Meiosis 1 set of chromosomes in daughter cells. Cell division – mitosis, meiosis

Lecture No. 3 MITOSIS. MEIOSIS. GAMETOGENESIS. FERTILIZATION. EMBRYONAL DEVELOPMENT

A cell goes through different states in its life: a growth phase and a phase of preparation for division and division. The cell cycle - the transition from division to the synthesis of substances that make up the cell, and then again to division - can be represented in the diagram as a cycle in which several phases are distinguished.

After division, the cell enters a phase of protein synthesis and growth, this phase is called G1. Some cells from this phase enter the G0 phase, these cells function and then die without dividing (for example, red blood cells). But most cells, having accumulated the necessary substances and restored their size, and sometimes without changing size after the previous division, begin preparations for the next division. This phase is called the S phase - the phase of DNA synthesis, then, when the chromosomes have doubled, the cell enters the G2 phase - the phase of preparation for mitosis. Then mitosis (cell division) occurs and the cycle repeats. Phases G1, G2, S are collectively called interphase (i.e. the phase between cell divisions).

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Cell life and the transition from one phase of the cell cycle to another is regulated by changes in protein concentrations cyclins , as shown in the figure.

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In preparation for division, DNA replication occurs and a copy is synthesized on each chromosome. Until these chromosomes separate after duplication, each chromosome in this pair is called a chromatid. After replication, the DNA condenses, the chromosomes become more compact, and in this state they can be seen in a light microscope. Between divisions, these chromosomes are not as condensed and are more unwoven. It is clear that in a condensed state it is difficult for them to function. The chromosome appears as an X only during one of the stages of mitosis. Previously, it was believed that between cell divisions, chromosomal DNA ( chromatin ) is in a completely untwisted state, but it now turns out that the chromosome structure is quite complex and the degree of chromatin decondensation between divisions is not very great.

Division process, in which an initially diploid cell gives rise to two daughter, also diploid, cells is called mitosis . The chromosomes present in the cell double, line up in the cell, forming a mitotic plate, spindle threads are attached to them, which stretch to the poles of the cell and the cell divides, forming two copies of the original set.

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during the formation of gametes, i.e. germ cells - sperm and eggs - cell division occurs, called meiosis. The original cell has a diploid set of chromosomes, which then double. But, if during mitosis the chromatids in each chromosome simply separate, then during meiosis a chromosome (consisting of two chromatids) is closely intertwined in its parts with another chromosome homologous to it (also consisting of two chromatids), and crossing over -exchange of homologous regions of chromosomes. Then new chromosomes with mixed “mother’s” and “father’s” genes diverge and cells with a diploid set of chromosomes are formed, but the composition of these chromosomes is already different from the original one; recombination . The first meiotic division is completed, and the second meiotic division occurs without DNA synthesis, so during this division the amount of DNA is halved. From initial cells with a diploid set of chromosomes, gametes with a haploid set arise. From one diploid cell four haploid cells are formed. The phases of cell division that follow interphase are called prophase, metaphase, anaphase, telophase, and after division again interphase.

In meiosis, the phases are also called, but it is indicated which division of meiosis it belongs to. Crossing over - the exchange of parts between homologous chromosomes - occurs in prophase of the first division of meiosis (prophase I), which includes the following stages: leptotene, zygotene, pachytene, diplotene, diakinesis. The processes occurring in the cell are described in detail in Makeev’s textbook, and you should know them.

BRIEF OVERVIEW OF GAMETHOGENESIS STAGES

Gametogenesis divided into spermatogenesis (the process of sperm formation in males) and oogenesis (process of egg formation). In terms of what happens to DNA, these processes are practically the same: one initial diploid cell gives rise to four haploid ones. However, in terms of what happens to the cytoplasm, these processes are radically different.

