Structure and functions of chromosomes

Chromosomes are cell structures that store and transmit hereditary information. A chromosome consists of DNA and protein. A complex of proteins bound to DNA forms chromatin. Proteins play an important role in packaging DNA molecules in the nucleus.

The DNA in chromosomes is packaged in such a way that it fits in the nucleus, the diameter of which usually does not exceed 5 microns (5-10-4 cm). The DNA packaging takes on the appearance of a loop structure, similar to the lampbrush chromosomes of amphibians or the polytene chromosomes of insects. The loops are maintained by proteins that recognize specific nucleotide sequences and bring them together. The structure of the chromosome is best seen in 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 is built from chromatin loops. Chromatin does not replicate. Only DNA is replicated.

When DNA replication begins, RNA synthesis stops. Chromosomes can be in two states: condensed (inactive) and decondensed (active).

The diploid set of chromosomes of an organism is called a karyotype. Modern research methods make it possible to identify each chromosome in a karyotype. To do this, take into account the distribution of light and dark bands visible under a microscope (alternating AT and GC pairs) in chromosomes treated with special dyes. The chromosomes of representatives of different species have transverse striations. Related species, such as humans and chimpanzees, have very similar patterns of alternating bands in their chromosomes.

Lecture No. 3

Topic: Organizing the flow of genetic information

Lecture outline

1. Structure and functions of the cell nucleus.

2. Chromosomes: structure and classification.

3. Cellular and mitotic cycles.

4. Mitosis, meiosis: cytological and cytogenetic characteristics, significance.

Structure and function of the cell nucleus

The main genetic information is contained in the cell nucleus.

Cell nucleus(lat. – nucleus; Greek – karyon) was described in 1831. Robert Brown. The shape of the nucleus depends on the shape and function of the cell. The size of the nuclei varies depending on the metabolic activity of the cells.

Interphase core shell (karyolemma) consists of outer and inner elementary membranes. Between them is perinuclear space. There are holes in the membranes - pores. Between the edges of the nuclear pore there are protein molecules that form pore complexes. The pore opening is covered with a thin film. During active metabolic processes in the cell, most of the pores are open. Through them there is a flow of substances - from the cytoplasm to the nucleus and back. Number of pores in one nucleus

Rice. Diagram of the structure of the cell nucleus

1 and 2 – outer and inner membranes of the nuclear envelope, 3

– nuclear pore, 4 – nucleolus, 5 – chromatin, 6 – nuclear juice

reaches 3-4 thousand. The outer nuclear membrane connects to the endoplasmic reticulum channels. It is usually located ribosomes. Proteins on the inner surface of the nuclear envelope form nuclear lamina. It maintains the constant shape of the nucleus and chromosomes are attached to it.

Nuclear juice - karyolymph, a colloidal solution in a gel state that contains proteins, lipids, carbohydrates, RNA, nucleotides, and enzymes. Nucleolus– a non-permanent component of the nucleus. It disappears at the beginning of cell division and is restored at the end of it. Chemical composition of nucleoli: protein (~90%), RNA (~6%), lipids, enzymes. Nucleoli are formed in the area of ​​secondary constrictions of satellite chromosomes. Function of nucleoli: assembly of ribosomal subunits.

X romatine nuclei are interphase chromosomes. They contain DNA, histone proteins and RNA in a ratio of 1:1.3:0.2. DNA combines with protein to form deoxyribonucleoprotein(DNP). During mitotic division of the nucleus, DNP spirals and forms chromosomes.

Functions of the cell nucleus:

1) stores the hereditary information of the cell;

2) participates in cell division (reproduction);

3) regulates metabolic processes in the cell.

Chromosomes: structure and classification

Chromosomes(Greek - chromo- color, soma– body) is a spiralized chromatin. Their length is 0.2 – 5.0 µm, diameter 0.2 – 2 µm.

