How is a chromosome formed? What is a chromosome in biology? A set of chromosomes. Levels of compaction of chromosomal DNA
Video lesson 1: Cell division. Mitosis
Video lesson 2: Meiosis. Phases of meiosis
Lecture: A cell is the genetic unit of a living thing. Chromosomes, their structure (shape and size) and functions
The cell is the genetic unit of life
The single cell is recognized as the basic unit of life. It is at the cellular level that processes occur that distinguish living matter from non-living matter. Each of the cells stores and intensively uses hereditary information about the chemical structure of proteins that must be synthesized in it, and therefore it is called the genetic unit of the living. Even nuclear-free erythrocytes at the initial stages of their existence have mitochondria and a nucleus. Only in their mature state do they lack the structures for protein synthesis.
To date, science does not know cells that would 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 present not only in the nuclei, its molecules are contained in chloroplasts and mitochondria, these organelles can multiply inside the cell.
DNA in a cell is in the form of chromosomes - complex protein-nucleic acid complexes. Eukaryotic chromosomes are located 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 (in humans) up to 8 microns in size;
Special histone proteins, whose role is to pack chromatin (the substance of the chromosome) into a familiar rod-shaped form;
Chromosomes, their structure (shape and size) and functions
This dense packing of genetic material is produced by the cell before division. It is at this moment that densely packed, formed chromosomes can be examined under a microscope. When DNA is folded into compact chromosomes called heterochromatin, messenger RNA synthesis is not possible. During the period of cell mass recruitment and its interphase development, chromosomes are in a less packed state, which is called interchromatin and mRNA is synthesized in it, DNA replication occurs.
The main elements of the structure of chromosomes are:
centromere. This is a part of the chromosome with a special sequence of nucleotides. It connects two chromatids together, participates in conjugation. It is to it that the protein filaments of the spindle tubes of cell division 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.
Points of initiation of DNA replication.
Chromosomes of prokaryotes are very different from eukaryotic ones, representing DNA-containing structures located in the cytoplasm. Geometrically, they represent a ring molecule.
The chromosome set of a cell has its own name - a karyotype. Each of the species of living organisms has its own composition, number and shape of chromosomes, characteristic only for it.
Somatic cells contain a diploid (double) chromosome set, half of each parent received.
Chromosomes responsible for encoding the same functional proteins are called homologous. The ploidy of cells can be different - as a rule, in animals, gametes are haploid. In plants, polyploidy is now a fairly common phenomenon, which is used to create new varieties as a result of hybridization. Disruption of the amount of ploidy in warm-blooded animals and humans causes serious congenital diseases such as Down's syndrome (the presence of three copies of the 21st chromosome). Most often, chromosomal abnormalities lead to the non-viability 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. The smallest set of chromosomes in the Australian black bulldog ants is only 1.
The life cycle of a cell. Phases of mitosis and meiosis
Interphase, in other words, the length of time between two divisions, is defined by science as life cycle cells.
During the interphase, vital chemical processes take place in the cell, it grows, develops, and accumulates reserve substances. Preparation for reproduction involves doubling the content - organelles, vacuoles with nutritional content, the volume of the cytoplasm. It is thanks to division, as a way to rapidly increase the number of cells, that long life, reproduction, an increase in the size of the body, its survival in case of injuries 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 increases, the volume of the reserve substance increases, proteins are synthesized. In the last part of the interphase, the chromosomes are ready for the 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, in its course, 2 cells are obtained from one, with an identical set of genetic material.
Meiosis is characteristic of gametogenesis. Prokaryotic cells have retained the ancient method of reproduction - direct division.
Mitosis consists of 5 main phases:
- Anaphase. The efforts of the microtubules break off the connections of the chromosomes in the centromere region, and with force stretch them to the poles of the cell. In this case, the chromosomes sometimes take a V-shape due to the resistance of the cytoplasm. A ring of protein fibers appears in the region of the metaphase plate.
- Telophase. Its beginning is considered the moment when the chromosomes reach the poles of division. The process of restoring the internal membrane structures of the cell begins - EPS, the Golgi apparatus, the nucleus. Chromosomes unpack. The nucleoli assemble and the synthesis of ribosomes begins. The spindle of division disintegrates.
- cytokinesis. The last phase, in which the protein ring that appeared in the central region of the cell begins to shrink, pushing the cytoplasm towards the poles. There is a division of the cell into two and the formation of the protein ring of the cell membrane in place.
Prophase. Its beginning is considered the moment when the chromosomes become so densely packed that they are visible under a microscope. Also, at this time, the nucleoli are destroyed, a division spindle is formed. Microtubules are activated, the duration of their existence decreases to 15 seconds, but the rate of formation also increases significantly. Centrioles diverge to opposite sides of the cell, forming a huge number of constantly synthesizing and decaying protein microtubules that extend from them to the centromeres of chromosomes. This is how the spindle is formed. Membrane structures such as ER and the Golgi apparatus disintegrate into separate vesicles and tubules 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 its own direction. At the same time, the synthesis and disintegration of microtubules continues, their kind of "bulkhead". This phase is the longest.
