Where are chromosomes located during division? Human chromosomes

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Chromosomes are intensely colored bodies consisting of a DNA molecule bound to histone proteins. Chromosomes are formed from chromatin at the beginning of cell division (in the prophase of mitosis), but they are best studied in the metaphase of mitosis. When chromosomes are located in the equatorial plane and are clearly visible under a light microscope, the DNA in them reaches maximum spiralization.

Chromosomes consist of 2 sister chromatids (duplicated DNA molecules) connected to each other in the region of the primary constriction - the centromere. The centromere divides the chromosome into 2 arms. Depending on the location of the centromere, chromosomes are divided into:

metacentric centromere is located in the middle of the chromosome and its arms are equal;

submetacentric centromere is displaced from the middle of the chromosomes and one arm is shorter than the other;

acrocentric - the centromere is located close to the end of the chromosome and one arm is much shorter than the other.

Some chromosomes have secondary constrictions that separate a region called a satellite from the chromosome arm, from which the nucleolus is formed in the interphase nucleus.

Chromosome rules

1. Constancy of number. Somatic cells of the body of each species have a strictly defined number of chromosomes (in humans - 46, in cats - 38, in Drosophila flies - 8, in dogs - 78, in chickens - 78).

2. Pairing. Each chromosome in somatic cells with a diploid set has the same homologous (identical) chromosome, identical in size and shape, but different in origin: one from the father, the other from the mother.

3. Individuality. Each pair of chromosomes differs from the other pair in size, shape, alternating light and dark stripes.

4. Continuity. Before cell division, the DNA doubles, resulting in 2 sister chromatids. After division, one chromatid at a time enters the daughter cells and, thus, the chromosomes are continuous - a chromosome is formed from a chromosome.

All chromosomes are divided into autosomes and sex chromosomes. Autosomes are all chromosomes in cells, with the exception of sex chromosomes, there are 22 pairs of them. Sexual chromosomes are the 23rd pair of chromosomes, which determine the formation of male and female organisms.

Somatic cells have a double (diploid) set of chromosomes, while sex cells have a haploid (single) set.

A certain set of cell chromosomes, characterized by the constancy of their number, size and shape, is called karyotype.

In order to understand the complex set of chromosomes, they are arranged in pairs as their size decreases, taking into account the position of the centromere and the presence of secondary constrictions. Such a systematic karyotype is called an idiogram.

For the first time, such a systematization of chromosomes was proposed at the Congress of Genetics in Denver (USA, 1960)

In 1971, in Paris, chromosomes were classified according to color and alternation of dark and light stripes of hetero- and euchromatin.

To study the karyotype, geneticists use the method of cytogenetic analysis, which can diagnose a number of hereditary diseases associated with disturbances in the number and shape of chromosomes.

1.2. Life cycle of a cell.

The life of a cell from the moment it arises as a result of division until its own division or death is called the life cycle of the cell. Throughout life, cells grow, differentiate and perform specific functions.

The life of a cell between divisions is called interphase. Interphase consists of 3 periods: presynthetic, synthetic and postsynthetic.

The presynthesis period immediately follows division. At this time, the cell grows intensively, increasing the number of mitochondria and ribosomes.

During the synthetic period, replication (doubling) of the amount of DNA occurs, as well as the synthesis of RNA and proteins.

During the post-synthesis period, the cell stores energy, spindle achromatin proteins are synthesized, and preparations for mitosis are underway.

There are different types of cell division: amitosis, mitosis, meiosis.

Amitosis is the direct division of prokaryotic cells and some cells in humans.

Mitosis is an indirect cell division during which chromosomes are formed from chromatin. Somatic cells of eukaryotic organisms divide through mitosis, as a result of which the daughter cells receive exactly the same set of chromosomes as the daughter cell had.

Mitosis

Mitosis consists of 4 phases:

Prophase is the initial phase of mitosis. At this time, DNA spiralization begins and chromosomes shorten, which from thin invisible strands of chromatin become short, thick, visible in a light microscope, and are arranged in the form of a ball. The nucleolus and nuclear membrane disappear, and the nucleus disintegrates, the centrioles of the cell center diverge to the poles of the cell, and the filaments of the spindle stretch between them.

