Meiosis and mitosis - difference, phases. The life cycle of a cell. Mitosis. Meiosis Mitosis and meiosis have in common

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 is a special method of 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 start of cell division in 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. In different types of organisms, the cell cycle takes different times: in bacteria - about 20 minutes, in ciliates - from 10 to 20 hours. Cells of tissues of multicellular organisms in the early stages of its development divide very often, and then 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. An obligatory component of each cell cycle is the 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 dormant periods 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.

Meiosis is a method of cell division in eukaryotes, in which haploid cells are formed. This is different from mitosis, which produces diploid cells.

In addition, meiosis proceeds in two successive divisions, which are called respectively the first (meiosis I) and the second (meiosis II). Already after the first division, the cells contain a single, i.e. haploid, set of chromosomes. Therefore, the first division is often called reduction. Although sometimes the term "reduction division" is used in relation to the entire meiosis.

The second division is called equational and similar in mechanism to mitosis. In meiosis II, sister chromatids diverge to the poles of the cell.

Meiosis, like mitosis, is preceded in interphase by DNA synthesis - replication, after which each chromosome already consists of two chromatids, which are called sister chromatids. Between the first and second divisions, DNA synthesis does not occur.

If as a result of mitosis two cells are formed, then as a result of meiosis - 4. However, if the body produces eggs, then only one cell remains, which has concentrated nutrients in itself.

The amount of DNA before the first division is usually denoted as 2n 4c. Here n denotes chromosomes, c denotes chromatids. This means that each chromosome has a homologous pair (2n), at the same time, each chromosome consists of two chromatids. Given the presence of a homologous chromosome, four chromatids are obtained (4c).

After the first and before the second division, the amount of DNA in each of the two daughter cells is reduced to 1n 2c. That is, homologous chromosomes diverge into different cells, but continue to consist of two chromatids.

After the second division, four cells are formed with a set of 1n 1c, i.e., each contains only one chromosome from a pair of homologous ones and it consists of only one chromatid.

The following is a detailed description of the first and second meiotic divisions. The designation of the phases is the same as in mitosis: prophase, metaphase, anaphase, telophase. However, the processes occurring in these phases, especially in prophase I, are somewhat different.

Meiosis I

Prophase I

This is usually the longest and most complex phase of meiosis. It takes much longer than with mitosis. This is due to the fact that at this time homologous chromosomes approach each other and exchange DNA segments (conjugation and crossing over occur).

Conjugation- the process of linking homologous chromosomes. Crossing over- exchange of identical regions between homologous chromosomes. Nonsister chromatids of homologous chromosomes can exchange equivalent regions. In places where such an exchange occurs, the so-called chiasma.

Paired homologous chromosomes are called bivalents, or tetrads. Communication is maintained until anaphase I and is provided by centromeres between sister chromatids and chiasmata between nonsister chromatids.

In prophase, chromosomes spiralize, so that by the end of the phase, the chromosomes acquire their characteristic shape and size.

In the later stages of prophase I, the nuclear envelope breaks up into vesicles and the nucleoli disappear. The meiotic spindle begins to form. Three types of spindle microtubules are formed. Some are attached to kinetochores, others - to tubules growing from the opposite pole (the structure acts as spacers). Still others form a stellate structure and are attached to the membrane skeleton, performing the function of a support.

Centrosomes with centrioles diverge towards the poles. Microtubules are introduced into the region of the former nucleus, attached to kinetochores located in the centromere region of chromosomes. In this case, the kinetochores of sister chromatids merge and act as a single whole, which allows the chromatids of one chromosome not to separate and subsequently move together to one of the poles of the cell.

Metaphase I

The fission spindle is finally formed. Pairs of homologous chromosomes are located in the plane of the equator. They line up opposite each other along the equator of the cell so that the equatorial plane is between pairs of homologous chromosomes.

Anaphase I

Homologous chromosomes separate and diverge to different poles of the cell. Due to the crossing over that occurred during prophase, their chromatids are no longer identical to each other.

Telophase I

The nuclei are restored. Chromosomes despiralize into thin chromatin. The cell is divided in two. In animals, by invagination of the membrane. Plants have a cell wall.

