Morphofunctional characteristics and classification of human chromosomes. Morpho-functional characteristics and classification of chromosomes

The term "chromosome" was proposed in 1888 by the German morphologist Waldeyr. In 1909, Morgan, Bridges and Sturtevant proved the relationship of hereditary material with chromosomes. Chromosomes play a leading role in the transfer of hereditary information from cell to cell, because they meet all the requirements:

1) Ability to doubling;

2) The constancy of presence in the cell;

3) Uniform distribution of genetic material between daughter cells.

The genetic activity of chromosomes depends on the degree of compaction and changes during the mitotic cycle of the cell.

The despiralized form of the existence of a chromosome in a non-dividing nucleus is called chromatin, its basis is protein and DNA, which form DNP (deoxyribonucleic complex).

Chemical composition of chromosomes.

Histone proteins H 1, H 2a, H 2c, H 3, H 4 - 50% - basic properties;

Non-histone proteins - acidic properties

RNA, DNA, lipids (40%)

Polysaccharides

metal ions

When a cell enters the mitotic cycle, the structural organization and functional activity of chromatin changes.

The structure of the metaphase chromosome (mitotic)

It consists of two chromatids connected by a central constriction, which divides the chromosome into 2 arms - p and q (short and long).

The position of the centromere along the length of the chromosome determines its shape:

Metacentric (p=q)

Submetacentric (p>q)

Acrometacentric (p

There are satellites that are connected by a secondary constriction to the main chromosome, in its region there are genes responsible for the synthesis of ribosomes (the secondary constriction is the nucleolar organizer).

At the ends of chromosomes there are telomeres that prevent the chromosomes from sticking together, and also contribute to the attachment of chromosomes to the nuclear envelope.

For accurate identification of chromosomes, the centromere index is used - the ratio of the length of the short arm to the length of the entire chromosome (and multiply by 100%).

The interphase form of the chromosome corresponds to the chromatin of the nuclei of interphase cells, which is visible under a microscope as a collection of more or less loosely located filamentous formations and clumps.

Interphase chromosomes are characterized by a despiralized state, i.e. they lose their compact shape, loosen, decondense.

DNP compactization levels

Compaction level	Compactization factor	fibril diameter
Nucleosomal. G 1 , S. Chromatin fibril, "string of beads". Formed: histone proteins of four classes - H 2a, H 2b, H 3, H 4 - which form a histone octanet (two molecules from each class). A DNA molecule is wound onto histone octamers (75 turns); free linker (binder) site. Characteristic of the synthetic period of interphase.	7 times	10 nm
Nucleomeric. G 2. Chromatin fibril - solenoid structure: due to the connection of neighboring nucleosomes, due to the incorporation of proteins into the linker region.	40 times	30 nm
Chromomeric. With the participation of non-histone proteins with the formation of loops (during compaction). Characteristic for the beginning of the prophase of mitosis. One chromosome has 1000 loops. One loop - 20000-80000 nucleotide pairs.	200-400 times	300 nm
chromonemic. Acidic proteins are involved. characteristic of the end of prophase.	1000 times	700 nm
Chromosomal. characteristic of the metaphase of mitosis. Participation of the histone protein H 1 . Maximum degree of spiralization.	10 4 -10 5 times	1400 nm

The degree of chromatin compaction affects its genetic activity. The lower the level of compactization, the greater the genetic activity and vice versa. At the nucleosomal and nucleomeric levels, chromatin is active, but in the metaphase it is inactive, and the chromosome performs the function of storing and distributing genetic information.

Faculty of Dentistry

Thematic plan of lectures for students of the Faculty of Dentistry

1 semester

1. A cell is an elementary genetic structural and functional unit of a living being. Organization of flows of energy, information and matter in the cell.

2. Cell cycle. Mitotic cycle. Mitosis. The structure of chromosomes. Dynamics of its structure in the cell cycle. Hetero- and euchromatin. Karyotype.

3. Gametogenesis. Meiosis. Gametes. Fertilization.

4. Subject, tasks and methods of genetics. Classification of genes. The main patterns of inheritance and the formation of signs. Chromosomal theory of heredity.

5. Molecular basis of heredity. DNA code system. The structure of genes in eukaryotes and prokaryotes.

6. Expression of genes. Transcription, Processing, Translation. Genetic Engineering.

7. Variation forms. modification variability. reaction rate. Modifications.

8. Mutational combinative variability. Mutations. Mutagenesis.

9. Genetic and chromosomal hereditary human diseases.

10. Ontogeny as a process of realization of hereditary information. Critical periods of development. Problems of ecology and itatogenesis.

