What set of chromosomes do daughter cells receive? §23

MEIOSIS The reproduction and individual development of organisms is based on the process of cell division. A special type of cell division that results in the formation of sex cells is called meiosis. Features of 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 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 homologous chromosome (also consisting of two chromatids), and crossing over occurs. 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 has occurred in them. First division of meiosis Phases Processes Prophase I Conjugation of homologous chromosomes (one of them is maternal, the other is paternal) Formation of a division spindle. Arrangement of homologous chromosomes along the equator Metaphase I Anaphase I Separation of pairs of chromosomes (consisting of two chromatids) and their movement to the poles. Telophase I Formation of daughter cells. The second division of meiosis Phases Prophase II Processes The daughter cells arising in telophase I undergo Metaphase II mitotic division. Centromeres divide, the chromatids of the chromosomes of both Anaphase II Telophase II daughter cells diverge to their poles. Formation of four haploid nuclei or cells. The second division of meiosis 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. As a result of meiosis, one diploid cell produces four haploid cells. The process of meiosis consists of two successive cell divisions - meiosis I (first division) 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. The second division of meiosis ends with the formation of germ cells Gametogenesis is the process of formation of male or female gametes (sex cells). A Brief Overview of the Stages of Gametogenesis Gametogenesis is divided into spermatogenesis (the process of producing sperm in males) and oogenesis (the process of producing eggs). 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. Biological significance of meiosis 1. A complete diploid set of chromosomes and a constant amount of DNA are ensured for each species. 2. A large number of qualitatively different germ cells arise, which contributes to hereditary variability. 3. Disruption of the meiosis process leads to severe disturbances in the development of the organism or to its death.

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Meiosis
Sexual reproduction of animals, plants and fungi is associated with the formation of specialized germ cells.
Meiosis- a special type of cell division that results in the formation of sex cells.
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 I(first division) and meiosis II(second division).
Doubling
DNA
and chromosomes occur only before meiosis I.
As a result of the first division of meiosis, called reductionist, cells are formed with a halved number of chromosomes. The second division of meiosis ends with the formation of germ cells. Thus, all somatic cells of the body contain double,
diploid (2n), a set of chromosomes, where each chromosome has a paired, homologous chromosome. Mature sex cells have only single, haploid (n), a set of chromosomes and, accordingly, half the amount of DNA.
Phases of meiosis
During prophase I 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 double chromosomes begin to push away from each other. At places where chromatids cross, transverse breaks and exchanges of their sections occur. This phenomenon is called crossing of chromosomes. 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.
Characteristic sign metaphase I- arrangement in the equatorial plane of the cell of homologous chromosomes lying in pairs. Following this comes anaphase I, during which entire homologous chromosomes, each consisting of two chromatids, move to opposite poles of the cell. 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 has 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 half 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 chromatids diverge to 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.
Gamete diversity
Consider meiosis of a cell that has three pairs of chromosomes ( 2n = 6). In this case, after two meiotic divisions, four 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 original mother cell is equally likely.

An even greater variety of gametes is provided by the conjugation and crossing of homologous chromosomes in meiotic prophase, which has a very large general biological significance.
Biological significance of meiosis
If the number of chromosomes did not decrease during the process of meiosis, 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, and during fertilization the characteristic number is restored this species diploid (2n) number.
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. From each pair of two homologous chromosomes
(maternal and paternal), which were part of the chromosome set of diploid organisms, the haploid set of an egg or sperm contains only one chromosome. It could be:
o paternal chromosome;
o maternal chromosome;
o paternal with maternal area;
o maternal with the paternal plot.
These processes of origin large quantity 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.