Meiosis II

The interphase between two meiotic divisions is called interkinesis, it is very short. Unlike interphase, DNA duplication does not occur. In fact, it is already doubled, just each of the two cells contains one of the homologous chromosomes. Meiosis II occurs simultaneously in two cells formed after meiosis I. The diagram below shows the division of only one cell out of two.

Prophase II

Short. The nuclei and nucleoli disappear again, and the chromatids spiralize. The spindle begins to form.

Metaphase II

Two spindle strands are attached to each chromosome, which consists of two chromatids. One thread from one pole, the other from the other. The centromeres are composed of two separate kinetochores. The metaphase plate is formed in a plane perpendicular to the equator of metaphase I. That is, if the parent cell in meiosis I divided along, now two cells will divide across.

Anaphase II

The protein that binds the sister chromatids separates, and they diverge to different poles. Sister chromatids are now called sister chromosomes.

Telophase II

Similar to telophase I. Despiralization of chromosomes occurs, the fission spindle disappears, the formation of nuclei and nucleoli, cytokinesis.

The meaning of meiosis

In a multicellular organism, only germ cells divide by meiosis. Therefore, the main meaning of meiosis is securitymechanismAsexual reproduction,which maintains the constancy of the number of chromosomes in the species.

Another meaning of meiosis is the recombination of genetic information that occurs in prophase I, i.e. combinative variability. New combinations of alleles are created in two cases. 1. When crossing over occurs, i.e., non-sister chromatids of homologous chromosomes exchange sites. 2. With independent divergence of chromosomes to the poles in both meiotic divisions. In other words, each chromosome can be in the same cell in any combination with other non-homologous chromosomes.

Already after meiosis I, cells contain different genetic information. After the second division, all four cells differ from each other. This is an important difference between meiosis and mitosis, in which genetically identical cells are formed.

Crossing over and random segregation of chromosomes and chromatids in anaphases I and II create new combinations of genes and are oneof the causes of hereditary variability of organisms which makes possible the evolution of living organisms.

The purpose of the lesson: repetition of material on the methods of cell reproduction.

Tasks

Educational: to form and consolidate knowledge about two types of cell division, about the significance of cell division for unicellular and multicellular organisms, about the processes occurring in different phases of mitosis and meiosis, about the differences between meiosis and mitosis.

Developing: development of skills to work in a group, characterize objects and phenomena, compare them, justify conclusions, apply knowledge, evaluate yourself and your knowledge; development of interest in the subject.

Educational: fostering a respectful attitude towards each other.

Equipment: sheets of drawing paper and paper, felt-tip pens, glue, adhesive tape, scissors, files with tasks, an instruction card for each team.

Preparing for the lesson

1. At the previous lesson, students should be familiarized with the principles and rules for conducting a workshop lesson.

2. Since the topic “Cell Division” was studied in the 9th grade and the students forgot a lot, as homework they had to repeat the material on the topic: “Cell Division”.

Dividing the class into teams

Students are asked to choose one of the following questions and write it down on a piece of paper. (Most likely, the student will choose the question, the answer to which he knows or assumes that he knows.)

What is the biological meaning of meiosis?
How is mitosis different from meiosis?
What is the biological meaning of mitosis?

From a piece of paper with a written question, you need to fold a paper airplane. Standing in a circle, students launch their airplanes (all at the same time at the teacher's command) and, picking up the airplane that has fallen nearby, repeat this operation 2 times. Having opened the airplanes, the students are divided into three teams - on the same questions.

Each team receives a file containing material for work: a list of terms, definitions, diagrams, historical reference.

instruction card

Select from the list of terms (Appendix 2) those that are related to the topic “Cell division. Mitosis. Meiosis". The chosen words of the team are read aloud.

Select definitions (Appendix 3) that correspond to the selected terms from the previous task. Be careful, some definitions have been replaced! To complete this task correctly, you need to find and ask the other team for their definition. Terms cannot be changed!

For the processes occurring in the cell during mitosis or meiosis, select the appropriate drawings (Appendix 4).

Stick words, definitions and pictures in a logical sequence on a piece of drawing paper. Prepare a short story about this biological process.

(Teams hang their work on the stand. Team members talk about the processes depicted on the paper.)

Answer the question that is written on your flyer. Write down the answer in your notebook. (When completing this task, you can use the original source. Each team reads its answer to the question aloud.)

