Meiosis, differences from mitosis. Phases of meiosis In which phase of meiosis is the nuclear membrane repaired?

Mitosis- the main method of division of eukaryotic cells, in which doubling first occurs, and then a uniform distribution of hereditary material between daughter cells.

Mitosis is a continuous process in which there are four phases: prophase, metaphase, anaphase, and telophase. Before mitosis, the cell prepares for division, or interphase. The period of cell preparation for mitosis and mitosis itself together make up mitotic cycle. Below is a brief description of the phases of the cycle.

Interphase consists of three periods: presynthetic, or postmitotic, - G 1, synthetic - S, postsynthetic, or premitotic, - G 2.

Presynthetic period (2n 2c, where n- the number of chromosomes, with- the number of DNA molecules) - cell growth, activation of biological synthesis processes, preparation for the next period.

Synthetic period (2n 4c) is DNA replication.

Postsynthetic period (2n 4c) - preparation of the cell for mitosis, synthesis and accumulation of proteins and energy for the upcoming division, an increase in the number of organelles, doubling of centrioles.

Prophase (2n 4c) - the dismantling of nuclear membranes, the divergence of centrioles to different poles of the cell, the formation of fission spindle threads, the "disappearance" of the nucleoli, the condensation of two-chromatid chromosomes.

metaphase (2n 4c) - alignment of the most condensed two-chromatid chromosomes in the equatorial plane of the cell (metaphase plate), attachment of the spindle fibers with one end to the centrioles, the other - to the centromeres of the chromosomes.

Anaphase (4n 4c) - the division of two-chromatid chromosomes into chromatids and the divergence of these sister chromatids to opposite poles of the cell (in this case, the chromatids become independent single-chromatid chromosomes).

Telophase (2n 2c in each daughter cell) - decondensation of chromosomes, the formation of nuclear membranes around each group of chromosomes, the disintegration of the fission spindle threads, the appearance of the nucleolus, the division of the cytoplasm (cytotomy). Cytotomy in animal cells occurs due to the fission furrow, in plant cells - due to the cell plate.

1 - prophase; 2 - metaphase; 3 - anaphase; 4 - telophase.

The biological significance of mitosis. The daughter cells formed as a result of this method of division are genetically identical to the mother. Mitosis ensures the constancy of the chromosome set in a number of cell generations. Underlies such processes as growth, regeneration, asexual reproduction, etc.

- This is a special way of dividing eukaryotic cells, as a result of which the transition of cells from a diploid state to a haploid one occurs. Meiosis consists of two consecutive divisions preceded by a single DNA replication.

First meiotic division (meiosis 1) called reduction, because it is during this division that the number of chromosomes is halved: from one diploid cell (2 n 4c) form two haploid (1 n 2c).

Interphase 1(at the beginning - 2 n 2c, at the end - 2 n 4c) - the synthesis and accumulation of substances and energy necessary for the implementation of both divisions, an increase in cell size and the number of organelles, doubling of centrioles, DNA replication, which ends in prophase 1.

Prophase 1 (2n 4c) - dismantling of nuclear membranes, divergence of centrioles to different poles of the cell, formation of fission spindle filaments, "disappearance" of nucleoli, condensation of two-chromatid chromosomes, conjugation of homologous chromosomes and crossing over. Conjugation- the process of convergence and interlacing of homologous chromosomes. A pair of conjugating homologous chromosomes is called bivalent. Crossing over is the process of exchanging homologous regions between homologous chromosomes.

Prophase 1 is divided into stages: leptotene(completion of DNA replication), zygotene(conjugation of homologous chromosomes, formation of bivalents), pachytene(crossing over, recombination of genes), diplotene(detection of chiasmata, 1 block of human oogenesis), diakinesis(terminalization of chiasma).

1 - leptotene; 2 - zygotene; 3 - pachytene; 4 - diplotene; 5 - diakinesis; 6 - metaphase 1; 7 - anaphase 1; 8 - telophase 1;
9 - prophase 2; 10 - metaphase 2; 11 - anaphase 2; 12 - telophase 2.

Metaphase 1 (2n 4c) - alignment of bivalents in the equatorial plane of the cell, attachment of the fission spindle threads at one end to the centrioles, the other - to the centromeres of the chromosomes.

Anaphase 1 (2n 4c) - random independent divergence of two-chromatid chromosomes to opposite poles of the cell (from each pair of homologous chromosomes, one chromosome moves to one pole, the other to the other), recombination of chromosomes.

Telophase 1 (1n 2c in each cell) - the formation of nuclear membranes around groups of two-chromatid chromosomes, the division of the cytoplasm. In many plants, a cell from anaphase 1 immediately transitions to prophase 2.

Second meiotic division (meiosis 2) called equational.

Interphase 2, or interkinesis (1n 2c), is a short break between the first and second meiotic divisions during which DNA replication does not occur. characteristic of animal cells.