Metaphase - chromosomes move towards the center, spindle threads are attached to them. Chromosomes are located in the equatorial plane.

Anaphase - sister chromatids (appearing in the synthetic period during DNA doubling) move towards the poles.

Telophase (telos in Greek - end) is the opposite of prophase: chromosomes change from short thick visible to thin to long invisible in a light microscope, the nuclear membrane and nucleolus are formed. Telophase ends with the division of the cytoplasm to form two daughter cells.

The biological significance of mitosis is as follows:

daughter cells receive exactly the same set of chromosomes that the mother cell had, therefore a constant number of chromosomes is maintained in all cells of the body (somatic).

All cells, except sex cells, divide:

the body grows in the embryonic and postembryonic periods;

all functionally obsolete cells of the body (epithelial cells of the skin, blood cells, cells of the mucous membranes, etc.) are replaced by new ones;

processes of regeneration (restoration) of lost tissues occur.

Mitosis diagram

When a dividing cell is exposed to unfavorable conditions, the spindle of division can unevenly stretch the chromosomes to the poles, and then new cells with a different set of chromosomes are formed, and pathology of somatic cells occurs (heteroploidy of autosomes), which leads to disease of tissues, organs, and the body.

Sometimes they give us amazing surprises. For example, do you know what chromosomes are and how they affect?

We propose to look into this issue in order to dot the i’s once and for all.

Looking at family photographs, you may have probably noticed that members of the same family resemble each other: children look like parents, parents look like grandparents. This similarity is passed on from generation to generation through amazing mechanisms.

All living organisms, from single-celled organisms to African elephants, contain chromosomes in the cell nucleus - thin, long threads that can only be seen with an electron microscope.

Chromosomes (ancient Greek χρῶμα - color and σῶμα - body) are nucleoprotein structures in the cell nucleus, in which most of the hereditary information (genes) is concentrated. They are designed to store this information, implement it and transmit it.

How many chromosomes does a person have

At the end of the 19th century, scientists discovered that the number of chromosomes in different species is not the same.

For example, peas have 14 chromosomes, y have 42, and in humans – 46 (that is, 23 pairs). Hence the temptation arises to conclude that the more there are, the more complex the creature that possesses them. However, in reality this is absolutely not the case.

Of the 23 pairs of human chromosomes, 22 pairs are autosomes and one pair are gonosomes (sex chromosomes). The sexes have morphological and structural (gene composition) differences.

In a female organism, a pair of gonosomes contains two X chromosomes (XX-pair), and in a male organism, one X-chromosome and one Y-chromosome (XY-pair).

The sex of the unborn child depends on the composition of the chromosomes of the twenty-third pair (XX or XY). This is determined by fertilization and the fusion of the female and male reproductive cells.

This fact may seem strange, but in terms of the number of chromosomes, humans are inferior to many animals. For example, some unfortunate goat has 60 chromosomes, and a snail has 80.

Chromosomes consist of a protein and a DNA (deoxyribonucleic acid) molecule, similar to a double helix. Each cell contains about 2 meters of DNA, and in total there are about 100 billion km of DNA in the cells of our body.

An interesting fact is that if there is an extra chromosome or if at least one of the 46 is missing, a person experiences a mutation and serious developmental abnormalities (Down's disease, etc.).

Chromosome is a thread-like structure containing DNA in the cell nucleus, which carries genes, units of heredity, arranged in a linear order. Humans have 22 pairs of regular chromosomes and one pair of sex chromosomes. In addition to genes, chromosomes also contain regulatory elements and nucleotide sequences. They house DNA-binding proteins that control DNA functions. Interestingly, the word "chromosome" comes from the Greek word "chrome", meaning "color". Chromosomes received this name because they have the ability to be colored in different tones. The structure and nature of chromosomes vary from organism to organism. Human chromosomes have always been a subject of constant interest to researchers working in the field of genetics. The wide range of factors that are determined by human chromosomes, the abnormalities for which they are responsible, and their complex nature have always attracted the attention of many scientists.

