Meiosis and its phases. Characteristics of the phases of meiosis. reproduction of organisms. Similarities between mitosis and meiosis. Meiosis, differences from mitosis Daughter cells are genetically identical mitosis or meiosis

The development and growth of living organisms is impossible without the process of cell division. In nature, there are several types and methods of division. In this article, we will briefly and clearly talk about mitosis and meiosis, explain the main meaning of these processes, and introduce how they differ and how they are similar.

Mitosis

The process of indirect fission, or mitosis, is most common in nature. It is based on the division of all existing non-sex cells, namely muscle, nerve, epithelial and others.

Mitosis consists of four phases: prophase, metaphase, anaphase and telophase. The main role of this process is the uniform distribution of the genetic code from the parent cell to two daughter cells. At the same time, the cells of the new generation are one to one similar to the mother ones.

Rice. 1. Scheme of mitosis

The time between fission processes is called interphase . Most often, interphase is much longer than mitosis. This period is characterized by:

• synthesis of protein and ATP molecules in the cell;
• duplication of chromosomes and the formation of two sister chromatids;
• an increase in the number of organelles in the cytoplasm.

Meiosis

The division of germ cells is called meiosis, it is accompanied by a halving of the number of chromosomes. The peculiarity of this process is that it takes place in two stages, which continuously follow each other.

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The interphase between the two stages of meiotic division is so short that it is almost imperceptible.

Rice. 2. Scheme of meiosis

The biological significance of meiosis is the formation of pure gametes that contain a haploid, in other words, a single set of chromosomes. Diploidy is restored after fertilization, that is, the fusion of the maternal and paternal cells. As a result of the fusion of two gametes, a zygote with a complete set of chromosomes is formed.

The decrease in the number of chromosomes during meiosis is very important, since otherwise the number of chromosomes would increase with each division. Due to reduction division, a constant number of chromosomes is maintained.

Comparative characteristics

The difference between mitosis and meiosis is the duration of the phases and the processes occurring in them. Below we offer you a table "Mitosis and meiosis", which shows the main differences between the two methods of division. The phases of meiosis are the same as those of mitosis. You can learn more about the similarities and differences between the two processes in a comparative description.

Rice. 3. Comparative scheme of mitosis and meiosis

What have we learned?

In nature, cell division differs depending on their purpose. So, for example, non-sex cells divide by mitosis, and sex cells - by meiosis. These processes have similar division schemes in some steps. The main difference is the presence of the number of chromosomes in the formed new generation of cells. So, during mitosis, the newly formed generation has a diploid set, and during meiosis, a haploid set of chromosomes. The time of the division phases also differ. Both methods of division play a huge role in the life of organisms. Without mitosis, not a single renewal of old cells, reproduction of tissues and organs takes place. Meiosis helps maintain a constant number of chromosomes in a newly formed organism during reproduction.

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

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

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

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

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

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

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

Cytokinesis I provides division of cytoplasms of daughter cells.

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

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

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

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

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

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

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

All organisms are made up of cells capable of growth, development and reproduction. Meiosis and mitosis are methods of cell division. With their help, cells multiply. Meiosis and mitosis are similar in many ways. Both processes consist of the same phases, before which there is a spiralization of chromosomes and an increase in their number by a factor of two. With the help of mitosis, somatic cells multiply, and with the help of meiosis, sex cells.

Mitosis

Mitosis is a universal method of indirect division of eukaryotic cells. With its help, the cells of animals, plants, fungi divide.

Meiosis

Meiosis is also a cell division process, but it results in the formation of gametes.

Similarities between mitosis and meiosis

Meiosis and mitosis contain the same phases, called prophase, metaphase, anaphase and telophase. In the interphase of both processes, the number of chromosomes doubles. Meiosis and mitosis are processes that ensure cell reproduction.

Comparison of the processes of mitosis and meiosis

How is mitosis different from meiosis?

biological significance

Mitosis provides a strictly identical division of carriers of hereditary information between daughter cells.

Meiosis maintains a constant number of chromosomes and promotes the emergence of new hereditary properties through conjugation.

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.

