Chromatin represents they are proteins (non-histone and histone) and a complex of nucleic acids (RNA and DNA), which together form highly ordered structures in space - eukaryotic chromosomes.
In chromatin, the ratio of protein and DNA is approximately 1:1, the bulk of the protein is represented by histones.
Types of chromatin
Chromatin is heterogeneous in its structure. Conventionally, all chromatin is divided into two functional categories:
1) inactive - heterochromatin - contains currently unreadable genetic information;
2) active - euchromatin - it is from it that genetic information is read.
The ratio of the content of heterochromatin and euchromatin is constantly in a mobile stage. Mature cells, for example blood, have nuclei characterized by condensed, densest chromatin lying lumps.
In the nuclei of somatic female cells, clumps of chromatin are close to the nuclear membrane - this is the female chromatin of the germ cell.
Sexual male chromatin is represented by a lump in male somatic cells, glowing when stained with fluorochromes. Sex chromatin makes it possible to determine the sex of the unborn child using cells obtained from the amniotic fluid of a pregnant woman.
The structure of chromatin
Chromatin - nucleoprotein of the cell nucleus, which is the main component of chromosomes.
Composition of chromatin:
Histones - 30-50%;
Non-histone proteins - 4-33%;
DNA - by weight 30-40%;
Depending on the nature of the object, as well as on the method of chromatin isolation, the sizes of DNA molecules, the number of RNA, non-histone proteins vary widely.
Functions of chromatin
Chromatin and chromosome do not differ from each other in chemical organization (complex of DNA with proteins), they pass mutually into each other.
In interphase, it is not possible to distinguish individual chromosomes. They are weakly spiralized, form loose chromatin, distributed throughout the entire volume of the nucleus. It is the loosening of the structure that is considered a required condition for transcription, the transfer of information of a hereditary nature present in DNA.
Karyotype
Karyotype (from karyo... and Greek tepos - sample, shape, type), chromosome set, a set of characteristics of chromosomes (their number, size, shape and details of the microscopic structure) in the cells of the body of an organism of one species or another. The concept of karyotype was introduced by Sov. geneticist G. A. Levitsky (1924). Karyotype is one of the most important genetic characteristics of a species, since each species has its own karyotype, which differs from the karyotype of related species (this is the basis of a new branch of taxonomy - the so-called karyosystematics).
8. Features of the morphological and functional structure of chromosomes. Hetero- and euchromatin. (one answer for 2 questions).
Chromosomes: structure and classification
Chromosomes(Greek - chromo- Colour, soma body) is a spiralized chromatin. Their length is 0.2 - 5.0 microns, diameter is 0.2 - 2 microns.
Metaphase chromosome consists of two chromatids, which are connected centromere (primary constriction). She divides the chromosome into two shoulder. Individual chromosomes have secondary constrictions. The area they separate is called satellite, and such chromosomes are satellite. The ends of chromosomes are called telomeres. Each chromatid contains one continuous DNA molecule in combination with histone proteins. Intensely stained sections of chromosomes are areas of strong spiralization (heterochromatin). Lighter areas are areas of weak spiralization (euchromatin).
Types of chromosomes are distinguished by the location of the centromere.
1. metacentric chromosomes- the centromere is located in the middle, and the arms are of the same length. The part of the shoulder near the centromere is called proximal, the opposite is called distal.
2. Submetacentric chromosomes- the centromere is displaced from the center and the arms have different lengths.
3. Acrocentric chromosomes- the centromere is strongly displaced from the center and one arm is very short, the second arm is very long.
In the cells of the salivary glands of insects (Drosophila flies) there are giant, polytene chromosomes(multistranded chromosomes).
For the chromosomes of all organisms, there are 4 rules:
1. The rule of constancy of the number of chromosomes. Normally, organisms of certain species have a constant number of chromosomes characteristic of the species. For example: a human has 46, a dog has 78, a fruit fly has 8.
2. pairing of chromosomes. In a diploid set, each chromosome normally has a paired chromosome - the same in shape and size.