11. Population structure of the species. Evolutionary factors. Micro- and macroevolution. Mechanisms of regularity of evolution of the organic world. Synthetic theory of evolution.

12. Features of human evolution. Population structure of mankind. People as an object of evolutionary factors. Genetic polymorphism of mankind.

Annotated lecture plan

1. Cell - an elementary genetic structural and functional unit of the living. Organization of flows of energy, information and matter in the cell.

Water as the primary environment of life, its role in intermolecular interactions. Molecular organization of hereditary material. Universal organization and functions of nucleic acids in the storage, transmission and implementation of hereditary information. Coding and realization of genetic information in a cell. DNA code system. Proteins are direct products and implementers of genetic information. Molecular organization and functions of proteins as a substrate of life. The biological role of polysaccharides and lipids, their properties. The biological role of polysaccharides, ATP in bioenergetics. A cell is an element of a biological system. A cell is an organism. The cell is an elementary genetic and structural and functional unit of multicellular organisms. The flow of substances, energy and information in the cell. Hierarchy of structural and functional levels of organization of the eukaryotic cell. Molecular, enzymatic and structural and functional complexes. Cell membranes, their role in the spatial and temporal organization of the cell. Cell surface receptors. Their chemical nature and significance. Features of the molecular organization of the epimembrane complex of bacteria, making them resistant to saliva lysozyme, phagocytes and antibiotics. Ion channels of the surface apparatus and their role in the analgesic effect during local anesthesia in surgical dentistry. Endomembrane system as the main component of spatial subcellular organization. Cell organoids, their morphofunctional organization and classification. The nucleus is the control system of the cell. Nuclear shell.

2. Cell cycle. Mitotic cycle. Mitosis. The structure of chromosomes. Dynamics of its structure in the cell cycle. Hetero- and euchromatin. Karyotype.

Morphofunctional characteristics and classification of chromosomes. Human karyotype. Temporal organization of the cell. Cell cycle, its periodization. Mitotic cycle, phases of autoreproduction and distribution of genetic material. The structure of the chromosome and the dynamics of its structure in the cell cycle. Hetero- and euchromatin. The value of mitosis for the reproduction of organisms and regeneration. Mitotic activity of tissues of the human oral cavity. mitotic ratio. Life cycles of cells, tissues and organs of the human oral cavity. Differences in the life cycles of normal and tumor cells. Regulation of the cell cycle and mitotic activity.

3. Gametogenesis. Meiosis. Gametes. Fertilization .

The evolution of reproduction. Biological role and forms of asexual reproduction. The sexual process as a mechanism for the exchange of hereditary information within the species. Gametogenesis. Meiosis, cytological and cytogenetic characteristics. Fertilization. Insemination. Sexual dimorphism: genetic, morphophysiological, endocrine and behavioral aspects. The biological aspect of human reproduction.

4. Subject, tasks and methods of genetics. Classification of genes. The main patterns of inheritance and the formation of signs. Chromosomal theory of heredity.

The general concept of genetic material and its properties: storage of information, change (mutation) of genetic information, repair, its transmission from generation to generation, implementation. The gene is a functional unit of heredity, its properties. Classification of genes (structural, regulatory, jumping). Localization of genes in chromosomes. The concept of allelism, homozygosity, heterozygosity. Genetic and cytological maps of chromosomes. Chromosomes as linkage groups of genes. Fundamentals of the chromosome theory of heredity. Hybridological analysis is a fundamental method of genetics. Types of inheritance. Monogenic inheritance as a mechanism for the transmission of qualitative traits to offspring. Monohybrid cross. The rule of uniformity of hybrids of the first generation. The rule of splitting hybrids of the second generation. Dominance and recissivity, Di- and polyhybrid crossing. Independent combination of non-allelic genes. Statistical nature of Mendelian patterns. Conditions for Mendelian signs, Mendelian signs of a person. Linked inheritance of traits and crossing over. Inheritance of sex-linked traits. Inheritance of traits controlled by human X and Y chromosome genes. Polygenic inheritance as a mechanism for the inheritance of quantitative traits. The role of group-specific substances of saliva in forensic medicine to establish blood groups.

5. Molecular basis of heredity. DNA code system. The structure of genes in eukaryotes and prokaryotes.

Covariant reproduction is a molecular mechanism of heredity and variability in living organisms. DNA sections with unique repeating nucleotide sequences, their functional significance. Molecular bases of heredity. Gene structure in prokaryotes and eukaryotes.

