Division of the cytoplasm during mitosis and meiosis. cell cycle

Meiosis- this is a method of indirect division of primary germ cells (2p2s), in which results in the formation of haploid cells (lnlc), most often sex.

Unlike mitosis, meiosis consists of two successive cell divisions, each preceded by an interphase (Fig. 2.53). The first division of meiosis (meiosis I) is called reduction, since in this case the number of chromosomes is halved, and the second division (meiosis II)-equational, since in its process the number of chromosomes is preserved (see Table 2.5).

Interphase I proceeds similarly to the interphase of mitosis. Meiosis I is divided into four phases: prophase I, metaphase I, anaphase I and telophase I. prophase I two major processes occur - conjugation and crossing over. Conjugation- this is the process of fusion of homologous (paired) chromosomes along the entire length. The pairs of chromosomes formed during conjugation are retained until the end of metaphase I.

Crossing over- mutual exchange of homologous regions of homologous chromosomes (Fig. 2.54). As a result of crossing over, the chromosomes received by the organism from both parents acquire new combinations of genes, which leads to the appearance of genetically diverse offspring. At the end of prophase I, as in the prophase of mitosis, the nucleolus disappears, the centrioles diverge towards the poles of the cell, and the nuclear envelope disintegrates.

ATmetaphase I pairs of chromosomes line up along the equator of the cell, spindle microtubules are attached to their centromeres.

AT anaphase I whole homologous chromosomes consisting of two chromatids diverge to the poles.

AT telophase I around clusters of chromosomes at the poles of the cell, nuclear membranes form, nucleoli form.

Cytokinesis I provides division of cytoplasms of daughter cells.

The daughter cells formed as a result of meiosis I (1n2c) are genetically heterogeneous, since their chromosomes, randomly dispersed to the poles of the cell, contain unequal genes.

Interphase II very short, since DNA doubling does not occur in it, that is, there is no S-period.

Meiosis II also divided into four phases: prophase II, metaphase II, anaphase II and telophase II. AT prophase II the same processes occur as in prophase I, with the exception of conjugation and crossing over.

AT metaphase II Chromosomes are located along the equator of the cell.

AT anaphase II Chromosomes split at the centromere and the chromatids stretch towards the poles.

AT telophase II nuclear membranes and nucleoli form around clusters of daughter chromosomes.

After cytokinesis II the genetic formula of all four daughter cells - 1n1c, however, they all have a different set of genes, which is the result of crossing over and a random combination of maternal and paternal chromosomes in daughter cells.

All organisms are made up of cells, they are capable of development, reproduction and growth. Mitosis and meiosis are methods of cell division. It is with their help that cells multiply. These 2 methods are very similar. Mitosis and meiosis consist of the same phases. Reproduce by meiosis sex cells, and with the help of mitosis - somatic.

Meiosis- the process of cell division, can lead to the formation of gametes.

Mitosis- the process of indirect division of eukaryotic cells. Thanks to him, the cells of fungi, plants, animals divide.

Similarities and differences between mitosis and meiosis:

• Mitosis can transfer information from cell to cell. Meiosis, in turn, passes from generation to generation.
• Mitosis is the universal way of reproduction of all body cells. Meiosis is the way eggs and sperm are produced.
• Meiosis produces 4 cells. Mitosis results in the formation of 2 cells.
• During meiosis, the cell receives a haploid set - a decrease in the number of chromosomes. During mitosis, cells remain diploid - the number of chromosomes does not change.
• Meiosis consists of two divisions, while mitosis consists of one.

1. How is mitosis different from meiosis?

Answer. Mitosis is a universal division of somatic cells, as a result of which 2 daughter cells are formed from the original (mother) cell, genetically identical to the mother.

Meiosis - special way division, as a result of which 4 cells are formed with a set of chromosomes halved compared to the mother (usually cells with a haploid set of chromosomes are formed), and all the resulting cells are genetically different from each other.

In meiosis, not one division occurs (as in mitosis), but two successive divisions - reduction and equational.

In meiosis (in the prophase of the first division), conjugation of homologous chromosomes and crossing over occurs, but in mitosis it does not occur.