3. Individuality of chromosomes. The chromosomes of different pairs differ in shape, structure and size.
4. Chromosome continuity. When the genetic material is duplicated, a chromosome is formed from a chromosome.
The set of chromosomes of a somatic cell, characteristic of an organism of a given species, is called karyotype .
1. Chromosomes that are the same in the cells of male and female organisms are called autosomes
idiogram
Classification of chromosomes is carried out according to different criteria.
1. Chromosomes that are the same in the cells of male and female organisms are called autosomes. The human karyotype has 22 pairs of autosomes. Chromosomes that are different in male and female cells are called heterochromosomes, or sex chromosomes. In men, these are X and Y chromosomes; in women, X and X.
2. The arrangement of chromosomes in descending order is called idiogram. This is a systematic karyotype. Chromosomes are arranged in pairs (homologous chromosomes). The first pair are the largest, the 22nd pair are the smallest, and the 23rd pair are the sex chromosomes.
3. In 1960 was proposed Denver classification chromosomes. It is built on the basis of their shape, size, centromere position, presence of secondary constrictions and satellites. An important indicator in this classification is centromeric index(CI). This is the ratio of the length of the short arm of the chromosome to its entire length, expressed as a percentage. All chromosomes are divided into 7 groups. Groups are designated by Latin letters from A to G.
Group A includes 1 - 3 pairs of chromosomes. These are large metacentric and submetacentric chromosomes. Their CI is 38-49%.
Group B. 4th and 5th pairs are large metacentric chromosomes. CI 24-30%.
Group C. Pairs of chromosomes 6 - 12: medium size, submetacentric. CI 27-35%. This group also includes the X chromosome.
Group D. 13 - 15th pairs of chromosomes. Chromosomes are acrocentric. CI about 15%.
Group E. Pairs of chromosomes 16 - 18. Relatively short, metacentric or submetacentric. CI 26-40%.
Group F. 19 - 20th pair. Short, submetacentric chromosomes. CI 36-46%.
Group G. 21-22 pairs. Small, acrocentric chromosomes. CI 13-33%. The Y chromosome also belongs to this group.
4. Parisian classification human chromosomes was established in 1971. With the help of this classification, it is possible to determine the localization of genes in a particular pair of chromosomes. Using special staining methods, a characteristic order of alternation of dark and light stripes (segments) is revealed in each chromosome. Segments are designated by the name of the methods that reveal them: Q - segments - after staining with quinacrine mustard; G - segments - Giemsa staining; R - segments - staining after heat denaturation and others. The short arm of the chromosome is denoted by the letter p, the long arm by the letter q. Each chromosome arm is divided into regions and numbered from centromere to telomere. The bands within the regions are numbered in order from the centromere. For example, the location of the D esterase gene - 13p14 - is the fourth band of the first region of the short arm of the 13th chromosome.
Function of chromosomes: storage, reproduction and transmission of genetic information during the reproduction of cells and organisms.
Term chromosome proposed in 1888. German morphologist W. Waldeyr. The work of D Morgan and his colleagues established the linearity of the location of genes along the length of the chromosome.
According to the chromosomal theory of heredity, the set of genes that make up one chromosome forms clutch group.
Chromosomes consist mainly of DNA and proteins that form a nucleoprotein complex. Proteins make up a significant part of the substance of chromosomes. They account for about 65% of the mass of these structures. All chromosomal proteins are divided into two groups: histones and nonhistone proteins. Chromosome RNA is mainly represented by transcription products that have not yet left the site of synthesis.
The regulatory role of the components of chromosomes is to "prohibit" or "permit" the reading of information from the DNA molecule.
In the first half of mitosis, the chromosomes consist of two chromatids. interconnected in the region of the primary constriction ( centromeres) a specially organized section of the chromosome common to both sister chromatids. In the second half of mitosis, chromatids separate from each other. They form single strands. daughter chromosomes, distributed among daughter cells.
Karyotype - a diploid set of chromosomes, characteristic of the somatic cells of organisms of a given species, which is a species-specific feature and is characterized by a certain number and structure of chromosomes. If the number of chromosomes in the haploid set of germ cells is denoted P, then the general karyotype formula will look like 2 P, where the number P different for different species.