Interesting facts about human chromosomes

Human cells contain 23 pairs of nuclear chromosomes. Chromosomes are made up of DNA molecules that contain genes. The chromosomal DNA molecule contains three nucleotide sequences required for replication. When chromosomes are stained, the banded structure of mitotic chromosomes becomes apparent. Each strip contains numerous DNA nucleotide pairs.

Humans are a sexually reproducing species with diploid somatic cells containing two sets of chromosomes. One set is inherited from the mother, while the other is inherited from the father. Reproductive cells, unlike body cells, have one set of chromosomes. Crossing over between chromosomes leads to the creation of new chromosomes. New chromosomes are not inherited from either parent. This accounts for the fact that not all of us exhibit traits that we receive directly from one of our parents.

Autosomal chromosomes are assigned numbers from 1 to 22 in descending order as their size decreases. Each person has two sets of 22 chromosomes, an X chromosome from the mother and an X or Y chromosome from the father.

An abnormality in the contents of a cell's chromosomes can cause certain genetic disorders in people. Chromosomal abnormalities in people are often responsible for the development of genetic diseases in their children. Those who have chromosomal abnormalities are often only carriers of the disease, while their children develop the disease.

Chromosomal aberrations (structural changes in chromosomes) are caused by various factors, namely deletion or duplication of part of a chromosome, inversion, which is a change in the direction of a chromosome to the opposite, or translocation, in which part of a chromosome is torn off and attached to another chromosome.

An extra copy of chromosome 21 is responsible for a very well known genetic disorder called Down syndrome.

Trisomy 18 results in Edwards syndrome, which can cause death in infancy.

Deletion of part of the fifth chromosome results in a genetic disorder known as Cri-Cat Syndrome. People affected by this disease often have mental retardation and their crying in childhood resembles that of a cat.

Disorders caused by sex chromosome abnormalities include Turner syndrome, in which female sexual characteristics are present but characterized by underdevelopment, as well as XXX syndrome in girls and XXY syndrome in boys, which cause dyslexia in affected individuals.

Chromosomes were first discovered in plant cells. Van Beneden's monograph on fertilized roundworm eggs led to further research. August Weissman later showed that the germ line was distinct from the soma and discovered that cell nuclei contained hereditary material. He also suggested that fertilization leads to the formation of a new combination of chromosomes.

These discoveries became cornerstones in the field of genetics. Researchers have already accumulated a significant amount of knowledge about human chromosomes and genes, but much remains to be discovered.

Video

Chromosomes are the main structural elements of the cell nucleus, which are carriers of genes in which hereditary information is encoded. Having the ability to reproduce themselves, chromosomes provide a genetic link between generations.

The morphology of chromosomes is related to the degree of their spiralization. For example, if at the stage of interphase (see Mitosis, Meiosis) the chromosomes are maximally unfolded, i.e., despiralized, then with the beginning of division the chromosomes intensively spiralize and shorten. Maximum spiralization and shortening of chromosomes is achieved at the metaphase stage, when relatively short, dense structures that are intensely stained with basic dyes are formed. This stage is most convenient for studying the morphological characteristics of chromosomes.

The metaphase chromosome consists of two longitudinal subunits - chromatids [reveals elementary threads in the structure of chromosomes (the so-called chromonemas, or chromofibrils) 200 Å thick, each of which consists of two subunits].

The sizes of plant and animal chromosomes vary significantly: from fractions of a micron to tens of microns. The average lengths of human metaphase chromosomes range from 1.5-10 microns.

The chemical basis of the structure of chromosomes are nucleoproteins - complexes (see) with the main proteins - histones and protamines.

Rice. 1. The structure of a normal chromosome.
A - appearance; B - internal structure: 1-primary constriction; 2 - secondary constriction; 3 - satellite; 4 - centromere.

