotype, it is important to explain that the chromosome set of healthy people consists of 22 autosomes, as well as one male and one female chromosome (the so-called sex chromosomes). A person’s karyotype can be determined without any problems by simply studying the totality of the characteristics of these units in one cell. If any abnormality is found in the karyotype, the person will face big health problems.
There can be several problems at the gene level. And all of them are considered separately, because they have a different clinical picture. Below are only those pathologies that modern medicine can successfully treat after a sick child is born:
These readings are considered a deviation from the norm and can be determined during fetal development. If it is possible that the child will be born with serious problems, doctors often recommend that the pregnant woman have an abortion. Otherwise, a woman dooms herself to life with a disabled person who will need additional education.
Abnormalities in chromosome sets
Sometimes the number of pairs does not meet the standard. A problem in intrauterine development can only be noticed by a geneticist if the expectant mother voluntarily undergoes a study. If the quantity is disturbed, then the following diseases are distinguished:
- Klinefelter's syndrome.
- Down's disease.
- Shereshevsky-Turner syndrome.
Conservative methods for replenishing the missing genetic series do not exist today. That is, such a diagnosis is considered incurable. If the problem was diagnosed during pregnancy, it is best to terminate it. Otherwise, a sick child appears with possible external deformities.
Down's disease
This disease was first diagnosed back in the 17th century. At that time, determining the number of chromosomes in a healthy person was an extremely problematic task. Therefore, the number of sick newborns was truly frightening. For every 1,000 babies, two were born with Down syndrome. After some time the illness was studied at the genetic level, which made it possible to determine how the chromosome set changes.
In Down syndrome, another pair is added to the 21st pair. That is, the total number is not 46, but 47 chromosomes. The pathology develops spontaneously, and its cause may be diabetes mellitus, the elderly age of the parents, an increased dose of radiation, or the presence of certain chronic diseases.
Outwardly, such a child differs from healthy peers. He has a narrow and wide forehead, a voluminous tongue, large ears, and his mental retardation is immediately obvious. The patient is also diagnosed with other health problems that affect many internal systems and organs.
By and large, the chromosomal sequence of the unborn baby is highly dependent on the genome of its mother. That is why before starting pregnancy planning it is necessary to undergo a full clinical examination. It will help identify hidden problems. If doctors find no contraindications, you can think about conceiving a child.
Patau syndrome
With this disorder, trisomy is observed in the thirteenth pair of structural units. This disease is much less common than Down syndrome. It occurs if an extra structural unit is attached or the structure of chromosomes and their redistribution are disrupted.
There are three main symptoms, by which this pathology is diagnosed:
- Reduced eye size or microphthalmia.
- Increased number of fingers (polydactyly).
- Cleft palate and lip.
With this disease, about 70% of infants die soon after birth (before three years of age). Children with Patau syndrome are often diagnosed with heart defects, as well as brain defects, and problems with many internal organs.
Edwards syndrome
This pathology is characterized by the presence of three chromosomes in the eighteenth pair. Most babies die soon after birth. They are born with pronounced malnutrition (they cannot gain weight due to digestive problems). They have low-set ears and wide-set eyes. Heart defects are often diagnosed.
In order to prevent the development of pathology, it is recommended that all parents who decide to conceive a child after 35 years of age undergo special examinations. There is also a greater likelihood of developing diseases in those whose parents had problems with the thyroid gland.
Poor ecology, life in constant stress, priority of career over family - all this has a bad effect on a person’s ability to bear healthy offspring. Sadly, about 1% of babies born with serious chromosome abnormalities grow up mentally or physically retarded. In 30% of newborns, deviations in the karyotype lead to the formation of congenital defects. Our article is devoted to the main issues of this topic.
The main carrier of hereditary information
As is known, a chromosome is a certain nucleoprotein (consisting of a stable complex of proteins and nucleic acids) structure inside the nucleus of a eukaryotic cell (that is, those living beings whose cells have a nucleus). Its main function is the storage, transmission and implementation of genetic information. It is visible under a microscope only during processes such as meiosis (division of a double (diploid) set of chromosome genes during the creation of germ cells) and mycosis (cell division during the development of the organism).