The egg accumulates nutrients necessary for the further development of the embryo, so the egg is a very large cell, and when it divides, the goal is to preserve nutrients for the future embryo, so the division of the cytoplasm is asymmetrical. In order to preserve all the reserves of the cytoplasm and at the same time get rid of unnecessary genetic material, polar bodies are separated from the cytoplasm, which contain very little cytoplasm, but allow the division of the chromosome set. Polar bodies are separated during the first and second divisions of meiosis (more information about what happens to the polar bodies of plants is in Makeev)

During spermatogenesis, the cytoplasm of the original first-order spermatocyte is divided (first division of meiosis) equally between the cells, giving rise to second-order spermatocytes. The second division of meiosis leads to the formation of second-order haploid spermatocytes. Maturation then occurs without cell division, most of the cytoplasm is discarded, and spermatozoa are obtained containing a haploid set of chromosomes with very little cytoplasm. Below is a photograph of a human sperm and a diagram of its structure.

Animal sperm have the same basic structure, but may differ in shape and size. The sperm has a head in which DNA is tightly packed. The head of the sperm is surrounded by a very thin layer of cytoplasm. At its anterior end is a structure called an acrosome. This structure contains enzymes that allow the sperm to penetrate the membrane of the egg. The sperm has a tail. The part of the tail adjacent to the head (“neck”) is surrounded by mitochondria. They are necessary to ensure that the tail beats and the sperm moves in the desired direction. The sperm has chemoreceptors similar to olfactory cells to select the direction of movement.

Sperm maturation occurs in the seminiferous tubules of the testicles. When the original cell, the spermatogonia, transforms into a spermatocyte, spermatids and mature sperm, the cell moves from the basement membrane of the spermatic cord to its cavity. After maturation, the sperm are separated, entering the lumen of the seminiferous tubules, and are ready to move in search of the egg and fertilization. The maturation process lasts approximately three months. In male mammals, the process of sperm maturation - spermatogenesis - begins at the age of puberty and then continues until old age.

The process of maturation of the egg - oogenesis - is significantly different. During the embryonic development of mammals, a large number of eggs appear, and by the birth of a female, her ovaries already contain about 200-300 thousand eggs that have stopped at the first stage of meiotic division. During puberty, eggs begin to respond to sex hormones. Regular cyclical changes in hormones subsequently cause the maturation of an egg, usually one, sometimes two or more. When a woman is given injections of sex hormones to induce the maturation of eggs to treat infertility, excess of these hormones can lead to the maturation of several eggs, and as a result, multiple pregnancies. The egg matures in a sac called a follicle.

Over the course of a lifetime, women in modern industrialized countries mature only 400-500 eggs, while women of traditional culture – in hunter-gatherer tribes – have less than 200 eggs. This is due to differences in the tradition of childbirth: European women give birth to an average of 1-2 children, whom she feeds for an average of 3-5 months (and it is known that lactation inhibits the restoration of monthly cycles after childbirth), that is, she has a longer period of time remains for the maturation of eggs and the passage of menstrual cycles; At the same time, Bushmen women give birth to an average of 5 children, they do not have abortions, unlike Western women, and they breastfeed for 3-4 years, while ovulation is inhibited, so they have 2 times fewer monthly cycles than Western women. A greater number of ovulatory cycles leads to an increased risk of diseases of the reproductive organs in women, since each ovulation is associated with cell division, and the more divisions, the more mutations can occur, leading to the appearance of malignant tumors.

A woman's monthly cycles are regulated by changes in hormone concentrations (top graph in the figure). Under the influence of hormones, one of the resting follicles (vesicles) with the egg begins to develop. After a few days, the follicle bursts and a mature egg is released. This process is called ovulation. The mucous membrane of the uterus (endometrium) grows, preparing to receive

fertilized egg. If pregnancy does not occur, degeneration and rejection of the upper layer of the endometrium occurs, accompanied by bleeding. During ovulation, a woman's so-called basal temperature (that is, the temperature measured rectally and vaginally immediately after waking up) increases by a few tenths of a degree (bottom graph in the figure), then it may fall or remain slightly elevated until the onset of menstruation. For each woman, fluctuations in basal temperature are individual, but more or less constant with a stable monthly cycle. Thus, by changing the temperature, you can roughly judge when ovulation occurs.