Rice. Types of chromosomes

Metaphase chromosome consists of two chromatid, which connect centromere (primary constriction). It 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 combined with histone proteins. Intensely stained areas 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 have the same length. The section of the arm near the centromere is called proximal, the opposite is called distal.

2. Submetacentric chromosomes– the centromere is offset 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(multi-stranded chromosomes).

There are 4 rules for the chromosomes of all organisms:

1. Rule of constant number of chromosomes. Normally, organisms of certain species have a constant, species-specific number of chromosomes. For example: a person has 46, a dog has 78, a Drosophila fly has 8.

2. Chromosome pairing. In a diploid set, each chromosome normally has a paired chromosome - identical in shape and size.

3. Individuality of chromosomes. Chromosomes of different pairs differ in shape, structure and size.

4. Chromosome continuity. When 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.

Chromosomes are classified according to different characteristics.

1. Chromosomes that are identical in the cells of male and female organisms are called autosomes. A person has 22 pairs of autosomes in their karyotype. Chromosomes that are different in the cells of male and female organisms are called heterochromosomes, or sex chromosomes. In a man these are the X and Y chromosomes, in a woman they are the X and X chromosomes.

2. The arrangement of chromosomes in decreasing order of magnitude is called idiogram. This is a systematic karyotype. Chromosomes are arranged in pairs (homologous chromosomes). The first pair are the largest ones, the 22nd pair are the small ones, and the 23rd pair are the sex chromosomes.

3. In 1960 Denver classification of chromosomes was proposed. It is built on the basis of their shape, size, position of the centromere, the 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 a 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. The 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. The chromosomes are acrocentric. CI is about 15%.

Group E. Pairs of chromosomes 16 – 18. Relatively short, metacentric or submetacentric. CI 26-40%.

Group F. 19th – 20th pairs. Short, submetacentric chromosomes. CI 36-46%.

Group G. 21-22nd pairs. Small, acrocentric chromosomes. CI 13-33%. The Y chromosome also belongs to this group.

4. The Paris classification of human chromosomes was created in 1971. Using this classification, it is possible to determine the localization of genes in a specific pair of chromosomes. Using special staining methods, a characteristic order of alternating dark and light stripes (segments) is identified in each chromosome. Segments are designated by the name of the methods that identify them: Q – segments – after staining with quinine mustard; G – segments – stained with Giemsa dye; R – segments – staining after heat denaturation and others. The short arm of the chromosome is designated by the letter p, the long arm by the letter q. Each chromosome arm is divided into regions and designated by numbers from centromere to telomere. Bands within regions are numbered in order from the centromere. For example, the location of the esterase D gene is 13p14 - 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.

Related information.

Video tutorial 1: Cell division. Mitosis

Video tutorial 2: Meiosis. Phases of meiosis

Lecture: A cell is the genetic unit of a living thing. Chromosomes, their structure (shape and size) and functions

Cell - genetic unit of living things

The basic unit of life is the individual cell. It is at the cellular level that processes occur that distinguish living matter from nonliving matter. In each of the cells, hereditary information about the chemical structure of proteins that must be synthesized in it is stored and intensively used, and therefore it is called the genetic unit of the living. Even anucleated red blood cells in the initial stages of their existence have mitochondria and a nucleus. Only in a mature state do they not have structures for protein synthesis.

To date, science does not know any cells that do not contain DNA or RNA as a carrier of genomic information. In the absence of genetic material, the cell is not capable of protein synthesis, and therefore life.

DNA is not only found in nuclei; its molecules are contained in chloroplasts and mitochondria; these organelles can multiply inside the cell.