Prophase 2 (1n 2c) - dismantling of nuclear membranes, divergence of centrioles to different poles of the cell, formation of spindle fibers.

Metaphase 2 (1n 2c) - alignment of two-chromatid chromosomes in the equatorial plane of the cell (metaphase plate), attachment of the spindle fibers with one end to the centrioles, the other - to the centromeres of the chromosomes; 2 block of oogenesis in humans.

Anaphase 2 (2n 2with) - the division of two-chromatid chromosomes into chromatids and the divergence of these sister chromatids to opposite poles of the cell (in this case, the chromatids become independent single-chromatid chromosomes), recombination of chromosomes.

Telophase 2 (1n 1c in each cell) - decondensation of chromosomes, the formation of nuclear membranes around each group of chromosomes, the disintegration of the fission spindle threads, the appearance of the nucleolus, the division of the cytoplasm (cytotomy) with the formation of four haploid cells as a result.

The biological significance of meiosis. Meiosis is the central event of gametogenesis in animals and sporogenesis in plants. Being the basis of combinative variability, meiosis ensures the genetic diversity of gametes.

Amitosis

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

cell cycle

cell cycle- the life of a cell from the moment of its appearance to division or death. An obligatory component of the cell cycle is the mitotic cycle, which includes a period of preparation for division and mitosis itself. In addition, there are periods of rest in the life cycle, during which the cell performs its own functions and chooses its further fate: death or return to the mitotic cycle.

Go to lectures №12"Photosynthesis. Chemosynthesis"

Go to lectures №14"Reproduction of Organisms"

It is known about living organisms that they breathe, eat, multiply and die, this is their biological function. But why is this all happening? Due to the bricks - cells that also breathe, feed, die and multiply. But how does it happen?

About the structure of cells

The house consists of bricks, blocks or logs. So the body can be divided into elementary units - cells. The whole variety of living beings consists of them, the difference lies only in their number and types. Muscles, bone tissue, skin, all internal organs are composed of them - they differ so much in their purpose. But regardless of what functions this or that cell performs, they are all arranged in approximately the same way. First of all, any "brick" has a shell and cytoplasm with organelles located in it. Some cells do not have a nucleus, they are called prokaryotic, but all more or less developed organisms consist of eukaryotic cells that have a nucleus in which genetic information is stored.

Organelles located in the cytoplasm are diverse and interesting, they perform important functions. In cells of animal origin, the endoplasmic reticulum, ribosomes, mitochondria, the Golgi complex, centrioles, lysosomes and motor elements are isolated. With the help of them, all the processes that ensure the functioning of the body take place.

cell vitality

As already mentioned, all living things eat, breathe, multiply and die. This statement is true both for whole organisms, that is, people, animals, plants, etc., and for cells. It's amazing, but each "brick" has its own life. Due to its organelles, it receives and processes nutrients, oxygen, and removes all excess to the outside. The cytoplasm itself and the endoplasmic reticulum perform a transport function, mitochondria are responsible, among other things, for respiration, as well as providing energy. The Golgi complex is involved in the accumulation and removal of cell waste products. Other organelles are also involved in complex processes. And at a certain stage, it begins to divide, that is, the process of reproduction takes place. It is worth considering in more detail.

cell division process

Reproduction is one of the stages in the development of a living organism. The same applies to cells. At a certain stage of the life cycle, they enter a state when they become ready for reproduction. they simply divide in two, lengthening, and then forming a partition. This process is simple and almost completely studied on the example of rod-shaped bacteria.

With everything is a little more complicated. They reproduce in three different ways, which are called amitosis, mitosis, and meiosis. Each of these pathways has its own characteristics, it is inherent in a particular type of cell. Amitosis

considered the simplest, it is also called direct binary fission. It doubles the DNA molecule. However, no fission spindle is formed, so this method is the most energy efficient. Amitosis is observed in unicellular organisms, while multicellular tissues reproduce by other mechanisms. However, it is sometimes observed in places where mitotic activity is reduced, for example, in mature tissues.

Sometimes direct division is isolated as a type of mitosis, but some scientists consider it a separate mechanism. The course of this process, even in old cells, is quite rare. Next, meiosis and its phases, the process of mitosis, as well as the similarities and differences of these methods, will be considered. Compared to simple division, they are more complex and perfect. This is especially true of the reduction division, so that the characteristics of the phases of meiosis will be the most detailed.

An important role in cell division is played by centrioles - special organelles, usually located next to the Golgi complex. Each such structure consists of 27 microtubules grouped in threes. The whole structure is cylindrical. Centrioles are directly involved in the formation of the cell division spindle in the process of indirect division, which will be discussed later.