Regulators of the process of mitosis are specific protein complexes. The result of mitotic division is a pair of cells with identical genetic information. In heterotrophic cells, mitosis proceeds 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, a different division mechanism is used by the body - meiosis.
It is associated with the need to produce germ cells; in multicellular organisms, it avoids a 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 is two divisions that follow each other without interruption.
The following phases of meiosis are defined:
Prophase I. Homologous chromosomes approach each other and unite longitudinally. Such an association is called a conjugation. Then there is a crossing over - double chromosomes cross their shoulders and exchange sections.
Metaphase I Chromosomes separate and take up positions at the equator of the cell spindle, taking on a V-shape due to microtubule tension.
Anaphase I Homologous chromosomes are stretched by microtubules to the poles of the cell. But unlike mitotic division, they diverge as whole, not as individual chromatids.
The result of the first division of meiosis is the formation of two cells with half the number of whole chromosomes. Between divisions of meiosis, interphase is practically absent, doubling of chromosomes does not happen, they are already two-chromatid.
Immediately following the first repeated meiotic division is completely similar 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 an unchanged number of them;
spermatogonia are capable of reproduction at any time during the reproductive period of the male body. Much more of them are generated than female gametes.
Gametogenesis of animal organisms occurs in the sex glands - gonads.
The process of transformation of spermatogonia into spermatozoa occurs in several stages:
Mitotic division transforms spermatogonia into spermatocytes of the 1st order.
As a result of a single meiosis, they turn into spermatocytes of the 2nd order.
The second meiotic division produces 4 haploid spermatids.
There is a period of formation. In the cell, the nucleus is compacted, the amount of cytoplasm decreases, and the flagellum is formed. Also, proteins are stored and the number of mitochondria increases.
The formation of eggs in an adult female body occurs as follows:
From the oocyte of the 1st order, of which there is a certain amount in the body, as a result of meiosis, with a decrease in the number of chromosomes by half, oocytes of the 2nd order are formed.
As a result of the second meiotic division, a mature egg and three small reduction bodies are formed.
This non-equilibrium distribution of nutrients between 4 cells is designed to provide a large resource of nutrients for a new living organism.
The eggs of ferns and mosses are produced in archegoniums. In more highly organized plants - in special ovules located in the ovary.
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They consist of two strands - chromatids
Arranged in parallel and interconnected at one point, called centromere
or primary constriction
On some chromosomes, one can see and secondary stretch.
If the secondary constriction is located close to the end of the chromosome, then the distal region bounded by it is called satellite.
The end sections of chromosomes have a special structure and are called telomeres
The section of a chromosome from the telomere to the centromere is called chromosome arm
Each chromosome has two arms. Depending on the ratio of the lengths of the arms, three types of chromosomes are distinguished: 1) metacentric (equal arms); 2) submetacentric (unequal shoulder); 3) acrocentric, in which one shoulder is very short and not always clearly distinguishable.
Along with the location of the centromere, the presence of a secondary constriction and a satellite, their length is important for determining individual chromosomes. For each chromosome of a certain set, its length remains relatively constant. The measurement of chromosomes is necessary to study their variability in ontogeny in connection with diseases, anomalies, and impaired reproductive function.
Fine structure of chromosomes.
Chemical analysis of the structure of chromosomes showed the presence of two main components in them: deoxyribonucleic acid (DNA) and proteins such as histones and protomite (in germ cells). Studies of the fine submolecular structure of chromosomes led scientists to the conclusion that each chromatid contains one thread - lameness. Each chromonema is made up of one DNA molecule. The structural basis of the chromatid is a strand of protein nature. The chromoneme is arranged in a chromatid in a shape close to a spiral. Evidence of this assumption was obtained, in particular, in the study of the smallest exchange particles of sister chromatids, which were located across the chromosome.
Karyotype
When analyzing sets of chromosomes in cells of different species, differences were revealed in the number of chromosomes or their structure, or both at the same time. The set of quantitative and structural features of the diploid set of chromosomes of the species received the name karyotype
By definition by S. G. Navashin, karyotype
This structure is a kind of formula of the species. The karyotype contains the genetic information of an individual, changes in which entail changes in the signs and functions of the organism of this individual or its offspring. Therefore, it is so important to know the features of the normal structure of chromosomes in order, if possible, to be able to identify changes in the karyotype.
DNA is a material carrier of the properties of heredity and variability and contains biological information - a program for the development of a cell, an organism, written using a special code.
Histones are represented by five fractions: HI, H2A, H2B, H3, H4. Being positively charged basic proteins, they are quite firmly attached to DNA molecules, which prevents the biological information contained in it from being read. these proteins perform a structural function, providing the spatial organization of DNA in chromosomes
Chromosome RNA is partly represented by transcription products that have not yet left the site of synthesis. Some fractions have a regulatory function.
The regulatory role of the components of chromosomes is to "prohibit" or "permit" the writing off of information from the DNA molecule.
The first level is the nucleosomal strand. DNA + histone proteins H2A, H2B, H3, H4. The degree of shortening is 6-7 times. Second: chromatin fibril. Nucleosome strand + histone H1 protein. Shortening by 42 times. Third: interphase chromosome. The chromatin fibril is folded into loops with the help of non-histone proteins. Shortening by 1600 times. Fourth. metaphase chromosome. supercondensation of chromatin. Shortening by 8000 times.