Sexual reproduction of animals, plants and fungi is associated with the formation of specialized germ cells.
Meiosis- a special type of cell division that results in the formation of sex cells.
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 I(first division) and meiosis II(second division).
DNA and chromosome duplication occurs only before meiosis I.
As a result of the first division of meiosis, called reductionist, cells are formed with a halved number of chromosomes. The second division of meiosis ends with the formation of germ cells. Thus, all somatic cells of the body contain double, diploid (2n), a set of chromosomes where each chromosome has a paired, homologous chromosome. Mature sex cells have only single, haploid (n), a set of chromosomes and, accordingly, half the amount of DNA.

Phases of meiosis

During prophase I Meiosis double chromosomes are clearly visible under a light microscope. Each chromosome consists of two chromotides, 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 double chromosomes begin to push away from each other. At places where chromatids cross, transverse breaks and exchanges of their sections occur. This phenomenon is called crossing of chromosomes. 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.
Characteristic sign metaphase I- arrangement in the equatorial plane of the cell of homologous chromosomes lying in pairs. Following this comes anaphase I, during which entire homologous chromosomes, each consisting of two chromatids, move to opposite poles of the cell. 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 has 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 half 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 diverge to 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.

Gamete diversity

Consider meiosis of a cell that has three pairs of chromosomes ( 2n = 6). In this case, after two meiotic divisions, four 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 original 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, which is of very great general biological importance.

Biological significance of meiosis

If there were no decrease in the number of chromosomes during the process of meiosis, 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:

• paternal chromosome;
• maternal chromosome;
• paternal with maternal area;
• 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.

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 has 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 diverge to 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 there were no decrease in the number of chromosomes during the process of meiosis, 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.

1. How many daughter cells and with what set of chromosomes are formed from one diploid cell as a result of: a) mitosis; b) meiosis?

Two haploid, two diploid, four haploid, four diploid.

a) As a result of mitosis - two diploid cells.

b) As a result of meiosis, there are four haploid cells.

2. What is chromosome conjugation? In what phase of meiosis does crossing over occur? What is the significance of this process?

Chromosome conjugation is observed in prophase of meiosis I. This is the process of bringing together homologous chromosomes. During conjugation, the chromatids of homologous chromosomes intersect in some places. Crossing over also occurs in prophase of meiosis I and is an exchange of regions between homologous chromosomes. Crossing over leads to recombination of hereditary material and is one of the sources of combinative variability, due to which descendants are not exact copies their parents and are different from each other.

3. What events occurring in meiosis ensure that the number of chromosomes in daughter cells is halved?

A decrease in the chromosome set occurs in anaphase I of meiosis due to the fact that not sister chromatids (as in anaphase of mitosis and anaphase II of meiosis), but bichromatid homologous chromosomes diverge to different poles of the dividing cell. Consequently, from each pair of homologous chromosomes only one will end up in the daughter cell. At the end of anaphase I, the set of chromosomes at each pole of the cell is already haploid (1n2c).

4. What is the biological significance of meiosis?

In animals and humans, meiosis leads to the formation of haploid germ cells - gametes. During the subsequent process of fertilization (fusion of gametes), the organism of the new generation receives a diploid set of chromosomes, which means it retains the karyotype inherent to this type of organism. Therefore, meiosis prevents the increase in the number of chromosomes during sexual reproduction. Without such a division mechanism, chromosome sets would double with each subsequent generation.

In plants, fungi and some protists, spores are formed through meiosis.

The processes occurring in meiosis (crossing over, independent divergence of chromosomes and chromatids) serve as the basis for the combinative variability of organisms.

5. Compare mitosis and meiosis, identify similarities and differences. What is the main difference between meiosis and mitosis?

The main difference is that as a result of meiosis, the set of chromosomes in daughter cells decreases by 2 times compared to the mother cell.

Similarities:

● They are methods of dividing eukaryotic cells and require energy.

● Accompanied by an accurate and uniform distribution of hereditary material between daughter cells.

● Similar processes of cell preparation for division (replication, doubling of centrioles, etc.).

● Similar processes occurring in the corresponding phases of division (spiralization of chromosomes, disintegration of the nuclear membrane, formation of the division spindle, etc.) and, as a consequence, the same names of the phases (prophase, metaphase, anaphase, telophase). The second division of meiosis proceeds by the same mechanism as mitosis of a haploid cell.