In the anaphase of the first division of meiosis, not chromatids diverge to the poles, but whole chromosomes

2. What phases of mitosis do you know?

Answer. There are four phases of mitosis: prophase, metaphase, anaphase, and telophase. In prophase, centrioles are clearly visible - formations located in the cell center and playing a role in the division of the daughter chromosomes of animals. Centrioles divide and diverge to different poles of the cell. Microtubules extend from the centrioles, forming spindle fibers, which regulate the divergence of chromosomes to the poles of the dividing cell.

At the end of prophase, the nuclear membrane disintegrates, the nucleolus gradually disappears, the chromosomes spiralize and as a result shorten and thicken, and they can already be observed under a light microscope. They are even better seen at the next stage of mitosis - metaphase.

In metaphase, chromosomes are located in the equatorial plane of the cell. It is clearly seen that each chromosome, consisting of two chromatids, has a constriction - the centromere. Chromosomes are attached by their centromeres to the spindle thread. After division of the centromere, each chromatid becomes an independent daughter chromosome.

Then comes the next stage of mitosis - anaphase, during which the daughter chromosomes (chromatids of one chromosome) diverge to different poles of the cell.

The next stage of cell division is telophase. It 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 (mitochondria, the Golgi complex, ribosomes, etc.) are more or less evenly distributed among the daughter cells.

Questions after §28

1. What is apoptosis?

Answer. In protozoa and bacteria, cell division is the main mode of reproduction. The amoeba, for example, does not undergo natural death, and instead of dying, it simply divides into two new cells. It is clear that the cells of a multicellular organism cannot divide indefinitely, otherwise all creatures, including humans, would become immortal. This does not happen because the DNA of the cell contains special "death genes" that are activated sooner or later. This leads to the synthesis of special proteins that kill this cell: it shrinks, its organelles and membranes are destroyed, but in such a way that their parts can be reused. This "programmed" cell death is called apoptosis. But from its "birth" to apoptosis, the cell goes through many normal cell cycles. At various kinds organisms, the cell cycle takes different time: in bacteria - about 20 minutes, in ciliates-shoes - from 10 to 20 hours. Cells of tissues of multicellular organisms on early stages its development is divided very often, and then the cell cycles are significantly lengthened. For example, immediately after birth, the neurons of animals divide frequently: 80% of the brain is formed at that time. However, most of these cells quickly lose their ability to divide, and some of them survive without dividing until the natural death of the animal from old age.

2. What cycle is called mitotic?

Answer. Mandatory component Each cell cycle is a mitotic cycle, which includes the preparation of the cell for the process of division and the division itself. In addition, the life cycle includes long or short periods rest, when the cell performs its functions in the body. After each of these periods, the cell must go either to the mitotic cycle or to apoptosis.

3. What processes occur in the cell during interphase?

Answer. The preparation of a cell for division is called interphase. It consists of three periods.

The presynthetic period (G1) is the longest part of the interphase. It can last in different types of cells from 2-3 hours to several days. This period immediately follows the previous division, during which the cell grows, accumulating energy and substances for subsequent DNA duplication.

The synthetic period (S), which usually lasts 6–10 hours, includes DNA duplication, the synthesis of proteins necessary for the formation of chromosomes, and an increase in the amount of RNA. By the end of this period, each chromosome already consists of two identical chromatids connected to each other at the centromere. In the same period, the centrioles double.

The postsynthetic period (G2) occurs after chromosome doubling. It lasts 2–5 hours; during this time, energy is accumulated for the upcoming mitosis and microtubule proteins are synthesized, which subsequently form the division spindle. Now the cell can start mitosis.

Before proceeding to a description of the methods of cell division, let us consider the process of DNA duplication, as a result of which sister chromatids are formed in the synthetic period.

4. During what period of interphase does DNA replication occur?

Answer. The duplication of a DNA molecule is also called replication or reduplication. During replication, a part of the "maternal" DNA molecule is untwisted into two strands with the help of a special enzyme, and this is achieved by breaking the hydrogen bonds between complementary nitrogenous bases: adenine - thymine and guanine - cytosine. Further, for each nucleotide of the divergent DNA strands, the DNA polymerase enzyme adjusts its complementary nucleotide. Thus, two double-stranded DNA molecules are formed, each of which includes one strand of the "parent" molecule and one newly synthesized ("daughter") strand. These two DNA molecules are absolutely identical.