The structure and functions of human metaphase chromosomes
Metaphase occupies a significant part of the mitosis period, and is characterized by a relatively stable state.
All this time, the chromosomes are held in the equatorial plane of the spindle due to the balanced tension forces of microtubules.
In metaphase, as well as during other phases of mitosis, active renewal of spindle microtubules continues through intensive assembly and depolymerization of tubulin molecules. By the end of the metaphase, a clear separation of sister chromatids is observed, the connection between which is preserved only in the centromeric regions. The arms of the chromatids are arranged parallel to each other, and the gap separating them becomes clearly visible.
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DNA helices in the nucleus packed into chromosomes. The human cell contains 46 chromosomes arranged in 23 pairs. Most of the genes that make up a pair on homologous chromosomes are almost or completely identical, and it is often heard that all genes in the human genome have their own pair, although this is not entirely correct.
Along with DNA Chromosomes contain a lot of protein most of which is represented by small positively charged histone molecules. They form many small, coil-like structures, which, one after the other, are wrapped around by short segments of DNA.
These structures play important role in the regulation of DNA activity, since they provide its dense “packing” and thus make it impossible to use it as a template for the synthesis of new DNA. There are also regulatory proteins that, on the contrary, decondense small portions of the histone packaging of DNA, thus enabling RNA synthesis.
Video: Mitosis. Cell mitosis. Phases of mitosis
Among the main chromosome components there are also non-histone proteins, which, on the one hand, are structural proteins of chromosomes, and on the other hand, they are activators, inhibitors, or enzymes in the composition of regulatory genetic systems.
Full replication of chromosomes begins a few minutes after the completion of DNA replication. During this time, newly synthesized DNA strands combine with proteins. Two newly formed chromosomes remain attached to each other until the very end of mitosis in a region close to their center and called the centromere. Chromosomes that separate but do not separate are called chromatids.
The process of division of the mother cell into two daughter cells is called mitosis. Following the replication of chromosomes with the formation of two chromatids, mitosis automatically begins within 1-2 hours.
One of the earliest changes in cytoplasm associated with mitosis occurs late in interphase and involves centrioles. Centrioles, like DNA and chromosomes, double during interphase—this usually occurs shortly before DNA replication. The centriole, about 0.4 µm long and about 0.15 µm in diameter, consists of nine parallel tube triplets assembled in the form of a cylinder. The centrioles of each pair lie at right angles to each other. A pair of centrioles together with the substance adjacent to it is called a centrosome.
Phases of cell mitosis
Shortly before the start mitosis both pairs of centrioles begin to move in the cytoplasm, moving away from each other. This movement is due to the polymerization of the protein of microtubules, which begin to grow from one pair of centrioles to another and, due to this, push them to opposite poles of the cell. At the same time, other microtubules begin to grow from each pair of centrioles, which increase in length and depart from them radially in the form of rays, forming the so-called astrosphere at each pole of the cell. Some of its rays penetrate the nuclear membrane, thus contributing to the separation of each pair of chromatids during mitosis. The group of microtubules between two pairs of centrioles is called the spindle of division, and the entire set of microtubules, together with the centrioles, is called the mitotic apparatus.
Prophase. As the spindle is formed in the nucleus, chromosomes begin to condense (in interphase they consist of two loosely connected chains), which, due to this, become clearly distinguishable.
prometaphase. Microtubules coming from the astrosphere destroy the nuclear envelope. At the same time, other microtubules extending from the astrosphere attach to the centromeres, which still connect all the chromatids in pairs, and begin to pull both chromatids of each pair to different poles of the cell.
Video: Phases of meiosis
metaphase. During metaphase, the astrospheres move further apart from each other.
It is believed that their movement is due to microtubules extending from them. These microtubules are woven together and form a spindle, which repels the centrioles from each other. It is also believed that molecules of small contractile proteins, or “motor molecules” (possibly similar to actin), are located between spindle microtubules, which ensure mutual sliding of microtubules in opposite directions, as occurs during muscle contraction. Microtubules attached to the centromeres pull the chromatids to the center of the cell and line them up in the form of a metaphase plate along the equator of the spindle.
Anaphase. During this phase, the two chromatids of each pair break away from each other at the centromere. All 46 pairs of chromatids separate and form two independent sets of 46 daughter chromosomes. Each set of chromosomes moves to opposite astrospheres, and the poles of the dividing cell at this time diverge further and further.
Telophase. In this phase, two sets of daughter chromosomes completely diverge, the mitotic apparatus is gradually destroyed, and a new nuclear envelope is formed around each set of chromosomes due to the membrane of the endoplasmic reticulum. Shortly thereafter, a constriction appears between the two new nuclei, dividing the cell into two daughter cells. Division is due to the formation of a ring of microfilaments of actin and, possibly, myosin (two contractile muscle proteins) in the constriction between daughter cells, which laces them from each other.
Educational video: cell mitosis and its stages
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Chemical composition of chromosomes
chromatin,
Proteins make up a significant part of the substance of chromosomes.
They account for about 65% of the mass of these structures. All chromosomal proteins are divided into two groups: histones and nonhistone proteins.
