First mitotic division. Mitosis - indirect division

Mitotic cell division

Mitosis(from the Greek. Mitos - thread), also called karyokinesis, or indirect cell division, is a universal mechanism for cell division. Mitosis follows the G2 period and completes the cell cycle.

It lasts 1-3 hours and provides uniform distribution genetic material into daughter cells. Mitosis has 4 main phases: prophase, metaphase, anaphase, and telophase.

Mitosis is one of the fundamental processes of ontogeny. Mitotic division ensures the growth of multicellular eukaryotes by increasing the populations of tissue cells.

As a result of mitotic division of meristem cells, the number of plant tissue cells increases. The fragmentation of a fertilized egg and the growth of most tissues in animals also occur through mitotic divisions.

Based morphological features Mitosis is conditionally divided into stages: prophase, prometaphase, metaphase, anaphase, telophase. The first descriptions of the phases of mitosis and the establishment of their sequence were undertaken in the 70-80s of the XIX century. In the late 1870s, the German histologist Walter Flemming coined the term "mitosis" to refer to the process of indirect cell division.

The average duration of mitosis is 1-2 hours. Mitosis of animal cells, as a rule, lasts 30-60 minutes, and plants - 2-3 hours. For 70 years, about 10 14 cell divisions are carried out in the human body in total.

The first incomplete descriptions concerning the behavior and changes of nuclei in dividing cells are found in the works of scientists in the early 1870s.

In the work of the Russian botanist Russov, dated 1872, metaphase and anaphase plates, consisting of individual chromosomes, are clearly described and depicted.

A year later, the German zoologist G.A. Schneider even more clearly and consistently, but, of course, not quite fully described mitotic division using the example of crushing eggs of the rectal turbellaria Mesostomum. In his work, in essence, the main phases of mitosis are described and illustrated in the correct sequence: prophase, metaphase, anaphase (early and late). In 1874, the Moscow botanist I.D. Chistyakov also observed individual phases of cell division in spores of club mosses and horsetails. Despite the first successes, neither Russov, nor Schneider, nor Chistyakov were able to give a clear and consistent description of mitotic division.

In 1875, works came out containing more than detailed descriptions mitoses. O. Buchli gave a description of cytological patterns in crushing eggs roundworms and molluscs and in insect spermatogenic cells.

E. Strasburger studied mitotic division in the cells of the green alga spirogyra, in the mother cells of onion pollen, and in the mother spore cells of the club moss. Referring to the work of O. Buechli and based on his own research, E. Strasburger drew attention to the unity of the processes of cell division in plant and animal cells.

By the end of 1878 - beginning of 1879 there appeared detailed works Schleicher and W. Flemming. In his work in 1879, Schleicher proposed the term "karyokinesis" to refer to complex processes cell division, implying movement constituent parts kernels. Walter Flemming was the first to introduce the term "mitosis" to refer to indirect cell division, which later became generally accepted. Flemming also owns the final formulation of the definition of mitosis as a cyclic process, culminating in the division of chromosomes between daughter cells.

In 1880 O.V. Baranetsky established the helical structure of chromosomes. In the course of further research, ideas about the spiralization and despiralization of chromosomes during the mitotic cycle were developed.

In the early 1900s, chromosomes were identified as carriers of hereditary information, which later provided an explanation biological role mitosis, which consists in the formation of genetically identical daughter cells.

In the 1970s, the deciphering and detailed study of the regulators of mitotic division began, thanks to a series of experiments on the fusion of cells located on different stages cell cycle. In those experiments, when a cell in the M phase was combined with a cell in any of the stages of interphase (G1, S, or G2), the interphase cells passed into the mitotic state (chromosome condensation began and the nuclear envelope disintegrated).

As a result, it was concluded that there is a factor (or factors) in the cytoplasmic cell that stimulates mitosis, or, in other words, the M-stimulating factor (MSF, from the English M-phase-promoting factor, MPF).

For the first time, the "mitosis stimulating factor" was discovered in mature unfertilized eggs of the clawed frog, which are in the M-phase of the cell cycle. The cytoplasm of such an egg, injected into the oocyte, led to a premature transition to the M-phase and to the onset of maturation of the oocyte (originally, the abbreviation MPF stood for Maturation Promoting Factor, which translates as "maturing promoting factor"). In the course of further experiments, the universal significance and, at the same time, a high degree of conservatism of the “mitosis stimulating factor” were established: extracts prepared from mitotic cells very diverse organisms, when introduced into clawed frog oocytes, they were transferred to the M-phase.

Subsequent studies revealed that the factor that stimulates mitosis is a heterodimeric complex consisting of a cyclin protein and a cyclin-dependent protein kinase. Cyclin is a regulatory protein and is found in all eukaryotes. Its concentration periodically increases during the cell cycle, reaching a maximum in the metaphase of mitosis. With the onset of anaphase, a sharp decrease in the concentration of cyclin is observed, due to its cleavage with the help of complex protein proteolytic complexes - proteosomes. Cyclin-dependent protein kinase is an enzyme (phosphorylase) that modifies proteins by transferring a phosphate group from ATP to the amino acids serine and threonine. Thus, with the establishment of the role and structure of the main regulator of mitotic division, studies of the subtle regulatory mechanisms of mitosis began, which continue to this day.

The development of a unified typology and classification of mitoses is complicated by a whole range of features that, in various combinations, create a variety and heterogeneity of patterns of mitotic division. At the same time, separate classification options developed for some taxa are unacceptable for others, since they do not take into account the specifics of their mitoses. For example, some variants of the classification of mitoses characteristic of animals or plant organisms, are unacceptable for algae.

One of the key features underlying the various typologies and classifications of mitotic division is the behavior of the nuclear envelope. If the formation of the spindle and the mitotic division itself proceeds inside the nucleus without destroying the nuclear membrane, then this type of mitosis is called closed. Mitosis with the collapse of the nuclear envelope, respectively, is called open, and mitosis with the collapse of the membrane only at the poles of the spindle, with the formation of "polar windows" - semi-closed.

one more hallmark is a type of symmetry of the mitotic spindle. In pleuromitosis, the division spindle is bilaterally symmetrical or asymmetrical and usually consists of two semi-spindles located in the metaphase-anaphase at an angle to each other. The category of orthomitoses is characterized by bipolar symmetry of the fission spindle, and in metaphase there is often a distinguishable equatorial plate.

Within the indicated signs, the most numerous is a typical open orthomitosis, on the example of which the principles and stages of mitotic division are discussed below. This type of mitosis is characteristic of animals, higher plants, and some protozoa.

Prophase begins with the condensation of chromosomes, which become visible under a light microscope as filamentous structures. Each chromosome consists of two parallel sister chromatids connected at the centromere. The nucleolus and nuclear envelope disappear by the end of the phase (the latter breaks up into membrane vesicles similar to the EPS elements, and the pore complex and lamina dissociate into subunits). Karyoplasm mixes with cytoplasm.

Centrioles migrate to opposite poles of the cell and give rise to the filaments of the mitotic (achromatin) spindle. In the region of the centromere, special protein complexes are formed - kinetochores, to which some spindle microtubules (kinetochore microtubules) are attached; it has been shown that kinetochores themselves are able to induce microtubule assembly and therefore can serve as microtubule organization centers. The rest of the spindle microtubules are called pole microtubules, as they extend from one pole of the cell to the other; the microtubules lying outside the spindle, diverging radially from the cell centers to the plasmalemma, received the name astral or microtubules (threads) of radiance.

Metaphase corresponds to the maximum level of condensation of chromosomes, which line up in the equatorial region of the mitotic spindle, forming a picture of the equatorial (metaphase) plate (side view) or the parent star (view from the poles). Chromosomes move to the equatorial plane and are held in it due to the balanced tension of the kinetochore microtubules. By the end of this phase, sister chromatids are separated by a gap, but are retained in the centromere region.

Anaphase begins with the synchronous splitting of all chromosomes into sister chromatids (in the centromere region) and the movement of daughter chromosomes to opposite poles of the cell, which occurs along the spindle microtubules at a speed of 0.2–0.5 µm/min. The signal for the onset of anaphase includes a sharp (by an order of magnitude) increase in the concentration of calcium cations in the hyaloplasm, secreted by membrane vesicles that form clusters at the spindle poles. The mechanism of chromosome movement in anaphase has not been fully elucidated, however, it has been established that in addition to actin, proteins such as myosin and dynein, as well as a number of regulatory proteins, are present in the spindle region. According to some observations, it is due to shortening (disassembly) of microtubules attached to kinetochores. Anaphase is characterized by elongation of the mitotic spindle due to some divergence of the cell poles. It ends with the accumulation of two identical sets of chromosomes at the poles of the cell, which form pictures of stars (the stage of daughter stars). At the end of anaphase, due to the contraction of actin microfilaments, concentrating around the circumference of the cell (contractile ring), a cell constriction begins to form, which, deepening, will lead to cytotomy in the next phase.