Preparation for ZNO. Biology.
Synopsis 34. cell cycle. Mitosis. Meiosis

cell cycle

cell cycle- the life of a cell from the moment of its appearance to division or death. An obligatory component of the cell cycle is the mitotic cycle, which includes a period of preparation for division and mitosis itself. There are two major phases: interphase and cell division mitosis or meiosis).
Interphase consists of three periods: presynthetic, or postmitotic, - G 1, synthetic - S, postsynthetic, or premitotic, - G 2.

Mitosis

Mitosis- the main method of division of eukaryotic cells, in which the hereditary material is first doubled, and then it uniform distribution between daughter cells.

There are four phases of mitosis: prophase , metaphase , anaphase and telophase . Before mitosis, the cell prepares for division, or interphase.
Presynthetic period (2n 2c, where n- the number of chromosomes, with- the number of DNA molecules) - cell growth, activation of biological synthesis processes, preparation for the next period.
Synthetic period (2n 4c) is DNA replication.
Postsynthetic period (2n 4c) - preparation of the cell for mitosis, synthesis and accumulation of proteins and energy for the upcoming division, an increase in the number of organelles, doubling of centrioles.
Prophase (2n 4c) - dismantling of nuclear membranes, divergence of centrioles to different poles of the cell, formation of fission spindle threads, "disappearance" of nucleoli, condensation of two-chromatid chromosomes.
metaphase (2n 4c) - alignment of the most condensed two-chromatid chromosomes in the equatorial plane of the cell (metaphase plate), attachment of the spindle fibers with one end to the centrioles, the other - to the centromeres of the chromosomes.
Anaphase (4n 4c) - the division of two-chromatid chromosomes into chromatids and the divergence of these sister chromatids to opposite poles of the cell (in this case, the chromatids become independent single-chromatid chromosomes).
Telophase (2n 2c in each daughter cell) - decondensation of chromosomes, the formation of nuclear membranes around each group of chromosomes, the disintegration of the fission spindle threads, the appearance of the nucleolus, the division of the cytoplasm (cytotomy). Cytotomy in animal cells occurs due to the fission furrow, in plant cells- due to the cell plate.
biological significance mitosis. The daughter cells formed as a result of this method of division are genetically identical to the mother. Mitosis ensures the constancy of the chromosome set in a number of cell generations. Underlies such processes as growth, regeneration, asexual reproduction, etc.

Meiosis

Meiosis- This is a special way of dividing eukaryotic cells, as a result of which the transition of cells from a diploid state to a haploid one occurs. Consists of two successive mitotic divisions preceded by a single DNA replication.
first meiotic division(meiosis 1) is called reduction, because it is during this division that the number of chromosomes is halved: from one diploid cell (2 n 4c) form two haploid (1 n 2c).
Interphase 1(at the beginning - 2 n 2c, at the end - 2 n 4c) - the synthesis and accumulation of substances and energy necessary for the implementation of both divisions, an increase in cell size and the number of organelles, doubling of centrioles, DNA replication, which ends in prophase 1.
Prophase 1 (2n 4c) - dismantling of nuclear membranes, divergence of centrioles to different poles of the cell, formation of fission spindle filaments, "disappearance" of nucleoli, condensation of two-chromatid chromosomes, conjugation of homologous chromosomes and crossing over. Conjugation- the process of convergence and interlacing of homologous chromosomes. A pair of conjugating homologous chromosomes is called bivalent. Crossing over- the process of exchange of homologous regions between homologous chromosomes.
Metaphase 1 (2n 4c) - alignment of bivalents in the equatorial plane of the cell, attachment of the spindle fibers with one end to the centrioles, the other - to the centromeres of the chromosomes.
Anaphase 1 (2n 4c) - random independent divergence of two-chromatid chromosomes to opposite poles of the cell (from each pair of homologous chromosomes, one chromosome moves to one pole, the other to the other), recombination of chromosomes.
Telophase 1 (1n 2c in each cell) - the formation of nuclear membranes around groups of two-chromatid chromosomes, the division of the cytoplasm. In many plants, a cell from anaphase 1 immediately transitions to prophase 2.
Second meiotic division (meiosis 2) is called equational.
Interphase 2 or interkinesis(1n 2c), is a short break between the first and second meiotic divisions during which DNA replication does not occur. characteristic of animal cells.
Prophase 2 (1n 2c) - dismantling of nuclear membranes, divergence of centrioles to different poles of the cell, formation of fission spindle filaments.
Metaphase 2 (1n 2c) - alignment of two-chromatid chromosomes in the equatorial plane of the cell (metaphase plate), attachment of the spindle fibers with one end to the centrioles, the other - to the centromeres of the chromosomes; 2 block of oogenesis in humans.
Anaphase 2 (2n 2c) - the division of two-chromatid chromosomes into chromatids and the divergence of these sister chromatids to opposite poles of the cell (in this case, the chromatids become independent single-chromatid chromosomes), recombination of chromosomes.
Telophase 2 (1n 1c in each cell) - decondensation of chromosomes, the formation of nuclear membranes around each group of chromosomes, the disintegration of the fission spindle threads, the appearance of the nucleolus, the division of the cytoplasm (cytotomy) with the formation of four haploid cells as a result.
The biological significance of meiosis. Meiosis is the central event of gametogenesis in animals and sporogenesis in plants. Being the basis of combinative variability, meiosis ensures the genetic diversity of gametes.

