• 1) In prophase, the volume of the nucleus increases, and due to the spiralization of chromatin, chromosomes are formed. By the end of prophase, each chromosome is seen to consist of two chromatids. Gradually, the nucleoli and nuclear membrane dissolve, and the chromosomes are randomly located in the cytoplasm of the cell. In the cytoplasm of the cell there is a small granular body called the centriole. At the beginning of prophase, the centriole divides, and the daughter centrioles move to opposite ends of the cell. Thin filaments in the form of rays depart from each centriole, forming a star; a spindle arises between the centrioles, consisting of a number of protoplasmic filaments called spindle filaments. These filaments are built from a protein similar in properties to the contractile proteins of muscle fibers. They are arranged in the form of two cones, folded base to base, so that the spindle is narrow at the ends, or poles, near the centrioles, and wide in the center, or at the equator. The threads of the spindle stretch from the equator to the poles; they consist of the denser protoplasm of the nucleus. The spindle is a specific structure: with the help of a micromanipulator, a thin needle can be inserted into the cell and the spindle can be moved with it. Spindles isolated from dividing cells contain protein, mostly one kind of protein, as well as a small amount of RNA. As the centrioles separate and the spindle forms, the chromosomes in the nucleus shorten, become shorter and thicker. If earlier it could not be seen that they consist of two elements, now it is clearly noticeable.
• 2) Prometaphase begins with the rapid disintegration of the nuclear envelope into small fragments indistinguishable from fragments of the endoplasmic reticulum. Chromosomes on each side of the centromere in prometaphase form special structures called kinetochores. They attach to a special group of microtubules called kinetochore filaments or kinetochore microtubules. These filaments extend from both sides of each chromosome, run in opposite directions, and interact with the filaments of the bipolar spindle. In this case, the chromosomes begin to move intensively.
• 3) Metaphase. Chromatids are attached to spindle fibrils by kinetochores. Once connected to both centrosomes, the chromatids move toward the equator of the spindle until their centromeres line up along the equator of the spindle perpendicular to its axis. This allows the chromatids to move freely towards their respective poles. The placement of chromosomes characteristic of metaphase is very important for chromosome segregation, i.e. segregation of sister chromatids. If an individual chromosome “slows down” in its movement towards the spindle equator, the onset of anaphase is usually delayed as well. Metaphase ends with the separation of sister chromatids.
• 4) Anaphase usually lasts only a few minutes. Anaphase begins with a sudden splitting of each chromosome, which is caused by the separation of sister chromatids at their junction point at the centromere.

This kinetochore-separating cleavage is independent of other mitotic events and occurs even in chromosomes not attached to the mitotic spindle. It allows the polar forces of the spindle acting on the metaphase plate to start moving each chromatid towards the respective spindle poles at a rate of about 1 µm/min. If there were no spindle threads, then the chromosomes would be pushed in all directions, but due to the presence of these threads, one complete set of daughter chromosomes is collected at one pole, and the other at the other. During the movement to the poles, the chromosomes usually take a V-shape, with their top facing the pole. The centromere is located at the top, and the force that makes the chromosome move towards the pole is applied to the centromere. Chromosomes that have lost their centromere during mitosis do not move at all.

5) Telophase begins after the daughter chromosomes, consisting of one chromatid, have reached the poles of the cell. At this stage, the chromosomes despiralize again and acquire the same form as they had before the cell division began in the interphase (long thin filaments). A nuclear envelope arises around them, and a nucleolus is formed in the nucleus, in which ribosomes are synthesized. In the process of cytoplasm division, all organelles are distributed more or less evenly between daughter cells. This completes nuclear division, also called karyokinesis; then the cell body divides, or cytokinesis.

Table 2. Phases of mitosis

In most cases, the entire process of mitosis takes from 1 to 2 hours. In plants, division occurs through the formation of a so-called cell plate that separates the cytoplasm; it arises in the equatorial region of the spindle, and then grows in all directions, reaching the cell wall. The material of the cell plate is produced by the endoplasmic reticulum. Then each of the daughter cells forms a cytoplasmic membrane on its side of the cell plate, and, finally, cellulose cell walls are formed on both sides of the plate.

The frequency of mitoses in different tissues and in different types sharply different. For example, in the human red bone marrow, where 10,000,000 red blood cells are formed every second, 10,000,000 mitoses should occur every second.

Section Code	Controlled element code	content elements, knowledge-verifiable CMMs
2	Cell as a biological system
		Chromosomes, their structure (shape and size) and functions. The number of chromosomes and their species constancy. Determination of the set of chromosomes in somatic and germ cells. Cell life cycle: interphase and mitosis. Mitosis is the division of somatic cells. Meiosis. Phases of mitosis and meiosis. The development of germ cells in plants and animals. Similarities and differences between mitosis and meiosis, their significance. Cell division is the basis for the growth, development and reproduction of organisms.