Telophase is the final stage of mitosis, during which the nuclei of daughter cells are reconstructed and their division is completed. Around the condensed chromosomes of daughter cells from membrane vesicles (according to other sources, from EPS), the karyolemma is restored, with which the emerging lamina is associated, nucleoli reappear, which are formed from sections of the corresponding chromosomes. The cell nuclei gradually increase, and the chromosomes progressively despiralize and disappear, being replaced by the chromatin pattern of the interphase nucleus. At the same time, the cell constriction deepens, and the cells remain connected for some time by a narrowing cytoplasmic bridge containing a bundle of microtubules (middle body). Further ligation of the cytoplasm ends with the formation of two daughter cells. In telophase, the distribution of organelles between daughter cells occurs; The uniformity of this process is facilitated by the fact that some organelles are quite numerous (for example, mitochondria), while others (like EPS and the Golgi complex) break up into small fragments and vesicles during mitosis.

Atypical mitoses occur when the mitotic apparatus is damaged and are characterized by an uneven distribution of genetic material between cells - aneuploidy (from the Greek an - not, eu - correct, ploon - add); in many cases, cytotomy is absent, resulting in the formation of giant cells. Atypical mitoses are characteristic of malignant tumors and irradiated tissues. The higher their frequency and the greater degree aneuploidy, the more malignant is the tumor. Violation of normal mitotic cell division can be caused by chromosome anomalies, which are called chromosomal aberrations (from Latin Aberratio - deviation). Variants of chromosomal aberrations are the adhesion of chromosomes, their break into fragments, the loss of a site, the exchange of fragments, the doubling of individual sections of chromosomes, etc. Chromosomal aberrations can occur spontaneously, but more often they develop due to the action of mutagens and ionizing radiation on cells.

Karyotyping - diagnostic study in order to assess the karyotype (set of chromosomes) is performed by examining the chromosomes in the metaphase plate. For karyotyping, a cell culture is obtained into which colchicine, a substance that blocks the formation of the mitotic spindle, is introduced. Chromosomes are extracted from such cells, which are further stained and identified. The normal human karyotype is represented by 46 chromosomes - 22 pairs of autosomes and two sex chromosomes (XY in men and XX in women). Karyotyping can diagnose a number of diseases associated with chromosomal abnormalities, in particular, Down syndrome (trisomy of the 21st chromosome), Edwards (trisomy of the 18th chromosome), Patau (trisomy of the 13th chromosome), as well as a number of syndromes associated with anomalies of the sex chromosomes - Klinefelter's syndrome (genotype - XXY) , Turner (genotype - XO) and others.

It is assumed that the complex mitotic process of higher organisms developed gradually from the mechanisms of prokaryotic division. This assumption is supported by the fact that prokaryotes appeared about a billion years earlier than the first eukaryotes. In addition, similar proteins are involved in eukaryotic mitosis and prokaryotic binary fission.

Possible intermediate stages between binary fission and mitosis can be traced in unicellular eukaryotes, in which the nuclear membrane is not destroyed during division. In most other eukaryotes, including plants and animals, the fission spindle is formed outside the nucleus, and the nuclear envelope is destroyed during mitosis. Although mitosis in single-celled eukaryotes is not yet well understood, it can be assumed that it originated from binary fission and eventually reached the level of complexity that exists in multicellular organisms.

In many protozoan eukaryotes, mitosis also remained a process associated with the membrane, but now it is no longer plasma, but nuclear.

The main regulatory mechanisms of mitosis are the processes of phosphorylation and proteolysis.

Reversible phosphorylation and dephosphorylation reactions allow for reversible mitotic events such as spindle assembly/disintegration or nuclear envelope disintegration/repair. Proteolysis underlies the irreversible events of mitosis, such as the separation of sister chromatids in anaphase or the breakdown of mitotic cyclins into late stages mitosis.

The division of all eukaryotic cells is associated with the formation of a special apparatus for cell division.

An active role in mitotic cell division is often assigned to cytoskeletal structures. The bipolar mitotic spindle, which consists of microtubules and associated proteins, is universal for both animal and plant cells. The spindle of division provides a strictly identical distribution of chromosomes between the poles of division, in the region of which the nuclei of daughter cells are formed in the telophase.

The process of mitosis ensures a strictly uniform distribution of chromosomes between two daughter nuclei, so that in a multicellular organism all cells have exactly the same (in number and character) sets of chromosomes.

Chromosomes contain genetic information encoded in DNA, and therefore a regular, ordered mitotic process also ensures the complete transfer of all information to each of the daughter nuclei; as a result, each cell has all the genetic information necessary for the development of all the characteristics of the organism. In this regard, it becomes clear why one cell taken from a fully differentiated adult plant can, under suitable conditions, develop into a whole plant. We have described mitosis in a diploid cell, but this process proceeds in a similar way in haploid cells, for example, in cells of the gametophyte generation of plants.

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Twin Method in the Study of Features with Continuous Distribution

There are two ways of division: 1) the most common, complete division - mitosis ( indirect division) and 2) amitosis (direct division). During mitotic division, the cytoplasm is restructured, the nuclear envelope is destroyed, and chromosomes are identified. In the life of a cell, there is a period of mitosis itself and an interval between divisions, which is called interphase. However, the period of interphase (non-dividing cells) in its essence can be different. In some cases, during interphase, the cell functions and simultaneously prepares for the next division. In other cases, cells enter interphase, function, but no longer prepare for division. As part of a complex multicellular organism, there are numerous groups of cells that have lost the ability to divide. These include, for example, nerve cells. Cell preparation for mitosis occurs in interphase. In order to imagine the main features of this process, remember the structure of the cell nucleus.

Onion cells in different phases of the cell cycle

Basic structural unit nuclei are chromosomes made up of DNA and protein. In the nuclei of living non-dividing cells, as a rule, individual chromosomes are indistinguishable, but most of the chromatin, which is found on stained preparations in the form of thin filaments or grains of various sizes, corresponds to the chromosomes. In some cells, individual chromosomes are also clearly visible in the interphase nucleus, for example, in rapidly dividing cells of a developing fertilized egg and in the nuclei of some protozoa. IN different periods During the life of a cell, chromosomes undergo cyclical changes that can be traced from one division to another. Chromosomes during mitosis are elongated dense bodies, along the length of which two strands can be distinguished - chromatids containing DNA, which are the result of chromosome doubling. Each chromosome has a primary constriction, or centromere. This narrowed part of the chromosome can be located either in the middle or closer to one of the ends, but for each particular chromosome its place is strictly constant. During mitosis, the chromosomes and chromatids are tightly coiled helical filaments (a spiralized or condensed state). In the interphase nucleus, the chromosomes are strongly elongated, i.e., despiralized, due to which they become difficult to distinguish. Consequently, the cycle of chromosome changes consists in spiralization, when they shorten, thicken and become clearly distinguishable, and despiralization, when they are strongly elongated, intertwined, and then it becomes impossible to distinguish each separately. Spiralization and despiralization are associated with the activity of DNA, since it functions only in a despiralized state. The release of information, the formation of RNA on DNA in a spiralized state, that is, during mitosis, stops. The fact that chromosomes are present in the nucleus of a non-dividing cell is also proved by the constancy of the amount of DNA, the number of chromosomes, and the preservation of their individuality from division to division.

Preparing a cell for mitosis. During interphase, a number of processes occur that enable mitosis. Let's name the most important of them: 1) centrioles are doubled, 2) chromosomes are doubled, i.e. the amount of DNA and chromosomal proteins, 3) proteins are synthesized from which the achromatin spindle is built, 4) energy is accumulated in the form of ATP, which is consumed during division, 5) cell growth ends. Of paramount importance in preparing a cell for mitosis is the synthesis of DNA and duplication of chromosomes. The doubling of chromosomes is associated primarily with the synthesis of DNA and the simultaneous synthesis of chromosome proteins. The doubling process lasts 6-10 hours and takes middle part interphases. Chromosome duplication proceeds in such a way that each old single strand of DNA builds a second one for itself. This process is strictly ordered and, starting at several points, spreads along the entire chromosome.