Amitosis

Amitosis- direct division of the interphase nucleus by constriction without the formation of chromosomes, outside the mitotic cycle. Described for aging, pathologically altered and doomed to death cells. After amitosis, the cell is unable to return to the normal mitotic cycle.

Target: students deepen their knowledge of the forms of reproduction of organisms; new concepts about mitosis and meiosis and their biological significance are being formed.

Equipment:

1. Educational visual aids: tables, posters
2. technical means training: interactive whiteboard, multimedia presentations, educational computer programs.

Lesson plan:

1. Organizing time
2. Repetition.
   1. What is reproduction?
   2. What types of reproduction do you know? Can you define them?
   3. List examples of asexual reproduction? Give examples.
   4. Biological significance of asexual reproduction?
   5. What kind of reproduction is called sexual?
   6. What sex cells do you know?
   7. How are gametes different from somatic cells?
   8. What is fertilization?
   9. What are the advantages of sexual reproduction over asexual reproduction?
3. Learning new material

During the classes

The transmission of hereditary information, reproduction, as well as growth, development and regeneration is based on the most important process - cell division. The molecular essence of division lies in the ability of DNA to self-doubling molecules.

Announcement of the topic of the lesson. Since the phases of mitosis and meiosis in in general terms we already studied in grade 9, the task of general biology is to consider this process at the molecular and biochemical level. Concerning Special attention we will focus on the change in chromosomal structures.

The cell is not only a unit of structure and function in living organisms, but also a genetic unit. This is a unit of heredity and variability, manifested in the process of cell division. The elementary carrier of the hereditary properties of the cell is the gene. A gene is a segment of a DNA molecule of several hundred nucleotides, which encodes the structure of one protein molecule and the manifestation of some hereditary trait of a cell. A DNA molecule in combination with a protein forms a chromosome. The chromosomes of the nucleus and the genes localized in them are the main carriers of the hereditary properties of the cell. At the start of cell division, the chromosomes shorten and stain more intensely so that they become visible individually.

In a dividing cell, the chromosome has the form of a double rod and consists of two halves or chromatids separated by a gap along the axis of the chromosome. Each chromatid contains one DNA molecule.

The internal structure of chromosomes, the number of DNA strands in them change in life cycle cells.

Recall: what is the cell cycle? What are the stages in the cell cycle? What happens at each stage?

Interphase includes three periods.

The presynthetic period G 1 occurs immediately after cell division. At this time, the cell synthesizes proteins, ATP, different types RNA and individual DNA nucleotides. The cell grows, and it intensively accumulates various substances. Each chromosome during this period is single-chromatid, the genetic material of the cell is designated 2n 1xp 2c (n is the set of chromosomes, xp is the number of chromatids, c is the amount of DNA).

In the synthetic period S, the reduplication of the DNA molecules of the cell is carried out. As a result of DNA doubling, each chromosome contains twice as much DNA as it had before the start of the S-phase, but the number of chromosomes does not change. Now the cell's genetic set is 2n 2xp 4c ( diploid set, chromosomes are bichromatid, the amount of DNA is 4).