Individual chromosomes (Fig. 1) are distinguished by the localization of the primary constriction, i.e., the location of the centromere (during mitosis and meiosis, spindle threads are attached to this place, pulling it towards the pole). When a centromere is lost, chromosome fragments lose their ability to separate during division. The primary constriction divides the chromosomes into 2 arms. Depending on the location of the primary constriction, chromosomes are divided into metacentric (both arms are equal or almost equal in length), submetacentric (arms of unequal length) and acrocentric (the centromere is shifted to the end of the chromosome). In addition to the primary one, less pronounced secondary constrictions may be found in chromosomes. A small terminal section of chromosomes, separated by a secondary constriction, is called a satellite.

Each type of organism is characterized by its own specific (in terms of the number, size and shape of chromosomes) so-called chromosome set. The totality of a double, or diploid, set of chromosomes is designated as a karyotype.

Rice. 2. Normal chromosome set of a woman (two X chromosomes in the lower right corner).

Rice. 3. The normal chromosome set of a man (in the lower right corner - X and Y chromosomes in sequence).

Mature eggs contain a single, or haploid, set of chromosomes (n), which makes up half of the diploid set (2n) inherent in the chromosomes of all other cells of the body. In the diploid set, each chromosome is represented by a pair of homologues, one of which is of maternal and the other of paternal origin. In most cases, the chromosomes of each pair are identical in size, shape and gene composition. The exception is sex chromosomes, the presence of which determines the development of the body in a male or female direction. The normal human chromosome set consists of 22 pairs of autosomes and one pair of sex chromosomes. In humans and other mammals, female is determined by the presence of two X chromosomes, and male by one X and one Y chromosome (Fig. 2 and 3). In female cells, one of the X chromosomes is genetically inactive and is found in the interphase nucleus in the form (see). The study of human chromosomes in health and disease is the subject of medical cytogenetics. It has been established that deviations in the number or structure of chromosomes from the norm that occur in reproductive organs! cells or in the early stages of fragmentation of a fertilized egg, cause disturbances in the normal development of the body, causing in some cases the occurrence of spontaneous abortions, stillbirths, congenital deformities and developmental abnormalities after birth (chromosomal diseases). Examples of chromosomal diseases include Down's disease (an extra G chromosome), Klinefelter's syndrome (an extra X chromosome in men) and (the absence of a Y or one of the X chromosomes in the karyotype). In medical practice, chromosomal analysis is carried out either directly (on bone marrow cells) or after short-term cultivation of cells outside the body (peripheral blood, skin, embryonic tissue).

Chromosomes (from the Greek chroma - color and soma - body) are thread-like, self-reproducing structural elements of the cell nucleus, containing factors of heredity - genes - in a linear order. Chromosomes are clearly visible in the nucleus during the division of somatic cells (mitosis) and during the division (maturation) of germ cells - meiosis (Fig. 1). In both cases, chromosomes are intensely stained with basic dyes and are also visible on unstained cytological preparations in phase contrast. In the interphase nucleus, the chromosomes are despiralized and are not visible in a light microscope, since their transverse dimensions exceed the resolution limits of the light microscope. At this time, individual sections of chromosomes in the form of thin threads with a diameter of 100-500 Å can be distinguished using an electron microscope. Individual non-despiralized sections of chromosomes in the interphase nucleus are visible through a light microscope as intensely stained (heteropyknotic) areas (chromocenters).

Chromosomes continuously exist in the cell nucleus, undergoing a cycle of reversible spiralization: mitosis-interphase-mitosis. The basic patterns of the structure and behavior of chromosomes in mitosis, meiosis and fertilization are the same in all organisms.

Chromosomal theory of heredity. Chromosomes were first described by I. D. Chistyakov in 1874 and E. Strasburger in 1879. In 1901, E. V. Wilson, and in 1902, W. S. Sutton, drew attention to parallelism in the behavior of chromosomes and Mendelian factors of heredity - genes - in meiosis and during fertilization and came to the conclusion that genes are located in chromosomes. In 1915-1920 Morgan (T.N. Morgan) and his collaborators proved this position, localized several hundred genes in Drosophila chromosomes and created genetic maps of the chromosomes. Data on chromosomes obtained in the first quarter of the 20th century formed the basis of the chromosomal theory of heredity, according to which the continuity of the characteristics of cells and organisms in a number of their generations is ensured by the continuity of their chromosomes.