Mitosis

Mitosis is a universal method of cell division in plants and animals, the main essence of which is the exact distribution of duplicated chromosomes between both formed daughter cells. The preparation of a cell for division, as we can see, occupies a significant part of the interphase, and mitosis begins only when the preparation in the nucleus and cytoplasm is completely completed. The whole process is divided into four phases. During the first of them - prophase - centrioles divide and begin to diverge in opposite directions. Around them, achromatin filaments are formed from the cytoplasm, which, together with centrioles, form an achromatin spindle. When the divergence of the centrioles ends, the whole cell is polar, both centrioles are located at opposite poles, and the middle plane can be called the equator. The filaments of the achromatin spindle converge at the centrioles and are widely distributed at the equator, resembling a spindle in shape. Simultaneously with the formation of a spindle in the cytoplasm, the nucleus begins to swell, and a ball of thickened threads - chromosomes - is clearly distinguished in it. During prophase, chromosomes spiralize, shortening and thickening. Prophase ends with the dissolution of the nuclear envelope, and the chromosomes are found to be lying in the cytoplasm. At this time, it can be seen that all chromosomes are already double. Then comes the second phase - metaphase. Chromosomes, randomly arranged at first, begin to move towards the equator. All of them are usually located in the same plane at an equal distance from the centrioles. At this time, part of the spindle threads is attached to the chromosomes, while the other part of them still stretches continuously from one centriole to another - these are the supporting threads. Pulling, or chromosomal, threads are attached to centromeres (primary constrictions of chromosomes), but it must be remembered that both chromosomes and centromeres are already double. Pulling threads from the poles are attached to those chromosomes that are closer to them. There is a short pause. This central part mitosis, after which the third phase begins - anaphase. During anaphase, the pulling filaments of the spindle begin to contract, stretching the chromosomes to different poles. In this case, the chromosomes behave passively, they, bending like a hairpin, move forward by centromeres, for which they are pulled by a spindle thread. At the beginning of anaphase, the viscosity of the cytoplasm decreases, which contributes to the rapid movement of chromosomes. Consequently, the threads of the spindle ensure the exact divergence of chromosomes (doubling even in interphase) to different poles of the cell. Mitosis is completed last stage- telophase. Chromosomes, approaching the poles, are closely intertwined with each other. At the same time, their stretching (despiralization) begins, and it becomes impossible to distinguish between individual chromosomes. Gradually, the nuclear envelope is formed from the cytoplasm, the nucleus swells, the nucleolus appears, and the previous structure of the interphase self is restored.

1. Define the life and mitotic cycles of a cell.
Life cycle- the time interval from the moment a cell appears as a result of division to its death or until the next division.
Mitotic cycle- a set of consecutive and interrelated processes during the preparation of the cell for division, as well as during mitosis itself.

2. Answer how the concept of "mitosis" differs from the concept of "mitotic cycle".
The mitotic cycle includes mitosis itself and the stages of preparing the cell for division, while mitosis is only cell division.

3. List the periods of the mitotic cycle.

2. DNA synthesis period (S)

4. mitosis.

4. Open biological significance mitosis.

Mitosis (indirect division) is the division of somatic cells (body cells). The biological significance of mitosis is the reproduction of somatic cells, the production of copy cells (with the same set of chromosomes, with exactly the same hereditary information). All somatic cells of the body are obtained from a single parent cell (zygote) by mitosis.

1) Prophase

chromatin spiralizes (twists, condenses) to the state of chromosomes
nucleoli disappear
the nuclear envelope breaks down
centrioles diverge towards the poles of the cell, the spindle of division is formed

2) Metaphase Chromosomes line up along the equator of the cell, forming a metaphase plate

3) Anphase- daughter chromosomes separate from each other (chromatids become chromosomes) and diverge towards the poles

4) Telophase

chromosomes despiralize (unwind, decondense) to the state of chromatin
nucleus and nucleoli appear
spindle fibers break down
cytokinesis occurs - the division of the cytoplasm of the mother cell into two daughter cells

The duration of mitosis is 1-2 hours.

cell cycle

This is the period of a cell's life from the moment of its formation by dividing the mother cell to its own division or death.

The cell cycle consists of two periods:

interphase(state when the cell is NOT dividing);
division (mitosis or meiosis).

Interphase consists of several phases:

presynthetic: the cell grows, active synthesis of RNA and proteins occurs in it, the number of organelles increases; in addition, there is a preparation for DNA duplication (accumulation of nucleotides)
synthetic: doubling (replication, reduplication) of DNA occurs
postsynthetic: the cell prepares for division, synthesizes the substances necessary for division, for example, fission spindle proteins.

MORE INFO: Mitosis, Differences between mitosis and meiosis, Cell cycle, DNA duplication (replication)
PART 2 ASSIGNMENTS: Mitosis

Tests and assignments

Install correct sequence processes that take place during mitosis. Write down the numbers under which they are indicated.
1) the collapse of the nuclear envelope
2) thickening and shortening of chromosomes
3) alignment of chromosomes in the central part of the cell
4) the beginning of the movement of chromosomes to the center
5) divergence of chromatids to the poles of the cell
6) the formation of new nuclear membranes

Choose the one most correct option. The process of cell reproduction different kingdoms wildlife is called
1) meiosis
2) mitosis
3) fertilization
4) crushing

All the features below, except for two, can be used to describe the processes of the interphase of the cell cycle. Identify two features that "drop out" of general list, and write down in the table the numbers under which they are indicated.
1) cell growth
2) divergence of homologous chromosomes
3) the location of chromosomes along the equator of the cell
4) DNA replication
5) synthesis of organic substances

Choose one, the most correct option. During what stage of life do chromosomes coil?
1) interphase
2) prophase
3) anaphase
4) metaphase

Choose three options.

What cell structures undergo biggest changes during mitosis?
1) core
2) cytoplasm
3) ribosomes
4) lysosomes
5) cell center
6) chromosomes

1. Establish the sequence of processes occurring in a cell with chromosomes in interphase and subsequent mitosis
1) location of chromosomes in the equatorial plane
2) DNA replication and the formation of two-chromatid chromosomes
3) spiralization of chromosomes
4) divergence of sister chromosomes to the poles of the cell

2. Set the sequence of processes occurring during interphase and mitosis. Write down the corresponding sequence of numbers.
1) spiralization of chromosomes, the disappearance of the nuclear membrane
2) divergence of sister chromosomes to the poles of the cell
3) the formation of two daughter cells
4) duplication of DNA molecules
5) placement of chromosomes in the plane of the cell equator

3. Set the sequence of processes occurring in interphase and mitosis. Write down the corresponding sequence of numbers.
1) dissolution of the nuclear membrane
2) DNA replication
3) destruction of the fission spindle
4) divergence to the poles of the cell of single-chromatid chromosomes
5) formation of a metaphase plate

Choose one, the most correct option. During cell division, the division spindle is formed
1) prophase
2) telophase
3) metaphase
4) anaphase

Choose one, the most correct option. Mitosis does NOT occur during prophase
1) dissolution of the nuclear envelope
2) spindle formation
3) duplication of chromosomes
4) dissolution of the nucleoli

Choose one, the most correct option. At what stage in life do chromatids become chromosomes?
1) interphase
2) prophase
3) metaphase
4) anaphase

Choose one, the most correct option. Despiralization of chromosomes during cell division occurs in
1) prophase
2) metaphase
3) anaphase
4) telophase

Choose one, the most correct option. In what phase of mitosis do pairs of chromatids attach with their centromeres to the fission spindle filaments
1) anaphase
2) telophase
3) prophase
4) metaphase

Establish a correspondence between the processes and phases of mitosis: 1) anaphase, 2) telophase. Write the numbers 1 and 2 in the correct order.
A) the nuclear envelope is formed
B) sister chromosomes diverge to the poles of the cell
C) the spindle of division finally disappears
D) chromosomes despiralize
D) centromeres of chromosomes are separated

All the features below, except for two, can be used to describe the processes occurring in the interphase. Identify two signs that “fall out” from the general list, and write down the numbers under which they are indicated in the table.
1) DNA replication
2) formation of the nuclear envelope
3) spiralization of chromosomes
4) ATP synthesis
5) synthesis of all types of RNA

How many cells are formed as a result of mitosis of one cell? Write down only the appropriate number in your answer.

