They can be numerical (a karyotype with an abnormal number of chromosomes as a result of the loss or addition of a chromosome) or structural, which refers to changes in the structure of individual chromosomes (loss, rearrangement or addition of chromosomal segments). Numerical and structural abnormalities can coexist in the same tumor cell.
A cell with a normal set of 46 structurally normal chromosomes is called diploid. Cells with 46 chromosomes but with numerical chromosomal abnormalities (such as the loss of one chromosome and the addition of another) are called pseudodiploid. An abnormal number of chromosomes is called aneuploidy, the presence of more than 46 chromosomes is called hyperdiploidy, and less than 46 chromosomes is called hypodiploidy.
Loss of one copy chromosomes leads to monosomy on this chromosome, the loss of both copies leads to nullisomy, the appearance of an additional copy of a chromosome leads to trisomy on this chromosome, and the more rare occurrence of two additional copies leads to tetrasomy. The addition and loss of chromosomes are indicated by a plus or minus sign. For example, 45,XY,-7 is a karyotype of a male cell with monosomy on chromosome 7, and 47,XX,+8 is a karyotype of a female cell with trisomy on chromosome 8.
Most common acquired trisomy on chromosome 8, which occur in acute myeloid leukemia, myelodysplastic syndromes and blast crisis of chronic myeloid leukemia. Other trisomies in myeloproliferative diseases include +4, +6, +9, +11, +13, +19, +21, in acute lymphoblastic leukemia - +4, +6, +10, +14, +17, +18, +20, +21 and +X.
Numerical chromosomal abnormalities They are especially common in acute lymphoblastic leukemia and have prognostic significance (hyperdiploidy - favorable, hypodiploidy - unfavorable). The most common cytogenetic abnormality in chronic lymphocytic leukemia, trisomy 12, is associated with a poor prognosis. In multiple myeloma, various variants of aneuploidy are identified in 90% of cases.
Structural chromosomal abnormalities
IN tumor cells In patients with oncohematological diseases, a wide variety of structural abnormalities can be found, which are defined in precise terms: deletions, isochromosomes, dicentric and isodicentric chromosomes, inversions, ring chromosomes, translocations, insertions, duplications, duplicated minichromosomes and marker chromosomes.
Chromosomal deletion(del) - loss of a chromosomal segment. There are interstitial and terminal deletions. With interstitial deletion, the internal chromosomal segment is lost, and the adjacent distal and proximal segments are connected. Interstitial deletion del(5)(ql3q33) indicates the loss of a region of the long arm of chromosome 5 between segments ql3 and q33.
With terminal deletions the end of the chromosome is missing, for example, the del(7)(q22) deletion means the loss of chromosomal material from the q22 segment of the long arm of chromosome 7 up to and including its telomere. Probably, the significance of chromosomal deletions in the development of oncohematological diseases is determined by the loss of tumor suppressor genes.
Isochromosome(i) - a structurally abnormal chromosome consisting of two identical arms oriented as a mirror image of one another. Isochromosomes can be monocentric (containing one centromere) and dicentric or isodicentric (two centromeres). For example, isochromosome i(17q), which is often found as a secondary cytogenetic abnormality in blast crisis of chronic myeloid leukemia, consists of two long arms.
Important corollary education i(17q) involves the loss of the short arm 17p, which contains the tumor suppressor gene p53.
Inversion(inv) - a structural chromosomal change consisting in a rotation of a chromosomal segment by 180°. There are pericentric and paracentric inversions. In pericentric inversion, the segment with the changed orientation contains the centromere. In paracentric inversion, the inverted segment is located within the short or long arm of the chromosome and does not include the centromere.
Pericentric inversion inv(16)(pl3q22) is often detected in the M4 variant of acute myeloid leukemia, and inv(3)(q21q26), found in the M7 variant, may serve as an example of paracentric inversion. The molecular consequences of inversions are the movement of genes to an unusual position and changes in their regulation.
Ring chromosome(r - from the English ring) is an anomalous chromosome, both arms of which, short and long, are torn, and the break points are connected together, forming a closed structure (ring). Ring chromosomes are rarely found in hematological cancers.
Chromosomal translocation(t) - exchange of genetic material between non-homologous chromosomes. There are reciprocal and non-reciprocal translocations. With reciprocal translocation, a mutual exchange of fragments occurs between two, less often three or more chromosomes, without loss of genetic material, in contrast to non-reciprocal translocations. A large number of translocations have been described in oncohematological diseases, and in many cases the associated molecular changes and mechanisms of malignant transformation have been identified.