Histones
Number of fractions nonhistone
chromosomes.
Morphology of chromosomes
centromeres daughter chromosomes,
Rice. 3.52. Chromosome shapes:
I- telocentric, II- acrocentric, III- submetacentric, IV- metacentric;
1 - centromere, 2 - satellite, 3 - short shoulder 4 - long shoulder, 5 - chromatids
chromosomal mutations or aberrations.About them - in the next lecture.
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Chemical composition of chromosomes
The study of the chemical organization of the chromosomes of eukaryotic cells showed that they consist mainly of DNA and proteins that form a nucleoprotein complex. chromatin, named for its ability to stain with basic dyes.
Proteins make up a significant part of the substance of chromosomes. They account for about 65% of the mass of these structures. All chromosomal proteins are divided into two groups: histones and nonhistone proteins.
Histones represented by five fractions: HI, H2A, H2B, H3, H4. Being positively charged basic proteins, they are quite firmly attached to DNA molecules, which prevents the biological information contained in it from being read. This is their regulatory role. In addition, these proteins perform a structural function, providing the spatial organization of DNA in chromosomes.
Number of fractions nonhistone proteins exceeds 100. Among them are enzymes for the synthesis and processing of RNA, replication and repair of DNA. Acidic proteins of chromosomes also play a structural and regulatory role. In addition to DNA and proteins, RNA, lipids, polysaccharides, and metal ions are also found in the chromosomes.
The regulatory role of the components of chromosomes is to "prohibit" or "permit" the writing off of information from the DNA molecule. Other components are found in small quantities.
Structural organization of chromatin
Chromatin changes its organization depending on the period and phase of the cell cycle. In the interphase with light microscopy, it is detected in the form of clumps scattered in the nucleoplasm of the nucleus. During the transition of the cell to mitosis, especially in metaphase, chromatin takes the form of well-distinguished individual intensely stained bodies - chromosomes.
The most common point of view is that chromatin (chromosome) is a spiral thread.
Morphology of chromosomes
In the first half of mitosis, they consist of two chromatids connected to each other in the region of the primary constriction ( centromeres) a specially organized section of the chromosome common to both sister chromatids. In the second half of mitosis, chromatids separate from each other. They form single strands. daughter chromosomes, distributed among daughter cells.
Depending on the location of the centromere and the length of the arms located on both sides of it, several forms of chromosomes are distinguished: equal-armed, or metacentric (with a centromere in the middle), unequal-armed, or submetacentric (with a centromere shifted to one of the ends), rod-shaped, or acrocentric (with a centromere located almost at the end of the chromosome), and dot - very small, the shape of which is difficult to determine (Fig.).
Thus, each chromosome is individual not only in terms of the set of genes contained in it, but also in terms of morphology and the nature of differential staining.
3.52. Chromosome shapes:
I- telocentric, II- acrocentric, III- submetacentric, IV- metacentric;
1 - centromere, 2 - satellite, 3 - short shoulder 4 - long shoulder, 5 - chromatids
Rice. 3.53. Location of loci in human chromosomes
with their differential staining:
p - short arm, q - long arm; 1-22 - sequence number of the chromosome; XY - sex chromosomes
At the chromosomal level of organization, which appears in the process of evolution in eukaryotic cells, the genetic apparatus must meet all the requirements for the substrate of heredity and variability: it must be able to reproduce itself, maintain the constancy of its organization and acquire changes that can be transmitted to a new generation of cells.
Despite the evolutionary proven mechanism that allows maintaining the constant physicochemical and morphological organization of chromosomes in a number of cell generations, this organization can change under the influence of various influences. Changes in the structure of the chromosome, as a rule, are based on the initial violation of its integrity - breaks, which are accompanied by various rearrangements called chromosomal mutations or aberrations.About them - in the next lecture.
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The concept of "chromosome" was introduced into science by Waldeimer in 1888. Chromosome - This component the cell nucleus, with the help of which the regulation of protein synthesis in the cell is carried out, i.e. transmission of hereditary information. Chromosomes are represented by complexes of nucleic acids and proteins. Functionally, a chromosome is a strand of DNA with a huge functional surface. The number of chromosomes is constant for each particular species.
Each chromosome is formed by two morphologically identical intertwined threads of the same diameter - chromatids. They are closely connected centromere- a special structure that controls the movement of chromosomes during cell division.
Depending on the position of the chromosome, the body of the chromosome is divided into 2 arms. This, in turn, determines the 3 main types of chromosomes.
1 type - acrocentric chromosome.
Its centromere is located closer to the end of the chromosome and one arm is long and the other very short.
2 type - submetacentric chromosome.
Its centromere is located closer to the middle of the chromosome and divides it into unequal arms: short and long.
3 type - metacentric chromosome.
Its centromere is located in the very center of the body of the chromosome and divides into equal arms.
The length of chromosomes varies in different cells from 0.2 to 50 µm, and the diameter varies from 0.2 to 2 µm. Representatives of the lily family have the largest chromosomes in plants, and some amphibians in animals. The length of most human chromosomes is 2-6 microns.