All of the features listed below, except for two, are used to describe the phase of mitosis depicted in the figure. Identify two signs that “fall out” from the general list, and write down the numbers under which they are indicated.
1) the nucleolus disappears
2) a fission spindle is formed
3) doubling of DNA molecules occurs
4) chromosomes are actively involved in the biosynthesis of proteins
5) chromosomes spiralize

Establish the sequence of processes occurring during mitosis. Write down the corresponding sequence of numbers.
1) spiralization of chromosomes
2) chromatid separation
3) formation of the fission spindle
4) despiralization of chromosomes
5) division of the cytoplasm
6) the location of chromosomes at the equator of the cell

Choose one, the most correct option. What causes the spiralization of chromosomes at the beginning of mitosis
1) the acquisition of a two-chromatid structure
2) active participation chromosomes in protein biosynthesis
3) doubling the DNA molecule
4) transcription amplification

Establish a correspondence between the processes and periods of interphase: 1) postsynthetic, 2) presynthetic, 3) synthetic. Write down the numbers 1, 2, 3 in the order corresponding to the letters.
A) cell growth
B) ATP synthesis for the fission process
C) ATP synthesis for DNA replication
D) protein synthesis for building microtubules
D) DNA replication
E) doubling of centrioles

1. All of the features listed below, except for two, can be used to describe the process of mitosis. Identify two signs that “fall out” from the general list, and write down the numbers under which they are indicated.
1) underlies asexual reproduction
2) indirect division
3) provides regeneration
4) reduction division
5) genetic diversity increases

2. All the above features, except for two, can be used to describe the processes of mitosis. Identify two signs that “fall out” from the general list, and write down the numbers under which they are indicated.
1) the formation of bivalents
2) conjugation and crossing over
3) the invariance of the number of chromosomes in cells
4) the formation of two cells
5) preservation of the structure of chromosomes

All of the features listed below, except for two, are used to describe the process depicted in the figure. Identify two signs that “fall out” from the general list, and write down the numbers under which they are indicated.
1) daughter cells have the same set of chromosomes as parent cells
2) uneven distribution of genetic material between daughter cells
3) provides growth
4) the formation of two daughter cells
5) direct division

All of the processes listed below, except for two, occur during indirect cell division. Identify two processes that "fall out" from the general list, and write down the numbers under which they are indicated.
1) two diploid cells are formed
2) four haploid cells are formed
3) somatic cell division occurs
4) conjugation and crossing over of chromosomes occurs
5) cell division is preceded by one interphase

Establish a correspondence between the stages of the cell life cycle and processes. Occurring during them: 1) interphase, 2) mitosis. Write down the numbers 1 and 2 in the order corresponding to the letters.
A) the spindle is formed
B) the cell grows, active synthesis of RNA and proteins occurs in it
B) cytokinesis is carried out
D) the number of DNA molecules doubles
D) chromosomes spiralize

What processes occur in a cell during interphase?
1) protein synthesis in the cytoplasm
2) spiralization of chromosomes
3) mRNA synthesis in the nucleus
4) reduplication of DNA molecules
5) dissolution of the nuclear envelope
6) divergence of the centrioles of the cell center to the poles of the cell

Determine the phase and type of division shown in the figure. Write down two numbers in the order indicated in the task, without separators (spaces, commas, etc.).
1) anaphase
2) metaphase
3) prophase
4) telophase
5) mitosis
6) meiosis I
7) meiosis II

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Mitosis in animal and plant cells

The most important event that occurs in mitosis is the even distribution of genetic material. Mitosis in animals and plant cells almost the same, but there are a number of differences, which are indicated in our table (Fig.

4). Plant cells do not have centrioles, but animal cage centrioles are present, a cell plate is formed in a plant cell, it is not formed in an animal cell.

Rice. 4. Comparison of the features of mitosis in animal and plant cells

In plant cells, no constriction is formed during cytokinesis, but in animals, a cell is formed. Mitoses in plant cells occur mainly in the meristems, while in animal cells mitoses occur in various tissues and parts of the body.

Mitosis is divided into four successive phases: prophase, metaphase, anaphase, and telophase (Fig. 5). Interphase - the main stage of the cell life cycle (see previous lesson), is a preparation for division or precedes cell death, therefore it is not a phase of mitosis.

Rice. 5. Interphase and the following phases of mitosis: prophase, metaphase, anaphase and telophase

In prophase, DNA coils in the nucleus and, looking at the cell through a microscope, one can see tightly twisted chromosomes (Fig. 6).

Rice. 6. Prophase of mitosis

It is usually seen that each chromosome consists of two chromatids and unifying regions - the centromere. The nucleoli at this stage disappear. in animal cells and lower plants centrioles diverge towards the poles of the cell.

Short microtubules extend from each centriole in the form of rays. They form a structure shaped like a star.

Rice. 7. Prophase of mitosis in animal and plant cells

By the end of prophase (Fig. 7), the nuclear envelope disintegrates or dissolves and microtubules begin to form a fission spindle (Fig. 8).

Rice. 8. Completion of prophase and transition to metaphase

The next phase is metaphase. Chromosomes are arranged in such a way that their centromeres are on the plane of the cell equator (Fig. 9).

9. Metaphase: spindle of division. At the equator is the metaphase plate.

The so-called metaphase plate is formed (Fig. 10), which consists of chromosomes. The spindle fibers are attached to the centromeres of each chromosome.

Rice. 10. Metaphase. Painted preparation. The spindle is formed by centromeres (blue), microfibrils (purple) and chromosomes of the metaphase plate - yellow.

Anaphase is a very short phase (Fig. 11). Each chromosome splits longitudinally into two identical chromatids, which diverge to opposite poles of the cell, now they are called daughter chromosomes (or chromatids).

Rice. 11. Anaphase of mitosis

Due to the identity of the daughter chromosomes, the two poles of the cell have the same genetic material. The same one that was in the cell before mitosis began. It should be noted that at the same time, near each pole of information carriers - DNA molecules compactly packed into chromosomes - are two times less than in the original cell.

Telophase is the last phase, the daughter chromosomes are despiralized at the poles of the cell and become available for transcription, protein synthesis begins, nuclear membranes and nucleoli are formed (Fig. 12).

Rice. 12. Telophase of mitosis in animal and plant cells

The filaments of the fission spindle disintegrate. This is where karyokinesis ends and cytokinesis begins (Fig. 13), while constriction occurs in animal cells in the equatorial plane. It deepens until the separation of two daughter cells occurs.

Rice. 13. Cytokinesis

In the formation of a constriction important role play structures of the cytoskeleton. Cytokinesis in plant cells occurs differently, since plants have a rigid cell wall, and they do not divide to form a constriction, but form an intracellular septum.

Mitosis, in the first place, gives genetic stability. As a result of mitosis, two nuclei are formed, which contain as many chromosomes as there were in the mother or parent cells.

These chromosomes are formed by exact replication of the DNA molecule of the parental chromosomes, as a result of which their genes contain exactly the same hereditary information.

Thus, daughter cells are genetically identical to the parent cell, since mitosis cannot introduce any changes in the hereditary information. Cell populations obtained by mitosis from parental cells are genetically stable.

Mitosis is necessary for normal growth and the development of multicellular organisms, since as a result of mitosis, the number of cells increases.

Mitosis is one of the main growth mechanisms of multicellular eukaryotes.

Mitosis underlies the asexual reproduction of many animals and plants, ensures the regeneration of lost parts (for example, the limbs of crustaceans), as well as the replacement of cells that occurs in a multicellular organism.

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§ 28. Cell division - Mamontova, Sonina Grade 9 (answers)

1. Define the life and mitotic cycles of a cell.

Life cycle - the period of time from the moment a cell appears as a result of division to its death or until the next division.

The mitotic cycle is a set of sequential and interrelated processes during the preparation of a cell for division, as well as during mitosis itself.

2. Answer how the concept of "mitosis" differs from the concept of "mitotic cycle".

The mitotic cycle includes mitosis itself and the stages of preparing the cell for division, while mitosis is only cell division.

List the periods of the mitotic cycle.

1. period of preparation for DNA synthesis (G1)

2. DNA synthesis period (S)

3. period of preparation for cell division (G2)

4. Expand the biological significance of mitosis.

During mitosis, daughter cells receive a diploid set of chromosomes identical to the mother cell. The constancy of the structure and the correct functioning of organs would be impossible without the preservation of the same set of genetic material in cell generations. Mitosis provides embryonic development, growth, tissue repair after damage, maintaining the structural integrity of tissues with constant loss of cells in the course of their functioning.

5. Indicate the phases of mitosis and make schematic drawings that reflect the events occurring in the cell at a certain phase of mitosis. Fill the table.