Part A

1. What cell structures are distributed strictly evenly between daughter cells during mitosis:

1) ribosomes 3) chloroplasts

2) mitochondria 4) chromosomes

2. Attachment of spindle fibers to chromosomes occurs in:

1) interphase 3) metaphase

2) prophase 4) anaphase

3. In the prophase of mitosisnot happening :

1) dissolution of the nuclear envelope

2) spindle formation

3) DNA doubling

4) dissolution of the nucleoli

4. The divergence of chromatids to the poles of the cell occurs in:

1) anaphase 3) prophase

2) telophase 4) metaphase

5. The chromosome set in the cells of the body is called:

1) karyotype 3) genotype

2) phenotype 4) genome

6. The cell center in the process of mitosis is responsible for:

1) protein biosynthesis

2) spiralization of chromosomes

3) movement of the cytoplasm

4) formation of the fission spindle

7. New somatic cells in a multicellular animal organism are formed as a result of:

1) meiosis 3) oogenesis

2) mitosis 4) spermatogenesis

8. DNA doubling and the formation of two chromatids occurs in:

1) prophase of the first division of meiosis

2) prophase of the second division of meiosis

3) interphase before the first division

4) interphase before the second division

9. The formation of two chromatids in chromosomes is based on the process:

1) DNA self-duplication 3) DNA helix

2) i-RNA synthesis 4) formation of ribosomes

10. Preservation of a constant number of chromosomes in cells during vegetative propagation is ensured by:

1) meiotic division 3) mitotic division

2) the movement of the cytoplasm 4) spermatogenesis

11. The divergence of homologous chromosomes occurs in:

1) anaphase of meiosisI3) metaphase of meiosisII

2) metaphase of meiosisI4) anaphase of meiosisII

12. By what signs can you recognize the anaphase of mitosis:

1) random arrangement of spiralized chromosomes in the cytoplasm

2) alignment of chromosomes in the equatorial plane of the cell

3) divergence of daughter chromatids to opposite poles of the cell

4) despiralization of chromosomes and the formation of nuclear membranes around two nuclei

13. In the telophase of mitosis, the following occurs:

1) DNA duplication

2) spiralization of chromosomes

3) divergence of homologous chromosomes

4) formation of nuclei of daughter cells

14. Meiosis differs from mitosis:

1) the process of crossing over and conjugation of chromosomes

2) the presence of prophase, metaphase, anaphase and telophase

3) shorter duration

4) the presence of a division spindle

15. In the anaphase of mitosis, the following occurs:

1) spiralization of homologous chromosomes

2) divergence of homologous chromosomes

3) division of the cytoplasm

4) DNA doubling

16. Spiralization of chromosomes during mitosis occurs in:

1) anaphase 3) telophase

2) metaphase 4) prophase

17. In the prophase of mitosisnot happening :

1) spiralization of chromosomes

2) restoration of the nuclear envelope

3) formation of the fission spindle

4) dissolution of the nuclear membrane

18. In the cell cycle, DNA replication occurs in:

1) interphase 3) metaphase

2) prophase 4) anaphase

19. Division by mitosis is not typical for cells:

1) red algae

2) hydras

3) E. coli

4) mukora

20. Chromosomes that are the same in females and males are called:

1) sex chromosomes 3) ribosomes

2) autosomes 4) lysosomes

21. During the first division of meiosis, the following cells diverge to the poles of the dividing cell:

1) whole chromosomes from homologous pairs

2) sister chromatids

3) fragments of chromosomes from homologous pairs

4) fragments of non-homologous chromosomes

22. During mitosis, chromosomes line up on the cell equator during:

1) telophase 3) metaphase

2) prophase 4) anaphase

23. Unlike mitosis, meiosis:

1) consists of two divisions

2) is not accompanied by spiralization of chromosomes

3) characteristic of bacterial cells

4) observed in viruses

24. The chromosome constriction connecting two chromatids is called:

1) centrosome 3) centromere

2) acrosome 4) centriole

25. Human somatic cells contain:

1) 46 pairs of chromosomes 3) 23 pairs of chromosomes

2) 92 pairs of chromosomes 4) 32 pairs of chromosomes

26. Prophase IMeiosis is different from prophase of mitosis:

1) spiralization of chromosomes

2) the presence of conjugation and crossing over

3) the formation of a fission spindle

4) destruction of chromosomes

27. Division by mitosis is not typical for cells:

1) protozoa 3) fungi

2) bacteria 4) plants

28. The sequence of stages of mitosis is as follows:

1) metaphase, telophase, prophase, anaphase 3) prophase, metaphase, telophase, anaphase

2) prophase, metaphase, anaphase, telophase 4) telophase, prophase, metaphase, anaphase.