Errors in determining the timing of ovulation based on basal temperature can occur due to temperature changes not related to the monthly cycle (for example, with the flu or other disease that causes a rise in temperature) or due to cycle disruptions that a woman may experience due to climate change or stress or under the influence of other factors. An example of temperature changes in one monthly cycle is shown in the figure:

After leaving the follicle, the egg remains viable for approximately 24-48 hours. Sperm, after entering the woman’s genital tract, are viable for up to 2-3 days, after which they can be mobile, but are not capable of fertilization. Therefore, fertilization is possible within 2-3 days before and 1-2 days after ovulation. The rest of the time, conception cannot occur. But in fact, the temperature jump does not occur exactly during ovulation, but when the concentration of hormones that cause ovulation changes, so the accuracy of determining the day of ovulation from the temperature chart is approximately 2 days. Therefore, fertilization can occur 3+2=5 days before ovulation and 2+2=4 days after ovulation days of the cycle. Cautious people add another 1-2 days on each side. The remaining days are considered “safe”. I would like to note that the cycle is subject to emotional regulation, for example, during the war, due to hard life and malnutrition, women stopped menstruating, this phenomenon is called “wartime amenorrhea.” However, cases are described when the husband came home from the front for 2 days, during these 2 days the woman ovulated, regardless of the phase of the cycle, and subsequently gave birth to a child. The fact that physiological processes can be quite strongly regulated by the nervous system is shown by the process of childbirth in monkeys. In humans, the first birth lasts approximately 24 hours, but in monkeys it is only a few hours, and it usually begins while the herd is at rest. That is, by the morning, when the herd is about to set off, the mother is ready to travel further with the newborn. If for some reason the birth process has not been completed by the morning, and the herd is already ready to move on, then the birth stops, since herd animals should not lag behind their relatives, and only then, at a new stop, the birth resumes.

The process of sperm entering the egg is called fertilization. The egg is surrounded by several membranes, the structure of which is such that only sperm of its own species can enter the egg. After fertilization, the shell of the egg changes and other sperm can no longer penetrate it.

In some species, several sperm can penetrate into the egg, but still participate in the fusion of nuclei only one of them. During fertilization, only the nucleus of the sperm penetrates into the egg, but the tail, together with the mitochondria, is discarded and does not enter the cell. Therefore, all animals inherit mitochondrial DNA only from their mother. A fertilized egg is called a zygote (from the Greek zygotos - joined together).

After fertilization, cell division occurs, restoring the diploid set of chromosomes. The first and several subsequent divisions of the egg occur without an increase in cell size, which is why the process is called cleavage of the egg.

Embryo(Greek “embryo”) - the early stage of development of a living organism from the beginning of the fragmentation of the egg until exit from the egg or from the mother’s body (in obstetrics, unlike embryology, the term embryo is used only for the first 8 weeks of development, after the 8th week it is called fruit).

Embryogenesis (embryonic development) is part of ontogenesis (individual development) - the development of an organism from the formation of a zygote to its death. Embryogenesis is the process in which presumptive rudiments take their definitive places.

You remember from school that during the development of a lancelet embryo, a blastula (hollow cell ball) is formed, from which a two-layer gastrula is formed by invagination (invagination) of one side of the blastula inward.

In mammals the process occurs in a slightly different way. The fragmentation of the egg in them leads to the formation of a lump of cells called a morula. The morula is divided into an internal part, from which the embryo itself then develops, and an external part, which forms a hollow vesicle called the trophoblast. Further development leads to the formation of a three-layer embryo, consisting of an inner layer - endoderm, an outer layer - ectoderm, and a third layer between them - mesoderm. From each layer, certain tissues and organs are subsequently formed.

Meiosis. Sexual reproduction of animals, plants and fungi is associated with the formation of specialized germ cells. A special type of cell division that results in the formation of sex cells is called meiosis. Unlike mitosis, in which the number of chromosomes received by daughter cells is maintained, during meiosis the number of chromosomes in daughter cells is halved.

The process of meiosis consists of two successive cell divisions - meiosis 1 (first division) and meiosis 2 (second division). Duplication of DNA and chromosomes occurs only before meiosis 1.

As a result of the first division of meiosis, cells are formed with the number of chromosomes reduced by half. The second division of meiosis ends with the formation of germ cells. Thus, all somatic cells of the body contain a double, diploid (2n) set of chromosomes, where each chromosome has a paired, homologous chromosome. Mature germ cells have only a single, haploid (n), set of chromosomes and, accordingly, half the amount of DNA.