DNA in a cell is found in the form of chromosomes - complex protein-nucleic acid complexes. Eukaryotic chromosomes are localized in the nucleus. Each of them is a complex structure of:

The only long DNA molecule, 2 meters of which is packed into a compact structure measuring (in humans) up to 8 microns;

Special histone proteins, whose role is to pack chromatin (the substance of the chromosome) into the familiar rod-shaped shape;

Chromosomes, their structure (shape and size) and functions

This dense packing of genetic material is produced by the cell before dividing. It is at this moment that the densely packed formed chromosomes can be examined under a microscope. When DNA is folded into compact chromosomes called heterochromatin, messenger RNA cannot be synthesized. During the period of cell mass gain and interphase development, the chromosomes are in a less packed state, which is called interchromatin and in it mRNA is synthesized and DNA replication occurs.

The main elements of chromosome structure are:

Centromere. This is a part of a chromosome with a special nucleotide sequence. It connects two chromatids and participates in conjugation. It is to this that the protein filaments of the cell division spindle tubes are attached.

Telomeres. These are the terminal sections of chromosomes that are not capable of connecting with other chromosomes; they play a protective role. They consist of repeating sections of specialized DNA that form complexes with proteins.

DNA replication initiation points.

Prokaryotic chromosomes are very different from eukaryotic ones, being DNA-containing structures located in the cytoplasm. Geometrically, they are a ring molecule.

The chromosome set of a cell has its own name - karyotype. Each type of living organism has its own characteristic composition, number and shape of chromosomes.

Somatic cells contain a diploid (double) chromosome set, half of which is received from each parent.

Chromosomes responsible for encoding the same functional proteins are called homologous. The ploidy of cells can be different - as a rule, gametes in animals are haploid. In plants, polyploidy is currently a fairly common phenomenon, used in the creation of new varieties as a result of hybridization. Violation of the amount of ploidy in warm-blooded animals and humans causes serious congenital diseases such as Down syndrome (the presence of three copies of chromosome 21). Most often, chromosomal abnormalities lead to the inability of the organism.

In humans, the complete chromosome set consists of 23 pairs. The largest known number of chromosomes, 1600, was found in the simplest planktonic organisms, radiolarians. Australian black bulldog ants have the smallest chromosome set - only 1.

Life cycle of a cell. Phases of mitosis and meiosis

Interphase, in other words, the period of time between two divisions is defined by science as the life cycle of a cell.

During interphase, vital chemical processes occur in the cell, it grows, develops, and accumulates reserve substances. Preparation for reproduction involves doubling the contents - organelles, vacuoles with nutritional contents, and the volume of the cytoplasm. It is thanks to division, as a way to quickly increase the number of cells, that long life, reproduction, an increase in the size of the body, its survival from wounds and tissue regeneration are possible. The following stages are distinguished in the cell cycle:

Interphase. Time between divisions. First, the cell grows, then the number of organelles, the volume of reserve substance increases, and proteins are synthesized. In the last part of interphase, the chromosomes are ready for subsequent division - they consist of a pair of sister chromatids.

Mitosis. This is the name of one of the methods of nuclear division, characteristic of bodily (somatic) cells, during which 2 cells are obtained from one, with an identical set of genetic material.

Gametogenesis is characterized by meiosis. Prokaryotic cells have retained the ancient method of reproduction - direct division.

Mitosis consists of 5 main phases:

Prophase. Its beginning is considered to be the moment when the chromosomes become so densely packed that they are visible under a microscope. Also, at this time, the nucleoli are destroyed and a spindle is formed. Microtubules are activated, the duration of their existence decreases to 15 seconds, but the rate of formation also increases significantly. The centrioles diverge to opposite sides of the cell, forming a huge number of protein microtubules that are constantly synthesized and disintegrated, which extend from them to the centromeres of the chromosomes. This is how the fission spindle is formed. Membrane structures such as the ER and Golgi apparatus break up into separate vesicles and tubes, randomly located in the cytoplasm. Ribosomes are separated from the ER membranes.

Metaphase. A metaphase plate is formed, consisting of chromosomes balanced in the middle of the cell by the efforts of opposite centriole microtubules, each pulling them in their own direction. At the same time, the synthesis and disintegration of microtubules continues, a kind of “bulkhead” of them. This phase is the longest.