29. The longest phase of mitosis is:

1) prophase 3) anaphase

2) metaphase 4) telophase.

30. During mitosis, the divergence of homologous chromosomes to the poles of the cell occurs in:

1) prophase 3) anaphase

2) metaphase 4) no correct answer

31. During mitosis, the division of the cytoplasm of the cell occurs in:

1) interphase 3) metaphase

2) prophase 4) telophase

32. Doubling of chromosomes occurs in:

1) interphase 3) metaphase

2) prophase 4) telophase

33. Reduction of the number of chromosomes occurs during:

1) anaphase of mitosis 3) II division of meiosis

2) I division of meiosis 4) in all the above cases.

34. Crossing of chromosomes occurs in the process:

1) mitosis 3) DNA replication

2) meiosis 4) transcription.

35. In the anaphase of mitosis, a divergence occurs:

1) daughter chromosomes 3) non-homologous chromosomes

2) homologous chromosomes 4) cell organelles

36. Bivalents are called:

1) constrictions in the chromosomes to which the fission spindle threads are attached

2) halves of chromosomes that diverge during mitosis

3) fused homologous chromosomes during meiosis

4) despiralized, invisible under a microscope chromosomes

37. The biological significance of meiosis is to ensure:

1) genetic stability

2) tissue regeneration and increase in the number of cells in the body

3) genetic variability

4) asexual reproduction

38. As a result of mitosis, the following are formed:

1) somatic cells

2) eggs

3) sperm

4) all listed cells

39. A set of chromosomes in which each chromosome has a paired homologous chromosome is called:

1) haploid

2) diploid

3) triploid

4) tetraploid

40. During the development of germ cells in animals, cell division occurs in the gonads in the reproduction zone6

1) meiosis

2) mitosis

3) amitosis

4) simple binary division

41. During the formation of gametes in humans, reduction division occurs at the stage:

1) reproduction 3) maturation

2) growth 4) formation

42. In animals, in the process of mitosis, unlike meiosis, cells are formed:

1) somatic

2) with a half set of chromosomes

3) sexual

4) spore

43. mitosis in a multicellular organism is the basis of:

1) gametogenesis

2) growth and development

3) metabolism

4) self-regulation processes

44. In the process of mitosis, each daughter cell receives the same set of chromosomes as the mother cell, because:

1) in prophase, chromosomes spiralize

2) despiralization of chromosomes occurs

3) in the interphase, DNA doubles itself, two chromatids are formed in each chromosome

4) each cell contains two homologous chromosomes

Part B

Choose three correct answers from six.

1. The biological significance of meiosis is:

1) reduction in the number of chromosomes

2) the formation of male and female gametes

3) the formation of somatic cells

4) creating opportunities for the emergence of new gene combinations

5) increase in the number of cells in the body

6) a multiple increase in the set of chromosomes

2. During mitosis does not occur:

1) spiralization of chromosomes

2) divergence of chromosomes to the poles of a dividing cell

3) crossing over

4) DNA replication

5) photolysis of water

6) the formation of a fission spindle

3.Oogenesis is characterized by:

1) the presence of the formation stage

2) accumulation nutrients in the oocyte of the first order

3) the formation of four germ cells

4) death of polar bodies

5) the occurrence of multiple mitotic divisions at the stage of maturation

6) the course of multiple meiotic divisions at the stage of maturation

4. Oogenesis in contrast to spermatogenesis:

1) has a more pronounced growth stage

2) does not contain a reproduction stage

3) does not contain a formation stage

4) ends with the formation of one sex cell

5) at the stage of maturation is represented by mitosis

6) in humans ends in the embryonic period

5. The egg, unlike the sperm, is characterized by:

1) haploid set of chromosomes

2) diploid set of chromosomes

3) a large supply of nutrients

4) larger sizes

5) immobility

6) active movement

Tasks for establishing the sequence of biological objects, processes, phenomena. Write the answer as a sequence of letters.

1. Specify the sequence of cell formation during spermatogenesis:

A) spermatids
B) spermatogonia
C) spermatocytes of the 2nd order
D) spermatozoa
D) primary germ cells
E) spermatocytes of the 1st order

2. Indicate the sequence of phenomena and processes occurring in preparation for mitosis and during it.

A) the divergence of daughter chromatids to the poles of the cell

B) spiralization of chromosomes

C) despiralization of chromosomes
D) duplication of cellular DNA
E) formation of interphase nuclei of daughter cells
E) attachment of chromosomes to the threads of the spindle of division

3. Indicate the sequence of phenomena and processes occurring in the process of meiosis.

A) separation of chromatids
B) conjugation of homologous chromosomes
C) the formation of four haploid cells
D) spiralization of chromosomes of a dividing diploid cell
D) divergence of homologous chromosomes
E) exchange of sites between homologous chromosomes

Matching tasks. The answer must be written as a sequence of numbers.