Both divisions of meiosis include the same phases as mitosis: prophase, metaphase, anaphase, telophase.

In prophase of the first division of meiosis, chromosome spiralization occurs. At the end of prophase, when spiralization ends, chromosomes acquire their characteristic shape and size. The chromosomes of each pair, i.e. homologous, connected to each other along the entire length and twisted. This process of connecting homologous chromosomes is called conjugation. During conjugation, sections called genes (crossing over) are exchanged between some homologous chromosomes, which means the exchange of hereditary information. After conjugation, homologous chromosomes are separated from each other.

When the chromosomes are completely separated, a spindle is formed, metaphase of meiosis occurs and the chromosomes are located in the equatorial plane. Then anaphase of meiosis begins, and not halves of each chromosome, including one chromatid, as in mitosis, but whole chromosomes, each consisting of two chromatids, go to the poles of the cell. Consequently, only one of each pair of homologous chromosomes ends up in the daughter cell. Following the first division, the second division of meiosis occurs, and this division is not preceded by DNA synthesis. The interphase before the second division is very short. Prophase 2 is short-lived. In metaphase 2 chromosomes line up in the equatorial plane of the cell. In anaphase 2, their centromeres separate and each chromatid becomes an independent chromosome. In telophase 2, the divergence of sister chromosomes to the poles is completed and cell division begins. As a result, four haploid daughter cells are formed from two haploid cells.

The crossover of chromosomes that occurs in meiosis, the exchange of sections, as well as the independent divergence of each pair of homologous chromosomes determines the patterns of hereditary transmission of a trait from parents to offspring. Of each pair of two homologous chromosomes (maternal and paternal) that were part of the chromosome set of diploid organisms, the haploid set of an egg or sperm contains only one chromosome. It can be: 1. paternal chromosome; 2. maternal chromosome; 3. paternal with maternal area; 4. maternal with paternal section.

These processes of the emergence of a large number of qualitatively different germ cells contribute to hereditary variability.

In some cases, due to disruption of the meiosis process, when homologous chromosomes do not diverge, germ cells may not have a homologous chromosome or, conversely, have both homologous chromosomes. This leads to severe disturbances in the development of the organism or to its death.

Sexual reproduction of animals, plants and fungi is associated with the formation of specialized germ cells - gametes, which fuse during fertilization, combining their nuclei. Naturally, in this case, the zygote contains twice as many chromosomes as in each of the gametes. The cells of the entire organism that grow from the zygote will have the same double set of chromosomes. Indeed, non-sexual, somatic (from the Greek “soma” - body), cells of most multicellular organisms have a double, diploid (2n) set of chromosomes, where each chromosome has a paired, homologous chromosome. Gametes have a single, haploid (n), set of chromosomes, in which all chromosomes are unique and do not have homologous pairs. A special type of cell division, which results in the formation of sex cells, is called meiosis (Fig. 30). Unlike mitosis, in which the number of chromosomes received by daughter cells is maintained, during meiosis the number of chromosomes in daughter cells is halved.

Rice. 30. Meiosis scheme

The process of meiosis consists of two successive cell divisions - meiosis I (first division) and meiosis II (second division). Duplication of DNA and chromosomes occurs only before meiosis I.

As a result of the first division of meiosis, called reduction, cells are formed with the number of chromosomes halved. After the second division, the formation of mature germ cells follows.

Phases of meiosis. During prophase I of meiosis, double chromosomes are clearly visible under a light microscope. Each chromosome consists of two chromatids, which are linked together by a single centromere. During the process of spiralization, double chromosomes are shortened. Homologous chromosomes are closely connected to each other longitudinally (chromatid to chromatid), or, as they say, conjugate. In this case, the chromatids often cross or twist around one another. Then the homologous chromosomes begin to push away from each other. At places where chromatids intersect, transverse breaks occur and the chromatids exchange sections. This phenomenon is called chromosome crossing (Fig. 31). At the same time, as in mitosis, the nuclear membrane disintegrates, the nucleolus disappears, and spindle filaments are formed. The difference between prophase I of meiosis and prophase of mitosis is the conjugation of homologous chromosomes and the mutual exchange of sections during the process of chromosome crossing.