6. Expression of genes. Transcription, Processing, Translation. Genetic Engineering.

Gene expression during protein biosynthesis. Phenomenon splicing. Hypothesis "one gene - one enzyme". Oncogenes. Genetic Engineering.

7. Forms of variability. modification variability. reaction rate. Modifications.

Variability as a property that provides the possibility of the existence of living systems in various states. Forms of variability: modification, combinative, mutational and their significance in ontogenesis and evolution. modification variability. The reaction rate of genetically determined traits. Phenocopies. Adaptive nature of modifications.

8. Mutational combinative variability. Mutations. Mutagenesis

Genotypic variability (combinative and mutational). Mechanisms of combinative variability. The value of combinative variability in ensuring the genotypic diversity of people. Mutational variability. Mutations are qualitative or quantitative changes in genetic material. Classification of mutations: gene, chromosomal, genomic. Mutations in sex and somatic cells. Polyploidy, heteroploidy and haploidy, the mechanisms behind them. Chromosomal mutations: deletion, inversion, duplication and tralocation. Spontaneous and induced mutations. Mutagenesis and genetic control. Repair of genetic material, mechanisms of DNA repair. Mutagens: physical, chemical and biological. Mutagenesis in humans. mutagenesis and carcinogenesis. Genetic danger of environmental pollution and

protection measures.

9. Genetic and chromosomal hereditary human diseases.

The concept of hereditary diseases, the role of the environment in their manifestation. Congenital and non-congenital hereditary diseases. Classification of hereditary diseases. Genetic hereditary diseases, mechanisms of their development, frequency, examples. Chromosomal diseases associated with changes in the number of chromosomes in humans, mechanisms of their development, examples. Chromosomal hereditary diseases associated with changes in the structure of chromosomes, mechanisms of their development, examples. Genetic engineering, its prospects in the treatment of genetic hereditary diseases. Prevention of hereditary diseases. Medical genetic counseling as a basis for the prevention of hereditary diseases. Medico-genetic prediction - determination of the risk of having a sick child in the whole family. Prenatal (antenatal) diagnosis, its methods and possibilities. Monogenically inherited autosomal dominant, autosomal recessive and sex-linked signs, diseases and syndromes in dentistry. Polygenic inherited diseases and syndromes in dentistry. Manifestation and role of mutations in human maxillofacial pathology. Diagnosis of chromosomal diseases and their manifestation in the face and dentition. Consequences of related marriages for the manifestation of hereditary maxillofacial pathology.

10. Ontogeny as a process of realization of hereditary information. Critical periods of development. Problems of ecology and itatogenesis.

Individual development (ontogenesis). Periodization of ontogenesis (pre-embryonic, embryonic and post-embryonic periods). Periodization and general characteristics of the embryonic period: prezygotic period, fertilization, zygote, cleavage, gastrulation, histo-and organogenesis. Implementation of hereditary information in the formation of the definitive phenotype. embryonic induction. Differentiation and integration in development. The role of heredity and environment in ontogeny. Critical periods of development. Hypothesis of differential activity of genes. Selective activity of genes in development; the role of cytoplasmic factors of the egg, contact interactions of cells, intertissue interactions, hormonal influences. Integrity of ontogeny. The laying, development and formation of the face, oral cavity and dentoalveolar system in human embryogenesis. Transformation of the gill apparatus. Hereditary and non-hereditary malformations of the face and dentition as a consequence of dysregulation of ontogenesis. Change of teeth. Age-related changes in the organs of the oral cavity and the dentoalveolar system of a person. The role of environmental factors in the development of caries and diseases of the digestive system.

11. Population structure of the species. Evolutionary factors. Micro- and macroevolution. Mechanisms of regularity of evolution of the organic world. Synthetic theory of evolution.