The chemical composition of chromosomes is determined mainly by DNA, as well as proteins - 5 types of histone and 2 types of non-histone, as well as RNA. Features of these chemical substances determine the important functions of chromosomes:
1. reduplication and transmission of genetic material from generation to generation;
2. protein synthesis and control of all biochemical processes that form the basis of the specificity of development and differentiation of the body's cellular systems. In addition, the composition of the chromosomes found: a complex residual protein, lipids, calcium, magnesium, iron.
The structural basis of chromosomes is the DNA-histone complex. In the chromosome, the DNA strand is packaged by histones into regularly repeating structures with a diameter of about 10 nm, called nucleosomes. The surface of histone molecules is positively charged, while the DNA helix is negatively charged. Nucleosomes are packed into filamentous structures called fibrils. They are made up of chromatids.
The main substrate in which the genetic information of an organism is recorded is the euchromatic regions of chromosomes. In contrast, there is inert heterochromatin. Unlike euchromatin, which contains unique genes, an imbalance in which negatively affects the phenotype of an organism, a change in the amount of heterochromatin has much less or no effect on the development of the organism's traits.
In order to make it easier to understand the complex complex of chromosomes that make up the karyotype, they can be arranged in the form of an idiogram compiled by S.G. Novashin. In an idiogram, chromosomes (except for sex chromosomes) are arranged in descending order of magnitude.
However, identification by size alone is difficult because a number of chromosomes are similar in size. The size of chromosomes is measured by their absolute or relative length in relation to the total length of all chromosomes of the haploid set. The largest human chromosomes are 4-5 times longer than the smallest chromosomes. In 1960, a classification of human chromosomes was proposed depending on morphological characteristics: size, shape, position of the centromere - in order of decreasing overall length. According to this classification, 22 pairs of chromosomes are combined into 7 groups:
1gr.1-3 pair of chromosomes - large, metacentric.
2 gr.4-5 pair of chromosomes - large, submetacentric.
3 gr.6-12 pair of chromosomes - medium size, submetacentric.
4 gr.13-15 pair of chromosomes - medium size, acrocentric.
5 gr.16-18 a pair of chromosomes is short, of which 16 are metacentric, 17 are submetacentric, 18 are acrocentric.
6 gr.19-20 pair of chromosomes - short, metacentric.
7 gr.21-22 pair of chromosomes - very short, acroentric.
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). Chromatin is heterogeneous, and some types of such heterogeneity are visible under a microscope. The fine structure of chromatin in the interphase nucleus, determined by the nature of DNA folding and its interaction with proteins, plays an important role in the regulation of gene transcription and DNA replication and, possibly, cellular differentiation.
The DNA nucleotide sequences that form genes and serve as a template for mRNA synthesis are distributed along the entire length of chromosomes (individual genes, of course, are too small to be seen under a microscope). By the end of the 20th century, for about 6,000 genes, it was established on which chromosome and in which part of the chromosome they are located and what is the nature of their linkage (that is, their position relative to each other).
The heterogeneity of metaphase chromosomes, as already mentioned, can be seen even with light microscopy. Differential staining of at least 12 chromosomes revealed differences in the width of some bands between homologous chromosomes (Fig. 66.3). Such polymorphic regions are composed of non-coding highly repetitive DNA sequences.
The methods of molecular genetics have made it possible to identify a huge number of smaller and therefore polymorphic DNA regions that are not detected by light microscopy. These sites are identified as restriction fragment length polymorphism, tandem repeats varying in number, and short tandem repeat polymorphism (mono-, di-, tri- and tetranucleotide). Such variability usually does not appear phenotypically.
However, polymorphism serves as a convenient tool for prenatal diagnosis due to the linkage of certain markers to disease-causing mutant genes (for example, in Duchenne myopathy), as well as in establishing twin zygosity, establishing paternity, and predicting transplant rejection.
It is difficult to overestimate the importance of such markers, especially highly polymorphic short tandem repeats that are widespread in the genome, for mapping the human genome. In particular, they make it possible to establish the exact order and nature of the interaction of loci, which play an important role in ensuring normal ontogeny and cell differentiation. This also applies to those loci, mutations in which lead to hereditary diseases.
Microscopically visible areas on the short arm of acrocentric autosomes (Fig. 66.1) provide rRNA synthesis and nucleolus formation, therefore they are called regions of the nucleolar organizer. In metaphase, they are uncondensed and do not stain. The regions of the nucleolar organizer are adjacent to the condensed sections of chromatin - satellites located at the end of the short arm of the chromosome. Satellites do not contain genes and are polymorphic regions.
In a small part of the cells, it is possible to identify other areas decondensed in metaphase, the so-called fragile areas, where "complete" breaks of the chromosome can occur. Of clinical importance are disorders in the only such site located at the end of the long arm of the X chromosome. Such disorders cause fragile X syndrome.
Other examples of specialized regions of chromosomes are telomeres and centromeres.