Chromosomes are cell structures that store and transmit hereditary information. A chromosome is made up of DNA and protein. The complex of proteins associated with DNA forms chromatin. Proteins play an important role in the packaging of DNA molecules in the nucleus.
DNA in chromosomes is packed in such a way that it fits in the nucleus, which usually does not exceed 5 microns (5-10-4 cm) in diameter. The packaging of DNA takes the form of a looped structure, similar to amphibian lampbrush chromosomes or insect polytene chromosomes. The loops are maintained by proteins that recognize specific nucleotide sequences and bring them closer together. The structure of the chromosome is best seen in the metaphase of mitosis.
The chromosome is a rod-shaped structure and consists of two sister chromatids, which are held by the centromere in the region of the primary constriction. Each chromatid a is built from chromatin loops. Chromatin does not replicate. Only DNA is replicated.
When DNA replication starts, RNA synthesis stops. Chromosomes can be in two states: condensed (inactive) and decondensed (active).
Diploid set of chromosomes an organism is called a karyotype. Modern research methods make it possible to determine each chromosome in the karyotype. For this, the distribution of light and dark bands visible under a microscope (alternation of AT and GC pairs) in chromosomes treated with special dyes is taken into account. Transverse striation is possessed by chromosomes of representatives of different species. In related species, for example, in humans and chimpanzees, the pattern of alternation of bands in the chromosomes is very similar.
Each species of organisms has a constant number, shape and composition of chromosomes. The human karyotype has 46 chromosomes - 44 autosomes and 2 sex chromosomes. Males are heterogametic (XY) and females are homogametic (XX). The Y chromosome differs from the X chromosome in the absence of some alleles (for example, the blood clotting allele). Chromosomes of one pair are called homologous. Homologous chromosomes at the same loci carry allelic genes.
A human karyotype is a complex of features of a whole set of chromosomes, which is inherent in all human cells. The study of the karyotype is an urgent problem for future parents who want to identify the likelihood of chromosomal diseases in their child. This is especially true when any of the relatives have Down syndrome or Patau syndrome.
Quite often, genetic analysis is carried out by parents in case of non-carrying of previous pregnancies and infertility. In some cases, in order to exclude chromosomal pathology, a study of the fetal karyotype is carried out. For the same purpose, an ultrasound of the TVP is additionally performed when the collar area is examined. Its enlarged size indicates the presence of a pathological process.
What is a karyotype
The concept of a karyotype became widespread at the stage of research in medicine of genetic diseases, when they began to actively study the structure and functions of chromosomes. Received the discovery of Edwards syndrome, Klinefelter's syndrome. The karyotype, which is a cellular chromosome complex, is permanent. In humans, the norm is the presence of chromosomes, the number of which is 46. Of these, 22 pairs are autosomes and two are sex chromosomes.
For female representatives, they are designated as XX, for male representatives - XY. The main feature of the chromosome set is the species specificity of the karyotype. The functions of chromosomes are that each of them is the carrier of genes that respond to heredity.
The normal male karyotype is a 46, XY karyotype. A normal female karyotype looks like a 46,XX karyotype. The set of chromosomes remains unchanged throughout life. Therefore, it is enough to pass a karyotype once in a lifetime.
Methods for studying the karyotype
The definition of a karyotype has some peculiarities. It is carried out at one of the stages of the cell cycle. This is due to the fact that during other stages of cell development, chromosomes are difficult to study.
For the karyotyping procedure, any cells in the process of division are used.
The normal human karyotype is studied in two ways:
- using mononuclear leukocytes, which are extracted from blood samples (their division is provoked using mitogens);
- using cells that divide rapidly in the normal state, such as skin cells.
The essence of the procedure is that the cells are fixed at the metaphase stage, then they are stained and photographed. From the complex of photographs taken, the geneticist compiles a systematic karyotype, which is also called an ideogram (karyogram). It is a numbered set of autosomal pairs. Chromosomal images are arranged vertically. Short shoulders are at the top. Numbers are assigned in descending order of size. At the end is a pair of sex chromosomes.