As already mentioned, a chromosome consists of deoxyribonucleic acid (DNA) and proteins (about 63% of its mass) on which its thread is wound. Numerous studies in the field of cytogenetics (the science of chromosomes) have proven that DNA is the main carrier of heredity. It contains information that is subsequently implemented in a new organism. This is a complex of genes responsible for hair and eye color, height, number of fingers, etc. Which genes will be passed on to the child are determined at the time of conception.
Formation of the chromosome set of a healthy organism
A normal person has 23 pairs of chromosomes, each of which is responsible for a specific gene. There are 46 in total (23x2) - how many chromosomes a healthy person has. We get one chromosome from our father, the other is passed on from our mother. The exception is 23 pairs. It is responsible for the gender of a person: female is designated as XX, and male as XY. When the chromosomes are in a pair, this is a diploid set. In germ cells they are separated (haploid set) before being subsequently united during fertilization.
The set of characteristics of chromosomes (both quantitative and qualitative) examined within one cell is called a karyotype by scientists. Violations in it, depending on the nature and severity, lead to the occurrence of various diseases.
Deviations in the karyotype
When classified, all karyotype abnormalities are traditionally divided into two classes: genomic and chromosomal.
With genomic mutations, an increase in the number of the entire set of chromosomes, or the number of chromosomes in one of the pairs, is noted. The first case is called polyploidy, the second - aneuploidy.
Chromosomal abnormalities are rearrangements both within and between chromosomes. Without going into scientific jungle, they can be described as follows: some sections of chromosomes may not be present or may be doubled to the detriment of others; The sequence of genes may be disrupted, or their location may be changed. Disturbances in structure can occur in every human chromosome. Currently, the changes in each of them are described in detail.
Let us take a closer look at the most well-known and widespread genomic diseases.
Down syndrome
It was described back in 1866. For every 700 newborns, as a rule, there is one baby with a similar disease. The essence of the deviation is that a third chromosome is added to the 21st pair. This happens when the reproductive cell of one of the parents has 24 chromosomes (with double 21). The sick child ends up with 47 chromosomes – that’s how many chromosomes a Down person has. This pathology is facilitated by viral infections or ionizing radiation suffered by parents, as well as diabetes.
Children with Down syndrome are mentally retarded. Manifestations of the disease are visible even in appearance: an overly large tongue, large, irregularly shaped ears, a skin fold on the eyelid and a wide bridge of the nose, whitish spots in the eyes. Such people live on average forty years, because, among other things, they are susceptible to heart disease, problems with the intestines and stomach, and undeveloped genitals (although women may be capable of childbearing).
The older the parents, the higher the risk of having a sick child. Currently, there are technologies that make it possible to recognize a chromosomal disorder at an early stage of pregnancy. Older couples need to undergo a similar test. It will not hurt young parents if one of them has had Down syndrome in their family. The mosaic form of the disease (the karyotype of some cells is damaged) is formed already at the embryonic stage and does not depend on the age of the parents.
Patau syndrome
This disorder is trisomy of the thirteenth chromosome. It occurs much less frequently than the previous syndrome we described (1 in 6000). It occurs when an extra chromosome is attached, as well as when the structure of chromosomes is disrupted and their parts are redistributed.
Patau syndrome is diagnosed by three symptoms: microphthalmos (reduced eye size), polydactyly (more fingers), cleft lip and palate.
The infant mortality rate for this disease is about 70%. Most of them do not live to be 3 years old. In individuals susceptible to this syndrome, heart and/or brain defects and problems with other internal organs (kidneys, spleen, etc.) are most often observed.
Edwards syndrome
Most babies with 3 eighteenth chromosomes die soon after birth. They have pronounced malnutrition (digestive problems that prevent the child from gaining weight). The eyes are set wide and the ears are low. Heart defects are often observed.
conclusions
To prevent the birth of a sick child, it is advisable to undergo special examinations. The test is mandatory for women giving birth after 35 years of age; parents whose relatives were exposed to similar diseases; patients with thyroid problems; women who have had miscarriages.