In the third period of interphase - postsynthetic G 2 - the synthesis of RNA, proteins and the accumulation of energy by the cell continue. At the end of interphase, the cell increases in size and begins to divide.

Cell division.

In nature, there are 3 ways of cell division - amitosis, mitosis, meiosis.

Prokaryotic organisms and some eukaryotic cells divide by amitosis, for example, Bladder, human liver, as well as old or damaged cells. First, the nucleolus divides in them, then the nucleus into two or more parts by constriction, and at the end of the division, the cytoplasm is laced into two or more daughter cells. The distribution of hereditary material and cytoplasm is not uniform.

Mitosis- a universal method of dividing eukaryotic cells, in which two similar daughter cells are formed from a diploid mother cell.

The duration of mitosis is 1-3 hours and there are 4 phases in its process: prophase, metaphase, anaphase and telophase.

Prophase. Usually the longest phase of cell division.

The volume of the nucleus increases, the chromosomes spiralize. At this time, the chromosome consists of two chromatids connected to each other in the region of the primary constriction or centromere. Then the nucleoli and the nuclear envelope dissolve - the chromosomes lie in the cytoplasm of the cell. Centrioles diverge to the poles of the cell and form between themselves the threads of the spindle of division, and at the end of the prophase, the threads are attached to the centromeres of the chromosomes. The genetic information of the cell is still as in interphase (2n 2xp 4c).

Metaphase. Chromosomes are located strictly in the zone of the equator of the cell, forming a metaphase plate. At the metaphase stage, the chromosomes are the shortest, since at this time they are highly spiralized and condensed. Since the chromosomes are clearly visible, counting and studying the chromosomes usually takes place during this period of division. In terms of duration, this is the shortest phase of mitosis, since it lasts for the moment when the centromeres of duplicated chromosomes are located strictly along the equatorial line. And in the next moment, the next phase begins.

Anaphase. Each centromere splits into two, and the spindle fibers pull the daughter centromeres to opposite poles. Centromeres pull the separated chromatids along with them. One chromatid from a pair comes to the poles - these are daughter chromosomes. The amount of genetic information at each pole is now (2n 1xp 2s).

Mitosis is completed telophase. The processes occurring in this phase are the reverse of the processes that were observed in the prophase. At the poles, despiralization of the daughter chromosomes occurs, they become thinner and become barely distinguishable. Nuclear membranes form around them, and then nucleoli appear. At the same time, the division of the cytoplasm takes place: in animal cells - by constriction, and in plants from the middle of the cell to the periphery. After the formation of the cytoplasmic membrane in plant cells, a cellulose membrane is formed. Two daughter cells are formed with a diploid set of single-chromatid chromosomes (2n 1xp 2c).

It should be noted that all processes occurring in the cell, including mitosis, are under genetic control. Genes control successive stages of DNA replication, movement, spiralization of chromosomes, etc.

The biological significance of mitosis:

1. The exact distribution of chromosomes and their genetic information between daughter cells.
2. Ensures the constancy of the karyotype and genetic continuity in all cellular manifestations; because otherwise it would not be possible to maintain the constancy of the structure and the correct functioning of the organs and tissues of a multicellular organism.
3. Provides critical processes life activity - embryonic development, growth, repair of tissues and organs, as well as asexual reproduction of organisms.

Meiosis

The formation of germ cells (gametes) occurs differently than the process of reproduction of somatic cells. If the formation of gametes followed the same path, then after fertilization (the fusion of male and female gametes), the number of chromosomes would double each time. However, this does not happen. Each species is characterized by a certain number and its own specific set of chromosomes (karyotype).

Meiosis is special kind division, when germ cells (gametes) are formed from diploid (2n) somatic cells of the genital organs in animals and plants or spores in spore plants with a haploid (n) set of chromosomes in these cells. Then, in the process of fertilization, the nuclei of germ cells merge, and the diploid set of chromosomes (n + n = 2n) is restored.

In the continuous process of meiosis, there are two successive divisions: meiosis I and meiosis II. In each division, the same phases as in mitosis, but different in duration and changes in the genetic material. As a result of meiosis I, the number of chromosomes in the resulting daughter cells is halved (reduction division), while during meiosis II, the cell haploidy is preserved (equatorial division).