Indications for the procedure
Spousal karyotyping is an important step in the family and child planning process. The benefits of the procedure are unambiguous, even in the absence of clear indications. Indeed, in some cases, a person may simply not be aware of the presence of various hereditary pathologies in his distant relatives, among which Down syndrome, Edwards syndrome, Klinefelter syndrome are common. When determining the karyotype, the specialist will identify the abnormal chromosome and calculate the percentage of the probability of having a baby with genetic diseases, which can be different.
Among the indications for the study are:
- age category;
- absence of children when the reason is not clear;
- previous IV procedures that ended in vain;
- a history of chromosomal pathology in a man or woman (Down syndrome, Edwards syndrome, Klinefelter syndrome);
- hormonal imbalance (when examining a karyotype in a woman);
- interaction with various reagents of a chemical nature, irradiation;
- bad habits of the expectant mother or her use of certain medications;
- the presence in the history of a woman of situations of spontaneous interruption of the process of bearing a child;
- marriage between close relatives;
- the birth of a child with hereditary diseases.
The karyotype of a married couple is usually examined before pregnancy. However, it is possible to carry out the procedure in the process of bearing a child. Often women wish to rule out Down syndrome. The structure of hereditary material can be studied in the fetus. This analysis is called prenatal karyotyping.
In addition, the likelihood of developing a chromosomal disease is determined by ultrasound examination of the NT zone when examining the collar space. The abbreviation TVP implies the thickness of the corresponding area. If its size is increased, additional studies of the fetus are necessary to confirm the diagnosis of the presence of pathology.
Features of preparation for the study
Deciphering the karyotype is carried out by a geneticist. The specialist issuing the referral will tell you about how to take an analysis, what are the rules for preparation, the features of the procedure itself. The study for the karyotype is carried out by taking blood cells. Before analysis, in order to avoid errors, it is necessary to exclude the influence of those factors that can change the data. Preparation starts two weeks in advance. The following points can change the indicators:
- an acute form of any disease or a period of exacerbation of a chronic disease;
- the use of medications;
- drinking alcohol or smoking.
Features of the manipulation
To study the karyotype of the spouses, venous blood is taken. In the laboratory, those lymphocytes are isolated from the blood, for which the division phase is relevant. For three days they are studied. Research methods include treatment of cells with a special substance - a mitogen. Its purpose is to increase the rate of cell division. During this process, the laboratory assistant can observe the chromosomes, but it is stopped with the help of a special impact.
The structural organization of the chromosome is better seen after staining. This allows you to see the structural features of each chromosome. After the staining procedure, the strokes performed are analyzed: the number and structure are determined.
A cytogenetic study is considered completed after the results obtained correlate with normal values.
Karyotype and idiogram are obligatory leaving studies of hereditary material. To study, it is enough to take at least 12 cells. In some cases, the karyotype with aberrations is studied when an extended examination of 100 cells is carried out.
What pathologies are detected
The human karyotype is normally represented by 46 chromosomes and is designated as 46XX or 46XY. When deviations are detected, the result looks different. An example would be the determination of a third extra chromosome 21 in a woman, which will be designated as 46XX21+.
The study of hereditary material reveals the following deviations from the norm:
- The presence of a third chromosome in the complex, which is called trisomy (Down's syndrome develops, in which the TVP index is increased). In the presence of trisomy on chromosome 13, Patau syndrome occurs. With an increase in the number of chromosome 18 - Edwards syndrome. The appearance of an extra X chromosome (47xxy or 48xxxy) in the karyotype of a man gives Klinefelter's syndrome (mosaic karyotype).
- Reducing the number of chromosomes in the karyotype, that is, the absence of one chromosome in a pair - monosomy;
- Lack of a section of a chromosome, which is called a deletion;
- Doubling of a separate region of the chromosome, that is, duplication;
- The reversal of the chromosomal region, called inversion;
- Movement of chromosomal regions - translocation;
Not always people attach importance to the study of heredity. Timely karyotyping will help assess the state of genes before planning children. The karyotype for the genotype represents the external design of the inherent features. The procedure for the study of hereditary material helps to identify the pathology in time. The genome for the karyotype carries half of the important information. Its knowledge is necessary for many couples who suffer from infertility or have a history of children suffering from genetic abnormalities.