Chemical composition and autoreproduction of chromosomes. As a result of cytochemical and biochemical studies of chromosomes in the 30s and 50s of the 20th century, it was established that they consist of constant components [DNA (see Nucleic acids), basic proteins (histones or protamines), non-histone proteins] and variable components (RNA and acidic protein associated with it). The basis of chromosomes is made up of deoxyribonucleoprotein threads with a diameter of about 200 Å (Fig. 2), which can be connected into bundles with a diameter of 500 Å.

The discovery by Watson and Crick (J. D. Watson, F. N. Crick) in 1953 of the structure of the DNA molecule, the mechanism of its autoreproduction (reduplication) and the nucleic code of DNA and the development of molecular genetics that arose after this led to the idea of genes as sections of the DNA molecule. (see Genetics). The patterns of autoreproduction of chromosomes were revealed [Taylor (J. N. Taylor) et al., 1957], which turned out to be similar to the patterns of autoreproduction of DNA molecules (semi-conservative reduplication).

Chromosome set- the totality of all chromosomes in a cell. Each biological species has a characteristic and constant set of chromosomes, fixed in the evolution of this species. There are two main types of sets of chromosomes: single, or haploid (in animal germ cells), denoted n, and double, or diploid (in somatic cells, containing pairs of similar, homologous chromosomes from the mother and father), denoted 2n.

The sets of chromosomes of individual biological species vary significantly in the number of chromosomes: from 2 (horse roundworm) to hundreds and thousands (some spore plants and protozoa). The diploid chromosome numbers of some organisms are as follows: humans - 46, gorillas - 48, cats - 60, rats - 42, fruit flies - 8.

The sizes of chromosomes also vary between species. The length of chromosomes (in metaphase of mitosis) varies from 0.2 microns in some species to 50 microns in others, and the diameter from 0.2 to 3 microns.

The morphology of chromosomes is well expressed in metaphase of mitosis. It is metaphase chromosomes that are used to identify chromosomes. In such chromosomes, both chromatids are clearly visible, into which each chromosome and the centromere (kinetochore, primary constriction) connecting the chromatids are longitudinally split (Fig. 3). The centromere is visible as a narrowed area that does not contain chromatin (see); the threads of the achromatin spindle are attached to it, due to which the centromere determines the movement of chromosomes to the poles in mitosis and meiosis (Fig. 4).

Loss of a centromere, for example when a chromosome is broken by ionizing radiation or other mutagens, leads to the loss of the ability of the piece of chromosome lacking the centromere (acentric fragment) to participate in mitosis and meiosis and to its loss from the nucleus. This can cause severe cell damage.

The centromere divides the chromosome body into two arms. The location of the centromere is strictly constant for each chromosome and determines three types of chromosomes: 1) acrocentric, or rod-shaped, chromosomes with one long and a second very short arm, resembling a head; 2) submetacentric chromosomes with long arms of unequal length; 3) metacentric chromosomes with arms of the same or almost the same length (Fig. 3, 4, 5 and 7).

Rice. 4. Scheme of chromosome structure in metaphase of mitosis after longitudinal splitting of the centromere: A and A1 - sister chromatids; 1 - long shoulder; 2 - short shoulder; 3 - secondary constriction; 4- centromere; 5 - spindle fibers.

Characteristic features of the morphology of certain chromosomes are secondary constrictions (which do not have the function of a centromere), as well as satellites - small sections of chromosomes connected to the rest of its body by a thin thread (Fig. 5). Satellite filaments have the ability to form nucleoli. The characteristic structure in the chromosome (chromomeres) is thickening or more tightly coiled sections of the chromosomal thread (chromonemas). The chromomere pattern is specific to each pair of chromosomes.