Population structure of the species. Populations: genetic and ecological characteristics. The gene pool (allele pool) of the population. Mechanisms of formation and factors of the temporal dynamics of the gene pool. Hardy-Weinberg rule: content and mathematical expression. Use to calculate the frequency of heterozygous allelic carriage in humans. A population is an elementary unit of evolution. The primary evolutionary phenomenon is a change in the gene pool (genetic composition) of a population. Elementary evolutionary factors: the mutation process and genetic combinatorics. Population waves, isolation, natural selection. Interaction of elementary evolutionary factors and their role in creating and fixing changes in the genetic composition of populations. Natural selection. Forms of natural selection. The creative role of natural selection in evolution. The adaptive nature of the evolutionary selection of the evolutionary process. Adaptation, its definition. Adaptation to a narrow-local and wide range of conditions of existence. Environment as an evolutionary concept. Micro-macroevolution. Characterization of mechanisms and main results. Types, forms and rules of evolution of groups. The organic world as a result of the process of evolution. Dialectical-materialistic understanding of the problem of the direction of the evolutionary process. Progressive nature of evolution. Biological and morpho-physiological progress: criteria, genetic bases. Phylogenetically determined defects of the face and dentition.

12. Features of human evolution. Population structure of mankind. People as an object of evolutionary factors. Genetic polymorphism of mankind.

Population structure of mankind. Demos. Isolates. People as an object of action of evolutionary factors. Influence of the mutation process, migration, isolation on the genetic constitution of people. Drift of genes and features of gene pools of isolates. Specificity of the action of natural selection in human populations. Examples of selection against heterozygotes and homozygotes. Selection and counter selection. Factors of control selection in relation to the sign of sickle cell erythrocytes. Population-genetic effects of the selection-counter-selection system: stabilization of the gene pools of populations, maintenance of the state of genetic polymorphism over time. Genetic polymorphism, classification. Adaptive and balanced polymorphism. Genetic polymorphism and adaptive potential of populations. Genetic load and its biological essence. Genetic polymorphism of mankind: scales, formation factors. The significance of genetic diversity in the past, present and future of mankind (medical-biological and social aspects). Genetic aspects of predisposition to diseases. The problem of genetic load. Mutation load. The frequency of hereditary diseases. Man as a natural result of the process of historical development of the organic world. The biosocial nature of man. The position of the species in the system of the animal world: the qualitative originality of man. The genetic and social inheritance of man. The ratio of biological and social factors in the development of man at different stages of anthropogenesis. Austrolopithecines, archanthropes, paleoanthropes, neoanthropes. Biological prehistory of mankind: morphological and physiological prerequisites for entering the social sphere. Biological human heritage as one of the factors that ensure the possibility of social development. Its importance in determining the health of people. The role of nutrition in the evolution of the human dentition. The role of geographic environmental factors, primary changes in the masticatory apparatus and the general structure and facial skeleton in the formation of races.

Note: lectures are given once a week

Examination No. 3

“Cell nucleus: main components of the nucleus, their structural and functional characteristics. The hereditary apparatus of the cell. Temporal organization of hereditary material: chromatin and chromosomes. The structure and functions of chromosomes. The concept of karyotype.

Patterns of cell existence in time. Reproduction at the cellular level: mitosis and meiosis. The concept of apoptosis»

Questions for self-preparation:

The role of the nucleus and cytoplasm in the transmission of hereditary information; Characterization of the nucleus as a genetic center. The role of chromosomes in the transmission of hereditary information. Chromosome rules; Cytoplasmic (extranuclear) heredity: plasmids, episomes, their significance in medicine; The main components of the nucleus, their structural and functional characteristics. Modern ideas about the structure of chromosomes: nucleosome model of chromosomes, levels of DNA organization in chromosomes; Chromatin as a form of existence of chromosomes (hetero - and euchromatin): structure, chemical composition; Karyotype. Classification of chromosomes (Denver and Parisian). Types of chromosomes; The life cycle of a cell, its periods, its variants (features in different types of cells). The concept of stem, resting cells. Mitosis is a characteristic of its periods. regulation of mitosis. Morphofunctional characteristics and dynamics of chromosome structure in the cell cycle. The biological significance of mitosis. The concept of apoptosis. Categories of cell complexes. mitotic index. The concept of mitogens and cytostatics.

PART 1. Independent work:

Task number 1. Key concepts of the topic

Select the appropriate terms from the list and distribute them in the left column of Table 1, according to the definitions.