Cell division is the central moment of reproduction.

In the process of division, two cells arise from one cell. A cell, based on the assimilation of organic and inorganic substances, creates its own kind with a characteristic structure and functions.

In cell division, two main points can be observed: nuclear division - mitosis and division of the cytoplasm - cytokinesis, or cytotomy. The main attention of geneticists is still riveted to mitosis, since, from the point of view of chromosome theory, the nucleus is considered the "organ" of heredity.

During mitosis, the following occurs:

doubling of the substance of the chromosomes;
change physical condition and chemical organization of chromosomes;
divergence of daughter, or rather sister, chromosomes to the poles of the cell;
subsequent division of the cytoplasm and full recovery two new nuclei in sister cells.

Thus, in mitosis, the entire life cycle nuclear genes: doubling, distribution and functioning; as a result of the completion of the mitotic cycle, sister cells end up with an equal “heritage”.

When dividing, the cell nucleus goes through five successive stages: interphase, prophase, metaphase, anaphase and telophase; some cytologists distinguish another sixth stage - prometaphase.

Diagram of the phases of mitosis in an animal cell

Between two successive cell divisions, the nucleus is in the interphase stage. During this period, the nucleus, during fixation and coloring, has a mesh structure formed by dyeing thin threads, which in the next phase form into chromosomes. Although the interphase is otherwise called the phase of the resting nucleus, on the body itself, metabolic processes in the nucleus during this period are performed with the greatest activity.

Prophase is the first stage in the preparation of the nucleus for division. In prophase, the network structure of the nucleus gradually turns into chromosome threads. From the earliest prophase, even in light microscope the dual nature of chromosomes can be observed. This suggests that in the nucleus, it is in the early or late interphase that the most important process mitosis - doubling, or reduplication, of chromosomes, in which each of the maternal chromosomes builds its own similar - daughter. As a result, each chromosome looks longitudinally doubled. However, these halves of chromosomes, which are called sister chromatids, do not diverge in prophase, as they are held together by one common site - the centromere; the centromeric region is divided later. In prophase, the chromosomes undergo a process of twisting along their axis, which leads to their shortening and thickening. It should be emphasized that in prophase each chromosome in the karyolymph is located randomly.

In animal cells, even in late telophase or very early interphase, doubling of the centriole occurs, after which, in prophase, the daughter centrioles begin to converge to the poles and the formation of the astrosphere and spindle, called the new apparatus. At the same time, the nucleoli dissolve. An essential sign of the end of prophase is the dissolution of the nuclear membrane, as a result of which the chromosomes are in the total mass of the cytoplasm and karyoplasm, which now form the myxoplasm. This ends the prophase; the cell enters metaphase.

IN Lately between prophase and metaphase, researchers began to distinguish an intermediate stage called prometaphase. Prometaphase is characterized by the dissolution and disappearance of the nuclear membrane and the movement of chromosomes towards the equatorial plane of the cell. But by this time, the formation of the achromatin spindle has not yet been completed.

Metaphase called the end stage of the arrangement of chromosomes at the equator of the spindle. The characteristic arrangement of chromosomes in the equatorial plane is called the equatorial, or metaphase, plate. The arrangement of chromosomes in relation to each other is random. In metaphase, the number and shape of chromosomes are well revealed, especially when considering the equatorial plate from the poles of cell division. The achromatin spindle is fully formed: the spindle filaments acquire a denser consistency than the rest of the cytoplasm and are attached to the centromeric region of the chromosome. The cytoplasm of the cell during this period has the lowest viscosity.

Anaphase called the next phase of mitosis, in which chromatids divide, which can now be called sister or daughter chromosomes, diverge towards the poles. In this case, first of all, the centromeric regions repel each other, and then the chromosomes themselves diverge towards the poles. It must be said that the divergence of chromosomes in anaphase begins at the same time - "as if on command" - and ends very quickly.

In telophase, the daughter chromosomes despiralize and lose their visible individuality. The shell of the nucleus and the nucleus itself are formed. The nucleus is reconstructed reverse order compared to the changes it underwent in prophase. In the end, the nucleoli (or nucleolus) are also restored, and in the amount in which they were present in the parent nuclei. The number of nucleoli is characteristic of each cell type.

At the same time, the symmetrical division of the cell body begins.

The nuclei of the daughter cells enter the state of interphase.

Scheme of cytokinesis of animal and plant cells

The figure above shows a diagram of the cytokinesis of animal and plant cells. In an animal cell, division occurs by ligation of the cytoplasm of the mother cell. In a plant cell, the formation of a cell septum occurs with areas of spindle plaques that form a septum in the plane of the equator, called a phragmoplast. This ends the mitotic cycle. Its duration seems to depend on the type of tissue, physiological state body, external factors (temperature, light regime) and lasts from 30 minutes to 3 hours. According to various authors, the speed of passage of individual phases is variable.

Both internal and external factors environments that affect the growth of the organism and its functional state affect the duration of cell division and its individual phases. Since the nucleus plays a huge role in the metabolic processes of the cell, it is natural to believe that the duration of the phases of mitosis can change in accordance with the functional state of the organ tissue. For example, it has been established that the mitotic activity of various tissues during rest and sleep in animals is significantly higher than during wakefulness. In a number of animals, the frequency of cell divisions decreases in the light, and increases in the dark. It is also assumed that hormones influence the mitotic activity of the cell.

The reasons that determine the readiness of the cell for division are still unclear. There are reasons to assume several such reasons:

doubling of the mass of cellular protoplasm, chromosomes and other organelles, due to which nuclear-plasma relations are violated; for division, a cell must reach a certain weight and volume characteristic of the cells of a given tissue;
duplication of chromosomes;
secretion by chromosomes and other cell organelles of special substances that stimulate cell division.

The mechanism of divergence of chromosomes to the poles in the anaphase of mitosis also remains unclear. An active role in this process is apparently played by spindle filaments, which are protein filaments organized and oriented by centrioles and centromeres.

The nature of mitosis, as we have already said, varies depending on the type and functional state fabrics. Cells of different tissues are characterized by different types of mitosis. In the described type of mitosis, cell division occurs in an equal and symmetrical manner. As a result of symmetrical mitosis, sister cells are hereditarily equivalent in respect of both nuclear genes and cytoplasm. However, in addition to symmetrical, there are other types of mitosis, namely: asymmetric mitosis, mitosis with delayed cytokinesis, division of multinucleated cells (syncytia division), amitosis, endomitosis, endoreproduction and polythenia.

In the case of asymmetric mitosis, sister cells are unequal in size, amount of cytoplasm, and also in relation to their future fate. An example of this is the unequal size sister (daughter) cells of the grasshopper neuroblast, animal eggs during maturation and during spiral fragmentation; during the division of nuclei in pollen grains, one of the daughter cells can further divide, the other cannot, etc.

Mitosis with a delay in cytokinesis is characterized by the fact that the cell nucleus divides many times, and only then does the division of the cell body occur. As a result of this division, multinucleated cells like syncytium are formed. An example of this is the formation of endosperm cells and the formation of spores.

Amitosis called direct fission of the nucleus without the formation of fission figures. In this case, the division of the nucleus occurs by "lacing" it into two parts; sometimes several nuclei are formed from one nucleus at once (fragmentation). Amitosis is constantly found in the cells of a number of specialized and pathological tissues, for example, in cancerous tumors. It can be observed under the influence of various damaging agents (ionizing radiation and high temperature).

Endomitosis called such a process when a doubling of nuclear fission occurs. In this case, the chromosomes, as usual, are reproduced in the interphase, but their subsequent divergence occurs inside the nucleus with the preservation of the nuclear envelope and without the formation of an achromatin spindle. In some cases, although the shell of the nucleus dissolves, however, the divergence of chromosomes to the poles does not occur, as a result of which the number of chromosomes in the cell multiplies even by several tens of times. Endomitosis occurs in cells of various tissues of both plants and animals. So, for example, A. A. Prokofieva-Belgovskaya showed that by endomitosis in the cells of specialized tissues: in the cyclops hypodermis, fat body, peritoneal epithelium and other tissues of the filly (Stenobothrus) - the set of chromosomes can increase 10 times. This increase in the number of chromosomes is associated with functional features differentiated tissue.