1. Establish a correspondence between the phase of mitosis and the events that occur during it:

2. Indicate the correspondence between the phase of gametogenesis and the events occurring during it:

Part C

1. What are the mechanisms that ensure the constancy of the number of chromosomes in offspring during sexual reproduction?

Answers.

1.-4 2.-3 3.-3 4.- 4 5.-1 6.-4 7.-2 8.-3 9.-1 10.-3

11.-1 12.-3 13.-4 14.-1 15.-2 16.-3 17.-2 18.-1 19.-3 20.-2

21.-1 22.-3 23.-4 24.-3 25.-3 26.-2 27.-4 28.-2 29.-1 30.-4

31.-4 32.-1 33.-2 34.-2 35.-1 36.-3 37.-3 38.-1 39.-2 40.-2

41.-3 42.-1 43.-2 44.-3

3 of 6:

Letter sequence:

B1- DBEVAG

B2- HBEAVD

B3- GBEDAV

For compliance:

C1:

The regular divergence of chromosomes during meiosis ensures the exact distribution of the haploid number of chromosomes among gametes.

During fertilization, the zygote restores a diploid set of chromosomes corresponding to the parental set.

Subsequent mitotic divisions provide the same number of chromosomes in the cells of the body of the offspring, including in the cells of the precursors of germ cells.

Cell reproduction is one of the most important biological processes. necessary condition the existence of all living things. Reproduction is carried out by dividing the original cell.

Cell- this is the smallest morphological unit of the structure of any living organism, capable of self-production and self-regulation. The time of its existence from division to death or subsequent reproduction is called the cell cycle.

Tissues and organs are made up of various cells that have their own period of existence. Each of them grows and develops to ensure the vital activity of the organism. The duration of the mitotic period is different: blood and skin cells enter the process of division every 24 hours, and neurons are capable of reproduction only in newborns, and then completely lose their ability to reproduce.

There are 2 types of division - direct and indirect. Somatic cells reproduce indirectly; gametes or germ cells are characterized by meiosis (direct division).

Mitosis - indirect division

Mitotic cycle

The mitotic cycle includes 2 consecutive stages: interphase and mitotic division.

Interphase(rest stage) - preparation of the cell for further division, where duplication of the source material is performed, followed by its uniform distribution among the newly formed cells. It includes 3 periods:

• • Presynthetic(G-1) G - from the English gar, that is, a gap, preparations are underway for the subsequent synthesis of DNA, the production of enzymes. The inhibition of the first period was experimentally carried out, as a result of which the cell did not enter the next phase.
  • Synthetic(S) - the basis of the cell cycle. Replication of chromosomes and centrioles of the cell center occurs. Only after that the cell can proceed to mitosis.
  • Postsynthetic(G-2) or pre-mitotic period - there is an accumulation of mRNA, which is needed for the onset of the actual mitotic stage. In the G-2 period, proteins (tubulins) are synthesized - the main component of the mitotic spindle.

After the end of the premitotic period, mitotic division. The process includes 4 phases:

1. Prophase- during this period, the nucleolus is destroyed, the nuclear membrane (nucleolema) dissolves, centrioles are located at opposite poles, forming an apparatus for division. It has two subphases:
   • early- thread-like bodies (chromosomes) are visible, they are not yet clearly separated from each other;
   • late- separate parts of chromosomes are traced.
2. metaphase- begins from the moment of destruction of the nucleolema, when the chromosomes lie randomly in the cytoplasm and only begin to move towards the equatorial plane. All pairs of chromatids are connected to each other at the centromere.
3. Anaphase- at one moment all the chromosomes are separated and move to opposite points of the cell. This is a short and very important phase, since it is in it that the exact division of the genetic material takes place.
4. Telophase- chromosomes stop, the nuclear membrane, the nucleolus, is formed again. A constriction is formed in the middle, it divides the body of the mother cell into two daughter cells, completing the mitotic process. In the newly formed cells, the G-2 period begins again.

Meiosis - direct division

Meiosis - direct division

There is a special process of reproduction that occurs only in germ cells (gametes) - this meiosis (direct division). hallmark for him is the absence of interphase. Meiosis from one original cell produces four, with a haploid set of chromosomes. The whole process of direct division includes two successive stages, which consist of prophase, metaphase, anaphase and telophase.

Before the start of prophase, the germ cells double the initial material, thus, it becomes tetraploid.

Prophase 1:

1. Leptotena- chromosomes are visible in the form of thin threads, they are shortened.
2. Zygoten- the stage of conjugation of homologous chromosomes, as a result, bivalents are formed. Conjugation important point In meiosis, chromosomes move as close as possible to each other in order to cross over.
3. Pachytene- there is a thickening of chromosomes, their increasing shortening, there is a crossing over (the exchange of genetic information between homologous chromosomes, this is the basis of evolution and hereditary variability).
4. Diploten- the stage of doubled strands, the chromosomes of each bivalent diverge, keeping the connection only in the area of ​​​​the decussation (chiasm).
5. diakinesis- DNA begins to condense, chromosomes become very short and diverge.