Reflection

Option 1(if there is a lot of time left before the end of the lesson).

Give two or three arguments in defense of the fact that the topic “Cell division. Mitosis and meiosis" must be studied in the course of general biology in high school.

Option 2(if there is not enough time).

Are you satisfied with the lesson, with your work at the lesson? Think about your emotional state. Write down the answer on a piece of paper and, when leaving, attach it to the stand.

Homework

Answer the following questions.

What factors cause the violation of mitosis and meiosis?
What consequences can this lead to?

Annex 1. Historical background

Flemming Walter (1843–1905), German histologist University professor in Prague (since 1873) and Kiel (1876–1901). His main works are on the histology of mollusks, tissue regeneration, the study of connective and adipose tissues, the structure of follicles, spinal ganglion cells, etc. His studies of the fine structure of the cell gained particular fame. Using the methods of fixation (Flemming's liquid) and staining developed by him, he studied the structure of the protoplasm, nucleus, centrosomes, and, especially in detail, the process of cell division (direct and indirect). These studies were of great importance for the development of cytology; his methods of fixation and staining are widely used in laboratory practice.

Strasburger Edward (1844–1912), German botanist, Pole by origin, member of the Polish Academy of Sciences in Krakow (1888). Studied in Warsaw, Bonn and Jena. He was an assistant professor at Warsaw (1867–1869), a professor at Jena (1869–1880) and Bonn (1880–1911) universities. Major works in the field of cytology, anatomy and embryology of plants. studied mitosis. Described meiosis in higher plants, explained the biological significance of the reduction in the number of chromosomes. He studied the process of fertilization, the phenomena of parthenogenesis and apogamy. The works of the scientist were of great importance for the preparation of the chromosome theory of heredity and the development of ideas about the genetic unity of higher plants. Improved the methodology of cytological studies. Co-author of a reprinted course in botany (Textbook of Botany, 1894; 30th ed. - 1971), translated into a number of languages, including Russian.

Chistyakov Ivan Dorofeevich (1843–1877), Russian botanist. He graduated from Moscow University (1868) and was left with him, since 1871 he was a professor and head of the Botanical Garden. Founder of the Moscow school of plant embryologists and cytologists. One of the first to observe and describe mitosis in plants (1874).

Annex 2. Terms

(The underlined words are the correct student choice.)

File #1 (blue)

Mitosis, prophase, metaphase, anaphase, telophase, amitosis, cell cycle, photosynthesis.

File #2 (green)

Meiosis, 1st division, prophase 1, metaphase 1, anaphase 1, telophase 1, crossing over, assimilation, dissimilation.

File #3 (red)

Meiosis, 2nd division, prophase 2, metaphase 2, anaphase 2, telophase 2, interphase, polymers.

Annex 3. Definitions

File #1 (blue)

Mitosis- This is a method of dividing eukaryotic cells, in which each of the two newly emerging cells receives the same genetic material as in the original cell.

Prophase- chromosomes spiral and become clearly visible in a light microscope, the nucleolus disappears, two centrioles diverge towards the poles of the cell, microtubules extending from them form a fission spindle, the nuclear envelope disintegrates.

Anaphase

Telophase- around the chromosomes assembled at the poles, a nuclear envelope is formed, the chromosomes are despiralized (from compact they turn into thin and long, indistinguishable in a light microscope). Nucleoli are formed. This stage ends with cytokinesis (separation of the cytoplasm) and the formation of two diploid cells.

Amitosis- direct nuclear fission by constriction, does not always end with cytokinesis, as a result, multinucleated cells usually appear. After amitosis, cells are unable to start mitotic division. This process is characteristic of dying cells.

cell cycle- the period of cell life from division to division, the main part of the life of the cell.

Interphase- the period between divisions (lat. inter- between). The cell grows intensively, the number of structures and substances in the cell increases, the number of chromosomes doubles.

(Definition interphase in this fileher, but the definition metaphase absent.)

File #2 (green)

Meiosis(gr. meiosis

1st division- the first division of meiosis.

Prophase 1 Chromosomes begin to condense and become visible under a light microscope. Then the homologous chromosomes begin to approach each other - to conjugate. A pair of conjugating chromosomes is called a bivalent (each bivalent is formed by 4 chromatids). DNA replication ends. The phase ends with the disappearance of the nuclear envelope and nucleolus.