Karyotype studies reveal the following deviations in the state of genes:
- mutations that cause thrombosis and abortion;
- changes in the Y chromosome;
- gene changes leading to detoxification, when the body is unable to neutralize toxic agents;
- Changes leading to the development of cystic fibrosis.
In addition, the human karyotype contains information about predisposition to various diseases (heart muscle infarction, diabetes mellitus, hypertension). The study of hereditary material will allow to start the prevention of these diseases in time and maintain a high quality of life for many years.
If deviations are found
When abnormalities in the karyotype are detected (for example, syndromes such as Edwards syndrome, Klinefelter syndrome), the doctor must explain the features of the pathology that has arisen and its impact on the likelihood of having a child with various genetic diseases. At the same time, the geneticist focuses on the incurability of chromosomal and gene anomalies. The decision to have a child when a karyotype pathology is detected at the stage of gestation is made by the parents themselves.
The doctor only provides all the necessary information, telling what the number of chromosomes is and the constancy of their composition. Detection of anomalies in a developing fetus is one of the medical indications for abortion. However, the final decision is made by the woman.
Unfortunately, karyotype pathologies cannot be cured. Therefore, its timely determination will help to avoid many problems with planning children. It should be remembered that geneticists can also make mistakes. Therefore, having received positive results about the presence of an anomaly, one should not give up. You can always retake the test. During pregnancy, an ultrasound scan and a TVP study are additionally performed. If the results were confirmed a second time, it is worth considering alternative ways of raising a child. For many, they become ways of fulfilling themselves as a parent.
In contact with
The set of chromosomes found in the nucleus of a somatic cell is called a karyotype. The number and morphology of chromosomes refer to species characteristics. Different types of organisms differ in karyotype, while such differences are not observed within the same species, and karyotype anomalies are most often associated with severe pathological conditions.
Karyotype - a set of features (number, size, shape, etc.) of a complete set of chromosomes, inherent in cells of a given biological species ( species karyotype), given organism ( individual karyotype) or line (clone) of cells. A karyotype is sometimes also called a visual representation of the complete chromosome set (karyograms). It includes all the features of the chromosome complex: the number of chromosomes, their shape, the presence of details of the structure of individual chromosomes visible under a light microscope. The number of chromosomes in a karyotype is always even. This is due to the fact that in somatic cells there are two chromosomes of the same shape and size - one from the paternal organism, the second from the maternal one. The number of chromosomes in humans is 46.
Somatic cells usually have two sex chromosomes. In the female karyotype, sex chromosomes are represented by large paired (homologous) chromosomes (XX). In the male karyotype, the pair of sex chromosomes includes one X chromosome and a small rod-shaped Y chromosome. Thus, the human chromosome set contains 22 pairs of autosomes, sex chromosomes, in which both sexes differ.
During the maturation of germ cells, as a result of meiosis, gametes receive a haploid set of chromosomes. All eggs have one X chromosome, and sperm will be of two varieties: half will receive a Y chromosome during spermatogenesis, the other half will receive an X chromosome. The sex that forms gametes that are identical on the sex chromosome is called homogametic, and the sex that forms different gametes is called heterogametic. The numerical ratio of males and females in most dioecious organisms is close to one, which is a direct result of the chromosomal mechanism of sex determination. The homogametic sex produces gametes of one type, the heterogametic sex produces two, and in equal numbers. Thus, the sex of most organisms is determined at the time of fertilization and depends on the chromosome set of the zygote.
In mammals (including humans), worms, crustaceans, most insects (including Drosophila), most amphibians, some fish, the female is homogametic, and the male is heterogametic.
In birds, reptiles, some amphibians and fish, parts of insects (butterflies and caddisflies) the female sex is heterogametic. In this case, other symbols are used to designate the sex chromosomes. For example, in chickens that have 78 chromosomes in somatic cells, the male chromosomal formula is 76A + ZZ, the female - 76A + ZW.