Rice. 31. Crossing of chromosomes in meiosis

A characteristic feature of metaphase I is the arrangement in the equatorial plane of the cell of homologous chromosomes lying in pairs. This is followed by anaphase I, during which entire homologous chromosomes (each consisting of two chromatids) move to opposite poles of the cell. (Note that during mitosis, chromatids diverged towards the division poles.) It is very important to emphasize one feature of chromosome divergence at this stage of meiosis: the homologous chromosomes of each pair diverge randomly, regardless of the chromosomes of other pairs. Each pole ends up with half as many chromosomes as there were in the cell at the beginning of division. Then comes telophase I, during which two cells are formed with the number of chromosomes halved.

Interphase is short because DNA synthesis does not occur. This is followed by the second meiotic division (meiosis II). It differs from mitosis only in that the number of chromosomes in metaphase II is half as large as the number of chromosomes in metaphase of mitosis in the same organism. Since each chromosome consists of two chromatids, in metaphase II the centromeres of the chromosomes divide, and the chromatids move towards the poles, which become daughter chromosomes. Only now does real interphase begin. From each initial cell four cells with a haploid set of chromosomes arise.

Diversity of gametes. Let's consider meiosis of a cell with 3 pairs of chromosomes (2n=6). After two meiotic divisions, 4 cells with a haploid set of chromosomes are formed (n=3). Since the chromosomes of each pair disperse into daughter cells independently of the chromosomes of other pairs, the formation of eight types of gametes with different combinations of chromosomes present in the mother cell is equally likely.

An even greater variety of gametes is provided by the conjugation and crossing of homologous chromosomes in the prophase of meiosis.

Biological significance of meiosis. If during the process of meiosis there was no decrease in the number of chromosomes, then in each subsequent generation, with the fusion of the nuclei of the egg and sperm, the number of chromosomes would increase indefinitely. Thanks to meiosis, mature germ cells receive a haploid (n) number of chromosomes, but upon fertilization, the diploid (2n) number characteristic of this species is restored. During meiosis, homologous chromosomes end up in different germ cells, and during fertilization, the pairing of homologous chromosomes is restored. Consequently, a complete diploid set of chromosomes and a constant amount of DNA are ensured for each species.

The crossover of chromosomes that occurs in meiosis, the exchange of sections, 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) that were part of the chromosome set of diploid organisms, the haploid set of an egg or sperm contains only one chromosome. It could be:

1. paternal chromosome;
2. maternal chromosome;
3. paternal with maternal area;
4. maternal with the paternal plot.

These processes of the emergence of a large number of qualitatively different germ cells contribute to hereditary variability.

In some cases, due to disruption of the meiosis process, with non-disjunction of homologous chromosomes, germ cells may not have a homologous chromosome or, conversely, have both homologous chromosomes. This leads to severe disturbances in the development of the organism or to its death.

1. Compare mitosis and meiosis, highlight similarities and differences.
2. Describe the concepts: meiosis, diploid set of chromosomes, haploid set of chromosomes, conjugation.
3. What is the significance of the independent segregation of homologous chromosomes in the first division of meiosis?
4. What is the biological significance of meiosis?

Remember from your zoology course how fertilization occurs in animals.

Cell division is the reproductive mechanism by which living organisms grow, develop, and produce offspring. Upon completion of mitosis, one cell divides into two daughter cells. A parent cell undergoing meiosis produces four daughter cells.

While mitosis is common to both prokaryotic and eukaryotic organisms, meiosis occurs in animals, plants and fungi.

Daughter cells in mitosis

Mitosis is the stage of the cell cycle that involves the division and separation of chromosomes. The division process ends with cytokinesis, when two different daughter cells divide and form. Before mitosis, the cell prepares to divide by replicating DNA, increasing its mass and quantity. Mitosis includes several phases: prophase, metaphase, anaphase and telophase. During these phases, chromosomes separate, move to opposite poles of the cell, and are incorporated into newly formed nuclei. At the end of the division process, the duplicated chromosomes are divided equally between the two cells. These daughter cells are genetically identical, having the same number and type of chromosomes.

Somatic cells are examples of cells that divide through mitosis. These include everything except germ cells.