Metaphase 1- bivalents line up in the equatorial plane of the cell. Spindle threads are attached to the centromeres.

Anaphase 1- the bivalent breaks up into two chromosomes, which go to different poles of the cell.

Telophase 1- chromosomes decondense (from compact they turn into thin and long, indistinguishable in a light microscope). A nuclear membrane forms around the chromosomes assembled at the poles. Nucleoli are formed. This stage ends with cytokinesis (separation of the cytoplasm) and the formation of two diploid cells.

metaphase

(Definition metaphase in this fileher, but the definition crossing over absent.)

File #3 (red)

Meiosis(gr. meiosis- reduction) is a method of dividing eukaryotic cells, in which a reduction (decrease) in the number of chromosomes occurs, i.e. from diploid (containing a double set of chromosomes) cells are formed haploid (containing a single set of chromosomes).

2nd division- the second division of meiosis.

Prophase 2- chromosomes spiralize and become clearly visible in a light microscope, the nucleolus disappears, two centrioles diverge towards the poles of the cell, microtubules extending from them form a division spindle.

Metaphase 2- all chromosomes line up in the equatorial plane of the cell, at this stage they can be well distinguished and counted in the cell.

Anaphase 2- the stage during which sister chromatids, which have become independent chromosomes, diverge to opposite poles of the cell.

Telophase 2- a nuclear membrane forms around the chromosomes assembled at the poles. Chromosomes despiralize (from compact they turn into thin and long, indistinguishable in a light microscope). Nucleoli are formed. This stage ends with cytokinesis (separation of the cytoplasm) and the formation of four haploid cells.

Crossing over(English) crossing-over- precross) - the exchange of identical sections of homologous chromosomes.

(Definition crossing over in this fileher, but the definition interphase absent.)

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 teaching aids: 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 we already studied the phases of mitosis and meiosis in general terms in grade 9, the task of general biology is to consider this process at the molecular and biochemical level. In this regard, we will pay special attention to changes 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 during the life cycle of the cell.

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 synthesis of proteins, ATP, various types of RNA and individual DNA nucleotides occurs in the cell. The cell grows, and various substances intensively accumulate in it. 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 of the chromosomes contains twice as much DNA as it had before the onset of the S-phase, but the number of chromosomes does not change. Now the genetic set of the cell is 2n 2xp 4c (diploid set, chromosomes are two-chromatid, 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.

Amitosis divides prokaryotic organisms and some eukaryotic cells, for example, the 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 the most important life processes - embryonic development, growth, restoration 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 a special type of division, when germ cells (gametes) in animals and plants or spores in spore plants with a haploid (n) set of chromosomes in these cells are formed from diploid (2p) somatic cells of the genital organs. 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).

IN 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 sex cells are formed in sexually mature individuals of 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 of possible combinations of 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.

Of each pair of two homologous chromosomes (maternal and paternal) included in the chromosome set of diploid organisms, the haploid set of an 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 out the table: "Comparative characteristics of mitosis and meiosis"

division types	Mitosis (indirect division)	Meiosis (reduction division)
Number of divisions	one division	two division
ongoing processes	Replication and transcription are absent	In prophase 1, conjugation of homologous chromosomes occurs and crossing over occurs.
	Chromatids diverge to the poles of the cell	In the first division, homologous chromosomes diverge to the poles of the cell.
Number of daughter cells	2	4
A set of chromosomes in daughter cells (n is a set of chromosomes, xp is chromatids, c is the number of DNA)	The number of chromosomes remains constant 2n 1xp 2c (single chromatid chromosomes)	The number of chromosomes is halved 1n 1xp 1c (single chromatid chromosomes)
Cells where division occurs	somatic cells	Somatic cells of the genital organs of animals; spore-forming plant cells
Meaning	Provides asexual reproduction and growth of living organisms	Serves for the formation of germ cells

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

Literature:

1. Yu.I. Polyansky. Textbook for 10-11 grades of high school. -M.: "Enlightenment", 1992.
2. I.N. Ponomareva, O.A. Kornilova, T.E. Loshchilin. Textbook "Biology" grade 11, basic level, -M .: "Ventana-Graf", 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.