Prophase ends with the destruction of the nucleolema and the formation of the spindle.

Metaphase 1: bivalents are located in the middle of the cell.

Anaphase 1: Doubled chromosomes move to opposite poles.

Telophase 1: the division process is completed, the cells receive 23 bivalents.

Without subsequent doubling of the material, the cell enters into second phase division.

Prophase 2: all the processes that were in prophase 1 are repeated again, namely the condensation of chromosomes, which are randomly located between the organelles.

Metaphase 2: two chromatids connected at the intersection (univalents) are located in the equatorial plane, creating a plate called metaphase.

Anaphase 2:- the univalent is divided into separate chromatids or monads, and they go to different poles of the cell.

Telophase 2: the division process is completed, the nuclear envelope is formed, and each cell receives 23 chromatids.

Meiosis is an important mechanism in the life of all organisms. As a result of this division, we get 4 haploid cells that have half desired set chromatids. During fertilization, two gametes form a complete diploid cell, retaining its inherent karyotype.

It is difficult to imagine our existence without meiotic division, otherwise all organisms with each subsequent generation would receive double sets of chromosomes.

Mitosis- this is a cell division in which the daughter cells are genetically identical to the parent and to each other. That is, during mitosis, chromosomes are doubled and distributed between daughter cells so that each receives one chromatid of each chromosome.

There are several stages (phases) in mitosis. However, mitosis itself is preceded by a long interphase. Mitosis and interphase together make up the cell cycle. In the process of interphase, the cell grows, organelles are formed in it, and synthesis processes are actively going on. In the synthetic period of the interphase, DNA is replicated, i.e., doubled.

After duplication of chromatids, they remain connected in the area centromeres, i.e., the chromosome consists of two chromatids.

In mitosis itself, four main stages are usually distinguished (sometimes more).

The first stage of mitosis is prophase. In this phase, the chromosomes spiral and acquire a compact twisted shape. Because of this, the processes of RNA synthesis become impossible. The nucleoli disappear, which means that ribosomes are also not formed, i.e., synthetic processes in the cell are suspended. Centrioles diverge towards the poles (at different ends) of the cell, the division spindle begins to form. At the end of prophase, the nuclear envelope disintegrates.

prometaphase- This is a stage that is not always isolated separately. The processes occurring in it can be attributed to late prophase or early metaphase. In prometaphase, the chromosomes are in the cytoplasm, randomly moving around the cell until they are connected to the spindle thread in the region of the centromere.

The filament is a microtubule built from the protein tubulin. It grows by attaching new tubulin subunits. In this case, the chromosome moves away from the pole. From the side of the other pole, the spindle thread also joins it and also pushes it away from the pole.

Second stage of mitosis metaphase. All chromosomes are located in the equatorial region of the cell nearby. Attached to their centromeres are two filaments of the spindle. In mitosis, metaphase is the longest stage.

The third stage of mitosis is anaphase. In this phase, the chromatids of each chromosome are separated from each other and, due to the threads pulling them, the spindles of division move to different poles. Microtubules no longer grow, but disassemble. Anaphase is a relatively fast phase of mitosis. With the divergence of chromosomes, the organelles of the cell in approximately equal numbers also diverge closer to the poles.

Fourth stage of mitosis telophase- in many respects the reverse of prophase. Chromatids gather at the poles of the cell and unwind, i.e., despiralize. Nuclear membranes form around them. Nucleoli are formed and RNA synthesis begins. The spindle of division begins to collapse. Then the cytoplasm divides cytokinesis. In animal cells, this occurs due to the invagination of the membrane inward and the formation of a constriction. In plant cells, the membrane begins to form inside in the equatorial plane and goes to the periphery.

Mitosis. Table

Phase	Processes
Prophase	Spiralization of chromosomes. Disappearance of nucleoli. Disintegration of the nuclear envelope. Beginning of spindle formation.
prometaphase	Attachment of chromosomes to the spindle threads and their movement to the equatorial plane of the cell.
metaphase	Each chromosome is stabilized in the equatorial plane by two strands coming from different poles.
Anaphase	Rupture of the centromeres of chromosomes. Each chromatid becomes an independent chromosome. Sister chromatids move to different poles of the cell.
Telophase	Despiralization of chromosomes and resumption of synthetic processes in the cell. Formation of nucleoli and nuclear envelope. Destruction of the fission spindle. doubling of centrioles. Cytokinesis is the division of the cell body in two.