The role of heterochromatin, which accounts for a significant part of the human genome, has not yet been precisely established. Heterochromatin is condensed during almost the entire cell cycle, it is inactive and replicates late. Most of the sites are condensed and inactive in all cells (), although others, such as the X chromosome, can be either condensed and inactive, or decondensed and active (facultative heterochromatin). If, due to chromosomal aberrations, genes are close to heterochromatin, then the activity of such genes can change or even be blocked. Therefore, manifestations of chromosomal aberrations, such as duplications or deletions, depend not only on the affected loci, but also on the type of chromatin in them. Many non-lethal chromosomal abnormalities affect inactive or inactivated regions of the genome. Perhaps this explains why trisomy for some chromosomes or monosomy for the X chromosome is compatible with life.
Manifestations of chromosomal abnormalities also depend on the new arrangement of structural and regulatory genes in relation to each other and to heterochromatin.
Fortunately, many structural features of chromosomes can be reliably detected by cytological methods. Currently, there are a number of methods for differential staining of chromosomes (Fig. 66.1 and Fig. 66.3). The location and width of the bands are identical in each pair of homologous chromosomes, with the exception of polymorphic regions, so staining can be used in clinical cytogenetics to identify chromosomes and detect structural abnormalities in them.
Chromosomes are the main structural elements of the cell nucleus, which are carriers of genes in which hereditary information is encoded. Possessing the ability to self-reproduce, 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 deployed, i.e., despiralized, then with the onset of division, the chromosomes intensively spiralize and shorten. The maximum spiralization and shortening of the chromosome is reached at the metaphase stage, when relatively short, dense, intensely stained with basic dye structures are formed. This stage is most convenient for studying the morphological characteristics of chromosomes.
The metaphase chromosome consists of two longitudinal subunits - chromatids [reveals in the structure of chromosomes elementary filaments (the so-called chromonema, or chromofibrils) 200 Å thick, each of which consists of two subunits].
The sizes of chromosomes of plants and animals fluctuate considerably: from fractions of a micron to tens of microns. The average lengths of human metaphase chromosomes lie in the range of 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). With the loss of the centromere, fragments of chromosomes lose their ability to disperse 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 of equal or almost equal length), submetacentric (arms of unequal length) and acrocentric (the centromere is shifted to the end of the chromosome). In addition to the primary, less pronounced secondary constrictions can occur in the chromosomes. A small terminal section of chromosomes, separated by a secondary constriction, is called a satellite.
Each type of organism is characterized by its specific (in terms of the number, size and shape of chromosomes) so-called chromosome set. The set of a double, or diploid, set of chromosomes is designated as a karyotype.
Rice. 2. Normal female chromosome set (two X-chromosomes in the lower right corner).
Rice. 3. Normal chromosomal set of a man (in the lower right corner - sequentially X- and Y-chromosomes).
Mature eggs contain a single, or haploid, set of chromosomes (n), which is half of the diploid set (2n) inherent in the chromosomes of all other cells of the body. In a diploid set, each chromosome is represented by a pair of homologues, one of which is maternal and the other paternal. In most cases, the chromosomes of each pair are identical in size, shape, and genetic makeup. The exception is the sex chromosomes, the presence of which determines the development of the organism in the 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, the female is determined by the presence of two X chromosomes, and the male is determined by the presence of 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 normal and pathological conditions 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 the sex! cells or early stages fragmentation of a fertilized egg, cause disturbances in the normal development of the body, causing in some cases the occurrence of spontaneous abortions, stillbirths, congenital deformities and developmental anomalies after birth ( chromosomal diseases). Examples of chromosomal diseases are Down's disease (an extra G chromosome), Klinefelter's syndrome (an extra X chromosome in men) and (absence of a Y or one of the X chromosomes in the karyotype). In medical practice, chromosomal analysis is carried out either by a direct method (on bone marrow cells) or after a short-term cultivation of cells outside the body (peripheral blood, skin, embryonic tissues).
Chromosomes (from the Greek chroma - color and soma - body) are thread-like, self-reproducing structural elements of the cell nucleus, containing heredity factors in a linear order - genes. 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, the 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 under a light microscope, since their transverse dimensions are beyond the resolving power of a light microscope. At this time, separate sections of chromosomes in the form of thin threads with a diameter of 100-500 Å can be distinguished using an electron microscope. Separate non-despiralized sections of chromosomes in the interphase nucleus are visible through a light microscope as intensely stained (heteropyknotic) sections (chromocenters).
Chromosomes continuously exist in the cell nucleus, undergoing a cycle of reversible spiralization: mitosis-interphase-mitosis. The main regularities of the structure and behavior of chromosomes in mitosis, meiosis and during fertilization are the same in all organisms.
Chromosomal theory of heredity. For the first time chromosomes were described by I. D. Chistyakov in 1874 and Strasburger (E. Strasburger) in 1879. In 1901, E. V. Wilson, and in 1902 W. S. Sutton paid 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 chromosomes. Data on chromosomes, obtained in the first quarter of the 20th century, formed the basis of the chromosome 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 permanent components [DNA (see Nucleic acids), basic proteins (histones or protamines), non-histone proteins] and variable components (RNA and associated acidic protein). Chromosomes are based on deoxyribonucleoprotein filaments 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. H. Crick) in 1953 of the structure of the DNA molecule, the mechanism of its auto-reproduction (reduplication) and the DNA nucleic code and the development of molecular genetics that arose after that led to the idea of genes as sections of the DNA molecule. (see Genetics). The regularities of autoreproduction of chromosomes [Taylor (J. N. Taylor) et al., 1957], which turned out to be similar to the regularities of autoreproduction of DNA molecules (semiconservative reduplication), were revealed.