• Anaphase. The forces of microtubules tear off chromosome connections in the centromere region and forcefully stretch them towards the poles of the cell. In this case, chromosomes sometimes take a V-shape due to the resistance of the cytoplasm. A ring of protein fibers appears in the area of ​​the metaphase plate.
• Telophase. Its beginning is considered to be the moment when the chromosomes reach the division poles. The process of restoration of the internal membrane structures of the cell begins - the ER, Golgi apparatus, and nucleus. The chromosomes are unpacked. Nucleoli assemble and ribosome synthesis begins. The fission spindle disintegrates.
• Cytokinesis. The last phase in which the protein ring that appears in the central region of the cell begins to shrink, pushing the cytoplasm towards the poles. The cell divides into two and a protein ring of the cell membrane is formed in place.

Regulators of the mitosis process are specific protein complexes. The result of mitotic division is a pair of cells with identical genetic information. In heterotrophic cells, mitosis occurs faster than in plant cells. In heterotrophs, this process can take from 30 minutes, in plants – 2-3 hours.

To generate cells with half the normal number of chromosomes, the body uses another division mechanism - meiosis.

It is associated with the need to produce germ cells; in multicellular organisms, it avoids the constant doubling of the number of chromosomes in the next generation and makes it possible to obtain new combinations of allelic genes. It differs in the number of phases, being longer. The resulting decrease in the number of chromosomes leads to the formation of 4 haploid cells. Meiosis consists of two divisions following each other without interruption.

The following phases of meiosis are defined:

Prophase I. Homologous chromosomes move closer to each other and unite longitudinally. This combination is called conjugation. Then crossing over occurs - double chromosomes cross their arms and exchange sections.

Metaphase I. Chromosomes separate and occupy positions at the equator of the cell spindle, taking on a V-shape due to the tension of the microtubules.

Anaphase I. Homologous chromosomes are stretched by microtubules towards the cell poles. But unlike mitotic division, they separate as whole chromatids rather than as separate ones.

The result of the first meiotic division is the formation of two cells with half the number of intact chromosomes. Between divisions of meiosis, interphase is practically absent, chromosome doubling does not occur, they are already bichromatid.

Immediately following the first, the repeated meiotic division is completely analogous to mitosis - in it, the chromosomes are divided into separate chromatids, distributed equally between new cells.

oogonia go through the stage of mitotic reproduction at the embryonic stage of development, so that the female body is already born with a constant number of them;

spermatogonia are capable of reproduction at any time during the reproductive period of the male body. A much larger number of them are generated than female gametes.

Gametogenesis of animal organisms occurs in the gonads - gonads.

The process of transformation of spermatogonia into spermatozoa occurs in several stages:

Mitotic division transforms spermatogonia into first-order spermatocytes.

As a result of a single meiosis, they turn into second-order spermatocytes.

The second meiotic division produces 4 haploid spermatids.

The period of formation begins. In the cell, the nucleus becomes compacted, the amount of cytoplasm decreases, and a flagellum forms. Also, proteins are stored and the number of mitochondria increases.

The formation of eggs in an adult female body occurs as follows:

From the 1st order oocyte, of which there is a certain number in the body, as a result of meiosis with a halving of the number of chromosomes, 2nd order oocytes are formed.

As a result of the second meiotic division, a mature egg and three small reduction bodies are formed.

This unbalanced distribution of nutrients between the 4 cells is intended to provide a large resource of nutrients for the new living organism.

Ovules in ferns and mosses are formed in archegonia. In more highly organized plants - in special ovules located in the ovary.

Heredity and variability in living nature exist thanks to chromosomes, genes, (DNA). It is stored and transmitted as a chain of nucleotides as part of DNA. What role do genes play in this phenomenon? What is a chromosome from the point of view of transmission of hereditary characteristics? Answers to questions like these provide insight into coding principles and genetic diversity on our planet. It largely depends on how many chromosomes are included in the set and on the recombination of these structures.