Cancer cells dividing through mitosis are capable of producing three or more daughter cells. These cells have either too many or not enough chromosomes due to irregular division.

Daughter cells in meiosis

In organisms capable of , daughter cells are produced by meiosis. Meiosis is a process consisting of two steps that produce. A dividing cell goes through prophase, metaphase, anaphase and telophase twice. At the end of meiosis and cytokinesis, four are produced from a single diploid cell. These haploid daughter cells have half the number of chromosomes of the parent cell and are not genetically identical to it. During sexual reproduction, haploid gametes unite and become a diploid zygote. The zygote continues to divide by mitosis and develops into a fully functioning organism.

Daughter cells and chromosomal movement

How do the children complete the division with the corresponding number? The answer to this question concerns the spindle structure, which is made up of microtubules and proteins that manipulate chromosomes during cell division. Spindle fibers attach to replicated chromosomes, moving and separating them when needed.

Mitotic and meiotic spindles move chromosomes to opposite poles of cells, ensuring that each daughter cell receives the correct number of chromosomes. The spindle also determines the location of the metaphase plate, the plane at which the cell ultimately divides.

Daughter cells and cytokinesis

The last step in cell division occurs in cytokinesis. This process begins during anaphase and ends after telophase. During cytokinesis, a dividing cell is divided into two daughter cells using a spindle. The spindle arrangement locates an important structure during cell division called the contractile ring. The contractile ring is formed from filaments, actin proteins and microtubules, including the motor protein myosin. Myosin compresses the ring of actin filaments, forming a deep groove called the cleavage groove. As the contractile ring continues to contract, it divides the cytoplasm and splits the cell into two along the cleavage groove.

The process of cytokinesis differs in. Plant cells do not contain asters, star-shaped microtubules that help locate the cleavage groove. In fact, in plant cell cytokinesis, no commissural groove is formed. Instead, daughter cells are separated by a cell plate formed by vesicles that are released from sister chromatid organelles, which occurs at anaphase of mitosis and anaphase II of meiosis. Daughter chromosomes develop from the replication of single-stranded chromosomes during the synthesis phase (S phase) of the cell cycle.

Single-stranded chromosomes become double-stranded chromosomes, which are held together in a region called . Double-stranded chromosomes are known as sister chromosomes. The sister chromatids eventually separate and are divided among the newly formed daughter cells. Each individual chromatid is known as a daughter chromosome.

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. Organizational moment
2. Repetition.
   1. What is reproduction?
   2. What types of reproduction do you know? Give them definitions?
   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

Lesson progress

The transfer 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-duplicate molecules.

Announcing the topic of the lesson. Since we already studied the phases of mitosis and meiosis in general terms in 9th grade, 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 a 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 characteristic of the cell. A DNA molecule combined 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 beginning of cell division, the chromosomes are shortened and stained more intensely so that they become individually visible.

In a dividing cell, the chromosome has the shape 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 and the number of DNA strands in them change during the life cycle of the cell.

Let's remember: 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, various types of RNA and individual DNA nucleotides. 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, chp is the number of chromatids, c is the amount of DNA).

In the synthetic period S, the cell's DNA molecules are reduplicated. 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 genetic set of the cell is 2n 2xp 4c (diploid set, chromosomes are dichromatid, the amount of DNA is 4).

In the third period of interphase - postsynthetic G2 - the synthesis of RNA, proteins and the accumulation of energy by the cell continues. At the end of interphase, the cell increases in size and begins to divide.

Cell division.

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

Amitosis involves prokaryotic organisms and some eukaryotic cells, for example, the bladder, human liver, as well as old or damaged cells. First, the nucleolus is divided into them, then the nucleus is divided into two or more parts by constrictions, 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, chromosomes spiral. 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 nuclear membrane dissolve - the chromosomes lie in the cytoplasm of the cell. The centrioles diverge to the cell poles and form spindle filaments between themselves, and at the end of prophase the filaments are attached to the centromeres of the chromosomes. The genetic information of the cell is still the same as in interphase (2n 2хр 4с).

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

Anaphase. Each centromere splits into two, and the spindle filaments pull the daughter centromeres to opposite poles. Centromeres pull along the chromatids that separate from one another. One chromatid from a pair comes to the poles - these are daughter chromosomes. The amount of genetic information at each pole is now equal to (2n 1хр 2с).