Chromosomal set is 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 chromosome sets: single, or haploid (in animal germ cells), denoted n, and double, or diploid (in somatic cells, containing pairs of similar, homologous chromosomes from mother and father), denoted 2n.
The sets of chromosomes of individual biological species differ significantly in the number of chromosomes: from 2 (horse roundworm) to hundreds and thousands (some spore plants and protozoa). The diploid numbers of chromosomes of some organisms are as follows: humans - 46, gorillas - 48, cats - 60, rats - 42, Drosophila - 8.
The size of the chromosomes in different species is also different. The length of chromosomes (in the metaphase of mitosis) varies from 0.2 microns in some species to 50 microns in others, and the diameter is from 0.2 to 3 microns.
Chromosome morphology is well expressed in the metaphase of mitosis. Metaphase chromosomes are used to identify chromosomes. In such chromosomes, both chromatids are clearly visible, into which each chromosome is split longitudinally and the centromere (kinetochore, primary constriction) connecting the chromatids (Fig. 3). The centromere is visible as the narrowed site which is not containing chromatin (see); 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 the centromere, for example, when a chromosome is broken by ionizing radiation or other mutagens, leads to the loss of the ability of a piece of chromosome devoid of a centromere (acentric fragment) to participate in mitosis and meiosis and to its loss from the nucleus. This can lead to severe cell damage.
The centromere divides the body of the chromosome 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 the 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. Fig. 4. Scheme of the structure of chromosomes in the 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. A characteristic structure in the chromosome (chromomeres) is thickening or more densely spiralized sections of the chromosome thread (chromonema). The chromomere pattern is specific for each pair of chromosomes.
Rice. 5. Scheme of chromosome morphology in the anaphase of mitosis (chromatid moving towards the pole). A - the appearance of the chromosome; B - the internal structure of the same chromosome with two chromonemes (semichromatids) that make it up: 1 - primary constriction with chromomeres that make up 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 totality of these features of a set of chromosomes is called a karyotype. A karyotype can be represented as a diagram called an idiogram (see human chromosomes below).
sex chromosomes. Sex-determining genes are localized in a special pair of chromosomes - the sex chromosomes (mammals, humans); in other cases, iol is determined by the ratio of the number of sex chromosomes and all the rest, 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 Ovogenesis) 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 the X chromosome, and the other half the Y chromosome. The sex of a child is determined by the random fertilization of an egg by a sperm that carries an X or Y chromosome. The result is a female (XX) or male (XY) fetus. In the interphase nucleus in females, one of the X chromosomes is visible as a lump of compact sex chromatin.
Chromosome Function and Nuclear Metabolism. Chromosomal DNA is a template for the synthesis of specific messenger RNA molecules. This synthesis occurs when a given region of the chromosome is despiralized. Examples of local activation of chromosomes are: the formation of despiralized loops of chromosomes in the oocytes of birds, amphibians, fish (the so-called X-lamp brushes) and swellings (puffs) of certain chromosome loci in multifilamentous (polytene) chromosomes of the salivary glands and other secretory organs of dipteran insects (Fig. 6). An example of the 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. Fig. 6. Polytene chromosomes of the dipteran insect Acriscotopus lucidus: A and B - the area bounded by dotted lines, in a state of intensive functioning (puff); B - the same site in a non-functioning state. Numbers indicate individual loci of chromosomes (chromomeres).
Rice. 7. Chromosomal set in the culture of male peripheral blood leukocytes (2n=46).
The discovery of the mechanisms of functioning of polytene chromosomes such as lampbrushes and other types of spiralization and despiralization of chromosomes is of decisive importance for understanding the reversible differential activation of genes.
human chromosomes. In 1922, T. S. Painter established the diploid number of human chromosomes (in spermatogonia) equal to 48. In 1956, Tio and Levan (N. J. Tjio, A. Levan) used a set of new methods for studying human chromosomes : cell culture; the study of chromosomes without histological sections on total cell preparations; colchicine, which leads to the arrest of mitosis 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 to give a description of the human karyotype. In 1960, in Denver (USA), an international commission developed a nomenclature of human chromosomes. According to the proposals of the commission, the term "karyotype" should be applied to a systematized set of chromosomes of a single cell (Fig. 7 and 8). The term "idiotram" is retained to represent a set of chromosomes in the form of a diagram built on the basis of measurements and a description of the morphology of the chromosomes of several cells.
Human chromosomes are numbered (somewhat serially) from 1 to 22 in accordance with 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 found between a number of diseases and birth defects in human development and 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 stage of anaphase of mitosis in shamrock microsporocytes; B - at the metaphase stage of the first division of meiosis in pollen mother cells in Tradescantia. In both cases, the helical structure of the chromosomes is visible.
Rice. Fig. 2. Elementary chromosome filaments with a diameter of 100 Å (DNA + histone) from the interphase nuclei of the calf thymus gland (electron microscopy): A - filaments isolated from the nuclei; B - thin section through the film of the same preparation.