From the history of the discovery of “particles of heredity”

Studying plant and animal cells under a microscope, many botanists and zoologists in the middle of the 19th century drew attention to the thinnest threads and the smallest ring-shaped structures in the nucleus. More often than others, the German anatomist Walter Flemming is called the discoverer of chromosomes. It was he who used aniline dyes to treat nuclear structures. Flemming called the discovered substance “chromatin” for its ability to stain. The term “chromosomes” was introduced into scientific use in 1888 by Heinrich Waldeyer.

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

Prerequisites for the chromosomal theory of heredity

American researcher Walter Sutton found out how many chromosomes are contained in the cell nucleus. The scientist considered these structures to be carriers of units of heredity, characteristics of the organism. Sutton discovered that chromosomes consist of genes through which properties and functions are passed on to offspring from their parents. The geneticist in his publications gave descriptions of chromosome pairs and their movement during the division of the cell nucleus.

Regardless of his American colleague, work in the same direction was carried out by Theodore Boveri. Both researchers in their works studied the issues of transmission of hereditary characteristics and formulated the main provisions on the role of chromosomes (1902-1903). Further development of the Boveri-Sutton theory took place in the laboratory of Nobel laureate Thomas Morgan. The outstanding American biologist and his assistants established a number of patterns of gene placement on the chromosome and developed a cytological basis that explains the mechanism of the laws of Gregor Mendel, the founding father of genetics.

Chromosomes in a cell

The study of the structure of chromosomes began after their discovery and description in the 19th century. These bodies and filaments are found in prokaryotic organisms (non-nuclear) and eukaryotic cells (in nuclei). Study under a microscope made it possible to establish what a chromosome is from a morphological point of view. It is a mobile filamentous body that is visible during certain phases of the cell cycle. In interphase, the entire volume of the nucleus is occupied by chromatin. During other periods, chromosomes are distinguishable in the form of one or two chromatids.

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

Chromosome shapes

Each chromosome has an individual structure and differs from others in its coloring features. When studying morphology, it is important to determine the position of the centromere, the length and placement of the arms relative to the constriction. The set of chromosomes usually includes the following forms:

• metacentric, or equal arms, which are characterized by a median location of the centromere;
• submetacentric, or unequal arms (the constriction is shifted towards one of the telomeres);
• acrocentric, or rod-shaped, in which the centromere is located almost at the end of the chromosome;
• dotted with a difficult-to-define shape.

Functions of chromosomes

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

Role of DNA and RNA

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

Nucleic acids are directly involved in the biosynthesis of amino acids and proteins and ensure the transmission of hereditary characteristics from generation to generation. DNA is contained in the nucleus of a eukaryotic cell, RNA is concentrated in the cytoplasm.

Genes

X-ray diffraction analysis showed that DNA forms a double helix, the chains of which consist of nucleotides. They represent the carbohydrate deoxyribose, a phosphate group, and one of four nitrogenous bases:

Regions of helical deoxyribonucleoprotein strands are genes that carry encoded information about the sequence of amino acids in proteins or RNA. During reproduction, hereditary characteristics from parents are transmitted to offspring in the form of gene alleles. They determine the functioning, growth and development of a particular organism. According to a number of researchers, those sections of DNA that do not encode polypeptides perform regulatory functions. The human genome can contain up to 30 thousand genes.

Set of chromosomes

The total number of chromosomes and their features are a characteristic feature of the species. In the Drosophila fly their number is 8, in primates - 48, in humans - 46. This number is constant for the cells of organisms that belong to the same species. For all eukaryotes there is the concept of “diploid chromosomes”. This is a complete set, or 2n, as opposed to haploid - half the number (n).