With polythenia, the number of chromosome threads multiplies: after reduplication along the entire length, they do not diverge and remain adjacent to each other. In this case, the number of chromosome threads within one chromosome is multiplied, as a result, the diameter of the chromosomes increases markedly. The number of such thin threads in a polytene chromosome can reach 1000-2000. In this case, the so-called giant chromosomes are formed. With polythenia, all phases of the mitotic cycle fall out, except for the main one - the reproduction of the primary strands of the chromosome. The phenomenon of polythenia is observed in the cells of a number of differentiated tissues, for example, in the tissue of the salivary glands of Diptera, in the cells of some plants and protozoa.

Sometimes there is a doubling of one or more chromosomes without any transformation of the nucleus - this phenomenon is called endoreproduction.

Thus, all phases of cell mitosis that make up the mitotic cycle are mandatory only for a typical process.

in some cases, mainly in differentiated tissues, the mitotic cycle undergoes changes. The cells of such tissues have lost the ability to reproduce the whole organism, and the metabolic activity of their nucleus is adapted to the function of the socialized tissue.

Embryonic and meristem cells that have not lost the function of reproducing the whole organism and related to undifferentiated tissues retain full cycle mitosis, on which asexual and vegetative reproduction is based.

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Lesson topic. Cell division. Mitosis

The purpose of the lesson: to characterize the main method of division of eukaryotic cells - mitosis, to reveal the features of the course of each phase of mitosis, to create an idea of amitosis.

Tasks:

to form knowledge about the significance of division for the growth, development, reproduction of the cell and the organism as a whole; consider the mechanism of mitosis;
characterize the main stages of the cell and mitotic cycle;
improve the skills of working with a microscope;
reveal the biological significance of mitosis.

Resources: computer, microscopes, microslides “Mitosis in onion root cells”, interactive whiteboard, multimedia presentation “Cell division. Mitosis”, disk – “laboratory workshop Biology grades 6-11”, video “Stages of mitosis”, dynamic manual “Mitosis”.

Lesson stages

1. Organizational moment.

Setting the goal of the lesson, defining the problem and topic of the lesson.

At the time of birth, a child weighs an average of 3-3.5 kg and is about 50 cm tall, a brown bear cub whose parents reach a weight of 200 kg or more weighs no more than 500 g, and a tiny kangaroo weighs less than 1 gram. A beautiful swan grows from a gray nondescript chick, a nimble tadpole turns into a sedate toad, and a huge oak tree grows from an acorn planted near the house, which a hundred years later pleases new generations of people with its beauty.

Problem question. Through what processes are all these changes possible? (Slide1)

All these changes are possible due to the ability of organisms to grow and develop. The tree will not turn into a seed, the fish will not return to the eggs - the processes of growth and development are irreversible. These two properties of living matter are inextricably linked with each other, and they are based on the ability of the cell to divide and specialize. . What is the topic of the lesson? (Slide 2)

The topic of the lesson is “Cell division. Mitosis" (Slide 3)

To start studying a new topic, we need to recall the previously studied material (Slides 4,5,6)

2. Learning new material.

TYPES OF CELL DIVISION (Slide 7)

One of the provisions of the cell theory is based on the conclusion of the German scientist Rudolf Virchow "Every cell from a cell." This was the beginning of the study of the processes of cell division, the main regularities of which were revealed at the end of the 19th century.

Reproduction is one of the most important properties of living organisms. All living organisms, without exception, are capable of reproduction, from bacteria to mammals. Methods of reproduction various organisms can be very different from each other, but cell division is the basis of any type of reproduction. The lifespan of a multicellular organism exceeds the lifespan of most of its constituent cells. So, nerve cells stop dividing even during prenatal development. Once having arisen, the cells no longer divide, forming striated muscle tissues in animals and storage tissues in plants. Multicellular organisms grow, develop, they undergo renewal of cells and tissues, even parts of the body (Remember regeneration) It is known that cells grow old and die. For example, liver cells live 18 months, erythrocytes - 4 months, intestinal epithelium 1-2 days (about 70 billion people die every day).

intestinal epithelial cells and 2 billion erythrocytes). This means that cells are constantly being renewed in the body. It is also known that, on average, 1 time in 7 years, cells are updated. Therefore, almost all cells of multicellular organisms must divide in order to replace dying cells. All new cells arise by division from an existing cell.

AMITOSIS. Direct division of the interphase nucleus by constriction without the formation of a fission spindle (chromosomes are generally indistinguishable in a light microscope). Such a division occurs in unicellular organisms (for example, polyploid large ciliate nuclei divide by amitosis), as well as in some highly specialized cells of plants and animals with weakened physiological activity, degenerating, doomed to death, or with various pathological processes such as malignant growth, inflammation, etc. After amitosis, the cell is not able to enter into mitotic division.

MITOSIS (from the Greek. Mitos-thread) indirect division, is the main way of dividing eukaryotic cells. Mitosis is the process of cell division, as a result of which daughter cells receive genetic material identical to that contained in the mother cell.

MEIOSIS (indirect division) is special way cell division, which results in a reduction (reduction) in the number of chromosomes by half. During meiosis, two cell divisions occur and one diploid cell(2n2c) four haploid (nc) sex cells are formed. In the course of the further process of fertilization (fusion of gametes), the organism of a new generation will again receive a diploid set of chromosomes, i.e., the karyotype of organisms of a given species remains constant in a number of generations.

Conclusion: There are three types of cell division, thanks to which organisms grow, develop, multiply (amitosis, mitosis, meiosis).

Mitosis is the main mode of cell division.

Mitosis (from the Greek mitos - thread) - indirect cell division. It ensures the uniform transmission of the hereditary information of the mother cell to two daughter cells.

It is thanks to this type of cell division that almost all cells of a multicellular organism are formed.

The mitotic (cellular) cycle consists of a preparatory stage (interphase) and the actual division - mitosis (prophase, metaphase, anaphase and telophase).

characteristics of mitosis.

To study the topic, we will work in pairs.

EXERCISE 1.

1. Study the features of the first phase of mitosis - prophase.

2. Write down the features of prophase in your notebook after discussing the answer. (Slide 9)

TASK 2.

1. Study the features of the second phase of mitosis - metaphase.

2. Write down the features of metaphase in your notebook after discussing the answer. (Slide 10)

TASK 3.

1. Study the features of the third phase of mitosis - anaphase.

2. Write down the features of anaphase in a notebook after discussing the answer. (Slide 11)

TASK 4.

1. Study the features of the fourth phase of mitosis - telophase.

2. Write down the features of telophase in a notebook after discussing the answer. (Slide 12)

Guys! Now your attention will be presented to the video "MITOSIS". You need to carefully review it, and then complete the task. (Slide 12)

EXERCISE. Determine and write down the names of the phase corresponding to its description. (Slide 13)

3. Consolidation of the studied material.

LABORATORY WORK №5.(Slide 14.15)

Topic: “Mitosis in onion root cells”.

Target: to study the process of mitosis in onion root cells.

Equipment: light microscopes, micropreparations "Mitosis in onion root cells".

Progress

1. Consider the finished micropreparation, if possible, find cells at all stages of mitosis.

2. Compare the image under the microscope with the photomicrograph in the presentation for the lesson (slide).
3. Determine the set of chromosomes in each phase of mitosis.
4. Describe the features of each observed stage of mitosis.
5. Draw a conclusion about the role of mitosis.
Questions for consolidation.(Slide 16, 17, 18)

1. The total mass of all DNA molecules in 46 chromosomes of one human somatic cell is 6-10 "9 mg. What will be the mass of DNA molecules in: a) the metaphase of mitosis; b) the telophase of mitosis?

2. Consider if the conditions can environment affect the process of mitosis. What consequences for the body can this lead to?

3. Why are daughter cells formed during mitosis with a set of chromosomes equal to the set of chromosomes in the mother cell? What is the significance of this in the life of organisms?

4. Consider whether environmental conditions can affect the process of mitosis. What consequences for the body can this lead to?

5. Why are daughter cells formed during mitosis with a set of chromosomes equal to the set of chromosomes in the mother cell? What is the significance of this in the life of organisms?

At the end of the lesson, the results are summarized.

Mitosis is very meaningful process, a lot of time and effort was spent by scientists to understand all the features of this process. For example, it was found that mitosis in plant and animal cells proceeds with certain differences, that there are factors that adversely affect its course.

In addition, in the literature you can see another form of division - direct or amitosis. Work with additional literature.