Metaphase chromosomes, Metacentric chromosomes, Acrocentric chromosomes; Meiosis; Sperm; spermatocyte; cytokinesis; Binary division; spermatogenesis; spermatogonia; Mitosis; monospermia; schizogony; Endogony; Ovogenesis; Amitosis; apoptosis; isogamy; gametogenesis; sporulation; gametes; Haploid set of chromosomes; cytokinesis; Ovogonia (oogonia); Anisogamy; Ovotida (ovum); Fertilization; Parthenogenesis; Ovogamia; Fragmentation; Hermaphroditism; The life cycle of a cell; Interphase; Cellular (mitotic cycle).

this is a reduction division that occurs during the maturation of germ cells; as a result of this division, haploid cells are formed, that is, having a single set of chromosomes

this is a direct cell division, in which there is no uniform distribution of hereditary material between daughter cells

part of the cell life cycle during which a differentiated cell performs its functions and prepares for division

division of the cytoplasm following the division of the nucleus.

chromosomes in which the primary constriction (centromere) is located close to the telomeric region;

replicated, maximally spiralized chromosomes at the metaphase stage, located in the equatorial plane of the cell;

chromosomes in which the primary constriction (centromere) is located in the middle and divides the body of the chromosome into two equal-length arms (equal-arm chromosomes);

Task number 2. "The degree of helix chromatin and localization of chromatin in the nucleus".

Based on the materials of the lecture and the textbook "Cytology" 1) study the chromatin depending on the degree of its spiralization and fill in the diagram:

2) study the chromatin depending on the localization in the nucleus and fill in the diagram:

PART 2. Practical work:

Task number 1. Study the person's karyogram below and answer the following questions in writing:

1) Chromosomal set of what sex (male or female) does the karyogram reflect? Explain the answer.

2) Specify the number of autosomes and sex chromosomes shown on the karyogram.

3) What type of chromosomes does the Y chromosome belong to?

Determine the gender and write the word in the box, explain your answer:

"Human Kariogram"

Answer with explanation:

PART 3. Problem-situational tasks:

1. The synthesis of histone proteins is impaired in the cell. What consequences can this have for the cell?

2. On the micropreparation, non-identical two- and multi-nuclear cells were found, some of which did not contain nuclei at all. What process underlies their formation? Define this process.

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).

Chromosome types are distinguished by the location of the centromere (Fig.).

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.

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 The Denver classification of chromosomes was proposed. 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. The Parisian classification of human chromosomes was created 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.

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 K. was introduced by owls. geneticist G. A. Levitsky (1924). K. is one of the most important genetic characteristics of the species, because. each species has its own K., which differs from K. of related species (a new branch of taxonomy is based on this - the so-called karyosystematics)

Depending on the period of the cell cycle, chromosomes can be in the nucleus in two states - condensed, partially condensed and fully condensed.

Previously, the term spiralization, despiralization was used to denote the packing of chromosomes. Currently, a more accurate term is used, condensation, decondensation. This term is more capacious and includes the process of chromosome spiralization, its folding and shortening.

During interphase expression (function, work) of genes is maximum and chromosomes look like thin threads. Those sections of the thread in which RNA synthesis occurs are decondensed, and those sections where synthesis does not occur, on the contrary, are condensed (Fig. 19).

During the division when the DNA in the chromosomes practically does not function, the chromosomes are dense bodies, similar to "X" or "Y". This is due to the strong condensation of DNA in the chromosomes.

It is especially necessary to understand that the hereditary material is presented differently in cells that are in interphase and at the time of division. In the interphase in the cell, the nucleus is clearly visible, the hereditary material in which it is represented by chromatin. Chromatin, in turn, consists of partially condensed strands of chromosomes. If we consider the cell during division, when the nucleus is no longer there, then all the hereditary material is concentrated in the chromosomes, which are maximally condensed (Fig. 20).

The totality of all strands of chromosomes, consisting of DNA and various proteins, in the nuclei of eukaryotic cells is called chromatin (see Fig. 19. B). Chromatin is further divided into euchromatin and heterochromatin. The first is weakly stained with dyes, because. contains thin uncondensed strands of chromosomes. Heterochromatin, on the contrary, contains a condensed, and therefore, well-stained chromosome thread. Non-condensed sections of chromatin contain DNA in which genes function (i.e., RNA synthesis occurs).

A B C

Rice. 19. Chromosomes in interphase.

A - isolated strand of a chromosome from the nucleus of a cell in interphase. 1- condensed area; 2 - non-condensed area.

B - isolated several strands of chromosomes from the nucleus of a cell in interphase. 1 - condensed area; 2 - non-condensed area. B - cell nucleus with strands of chromosomes in interphase. 1 - condensed area; 2 - non-condensed area; 1 and 2, nuclear chromatin.

cell in interphase cell during division

Chromosome nucleus

Rice. 20. Two states of hereditary material in cells in the cell cycle: A - in interphase, the hereditary material is located in the chromosomes, which are partially decondensed and located in the nucleus; B - during cell division, the hereditary material leaves the nucleus, the chromosomes are located in the cytoplasm.