Rice. 5. Scheme of chromosome morphology in anaphase of mitosis (chromatid extending to the pole). A - appearance of the chromosome; B - internal structure of the same chromosome with its two constituent chromonemas (hemichromatids): 1 - primary constriction with chromomeres constituting the centromere; 2 - secondary constriction; 3 - satellite; 4 - satellite thread.

The number of chromosomes, their size and shape at the metaphase stage are characteristic of each type of organism. The combination of these characteristics of a set of chromosomes is called a karyotype. A karyotype can be represented in a diagram called an idiogram (see human chromosomes below).

Sex chromosomes. Genes that determine sex are localized in a special pair of chromosomes - sex chromosomes (mammals, humans); in other cases, the iol is determined by the ratio of the number of sex chromosomes and all others, called autosomes (Drosophila). In humans, as in other mammals, the female sex is determined by two identical chromosomes, designated as X chromosomes, the male sex is determined by a pair of heteromorphic chromosomes: X and Y. As a result of reduction division (meiosis) during the maturation of oocytes (see Oogenesis) in women all eggs contain one X chromosome. In men, as a result of the reduction division (maturation) of spermatocytes, half of the sperm contains an X chromosome, and the other half a Y chromosome. The sex of a child is determined by the accidental fertilization of an egg by a sperm carrying an X or Y chromosome. The result is a female (XX) or male (XY) embryo. In the interphase nucleus of women, one of the X chromosomes is visible as a clump of compact sex chromatin.

Chromosome functioning and nuclear metabolism. Chromosomal DNA is the template for the synthesis of specific messenger RNA molecules. This synthesis occurs when a given region of the chromosome is despiraled. Examples of local chromosome activation are: the formation of despiralized chromosome loops in the oocytes of birds, amphibians, fish (the so-called X-lamp brushes) and swellings (puffs) of certain chromosome loci in multi-stranded (polytene) chromosomes of the salivary glands and other secretory organs of dipteran insects (Fig. 6). An example of inactivation of an entire chromosome, i.e., its exclusion from the metabolism of a given cell, is the formation of one of the X chromosomes of a compact body of sex chromatin.

Rice. 6. Polytene chromosomes of the dipteran insect Acriscotopus lucidus: A and B - area limited by dotted lines, in a state of intensive functioning (puff); B - the same area in a non-functioning state. The numbers indicate individual chromosome loci (chromomeres).
Rice. 7. Chromosome set in a culture of male peripheral blood leukocytes (2n=46).

Revealing the mechanisms of functioning of lampbrush-type polytene chromosomes and other types of chromosome spiralization and despiralization is crucial for understanding reversible differential gene activation.

Human chromosomes. In 1922, T. S. Painter established the diploid number of human chromosomes (in spermatogonia) to be 48. In 1956, Tio and Levan (N. J. Tjio, A. Levan) used a set of new methods for studying human chromosomes : cell culture; study of chromosomes without histological sections on whole cell preparations; colchicine, which leads to the arrest of mitoses at the metaphase stage and the accumulation of such metaphases; phytohemagglutinin, which stimulates the entry of cells into mitosis; treatment of metaphase cells with hypotonic saline solution. All this made it possible to clarify the diploid number of chromosomes in humans (it turned out to be 46) and provide a description of the human karyotype. In 1960, in Denver (USA), an international commission developed a nomenclature for human chromosomes. According to the commission's proposals, the term "karyotype" should be applied to the systematic set of chromosomes of a single cell (Fig. 7 and 8). The term "idiotram" is retained to represent the set of chromosomes in the form of a diagram constructed from measurements and descriptions of the chromosome morphology of several cells.

Human chromosomes are numbered (somewhat serially) from 1 to 22 in accordance with the morphological features that allow their identification. Sex chromosomes do not have numbers and are designated as X and Y (Fig. 8).

A connection has been discovered between a number of diseases and birth defects in human development with changes in the number and structure of its chromosomes. (see Heredity).