Rice. 3. Chromosomal set of Vicia faba (horse beans) in the metaphase stage.
Rice. 8. Chromosomes of the same as in fig. 7, sets classified according to Denver nomenclature into pairs of homologues (karyotype).
Chromosomes are the nucleoprotein structures of a eukaryotic cell that store most of the hereditary information. Due to their ability to self-reproduce, it is the chromosomes that provide the genetic link between generations. Chromosomes are formed from a long DNA molecule, which contains a linear group of many genes, and all the genetic information, whether it be about a person, animal, plant, or any other living being.
The morphology of chromosomes is related to the level of their spiralization. So, if during the interphase stage the chromosomes are maximally deployed, then with the onset of division, the chromosomes actively spiralize and shorten. They reach their maximum shortening and spiralization during the metaphase stage, when new structures are formed. This phase is most convenient for studying the properties of chromosomes and their morphological characteristics.
The history of the discovery of chromosomes
Back in the middle of the nineteenth century before last, many biologists, studying the structure of plant and animal cells, drew attention to thin filaments and the smallest ring-shaped structures in the nucleus of some cells. And now the German scientist Walter Fleming, using aniline dyes to process the nuclear structures of the cell, what is called "officially" opens the chromosomes. More precisely, the discovered substance was called “chromatid” by him for its ability to stain, and the term “chromosome” was introduced into use a little later (in 1888) by another German scientist, Heinrich Wilder. The word "chromosome" comes from the Greek words "chroma" - color and "somo" - body.
Chromosomal theory of heredity
Of course, the history of the study of chromosomes did not end with their discovery, so in 1901-1902, American scientists Wilson and Saton, independently of each other, drew attention to the similarity in the behavior of chromosomes and Mendeleian factors of heredity - genes. As a result, scientists came to the conclusion that genes are located on chromosomes and it is through them that genetic information is transmitted from generation to generation, from parents to children.
In 1915-1920, the participation of chromosomes in the transmission of genes was proven in practice in a whole series of experiments made by the American scientist Morgan and his laboratory staff. They managed to localize several hundred hereditary genes in the chromosomes of the Drosophila fly and create genetic maps of the chromosomes. Based on these data, the chromosome theory of heredity was created.
The structure of chromosomes
The structure of chromosomes varies depending on the species, so the metaphase chromosome (formed in the metaphase stage during cell division) consists of two longitudinal threads - chromatids, which are connected at a point called the centromere. The centromere is the part of the chromosome that is responsible for the separation of sister chromatids into daughter cells. She also divides the chromosome into two parts, called the short and long arm, she is also responsible for the division of the chromosome, since it contains a special substance - the kinetochore, to which the division spindle structures are attached.
Here the picture shows a visual structure of the chromosome: 1. chromatids, 2. centromere, 3. short arm of chromatids, 4. long arm of chromatids. At the ends of chromatids are telomeres, special elements that protect the chromosome from damage and prevent fragments from sticking together.
Shapes and types of chromosomes
The sizes of chromosomes of plants and animals vary considerably: from fractions of a micron to tens of microns. The average lengths of human metaphase chromosomes range from 1.5 to 10 microns. Depending on the type of chromosome, its ability to stain also differs. Depending on the location of the centromere, the following forms of chromosomes are distinguished:
- Metacentric chromosomes, which are characterized by a median location of the centromere.
- Submetacentric, they are characterized by an uneven arrangement of chromatids, when one shoulder is longer and the second is shorter.
- Acrocentric or rod-shaped. Their centromere is located almost at the very end of the chromosome.
Functions of chromosomes
The main functions of chromosomes, both for animals and for plants and in general for all living beings, are the transfer of hereditary, genetic information from parents to children.
Set of chromosomes
The importance of chromosomes is so great that their number in cells, as well as the characteristics of each chromosome, determine feature one biological species or another. So, for example, the fruit fly has 8 chromosomes, the y - 48, and the human chromosome set is 46 chromosomes.
In nature, there are two main types of chromosome sets: single or haploid (contained in germ cells) and double or diploid. The diploid set of chromosomes has a paired structure, that is, the entire set of chromosomes consists of chromosome pairs.
Human chromosome set
As we wrote above, the cells human body contain 46 chromosomes, which are combined into 23 pairs. Together they make up the human chromosome set. The first 22 pairs of human chromosomes (they are called autosomes) are common for both men and women, and only 23 pairs - sex chromosomes - differ in different sexes, it also determines the gender of a person. The totality of all pairs of chromosomes is also called a karyotype.
This species has a human chromosome set, 22 pairs of double diploid chromosomes contain all our hereditary information, and the last pair is different, in men it consists of a pair of conditional X and Y sex chromosomes, while in women there are two X chromosomes.
All animals have a similar structure of the chromosome set, only the number of non-sex chromosomes in each of them is different.
Genetic diseases associated with chromosomes
Violation of the chromosomes, or even their very wrong number is the cause of many genetic diseases. For example, Down syndrome appears due to the presence of an extra chromosome in the human chromosome set. And such genetic diseases as color blindness, hemophilia are caused by malfunctions of existing chromosomes.
Chromosomes, video
And in conclusion, an interesting educational video about chromosomes.
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