Cell division through meiosis occurs in two main stages: meiosis I and meiosis II. At the end of the meiotic process, four are formed. Before a dividing cell enters meiosis, it goes through a period called interphase.

Interphase

• Phase G1: stage of cell development before DNA synthesis. At this stage, the cell, preparing for division, increases in mass.
• S-phase: the period during which DNA is synthesized. For most cells, this phase takes a short period of time.
• Phase G2: the period after DNA synthesis, but before the onset of prophase. The cell continues to synthesize additional proteins and grow in size.

In the last phase of interphase, the cell still has nucleoli. surrounded by a nuclear membrane, and the cellular chromosomes are duplicated, but are in the form. The two pairs formed from the replication of one pair are located outside the nucleus. At the end of interphase, the cell enters the first stage of meiosis.

Meiosis I:

Prophase I

In prophase I of meiosis, the following changes occur:

• Chromosomes condense and attach to the nuclear envelope.
• Synapsis occurs (pairwise convergence of homologous chromosomes) and a tetrad is formed. Each tetrad consists of four chromatids.
• Genetic recombination may occur.
• Chromosomes condense and detach from the nuclear envelope.
• Likewise, the centrioles migrate away from each other, and the nuclear envelope and nucleoli are destroyed.
• Chromosomes begin to migrate to the metaphase (equatorial) plate.

At the end of prophase I, the cell enters metaphase I.

Metaphase I

In metaphase I of meiosis, the following changes occur:

• The tetrads are aligned on the metaphase plate.
• homologous chromosomes are oriented to opposite poles of the cell.

At the end of metaphase I, the cell enters anaphase I.

Anaphase I

In anaphase I of meiosis, the following changes occur:

• Chromosomes move to opposite ends of the cell. Similar to mitosis, kinetochores interact with microtubules to move chromosomes to the poles of the cell.
• Unlike mitosis, they stay together after they move to opposite poles.

At the end of anaphase I, the cell enters telophase I.

Telophase I

In telophase I of meiosis, the following changes occur:

• The spindle fibers continue to move homologous chromosomes to the poles.
• Once movement is complete, each pole of the cell has a haploid number of chromosomes.
• In most cases, cytokinesis ( division) occurs simultaneously with telophase I.
• At the end of telophase I and cytokinesis, two daughter cells are formed, each with half the number of chromosomes of the original parent cell.
• Depending on the type of cell, various processes may occur in preparation for meiosis II. However, the genetic material does not replicate again.

At the end of telophase I, the cell enters prophase II.

Meiosis II:

Prophase II

In prophase II of meiosis, the following changes occur:

• The nuclear and nuclei are destroyed until the fission spindle appears.
• Chromosomes no longer replicate in this phase.
• Chromosomes begin to migrate to the metaphase plate II (on the cell equator).

At the end of prophase II, cells enter metaphase II.

Metaphase II

In metaphase II of meiosis, the following changes occur:

• Chromosomes line up on the metaphase plate II in the center of the cells.
• Kinetochore strands of sister chromatids diverge to opposite poles.

At the end of metaphase II, cells enter anaphase II.

Anaphase II

In anaphase II of meiosis, the following changes occur:

• Sister chromatids separate and begin to move to opposite ends (poles) of the cell. Spindle fibers that are not associated with chromatids are stretched and elongate the cells.
• Once paired sister chromatids are separated from each other, each of them is considered a complete chromosome, called.
• In preparation for the next stage of meiosis, the two poles of the cells also move away from each other during anaphase II. At the end of anaphase II, each pole contains a complete compilation of chromosomes.

After anaphase II, cells enter telophase II.

Telophase II

In telophase II of meiosis, the following changes occur:

• Separate nuclei are formed at opposite poles.
• Cytokinesis occurs (division of the cytoplasm and the formation of new cells).
• At the end of meiosis II, four daughter cells are produced. Each cell has half the number of chromosomes of the original parent cell.

meiosis result

The end result of meiosis is the production of four daughter cells. These cells have two fewer chromosomes than the parent. During meiosis, only sex cells are produced. Others divide by mitosis. When the genitals unite during fertilization, they become. Diploid cells have a complete set of homologous chromosomes.