Mitosis ends telophase. The processes occurring in this phase are the opposite of the processes observed in prophase. At the poles, the daughter chromosomes despiralize, they become thinner and become indistinguishable. Nuclear membranes form around them, and then nucleoli appear. At the same time, the cytoplasm is divided: in animal cells - by a constriction, and in plants - from the middle of the cell to the periphery. After the formation of the cytoplasmic membrane in plant cells, the cellulose membrane is formed. Two daughter cells are formed with a diploid set of single-chromatid chromosomes (2n 1хр 2с).

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

Biological significance of mitosis:

1. Precise distribution of chromosomes and their genetic information between daughter cells.
2. Ensures karyotype constancy and genetic continuity in all cellular manifestations; because otherwise, the constancy of the structure and correct functioning of the organs and tissues of a multicellular organism would not be possible.
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 proceeded in the same way, 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 has a certain number and its own specific set of chromosomes (karyotype).

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

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

Prophase of meiosis I– homologous chromosomes duplicated in interphase come closer in pairs. In this case, individual chromatids of homologous chromosomes intertwine, cross each other and can break in the same places. During this contact, homologous chromosomes can exchange corresponding sections (genes), i.e. crossing over is underway. Crossing over causes a recombination of the cell's genetic material. After this process, homologous chromosomes are separated again, the shells of the nucleus and nucleoli are dissolved, and a spindle is formed. The genetic information of a cell in prophase is 2n 2хр 4с (diploid set, dichromatid chromosomes, number of DNA molecules - 4).

Meiosis metaphase I – chromosomes are located in the equatorial plane. But if in the metaphase of mitosis homologous chromosomes have a position independent of each other, then in meiosis they lie next to each other - in pairs. The genetic information is the same (2n 2хр 4с).

Anaphase I – It is not halves of chromosomes from one chromatid that disperse to the cell poles, but whole chromosomes consisting of two chromatids. This means that from each pair of homologous chromosomes, only one, but bichromatid, 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 2хр 2с).

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

Following the first, the second meiotic division occurs, but it is not preceded by DNA synthesis. After a short prophase of meiosis II, bichromatid chromosomes in metaphase of meiosis II are located in the equatorial plane and are attached to the spindle threads. Their genetic information is the same – (1n 2хр 2с).

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

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, 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 = 8 388 608. The number of possible combinations in one parental pair is 2 23 x 2 23

3. Crossing of chromosomes, exchange of sections occurring in meiosis, as well as 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 contains only one chromosome. Moreover, it can be: 1) the paternal chromosome; 2) maternal chromosome; 3) paternal with a section of the maternal chromosome; 4) maternal with a paternal section. These processes lead to effective recombination of hereditary material in gametes formed by the body. As a result, genetic heterogeneity of gametes and offspring is determined.

When explaining, students fill out the table: “Comparative characteristics of mitosis and meiosis”

Types of division	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 and crossing over occurs
	Chromatids move towards the poles of the cell	In the first division, homologous chromosomes diverge to the poles of the cell
Number of daughter cells	2	4
Set of chromosomes in daughter cells (n – set of chromosomes, xp – chromatids, c – number of DNA)	The number of chromosomes remains constant 2n 1хр 2c (monochromatid chromosomes)	The number of chromosomes is halved 1n 1хр 1c (single-chromatid chromosomes)
Cells where division occurs	Somatic cells	Somatic cells of animal reproductive organs; 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 secondary school. –M.: “Enlightenment”, 1992.
2. I.N. Ponomareva, O.A. Kornilova, T.E. Loschilina. Textbook “Biology” 11th grade, basic level, – M.: “Ventana-Graf”, 2010.
3. S.G. Mamontov Biology for those entering universities. –M.: 2002.
4. N. Green, W. Stout, D. Taylor. Biology in 3 volumes – M.: “Mir”, 1993.
5. N.P. Dubinina. General biology. Teacher's manual. –M.: 1990.
6. N.N. Prikhodchenko, T.P. Shkurat “Fundamentals of Human Genetics.” Uch.pos. – Rostov n/a: “Phoenix”, 1997.