Group 1: task "Amitosis"

Select "reference" points from the text, i.e. in 4-5 positions indicate the main signs of amitosis. “Mitosis is the most common, but not the only type of cell division. Almost all eukaryotes have the so-called direct nuclear fission, or amitosis. During amitosis, there is no condensation of chromosomes and no spindle is formed, and the nucleus is divided by constriction or fragmentation, remaining in the interphase state. Cytokinesis always follows nuclear division, resulting in the formation of a multinucleated cell. Amitotic division is typical for cells that complete development: dying epithelial, follicular cells of the ovaries ... Amitosis also occurs in pathological processes: inflammation, malignant neoplasm… after it the cells are not capable of mitotic division.”

Group 2: task "violation of mitosis"

Make logical pairs: type of impact - consequences.

“The correct course of mitosis can be disturbed by various external factors: high doses radiation, some chemicals. For example, under the influence x-rays The DNA of a chromosome can break, and the chromosomes break as well. Such chromosomes are not able to move, for example, in anaphase. Some chemical substances, not characteristic of living organisms (alcohols, phenols) violate the consistency of mitotic processes. Some chromosomes move faster, others slower. Some of them may not be included in child kernels at all. There are substances that prevent the formation of fission spindle filaments. They are called cytostatics, for example, colchicine and colcemide. By acting on the cell, division can be stopped at the prometaphase stage. As a result of such an impact, a double set of chromosomes appears in the nucleus.

Conclusions. (Slide 19)

Today the lesson was devoted to the most important process - mitosis. We devoted enough time to the process itself, its features and problems. Most importantly, this process ensures the genetic stability of the species, as well as the processes of regeneration, growth, and asexual (vegetative) reproduction. The process is complex, multistage and very sensitive to environmental factors.

Homework.

1. Study § 29

2. Fill in the table “Mitotic cell cycle”

Explain what determines the number of chromosomes in DNA at different stages of mitosis.

mitotic cell cycle

It is a continuous process, each stage of which imperceptibly passes into the next after it. There are four stages of mitosis: prophase, metaphase, anaphase and telophase (Fig. 1). The study of mitosis focuses on the behavior of chromosomes.

Prophase . At the beginning of the first stage of mitosis - prophase - cells retain the same appearance as in interphase, only the nucleus noticeably increases in size, and chromosomes appear in it. In this phase, it is seen that each chromosome consists of two chromatids, spirally twisted relative to each other. Chromatids shorten and thicken as a result of the process of internal spiralization. A weakly colored and less condensed region of the chromosome begins to be revealed - the centromere, which connects two chromatids and is located in a strictly defined place in each chromosome.

During prophase, the nucleoli gradually disintegrate: the nuclear membrane is also destroyed, and the chromosomes are in the cytoplasm. In the late prophase (prometaphase), the mitotic apparatus of the cell is intensively formed. At this time, the centriole divides, and the daughter centrioles diverge to opposite ends of the cell. Thin filaments in the form of rays depart from each centriole; spindle fibers form between the centrioles. There are two types of filaments: pulling filaments of the spindle, attached to the centromeres of chromosomes, and supporting filaments, connecting the poles of the cell.

When the reduction of chromosomes reaches its maximum degree, they turn into short rod-shaped bodies and go to the equatorial plane of the cell.

metaphase . In metaphase, the chromosomes are completely located in the equatorial plane of the cell, forming the so-called metaphase or equatorial plate. The centromere of each chromosome, which holds both chromatids together, is located strictly in the region of the equator of the cell, and the arms of the chromosomes are extended more or less parallel to the spindle threads.

In metaphase, the shape and structure of each chromosome is well revealed, the formation of the mitotic apparatus is completed, and the pulling threads are attached to the centromeres. At the end of metaphase, the simultaneous division of all the chromosomes of a given cell occurs (and the chromatids turn into two completely separate daughter chromosomes).

Anaphase. Immediately after the division of the centromere, the chromatids repel each other and diverge to opposite poles of the cell. All chromatids begin to move towards the poles at the same time. Centromeres play an important role in the oriented movement of chromatids. In anaphase, the chromatids are called sister chromosomes.

The movement of sister chromosomes in anaphase occurs due to the interaction of two processes: contraction of the pulling and lengthening of the supporting threads of the mitotic spindle.

Telophase. At the beginning of telophase, the movement of sister chromosomes ends, and they are concentrated at the poles of the cell in the form of compact formations and clots. Chromosomes despiralize and lose their visible individuality. A nuclear envelope is formed around each daughter nucleus; the nucleoli are restored in the same amount as they were in the mother cell. This completes the division of the nucleus (karyokinesis), cell wall. Simultaneously with the formation of daughter nuclei in telophase, the entire contents of the original mother cell are separated, or cytokinesis.

When a cell divides, a constriction or groove appears on its surface near the equator. It gradually deepens and divides the cytoplasm into

two daughter cells, each with a nucleus.

In the process of mitosis, two daughter cells arise from one mother cell, containing the same set of chromosomes as the original cell.

Figure 1. Scheme of mitosis

The biological significance of mitosis . The main biological significance of mitosis is the precise distribution of chromosomes between two daughter cells. A regular and orderly mitotic process ensures the transfer of genetic information to each of the daughter nuclei. As a result, each daughter cell contains genetic information about all the characteristics of the organism.

Meiosis is a special division of the nucleus, which ends with the formation of a tetrad, i.e. four cells with a haploid set of chromosomes. Sex cells divide by meiosis.

Meiosis consists of two cell divisions in which the number of chromosomes is halved so that the gametes receive half as many chromosomes as the rest of the cells in the body. When two gametes unite at fertilization, the normal number of chromosomes is restored. The decrease in the number of chromosomes during meiosis does not occur randomly, but quite naturally: the members of each pair of chromosomes diverge into different daughter cells. As a result, each gamete contains one chromosome from each pair. This is carried out by pairwise connection of similar or homologous chromosomes (they are identical in size and shape and contain similar genes) and the subsequent divergence of the members of the pair, each of which goes to one of the poles. During the convergence of homologous chromosomes, crossing over can occur, i.e. mutual exchange of genes between homologous chromosomes, which increases the level of combinative variability.

In meiosis, a number of processes occur that are important in the inheritance of traits: 1) reduction - a halving of the number of chromosomes in cells; 2) conjugation of homologous chromosomes; 3) crossing over; 4) random segregation of chromosomes into cells.

Meiosis consists of two successive divisions: the first, which results in the formation of a nucleus with a haploid set of chromosomes, is called reduction; the second division is called equational and proceeds according to the type of mitosis. In each of them, prophase, metaphase, anaphase and telophase are distinguished (Fig. 2). The phases of the first division are usually denoted by the number Ι, the second - P. Between Ι and P divisions, the cell is in a state of interkinesis (lat. inter - between + gr. kinesis - movement). In contrast to interphase, DNA is not re(du) replicated in interkinesis and chromosome material is not duplicated.

Figure 2. Scheme of meiosis

Reduction division

Prophase Ι

The phase of meiosis during which complex structural transformations of chromosomal material occur. It is longer and consists of a number of successive stages, each of which has its own distinctive properties:

- leptotena - the stage of leptonema (connection of threads). Individual threads - chromosomes - are called monovalents. Chromosomes in meiosis are longer and thinner than chromosomes in the earliest stage of mitosis;

- zygotene - the stage of zygonema (connection of threads). There is a conjugation, or synapsis (connection in pairs), of homologous chromosomes, and this process is carried out not just between homologous chromosomes, but between exactly corresponding individual points of homologues. As a result of conjugation, bivalents are formed (complexes of pairwise homologous chromosomes connected in pairs), the number of which corresponds to the haploid set of chromosomes.

Synapsis is carried out from the ends of chromosomes, therefore, the localization sites of homologous genes in one or another chromosome coincide. Since the chromosomes are doubled, there are four chromatids in the bivalent, each of which eventually turns out to be a chromosome.

- pachytene - the stage of pachinema (thick filaments). The size of the nucleus and nucleolus increase, the bivalents shorten and thicken. The connection of homologues becomes so close that it is already difficult to distinguish between two separate chromosomes. At this stage, crossing over occurs, or chromosomes cross over;

- diplotene - the stage of diplonema (double strands), or the stage of four chromatids. Each of the homologous chromosomes of the bivalent splits into two chromatids, so that the bivalent contains four chromatids. Although the tetrads of chromatids move away from each other in some places, they are in close contact in other places. In this case, the chromatids of different chromosomes form X-shaped figures, called chiasms. The presence of the chiasma holds the monovalents together.