Mitosis, its phases, biological significance

The most important component of the cell cycle is the mitotic (proliferative) cycle. It is a complex of interrelated and coordinated phenomena during cell division, as well as before and after it. The mitotic cycle is a set of processes occurring in a cell from one division to the next and ending with the formation of two cells of the next generation. In addition, in the concept life cycle also includes the period of the cell performing its functions and periods of rest. At this time, the further cell fate is uncertain: the cell may begin to divide (enter mitosis) or begin to prepare to perform specific functions.

Main stages of mitosis.

1.Reduplication (self-doubling) of the genetic information of the mother cell and its uniform distribution between the daughter cells. This is accompanied by changes in the structure and morphology of chromosomes, in which more than 90% of the information of a eukaryotic cell is concentrated.

2. The mitotic cycle consists of four successive periods: presynthetic (or postmitotic) G1, synthetic S, postsynthetic (or premitotic) G2, and mitosis itself. They constitute the autocatalytic interphase (preparatory period).

Phases of the cell cycle:

1) presynthetic (G1). Occurs immediately after cell division. DNA synthesis has not yet taken place. The cell actively grows in size, stores the substances necessary for division: proteins (histones, structural proteins, enzymes), RNA, ATP molecules. There is a division of mitochondria and chloroplasts (i.e., structures capable of autoreproduction). The features of the organization of the interphase cell are restored after the previous division;

2) synthetic (S). Genetic material is duplicated by DNA replication. It occurs in a semi-conservative way, when the double helix of the DNA molecule diverges into two strands and a complementary strand is synthesized on each of them.

As a result, two identical DNA double helixes are formed, each of which consists of one new and one old DNA strand. The amount of hereditary material is doubled. In addition, the synthesis of RNA and proteins continues. Also, a small part of mitochondrial DNA undergoes replication (its main part is replicated in the G2 period);

3) postsynthetic (G2). DNA is no longer synthesized, but there is a correction of the shortcomings made during its synthesis in the S period (repair). Energy and nutrients are also accumulated, the synthesis of RNA and proteins (mainly nuclear) continues.

S and G2 are directly related to mitosis, so they are sometimes isolated in a separate period - preprophase.

This is followed by mitosis itself, which consists of four phases. The division process includes several successive phases and is a cycle. Its duration is different and ranges from 10 to 50 hours in most cells. At the same time, in cells of the human body, the duration of mitosis itself is 1-1.5 hours, the G2 period of interphase is 2-3 hours, the S-period of interphase is 6-10 hours .

stages of mitosis.

The process of mitosis is usually divided into four main phases: prophase, metaphase, anaphase, and telophase (Fig. 1–3). Since it is continuous, the phase change is carried out smoothly - one imperceptibly passes into another.

In prophase, the volume of the nucleus increases, and due to the spiralization of chromatin, chromosomes are formed. By the end of prophase, each chromosome is seen to consist of two chromatids. Gradually, the nucleoli and nuclear membrane dissolve, and the chromosomes are randomly located in the cytoplasm of the cell. The centrioles move towards the poles of the cell. An achromatin spindle is formed, some of the threads of which go from pole to pole, and some are attached to the centromeres of chromosomes. The content of genetic material in the cell remains unchanged (2n2хр).

Characteristics of the phases of mitosis

The main events of prophase include the condensation of chromosomes within the nucleus and the formation of a fission spindle in the cytoplasm of the cell. The disintegration of the nucleolus in prophase is a characteristic, but not obligatory feature for all cells.

Conventionally, the moment of occurrence of microscopically visible chromosomes due to the condensation of intranuclear chromatin is taken as the beginning of prophase. Compaction of chromosomes occurs due to the multilevel helixing of DNA. These changes are accompanied by an increase in the activity of phosphorylases that modify histones that are directly involved in DNA assembly. As a result, the transcriptional activity of chromatin sharply decreases, nucleolar genes are inactivated, and most of the nucleolar proteins dissociate. Condensing sister chromatids in early prophase remain paired along their entire length with the help of cohesin proteins, however, by the beginning of prometaphase, the connection between chromatids is preserved only in the centromere region. By the late prophase, mature kinetochores are formed on each centromere of sister chromatids, which are necessary for chromosomes to attach to spindle microtubules in prometaphase.

Along with the processes of intranuclear condensation of chromosomes, the mitotic spindle begins to form in the cytoplasm - one of the main structures of the cell division apparatus responsible for the distribution of chromosomes between daughter cells. In the formation of the spindle of division in all eukaryotic cells, polar bodies, microtubules and kinetochores of chromosomes take part.