Chromosomes in one pair are homologous, identical in shape, structure, location of centromeres and other elements. Homologues have their own characteristic features that distinguish them from other chromosomes in the set. Staining with basic dyes allows you to examine and study the distinctive features of each pair. is present in the somatic ones - in the reproductive ones (the so-called gametes). In mammals and other living organisms with a heterogametic male sex, two types of sex chromosomes are formed: the X chromosome and the Y. Males have a set of XY, females have a set of XX.

Human chromosome set

The cells of the human body contain 46 chromosomes. All of them are combined into 23 pairs that make up the set. There are two types of chromosomes: autosomes and sex chromosomes. The first form 22 pairs - common for women and men. What differs from them is the 23rd pair - sex chromosomes, which are non-homologous in the cells of the male body.

Genetic traits are associated with gender. They are transmitted by a Y and an X chromosome in men and two X chromosomes in women. Autosomes contain the rest of the information about hereditary traits. There are techniques that allow you to individualize all 23 pairs. They are clearly distinguishable in the drawings when painted in a certain color. It is noticeable that the 22nd chromosome in the human genome is the smallest. Its DNA, when stretched, is 1.5 cm long and has 48 million nitrogen base pairs. Special histone proteins from the composition of chromatin perform compression, after which the thread takes up thousands of times less space in the cell nucleus. Under an electron microscope, the histones in the interphase core resemble beads strung on a strand of DNA.

Genetic diseases

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

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

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

History of the discovery of chromosomes

Drawing from W. Flemming’s book depicting different stages of salamander epithelial cell division (W. Flemming. Zellsubstanz, Kern und Zelltheilung. 1882)

In different articles and books, priority for the discovery of chromosomes is given to different people, but most often the year of discovery of chromosomes is called 1882, and their discoverer is the German anatomist W. Fleming. However, it would be fairer to say that he did not discover chromosomes, but in his fundamental book “Zellsubstanz, Kern und Zelltheilung” (German) he collected and organized information about them, supplementing them with the results of his own research. The term "chromosome" was proposed by the German histologist Heinrich Waldeyer in 1888; "chromosome" literally means "colored body", since the basic dyes are well bound by chromosomes.

Now it is difficult to say who made the first description and drawing of chromosomes. In 1872, the Swiss botanist Carl von Negili published a work in which he depicted certain bodies that appear in place of the nucleus during cell division during the formation of pollen in a lily ( Lilium tigrinum) and Tradescantia ( Tradescantia). However, his drawings do not allow us to unequivocally state that K. Negili saw exactly chromosomes. In the same 1872, the botanist E. Russov presented his images of cell division during the formation of spores in a fern of the genus Zhovnik ( Ophioglossum) and lily pollen ( Lilium bulbiferum). In his illustrations it is easy to recognize individual chromosomes and stages of division. Some researchers believe that the German botanist Wilhelm Hofmeister was the first to see chromosomes long before K. Negili and E. Russow, back in 1848-1849. At the same time, neither K. Negili, nor E. Russov, nor even more so V. Hofmeister realized the significance of what they saw.

After the rediscovery of Mendel's laws in 1900, it took only one or two years for it to become clear that chromosomes behaved exactly as expected from “particles of heredity.” In 1902 T. Boveri and in 1902-1903 W. Setton ( Walter Sutton) independently of each other were the first to put forward a hypothesis about the genetic role of chromosomes. T. Boveri discovered that the sea urchin embryo Paracentrotus lividus can develop normally only if there is at least one, but complete set of chromosomes. He also found that different chromosomes are not identical in composition. W. Setton studied gametogenesis in the locust Brachystola magna and realized that the behavior of chromosomes in meiosis and during fertilization fully explains the patterns of divergence of Mendelian factors and the formation of their new combinations.

Experimental confirmation of these ideas and the final formulation of the chromosome theory was made in the first quarter of the 20th century by the founders of classical genetics, who worked in the USA with the fruit fly ( D. melanogaster): T. Morgan, K. Bridges ( C.B.Bridges), A. Sturtevant ( A.H. Sturtevant) and G. Möller. Based on their data, they formulated the “chromosomal theory of heredity,” according to which the transmission of hereditary information is associated with chromosomes, in which genes are localized linearly, in a certain sequence. These findings were published in 1915 in the book The Mechanisms of Mendelian Heredity.