Some insects (for example, water bugs, grasshoppers, etc.) have no Y chromosome at all. In these cases, males have only one X chromosome. As a result, half of the spermatozoa have a sex chromosome, and the other is deprived of it.
Bees and ants do not have sex chromosomes: females are diploid, males are haploid. Females develop from fertilized eggs, drones from unfertilized ones.
Each biological species has its own set of chromosomes; a person has forty-six.
The totality of all structural and quantitative features of the complete set of chromosomes characteristic of cells of a given type of living organisms is called karyotype.
The karyotype of the future organism is formed in the process of fusion of 2 germ cells - an egg and a sperm. In this case, chromosome sets are combined.
Fig. To compile a karyotype, dividing cells are distributed on a plate so that their chromosomes are clearly visible, and photographed (a). The homologous chromosomes in the photograph are then paired and arranged in size so that they are much easier to study.
The nucleus of a mature cell contains half of the set of chromosomes - 23 - a single set of chromosomes is called haploid; when fertilized into the body, a karyotype specific to this species is recreated. The complete set of chromosomes (46) of an ordinary somatic cell is diploid (2p)
Human chromosomes, like many animals, can be divided into pairs. Forty-six human chromosomes form 23 pairs (Fig. 5.36). Arranging them in the photo in order, we get a karyotype, that is, a set of chromosomes with which you can diagnose some genetic diseases.
Two identical chromosomes are called homologous (they are not only similar in appearance, but also contain genes responsible for the same traits).
If we arrange them in order, starting with the longest, then we will come to the shortest pair, on which the difference between men and women depends.
Women have exactly 23 pairs of chromosomes, but in men the last two chromosomes remain unpaired, and one of them is extremely short.
This short chromosome is called Y-chromosome, and the longer one X chromosome.
In women, the 23rd pair contains two X chromosomes.
It is clear that the X and Y chromosomes determine the sex of a person (sex). The remaining 22 pairs of homologous chromosomes are called autosomes.
Obviously, each person has two identical chromosomes, because everyone has two parents.
The development of the human body begins with the fertilization of an egg by a spermatozoon; each gamete contains 23 chromosomes, one of each type, and the resulting zygote already contains two chromosomes of each type.
All autosomes are divided into 7 groups: A (1,2,3), B (4,5), C (6-12), D (13-15), E (16-18), F (19-20) , G (21-22).
The hereditary information of an organism is strictly ordered according to individual chromosomes. A cryotype is a species passport. The human karyotype is represented by 24 chromosomes, 22 autosomes, x and y chromosomes.
Karyotype analysis reveals disorders that can lead to developmental anomalies, hereditary diseases, or death of the fetus and embryo in the early stages of development. Those. for normal development, a set of genes of a complete chromosome set is required.
Mitosis, its essence. Pathology of mitosis
The behavior of chromosomes during mitosis ensures a strictly equal distribution of hereditary material between daughter and mother cells.
Mitosis is a continuous process with 4 stages:
Prophase- chromatin threads begin to twist, spiralize. Chromosomes shorten and thicken, becoming available for microscopy. The nucleolus disappears, the nuclear envelope disintegrates. Centrisome divides into 2 centrioles, which move to different poles of the cell. From protein t ubulina microtubules are formed - threads of the achromatin spindle. Chromosomes are concentrated in the center.
metaphase- Chromosomes max are spiralized and located in the plane of the cell equator - it is convenient to view in a light microscope. Spindle threads from different poles are attached to the centromeres of all chromosomes.
Anaphase- characterized by the division of chromosomes in the centromere region into 2 chromatids. The spindle fibers contract and pull the chromatids of each chromosome to different poles of the cell. The shortest phase of mitosis.
Telophase- despiralization of chromosomes, turning them back into thin filaments of chromatin, invisible in a light microscope. Around each group of daughter cells, a nuclear envelope is formed, nucleoli appear. The fission spindle filaments disintegrate
The division of the cytoplasm in animal cells is preceded by the appearance of a CMP constriction.
Mitosis ends with the formation of 2 cells quantitatively and qualitatively identical to the mother cell.