With the beginning of the formation of the mitotic spindle in prophase, dramatic changes in the dynamic properties of microtubules are associated. The half-life of an average microtubule decreases by about 20 times from 5 minutes to 15 seconds. However, their growth rate increases by about 2 times compared to the same interphase microtubules. Polymerizing plus ends are "dynamically unstable" and abruptly transition from uniform growth to rapid shortening, which often depolymerizes the entire microtubule. It is noteworthy that for the proper functioning of the mitotic spindle, a certain balance is required between the processes of assembly and depolymerization of microtubules, since neither stabilized nor depolymerized spindle microtubules are able to move chromosomes.

Along with the observed changes in the dynamic properties of the microtubules that make up the spindle filaments, fission poles are formed in the prophase. Centrosomes replicated in the S phase diverge in opposite directions due to the interaction of pole microtubules growing towards each other. With their minus ends, the microtubules are immersed in the amorphous substance of centrosomes, and the polymerization processes proceed from the side of the plus ends facing the equatorial plane of the cell. In this case, the probable mechanism of pole separation is explained as follows: dynein-like proteins orient the polymerizing plus-ends of pole microtubules in a parallel direction, and kinesin-like proteins, in turn, push them towards the division poles.

In parallel with the condensation of chromosomes and the formation of the mitotic spindle, during prophase, fragmentation of the endoplasmic reticulum occurs, which breaks up into small vacuoles, which then divergent to the cell periphery. At the same time, ribosomes lose contact with ER membranes. The cisternae of the Golgi apparatus also change their perinuclear localization, disintegrating into separate dictyosomes, distributed in the cytoplasm in no particular order.

prometaphase

prometaphase

The end of prophase and the onset of prometaphase are usually marked by the disintegration of the nuclear membrane. A number of lamina proteins are phosphorylated, as a result of which the nuclear envelope is fragmented into small vacuoles, and the pore complexes disappear. After the destruction of the nuclear membrane, the chromosomes are randomly arranged in the region of the nucleus. However, soon they all start to move.

In prometaphase, intensive but random movement of chromosomes is observed. Initially, individual chromosomes rapidly drift towards the nearest pole of the mitotic spindle at a rate of up to 25 µm/min. Near the division poles, the probability of interaction of newly synthesized plus-ends of spindle microtubules with chromosome kinetochores increases. As a result of this interaction, kinetochore microtubules are stabilized from spontaneous depolymerization, and their growth partly ensures the distance of the chromosome connected to them in the direction from the pole to the equatorial plane of the spindle. On the other hand, the chromosome is overtaken by strands of microtubules coming from the opposite pole of the mitotic spindle. Interacting with the kinetochore, they also participate in the movement of the chromosome. As a result, sister chromatids are associated with opposite poles of the spindle. The force developed by microtubules from different poles not only stabilizes the interaction of these microtubules with kinetochores, but also, ultimately, brings each chromosome into the plane of the metaphase plate.

In mammalian cells, prometaphase proceeds, as a rule, within 10-20 minutes. In grasshopper neuroblasts, this stage takes only 4 minutes, while in Haemanthus endosperm and newt fibroblasts it takes about 30 minutes.

metaphase

metaphase

At the end of prometaphase, the chromosomes are located in the equatorial plane of the spindle approximately at an equal distance from both division poles, forming a metaphase plate. The morphology of the metaphase plate in animal cells, as a rule, is distinguished by an ordered arrangement of chromosomes: the centromeric regions face the center of the spindle, and the shoulders face the periphery of the cell. In plant cells, the chromosomes often lie in the equatorial plane of the spindle without a strict order.

Metaphase occupies a significant part of the mitosis period, and is characterized by a relatively stable state. All this time, the chromosomes are held in the equatorial plane of the spindle due to the balanced tension forces of the kinetochore microtubules, making oscillatory movements with a small amplitude in the plane of the metaphase plate.

In metaphase, as well as during other phases of mitosis, active renewal of spindle microtubules continues through intensive assembly and depolymerization of tubulin molecules. Despite some stabilization of bundles of kinetochore microtubules, there is a constant sorting of interpolar microtubules, the number of which in the metaphase reaches a maximum.

By the end of the metaphase, a clear separation of sister chromatids is observed, the connection between which is preserved only in the centromeric regions. The arms of the chromatids are arranged parallel to each other, and the gap separating them becomes clearly visible.

Anaphase is the shortest stage of mitosis, which begins with the sudden separation and subsequent separation of sister chromatids towards opposite poles of the cell. The chromatids separate at a uniform rate of up to 0.5-2 µm/min, and they often take on a V-shape. Their movement is due to the action of significant forces, estimated at 10 dynes per chromosome, which is 10,000 times greater than the force required to simply move the chromosome through the cytoplasm at the observed speed.

As a rule, chromosome segregation in anaphase consists of two relatively independent processes called anaphase A and anaphase B.