In 1933, T. Morgan received the Nobel Prize in Physiology or Medicine for his discovery of the role of chromosomes in heredity.

Eukaryotic chromosomes

The basis of the chromosome is a linear (not closed in a ring) macromolecule of deoxyribonucleic acid (DNA) of considerable length (for example, in the DNA molecules of human chromosomes there are from 50 to 245 million pairs of nitrogenous bases). When stretched, the length of a human chromosome can reach 5 cm. In addition to it, the chromosome includes five specialized proteins - H1, H2A, H2B, H3 and H4 (the so-called histones) and a number of non-histone proteins. The amino acid sequence of histones is highly conserved and practically does not differ in the most diverse groups of organisms.

Primary constriction

Chromosome constriction (X. n.), in which the centromere is localized and which divides the chromosome into arms.

Secondary constrictions

A morphological feature that allows the identification of individual chromosomes in a set. They differ from the primary constriction by the absence of a noticeable angle between the chromosome segments. Secondary constrictions are short and long and are localized at different points along the length of the chromosome. In humans, these are chromosomes 9, 13, 14, 15, 21 and 22.

Types of chromosome structure

There are four types of chromosome structure:

• telocentric(rod-shaped chromosomes with a centromere located at the proximal end);
• acrocentric(rod-shaped chromosomes with a very short, almost invisible second arm);
• submetacentric(with shoulders of unequal length, resembling the letter L in shape);
• metacentric(V-shaped chromosomes with arms of equal length).

The chromosome type is constant for each homologous chromosome and may be constant in all members of the same species or genus.

Satellites

Satellite- this is a round or elongated body, separated from the main part of the chromosome by a thin chromatin thread, with a diameter equal to or slightly smaller than the chromosome. Chromosomes with a satellite are usually referred to as SAT chromosomes. The shape, size of the satellite and the thread connecting it are constant for each chromosome.

Nucleolar zone

Zones of the nucleolus ( nucleolar organizers) - special areas with which the appearance of some secondary constrictions is associated.

Chromonema

Chromonema is a helical structure that can be seen in decompacted chromosomes through an electron microscope. It was first observed by Baranetsky in 1880 in the chromosomes of Tradescantia anther cells, the term was introduced by Veidovsky. A chromonema can consist of two, four or more threads, depending on the object being studied. These threads form two types of spirals:

• paranemic(spiral elements are easy to separate);
• plectonemic(the threads are tightly intertwined).

Chromosomal rearrangements

Violation of the structure of chromosomes occurs as a result of spontaneous or provoked changes (for example, after irradiation).

• Gene (point) mutations (changes at the molecular level);
• Aberrations (microscopic changes visible using a light microscope):

Giant chromosomes

Such chromosomes, which are characterized by their enormous size, can be observed in some cells at certain stages of the cell cycle. For example, they are found in the cells of some tissues of dipteran insect larvae (polytene chromosomes) and in the oocytes of various vertebrates and invertebrates (lampbrush chromosomes). It was on preparations of giant chromosomes that signs of gene activity were revealed.

Polytene chromosomes

Balbiani were first discovered in th, but their cytogenetic role was revealed by Kostov, Paynter, Geitz and Bauer. Contained in the cells of the salivary glands, intestines, tracheas, fat body and Malpighian vessels of dipteran larvae.

Lamp brush chromosomes

There is evidence that bacteria have proteins associated with nucleoid DNA, but histones have not been found in them.

Human chromosomes

Each nucleated human somatic cell contains 23 pairs of linear chromosomes, as well as numerous copies of mitochondrial DNA. The table below shows the number of genes and bases in human chromosomes.