Simultaneously with the continuing shortening and, accordingly, thickening of the chromosomes of the bivalent, their mutual repulsion occurs - divergence. The connection is preserved only in the plane of the intersection - in the chiasms. The exchange of homologous regions of chromatids is completed;

- diakinesis is characterized by the maximum shortening of diploten chromosomes. Bivalents of homologous chromosomes go to the periphery of the nucleus, so they are easy to count. The nuclear envelope is fragmented, the nucleoli disappear. This completes prophase 1.

Metaphase Ι

- begins with the disappearance of the nuclear envelope. The formation of the mitotic spindle is completed, the bivalents are located in the cytoplasm in the equatorial plane. Chromosome centromeres attach to the pulling filaments of the mitotic spindle but do not divide.

Anaphase Ι

- is distinguished by the complete termination of the relationship of homologous chromosomes, their repulsion from one another and the divergence to different poles.

Note that during mitosis, single-chromatid chromosomes diverged to the poles, each of which consists of two chromatids.

Thus, it is anaphase that reduction occurs - the preservation of the number of chromosomes.

Telophase Ι

- it is very short-term and weakly isolated from the previous phase. Telophase 1 produces two daughter nuclei.

Interkinesis

This is a short resting state between 1 and 2 divisions. Chromosomes are weakly despiralized, DNA replication does not occur, since each chromosome already consists of two chromatids. After interkinesis, the second division begins.

The second division occurs in both daughter cells in the same way as in mitosis.

Prophase P

In the nuclei of cells, chromosomes are clearly manifested, each of which consists of two chromatids connected by a centromere. They look like rather thin filaments located along the periphery of the nucleus. At the end of prophase P, the nuclear envelope fragments.

Metaphase P

In each cell, the formation of a division spindle is completed. Chromosomes are located along the equator. Spindle filaments are attached to the centromeres of chromosomes.

Anaphase P

The centromeres divide and the chromatids usually move rapidly to opposite poles of the cell.

Telophase P

Sister chromosomes concentrate at the poles of the cell and despiralize. The nucleus and cell membrane are formed. Meiosis ends with the formation of four cells with a haploid set of chromosomes.

The biological significance of meiosis

Like mitosis, meiosis ensures the precise distribution of genetic material into daughter cells. But, unlike mitosis, meiosis is a means of increasing the level of combinative variability, which is explained by two reasons: 1) there is a free, based on chance, combination of chromosomes in cells; 2) crossing over, leading to the emergence of new combinations of genes within chromosomes.

In each next generation of dividing cells, as a result of the action of these causes, new combinations of genes in gametes are formed, and during the reproduction of animals, new combinations of parental genes in their offspring are formed. This each time opens up new possibilities for the action of selection and the creation of genetically different forms, which allows a group of animals to exist in variable environmental conditions.

Thus, meiosis turns out to be a means of genetic adaptation that increases the reliability of the existence of individuals in generations.

Cell division is the central moment of reproduction.

In the process of division, two cells arise from one cell. A cell, based on the assimilation of organic and inorganic substances, creates its own kind with a characteristic structure and functions.

During mitosis, the following occurs:

doubling of the substance of the chromosomes;
changes in the physical state and chemical organization of chromosomes;
divergence of daughter, or rather sister, chromosomes to the poles of the cell;
the subsequent division of the cytoplasm and the complete restoration of two new nuclei in sister cells.

Thus, the entire life cycle of nuclear genes is laid down in mitosis: duplication, distribution, and functioning; as a result of the completion of the mitotic cycle, sister cells end up with an equal “heritage”.

When dividing, the cell nucleus goes through five successive stages: interphase, prophase, metaphase, anaphase and telophase; some cytologists distinguish another sixth stage - prometaphase.

Between two successive cell divisions, the nucleus is in the interphase stage. During this period, the nucleus, during fixation and coloring, has a mesh structure formed by dyeing thin threads, which in the next phase form into chromosomes. Although the interphase is called differently resting nucleus phase, on the body itself, metabolic processes in the nucleus during this period are performed with the greatest activity.

Prophase is the first stage in the preparation of the nucleus for division. In prophase, the network structure of the nucleus gradually turns into chromosome threads. From the earliest prophase, even in a light microscope, one can observe the dual nature of chromosomes. This suggests that in the nucleus, it is in the early or late interphase that the most important process of mitosis takes place - doubling, or reduplication, of chromosomes, in which each of the maternal chromosomes builds a similar one - a daughter one. As a result, each chromosome looks longitudinally doubled. However, these halves of chromosomes, which are called sister chromatids, do not diverge in prophase, as they are held together by one common area - the centromere; the centromeric region is divided later. In prophase, the chromosomes undergo a process of twisting along their axis, which leads to their shortening and thickening. It should be emphasized that in prophase each chromosome in the karyolymph is located randomly.

Recently, between prophase and metaphase, researchers have begun to distinguish an intermediate stage called prometaphase. Prometaphase is characterized by the dissolution and disappearance of the nuclear membrane and the movement of chromosomes towards the equatorial plane of the cell. But by this time, the formation of the achromatin spindle has not yet been completed.

In telophase, the daughter chromosomes despiralize and lose their visible individuality. The shell of the nucleus and the nucleus itself are formed. The nucleus is reconstructed in the reverse order compared to the changes that it underwent in prophase. In the end, the nucleoli (or nucleolus) are also restored, and in the amount in which they were present in the parent nuclei. The number of nucleoli is characteristic of each cell type.

At the same time, the symmetrical division of the cell body begins. The nuclei of the daughter cells enter the state of interphase.

The figure above shows a diagram of the cytokinesis of animal and plant cells. In an animal cell, division occurs by ligation of the cytoplasm of the mother cell. In a plant cell, the formation of a cell septum occurs with areas of spindle plaques that form a septum in the plane of the equator, called a phragmoplast. This ends the mitotic cycle. Its duration apparently depends on the type of tissue, the physiological state of the organism, external factors (temperature, light regimen) and lasts from 30 minutes to 3 hours. According to various authors, the speed of passage of individual phases is variable.

Both internal and external environmental factors affecting the growth of the organism and its functional state affect the duration of cell division and its individual phases. Since the nucleus plays a huge role in the metabolic processes of the cell, it is natural to believe that the duration of the phases of mitosis can change in accordance with the functional state of the organ tissue. For example, it has been established that the mitotic activity of various tissues during rest and sleep in animals is significantly higher than during wakefulness. In a number of animals, the frequency of cell divisions decreases in the light, and increases in the dark. It is also assumed that hormones influence the mitotic activity of the cell.

The reasons that determine the readiness of the cell for division are still unclear. There are reasons to assume several such reasons:

doubling of the mass of cellular protoplasm, chromosomes and other organelles, due to which nuclear-plasma relations are violated; for division, a cell must reach a certain weight and volume characteristic of the cells of a given tissue;
duplication of chromosomes;
secretion by chromosomes and other cell organelles of special substances that stimulate cell division.

The nature of mitosis, as we have already said, varies depending on the type and functional state of the tissue. Cells of different tissues are characterized by different types of mitosis. In the described type of mitosis, cell division occurs in an equal and symmetrical manner. As a result of symmetrical mitosis, sister cells are hereditarily equivalent in respect of both nuclear genes and cytoplasm. However, in addition to symmetrical, there are other types of mitosis, namely: asymmetric mitosis, mitosis with delayed cytokinesis, division of multinucleated cells (syncytia division), amitosis, endomitosis, endoreproduction and polythenia.

Endomitosis called such a process when a doubling of nuclear fission occurs. In this case, the chromosomes, as usual, are reproduced in the interphase, but their subsequent divergence occurs inside the nucleus with the preservation of the nuclear envelope and without the formation of an achromatin spindle. In some cases, although the shell of the nucleus dissolves, however, the divergence of chromosomes to the poles does not occur, as a result of which the number of chromosomes in the cell multiplies even by several tens of times. Endomitosis occurs in cells of various tissues of both plants and animals. So, for example, A. A. Prokofieva-Belgovskaya showed that by endomitosis in the cells of specialized tissues: in the cyclops hypodermis, fat body, peritoneal epithelium and other tissues of the filly (Stenobothrus) - the set of chromosomes can increase 10 times. This multiplication of the number of chromosomes is associated with the functional features of the differentiated tissue.

Sometimes there is a duplication of one or more chromosomes without any transformation of the nucleus - this phenomenon is called endoreproduction.

So, all phases of cell mitosis that make up are mandatory only for a typical process.

Embryonic and meristematic cells, which have not lost the function of reproducing the whole organism and belonging to undifferentiated tissues, retain the full cycle of mitosis, on which asexual and vegetative reproduction is based.