Differences:

● As a result of mitosis, daughter cells retain the set of chromosomes inherent in the mother cell. As a result of meiosis, the set of chromosomes in daughter cells decreases by 2 times.

● Mitosis is one cell division, and meiosis is two successive cell divisions (meiosis I and meiosis II). Therefore, as a result of mitosis, two daughter cells are formed from one mother cell, and as a result of meiosis, four are formed.

● Unlike mitosis, meiosis involves conjugation of homologous chromosomes and crossing over. Note: in fact, mitotic crossing over also exists (discovered by K. Stern in 1936), but its study is not included in the school curriculum.

● In anaphase of mitosis, sister chromatids diverge to different poles of the cell, and in anaphase I of meiosis, homologous chromosomes diverge.

And (or) other significant features.

6. A birch root cell contains 18 chromosomes.

1) The diploid cell of the birch anther has undergone meiosis. The resulting microspores divided by mitosis. How many cells were formed? How many chromosomes does each of them contain?

2) Determine the number of chromosomes and total quantity chromatids in birch cells during meiotic division:

a) in the equatorial plane of the cell in metaphase I;

b) in metaphase II;

c) at each cell pole at the end of anaphase I;

d) at each cell pole at the end of anaphase II.

1) The birch root cell is somatic, which means that birch has 2n = 18. As a result of meiosis, 4 cells are formed from one mother cell with a halved set of chromosomes. Consequently, 4 haploid microspores (n = 9) were formed from the diploid anther cell.

Each microspore then divided by mitosis. As a result of mitosis, two daughter cells with the same set of chromosomes were formed from each microspore. Thus, a total of 8 haploid cells were formed.

Answer: 8 cells were formed, each containing 9 chromosomes.

2) The formula of the hereditary material located in the equatorial plane of the cell in metaphase I is 2n4c, which for birch is 18 chromosomes, 36 chromatids. A cell in metaphase II has a set of 1n2c - 9 chromosomes, 18 chromatids. At the end of anaphase I, at each pole of the cell there is a set of 1n2c - 9 chromosomes, 18 chromatids, and at the end of anaphase II - 1n1c - 9 chromosomes, 9 chromatids.

Answer: a) 18 chromosomes, 36 chromatids; b) 9 chromosomes, 18 chromatids; c) 9 chromosomes, 18 chromatids; d) 9 chromosomes, 9 chromatids.

7. Why is meiosis not observed in organisms that are not characterized by sexual reproduction?

In the development cycle of all organisms that are characterized by sexual reproduction, the process of fertilization takes place - the fusion of two cells (gametes) into one (zygote). In fact, fertilization doubles the chromosome number. Therefore, there must also be a mechanism that reduces the number of chromosomes by 2 times, and this mechanism is meiosis. Without meiosis, chromosome sets would double with each successive generation.

Organisms that do not reproduce sexually do not undergo fertilization. Therefore, they do not have meiosis, there is no need for it.

8. Why is the second division of meiosis necessary, since a decrease in the number of chromosomes by 2 times has already occurred as a result of the first division?

Daughter cells formed as a result of the first meiotic division have a set of 1n2c, i.e. are already haploid. However, each chromosome of such a cell consists not of one chromatid, as it should be in a young cell entering a new cell cycle, but of two, as in a mature cell ready to divide. Consequently, cells with the 1n2c set will not be able to normally go through the cell cycle (and, above all, replication in the S period). Therefore, almost immediately after the first meiotic division, the second begins, during which sister chromatids diverge with the formation of “normal” single-chromatid chromosomes, characteristic of young daughter cells.

In addition, as a result of meiosis, gametes are formed in animals and humans, and spores are formed in plants. Due to the fact that meiosis is not one, but two successive divisions, the number of gametes (or spores) formed increases by 2 times.