Doubling of chromosomes and in the interphase of mitosis, uniform distribution of chromatids between daughter cells and cells ensures the maintenance of the constancy of genetic information in a number of generations of cells, serves as the basis for the growth and development of the organism.
Pathology of mitosis
Various environmental factors can disrupt the process of mitosis and lead to the appearance of abnormal cells.
There are 3 types of violations:
Change in the structure of chromosomes –
BUT) the appearance of chromosome breaks, the presence of small chromosomal fragments. Occurs under the influence of radiation, chemicals, viruses, as well as in cancer cells (mutations).
B) chromosomes can lag behind others in anaphase and not get into their cell. This will lead to a change in the number of chromosomes in daughter cells - aneuploidy.
Spindle damage- the function of distribution of chromosomes between daughter cells is disturbed - the appearance of cells containing a significant excess of chromosomes (for example, 92) is possible. A similar action is typical for anticancer drugs - this is how the growth of tumor cells is inhibited.
Violation of cytotomy– i.e. the absence of division of the cytoplasm of the cell during the telophase period. This is how binucleated cells are formed
The pathology of mitosis can lead to the appearance mosaicism- in one organism, clones of cells with a different set of chromosomes can be found (for example, some cells contain 46 chromosomes, and others - 47).
Mosaicism is formed in the early stages of cleavage of germ cells.
As a rule, karyotype disorders in humans are accompanied by multiple malformations; most of these anomalies are incompatible with life and lead to spontaneous abortions in the early stages of pregnancy.
However, a fairly large number of fetuses (~2.5%) with abnormal karyotypes endure until the end of pregnancy.
Meiosis
A type of division in which the number of chromosomes is reduced by half from diploid to haploid, consisting of 2 consecutive divisions of the nucleus.
called meiosis. With each fertilization, the original number of chromosomes is restored.
Sexual reproduction can thus be thought of as the following great cycle of events:
In the sex glands (gonads) of an adult organism - the testes and ovaries - some cells multiply through meiosis, forming respectively spermatozoa and eggs, that is, haploid cells. These gametes each contain one set of 23 chromosomes. During fertilization, a zygote with a double set of chromosomes is formed; and during mitotic division, an adult organism grows out of it, and the cycle begins anew.
The division mechanism - the formation of a centriole, spindle, etc. - is the same during meiosis as during mitosis, only the chromosomes behave somewhat differently.
Meiosis
Rice. 5.4. meiosis process(in general terms) in a cell with two pairs of chromosomes; one of the paired chromosomes is indicated by a thick line, the other by a dotted line.
Prophase I: Chromosomes become visible and form pairs.
Metaphase I: paired chromosomes line up opposite each other in the middle of the cell.
Anaphase I: each of the paired homologous chromosomes completely departs to one of the poles of the cell. Note that the chromatids do not separate and are still connected by centromeres.
Telophase I: The initial division is completed.
Prophase II: Chromosomes become visible again, as in mitotic division.
Metaphase II: Chromosomes line up again in the middle of the cell.
Anaphase II: This time the chromatids separate from each other and diverge towards opposite poles.
Telophase II: division ends with the formation of four haploid cells
The biological significance of meiosis:
Sexual reproduction - this process ensures the constancy of the number of chromosomes in a series of generations of sexually reproducing organisms.
Genetic variability - creates the possibility for new gene combinations. This leads to changes in the genotype and phenotype of the offspring.
Pathology of meiosis: under the influence of external damaging factors: simple, sequential and double nondisjunction.
Simple nondisjunction:
With the pathology of meiosis 1, all mature gametes will have a pathological set of chromosomes
Meiosis 2 - the number of chromosomes changes only in part of the gametes.
Sequential nondisjunction - affects both divisions 1 and 2, normal gametes are not formed.
Double nondisjunction- extremely rare - meiosis is damaged in both parents.
It is also possible to distinguish primary, secondary and tertiary nondisjunction of chromosomes.
The process of meiosis can be disturbed under the influence of various external adverse factors.
Balanced changes in chromosomes in the human karyotype do not affect the state of human health.