It must be remembered that if the gene is functioning, then the DNA in this region is decondensed. Conversely, gene DNA condensation indicates blockade of gene activity. The phenomenon of condensation and decondensation of DNA sections can often be detected when the activity (turning on or off) of genes is regulated in the cell.

The submolecular structure of chromatin (hereinafter we will call them interphase chromosomes) and chromosomes of a dividing cell (hereinafter we will call them metaphase chromosomes) has not yet been fully elucidated. However, it is clear that under different cell conditions (interphase and division), the organization of the hereditary material is different. Interphase (IC) and metaphase chromosomes (MX) are based on nucleosome . The nucleosome consists of a central protein portion around which a strand of DNA is wrapped. The central part is formed by eight histone protein molecules - H2A, H2B, H3, H4 (each histone is represented by two molecules). In this regard, the core of the nucleosome is called tetramer, octamer or core. A DNA molecule in the form of a helix wraps around the core 1.75 times and goes to the neighboring core, wraps around it and goes to the next one. Thus, a peculiar figure is created, resembling a thread (DNA) with beads (nucleosomes) strung on it.

Between the nucleosomes lies DNA called linker. Another histone, H1, can bind to it. If it binds to the linker site, then the DNA bends and coils into a spiral (Fig. 21. B). Histone H1 is involved in the complex process of DNA condensation, in which the string of beads coils into a 30 nm thick helix. This spiral is called solenoid. The strands of chromosomes in interphase cells consist of strands of beads and solenoids. In metaphase chromosomes, the solenoid coils into a supercoil, which connects to a mesh structure (made of proteins), forming loops that fit already in the form of a chromosome. Such packaging leads to almost 5000-fold compaction of DNA in the metaphase chromosome. Figure 23 shows the sequential chromatin folding scheme. It is clear that the process of DNA helixing in IC and MX is much more complicated, but what has been said makes it possible to understand the most general principles of chromosome packing.

Rice. 21. Structure of nucleosomes:

A - in an uncondensed chromosome. Histone H1 is not associated with linker DNA. B - in the condensed chromosome. Histone H1 is associated with linker DNA.

It should be noted that each chromosome in metaphase consists of two chromatids held together by centromeres(primary constriction). Each of these chromatids is based on daughter DNA molecules packed separately. After the process of compaction, they become clearly distinguishable in a light microscope as chromatids of one chromosome. At the end of mitosis, they disperse into daughter cells. Since the separation of the chromatids of one chromosome from each other, they are already called chromosomes, that is, the chromosome contains either two chromatids before division, or one (but it is already called a chromosome) after division.

Some chromosomes, in addition to the primary constriction, have a secondary constriction. She is also called nucleolar organizer. This is a thin thread of a chromosome, at the end of which a satellite is placed. The secondary constriction, like the main chromosome, consists of DNA, on which the genes responsible for the synthesis of ribosomal RNA are located. At the end of a chromosome is a region called telomere. It seems to "seal" the chromosome. If the telomere accidentally breaks off, a "sticky" end is formed, which can connect with the same end of another chromosome.

Cell in interphase Dividing cell

Chromosome strand

Nucleosome Histone H1

Rice. 22. Model of chromosome packaging in cells in interphase and mitosis.

located in the middle, the chromosome has equal arms. In submetacentric chromosomes, the centromere is slightly shifted towards one end. The arms of the chromosome are not the same in length - one is longer than the other. In acrocentric chromosomes, the centromere is located almost at the end of the chromosome and the short arms are difficult to distinguish. The number of chromosomes is constant for each species. Thus, the human karyotype contains 46 chromosomes. Drosophila has 8 of them, and in a wheat cell - 14.

The totality of all metaphase chromosomes of a cell, their shape and morphology is called karyotype. Three types of chromosomes are distinguished by shape - metacentric, submetacentric and acrocentric (Fig. 23). In metacentric chromosomes, the centromere

nucleolus

This is a dense, well-stained body located inside the nucleus. It contains DNA, RNA and proteins. The basis of the nucleolus is nucleolar organizers - DNA sections that carry multiple copies of rRNA genes. Synthesis of ribosomal RNA occurs on the DNA of nucleolar organizers. Proteins attach to them and a complex formation is formed - ribonucleoprotein (RNP) particles. These are the precursors (or semi-finished products) of the small and large subunits of ribosomes. The process of RNP formation mainly occurs in the peripheral part of the nucleoli. The predecessors of ri-

Satellite