Prophase of meiosis I- Homologous chromosomes doubled in interphase approach in pairs. In this case, individual chromatids of homologous chromosomes intertwine, intersect each other and can break in the same places. During this contact, homologous chromosomes can exchange corresponding regions (genes), i.e. there is a crossover. Crossing over causes the recombination of a cell's genetic material. After this process, the homologous chromosomes separate again, the membranes of the nucleus and nucleoli dissolve, and a division spindle is formed. The genetic information of a cell in prophase is 2n 2xp 4c (diploid set, two-chromatid chromosomes, the number of DNA molecules is 4).

Metaphase of meiosis I - Chromosomes are located in the plane of the equator. But if in the metaphase of mitosis homologous chromosomes have a position independent of each other, then in meiosis they lie side by side - in pairs. Genetic information is the same (2n 2xp 4c).

Anaphase I- not halves of chromosomes from one chromatid diverge to the poles of the cell, but whole chromosomes consisting of two chromatids. This means that from each pair of homologous chromosomes, only one, but two-chromatid, chromosome will get into the daughter cell. Their number in new cells will decrease by half (reduction in the number of chromosomes). The amount of genetic information at each pole of the cell becomes smaller (1n 2xp 2s).

AT telophase the first division of meiosis, nuclei, nucleoli are formed and the cytoplasm is divided - two daughter cells are formed with a haploid set of chromosomes, but these chromosomes consist of two chromatids (1n 2xp 2c).

Following the first, the second division of meiosis occurs, but it is not preceded by DNA synthesis. After a short prophase of meiosis II, the two-chromatid chromosomes in the metaphase of meiosis II are located in the plane of the equator and are attached to the spindle fibers. Their genetic information is the same - (1n 2xp 2s).

In the anaphase of meiosis II, the chromatids diverge to opposite poles of the cell, and in the telophase of meiosis II, four haploid cells with single chromatid chromosomes (1n 1xp 1c) are formed. Thus, in sperm and eggs, the number of chromosomes is halved. Such germ cells are formed in sexually mature individuals various organisms. The process of gamete formation is called gametogenesis.

The biological significance of meiosis:

1. Formation of cells with a haploid set of chromosomes. During fertilization, a constant set of chromosomes and a constant amount of DNA are provided for each species.

2. During meiosis, a random segregation of non-homologous chromosomes occurs, which leads to a large number possible combinations chromosomes in gametes. In humans, the number of possible combinations of chromosomes in gametes is 2 n, where n is the number of chromosomes of the haploid set: 2 23 \u003d 8 388 608. The number of possible combinations in one parental pair is 2 23 x 2 23

3. Occurring in meiosis, the crossover of chromosomes, the exchange of sites, as well as the independent divergence of each pair of homologous chromosomes

determine the patterns of hereditary transmission of a trait from parents to offspring.

From each pair of two homologous chromosomes (maternal and paternal) included in chromosome set diploid organisms, the haploid set of the egg or sperm cell contains only one chromosome. Moreover, it can be: 1) the paternal chromosome; 2) maternal chromosome; 3) paternal with a portion of the maternal chromosome; 4) maternal with paternal plot. These processes lead to efficient recombination of the hereditary material in the gametes formed by the organism. As a result, the genetic heterogeneity of gametes and offspring is determined.

When explaining, students fill in the table: " Comparative characteristics mitosis and meiosis

Consolidation of the studied material (according to the table, test work).

Literature:

1. Yu.I. Polyansky. Textbook for grades 10-11 high school. -M.: "Enlightenment", 1992.
2. I.N. Ponomareva, O.A. Kornilova, T.E. Loshchilin. Textbook "Biology" Grade 11, a basic level of, -M .: "Ventana-Count", 2010.
3. S.G. Mamontov Biology for applicants to universities. –M.: 2002.
4. N. Green, W. Stout, D. Taylor. Biology in 3 vols. -M .: "Mir", 1993.
5. N.P. Dubinin. General biology. A guide for the teacher. –M.: 1990.
6. N.N. Prikhodchenko, T.P. Shkurat Fundamentals of human genetics. Uch.pos. - Rostov n / a: "Phoenix", 1997.