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Cell division. Mitosis and meiosis

Updating knowledge 1 . The internal semi-liquid environment of the cell, in which the organelles and the nucleus are located, is: A - vacuole B - cytoplasm C - Golgi apparatus D - mitochondria 2. The main structural component of the nucleus is: A - chromosomes B - ribosomes C - mitochondria D - chloroplasts

3. Have their own DNA: A- Golgi complex B- lysosome B- endoplasmic reticulum G- mitochondria 4. An animal cell is classified as a eukaryotic cell, since it has: A- chloroplasts cytoplasm sheath

5. Prokaryotic cells, unlike eukaryotic cells, do NOT have: A - chromosomes B - cell membrane C - nuclear membrane D - plasma membrane 6. All prokaryotic and eukaryotic cells have: A - mitochondria and nucleus B - vacuoles and Golgi complex C - nuclear membrane and chloroplasts D - plasma membrane and ribosomes

7. Animal cells, unlike plant cells, do not have: A- cell membrane and cytoplasm B- mitochondria and ribosomes C- shaped nucleus and nucleolus D- plastids, vacuoles with cell sap, fiber membrane 8. Somatic cells, unlike sex cells, contain : A - double set of chromosomes B - single set of chromosomes C - cytoplasm D - plasma membrane.

9. What cellular organelle contains RNA? A- vacuole B- ribosome D- chloroplast D- lysosome 10. The cell stores hereditary information about the characteristics of the organism, therefore it is called the unit of the living:

Cell division. Mitosis and meiosis

The time interval from the moment a cell appears as a result of division to its death or to the next division is the life cycle of the cell.

The mitotic cycle is a set of sequential and interrelated processes during the preparation of a cell for division, as well as during mitosis itself.

Interphase - the period of cell preparation for division, the period of preparation for DNA synthesis (G1) - RNA and proteins are intensively formed, the activity of enzymes involved in biosynthesis increases. DNA synthesis or its reduplication - doubling. After completion of DNA synthesis (S phase), the cell does not immediately begin to divide. cell preparation for mitosis (G2) - doubling of centrioles, protein synthesis, from which the division spindle is built. Cell growth is completed

Mitosis (indirect division) is the main method of division of eukaryotic cells Prophase Metaphase Anaphase Telophase

Prophase Chromosomes twist, spiralize, become visible; two centrioles diverge towards the poles of the cell; the spindle of division is formed; the nucleolus, the nuclear membrane, disappears.

The metaphase chromosome consists of two sister chromatids connected in centromeric regions; chromosomes line up in the equatorial plane of the cell; spindle fibers are attached to each chromosome at the centromere.

Anaphase chromatids, doubled in interphase, become independent chromosomes, and diverge to the poles of the cell.

Telophase Chromosomes, gathered at the poles of the cell, unwind, despiralize; the nuclear membrane is formed, the nucleus is formed; division of the cytoplasm occurs; organelles are distributed between two cells; two cells separate from each other.

Name the phases the cells are in

Gametogenesis (Greek “gametes” - spouse, “genesis” - origin) - the development of eggs and spermatozoa Ovogenesis Spermatogenesis

breeding period. Primary germ cells divide by mitosis, as a result of which their number increases (flows more intensively). Growth period (interphase). Accumulation of nutrients and energy, duplication of chromosomes. Future eggs - oocytes - increase in size hundreds and thousands, even millions of times. During maturation, division occurs by meiosis - four cells with a haploid set of chromosomes are formed.

Meiosis - reproductive cell division - the formation of germ cells Interphase - as in mitosis Prophase 1 Metaphase 1 Anaphase 1 Telophase 1 Interkinesis Prophase 2 Metaphase 2 Anaphase 2 Telophase 2

Prophase 1 Conjugation - convergence of homologous chromosomes Crossing over - exchange of homologous regions, the shell and nucleoli dissolve, a division spindle is formed

Biological role Mitosis accuracy of transmission of hereditary information to daughter cells; the process of growth, development and regeneration + asexual reproduction Meiosis preservation of a constant set of chromosomes and the amount of DNA for each species;

Fill in the table "Comparative characteristics of mitosis and meiosis" Questions for comparison MITOSIS MEIOSIS What changes occur in the nucleus before division begins (in interphase)? 2) What are the phases of division? 3) Is conjugation of homologous chromosomes typical? 4) How many chromosomes does each daughter cell receive? 5) Where does this process (period) take place? 6) What is the significance for the existence of the species?

Homework paragraph 28, 31, learn definitions, mitosis, meiosis, mitotic cycle, life cycle, phases of mitosis and meiosis, be able to characterize and show in the figure. Creative task: compose a poem about mitosis, compose a poem about meiosis