Association of certain chromosomal translocations with certain forms of malignant tumors is well known in hemoblastoses. Translocations in human leukemias and lymphomas either activate cellular proto-oncogenes or lead to the formation of fused, “chimeric” genes that promote the malignant transformation of hematopoietic cells. Molecular genetic analysis of breakpoints shows that genetic translocations alter the structure or regulation of genes important for the growth and/or differentiation of the corresponding cell type.
In this regard, they can be used for the differential diagnosis of myeloproliferative and lymphoproliferative diseases.
An example of a translocation that activates cellular proto-oncogene as a result of its movement under the control of the regulatory element of another gene located on another chromosome - t(14;18)(q32;q21), which is naturally detected in follicular non-Hodgkin lymphomas and has pathogenetic significance. Chromosome breakpoints are located in segments q32 of chromosome 14 and q21 of chromosome 18; as a result, an exchange of chromosomal fragments occurs between chromosomes 14 and 18 with the transfer of the bcl-2 oncogene from chromosome 18 to chromosome 14.
It leads to dysregulation and uncontrolled expression of the anti-apoptotic gene bcl-2, the accumulation of long-lived centrocytes and promotes malignant transformation.
Translocation t(9;22)(q34;qll) is an example of the formation of a chimeric bcr/abl gene, which is formed by the fusion of the bcr gene from the 22qll locus and the abl gene from the 9q34 locus. The new gene is expressed to form bcr/abl-mRNA and a protein with increased tyrosine kinase activity and the ability to induce unlimited cell proliferation. This chromosomal rearrangement is detected in 95-97% of patients with chronic myeloid leukemia.
An example of a complex translocations involving three chromosomes - translocation t(3;9;22)(ql3;q34;qll), which occurs between loci 3ql3, 9q34 and 22qll also with the formation of the chimeric gene bcr/abl.
Dicentric chromosome(die) is a structurally abnormal chromosome with two centromeres, which is the result of a reciprocal translocation and contains centromeres of both chromosomes involved in the translocation. The dicentric chromosome dic(7;12)(pll;pll) occurs in acute lymphoblastic leukemia.
Addition of chromosomal material(add - from the English addition) - the addition of chromosomal material of unknown origin, which is indicated by a plus sign. For example, 14q+ means the presence of additional genetic material of unknown origin on the long arm of chromosome 14.
Insertion(ins - from the English insertion) - the presence of a chromosomal segment in a new position in the same or another homologous chromosome (rare). Some insertions have been previously described as translocations, for example ins(3;3)(q26;q21q26) - an insertion of a segment located between the q21 and q26 loci of chromosome 3 into the q26 locus of another chromosome 3.
Duplication(dup) - the presence of an additional copy of a chromosome segment next to the first copy with the formation of a tandem of two copies of the duplicated segment. An example is the secondary chromosomal abnormality dup(l)(pl2->q31) in acute lymphoblastic leukemia. Unlike chromosomal duplications, molecular microduplications, such as duplication of part of the ALL1 gene, can only be determined by molecular methods.
Duplicated mini chromosomes(dmin) - marker chromosomes without centromeres, which are usually the result of gene amplification. These small, spherical, paired diplococcus-like structures are more common in solid tumors than in hematologic tumors.
Marker chromosomes(mar - from the English marker) - the term is used to describe structurally abnormal chromosomes that do not have identifying characteristics. The karyotype may include one or more markers. The presence of one marker chromosome in the karyotype is indicated by the symbol +mar, several different ones - +marl, +mar2, +mar3, etc., several copies of one marker - +marl x2, +marl x3, etc.
Congenital and acquired chromosomal changes
Numerical and structural chromosomal abnormalities may be congenital or acquired. Congenital chromosomal abnormalities are present in all or almost all cells of the body already at the earliest stages of embryogenesis. Acquired chromosomal abnormalities occur in somatic cells and are usually associated with malignant transformation. Congenital chromosomal abnormalities are associated with hereditary genetic syndromes (for example, trisomy 21 - with Down syndrome) or are a normal variant.
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.).
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.
Scheme of chromosome structure in late prophase and metaphase of mitosis. 1 chromatid; 2 centromeres; 3 short shoulder; 4 long shoulder ... Wikipedia
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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. A small terminal section of 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).