Anaphase A is characterized by the separation of sister chromatids to opposite poles of cell division. In this case, the same forces that previously held the chromosomes in the plane of the metaphase plate are responsible for their movement. The process of chromatid separation is accompanied by a shortening of the length of depolymerizing kinetochore microtubules. Moreover, their decay is observed mainly in the region of kinetochores, from the side of the plus ends. Probably, the depolymerization of microtubules at kinetochores or in the region of division poles is a necessary condition for the movement of sister chromatids, since their movement stops when taxol or heavy water is added, which have a stabilizing effect on microtubules. The mechanism underlying chromosome segregation in anaphase A is still unknown.

During anaphase B, the poles of cell division themselves diverge, and, unlike anaphase A, this process occurs due to the assembly of polar microtubules from the side of the plus ends. The polymerizing antiparallel threads of the spindle, when interacting, partly create the force that pushes the poles apart. The magnitude of the relative movement of the poles in this case, as well as the degree of overlap of the pole microtubules in the equatorial zone of the cell, varies greatly in individuals of different species. In addition to repulsive forces, the division poles are affected by pulling forces from astral microtubules, which are created as a result of interaction with dynein-like proteins on the plasma membrane of the cell.

The sequence, duration, and relative contribution of each of the two processes that make up the anaphase can be extremely different. Thus, in mammalian cells, anaphase B begins immediately after the beginning of the chromatid divergence to opposite poles and continues until the lengthening of the mitotic spindle by 1.5–2 times compared to the metaphase one. In some other cells, anaphase B begins only after the chromatids have reached the division poles. In some protozoa, during anaphase B, the spindle lengthens 15 times compared to metaphase. Anaphase B is absent in plant cells.

Telophase

Telophase

Telophase is regarded as the final stage of mitosis; its beginning is taken as the moment when the separated sister chromatids stop at the opposite poles of cell division. In the early telophase, decondensation of chromosomes is observed and, consequently, their increase in volume. Near the grouped individual chromosomes, the fusion of membrane vesicles begins, which gives rise to the reconstruction of the nuclear membrane. The material for the construction of the membranes of the newly formed daughter nuclei are fragments of the initially decayed nuclear membrane of the mother cell, as well as elements of the endoplasmic reticulum. In this case, individual vesicles bind to the surface of the chromosomes and merge together. The outer and inner nuclear membranes are gradually restored, the nuclear lamina and nuclear pores are restored. In the process of nuclear envelope repair, discrete membrane vesicles probably connect to the surface of chromosomes without recognizing specific nucleotide sequences, since experiments have shown that nuclear membrane repair occurs around DNA molecules borrowed from any organism, even from a bacterial virus. Inside the newly formed cell nuclei, chromatin passes into a dispersed state, RNA synthesis resumes, and the nucleoli become visible.

In parallel with the processes of formation of the nuclei of daughter cells in the telophase, the disassembly of microtubules of the fission spindle begins and ends. Depolymerization proceeds in the direction from the division poles to the equatorial plane of the cell, from the minus ends to the plus ends. At the same time, microtubules are stored the longest in the middle part of the spindle, which form the residual Fleming body.

The end of telophase mainly coincides with the division of the body of the mother cell - cytokinesis. In this case, two or more daughter cells are formed. The processes leading to the division of the cytoplasm begin as early as the middle of the anaphase and may continue after the end of the telophase. Mitosis is not always accompanied by division of the cytoplasm, so cytokinesis is not classified as a separate phase of mitotic division and is usually considered as part of the telophase.

There are two main types of cytokinesis: division by the transverse constriction of the cell and division by the formation of a cell plate. The plane of cell division is determined by the position of the mitotic spindle and runs at right angles to the long axis of the spindle.

When dividing by a transverse constriction of the cell, the site of division of the cytoplasm is laid down beforehand during the anaphase period, when a contractile ring of actin and myosin filaments appears in the plane of the metaphase plate under the cell membrane. Subsequently, due to the activity of the contractile ring, a fission furrow is formed, which gradually deepens until the cell is completely divided. At the end of cytokinesis, the contractile ring completely disintegrates, and the plasma membrane contracts around the residual Fleming body, which consists of an accumulation of remnants of two groups of pole microtubules closely packed together with dense matrix material.

Division by the formation of a cell plate begins with the movement of small membrane-limited vesicles towards the equatorial plane of the cell. Here they fuse to form a disk-shaped, membrane-enclosed structure, the early cell plate. Small vesicles originate primarily from the Golgi apparatus and travel toward the equatorial plane along the residual pole microtubules of the spindle, forming a cylindrical structure called the phragmoplast. As the cell plate expands, the microtubules of the early phragmoplast simultaneously move to the cell periphery, where, due to new membrane vesicles, the growth of the cell plate continues until its final fusion with the membrane of the mother cell. After the final separation of the daughter cells, cellulose microfibrils are deposited in the cell plate, completing the formation of a rigid cell wall.