Mitosis

The lifespan of cells varies. Some live for a couple of days, and some can be attributed to centenarians, since their complete change occurs very rarely. And almost all of these cells reproduce by mitosis. For most of them, an average of 10-24 hours passes between periods of division. Mitosis itself takes a short period of time - in animals about 0.5-1

hour, and in plants about 2-3. This mechanism ensures the growth of the cell population and the reproduction of units identical in their genetic content. This is how the continuity of generations is observed at the elementary level. The number of chromosomes remains unchanged. It is this mechanism that is the most common variant of the reproduction of eukaryotic cells.

The significance of this type of division is great - this process helps to grow and regenerate tissues, due to which the development of the whole organism occurs. In addition, it is mitosis that underlies asexual reproduction. And another function is the movement of cells and the replacement of obsolete ones. Therefore, it is wrong to assume that due to the fact that the stages of meiosis are more complicated, its role is much higher. Both of these processes perform different functions and are important and irreplaceable in their own way.

Mitosis consists of several phases that differ in their morphological features. The state in which the cell is, being ready for indirect division, is called interphase, and the process itself is divided into 5 more stages, which need to be considered in more detail.

Phases of mitosis

Being in interphase, the cell prepares for division: the synthesis of DNA and proteins occurs. This stage is divided into several more, during which the entire structure grows and the chromosomes are duplicated. In this state, the cell stays up to 90% of the entire life cycle.

The remaining 10% is occupied directly by the division, which is divided into 5 stages. During mitosis of plant cells, preprophase is also released, which is absent in all other cases. New structures are formed, the nucleus moves to the center. A preprophase tape is formed, marking the proposed place of the future division.

In all other cells, the process of mitosis proceeds as follows:

Table 1

Stage name	Characteristic
Prophase	The nucleus increases in size, the chromosomes in it spiralize, become visible under a microscope. The spindle is formed in the cytoplasm. The nucleolus often breaks down, but this does not always happen. The content of genetic material in the cell remains unchanged.
prometaphase	The nuclear membrane breaks down. Chromosomes begin active, but random movement. Ultimately, they all come to the plane of the metaphase plate. This step lasts up to 20 minutes.
metaphase	Chromosomes line up along the equatorial plane of the spindle at about equal distance from both poles. The number of microtubules that hold the entire structure in a stable state reaches a maximum. Sister chromatids repel each other, keeping the connection only in the centromere.
Anaphase	The shortest stage. The chromatids separate and repel each other towards the nearest poles. This process is sometimes singled out separately and is called anaphase A. In the future, the division poles themselves diverge. In the cells of some protozoa, the division spindle increases in length up to 15 times. And this sub-stage is called anaphase B. The duration and sequence of processes at this stage is variable.
Telophase	After the end of the divergence to opposite poles, the chromatids stop. Decondensation of chromosomes occurs, that is, their increase in size. The reconstruction of the nuclear membranes of future daughter cells begins. Spindle microtubules disappear. Nuclei are formed, RNA synthesis resumes.

After the completion of the division of genetic information, cytokinesis or cytotomy occurs. This term refers to the formation of bodies of daughter cells from the body of the mother. In this case, the organelles, as a rule, are divided in half, although exceptions are possible, a partition is formed. Cytokinesis is not distinguished into a separate phase, as a rule, considering it within the telophase.

So, the most interesting processes involve chromosomes that carry genetic information. What are they and why are they so important?

About chromosomes

Still not having the slightest idea about genetics, people knew that many qualities of the offspring depend on the parents. With the development of biology, it became obvious that information about a particular organism is stored in every cell, and part of it is transmitted to future generations.

At the end of the 19th century, chromosomes were discovered - structures consisting of a long

DNA molecules. This became possible with the improvement of microscopes, and even now they can only be seen during the division period. Most often, the discovery is attributed to the German scientist W. Fleming, who not only streamlined everything that was studied before him, but also made his contribution: he was one of the first to study the cellular structure, meiosis and its phases, and also introduced the term "mitosis". The very concept of "chromosome" was proposed a little later by another scientist - the German histologist G. Waldeyer.

The structure of chromosomes at the moment when they are clearly visible is quite simple - they are two chromatids connected in the middle by a centromere. It is a specific sequence of nucleotides and plays an important role in the process of cell reproduction. Ultimately, the chromosome is externally in prophase and metaphase, when it can be best seen, resembles the letter X.

In 1900, describing the principles of the transmission of hereditary traits were discovered. Then it became finally clear that chromosomes are exactly what genetic information is transmitted with. In the future, scientists conducted a series of experiments proving this. And then the subject of study was the effect that cell division has on them.

Meiosis

Unlike mitosis, this mechanism eventually leads to the formation of two cells with a set of chromosomes 2 times less than the original one. Thus, the process of meiosis serves as a transition from the diploid phase to the haploid one, and in the first place

we are talking about the division of the nucleus, and already in the second - the whole cell. Restoration of the full set of chromosomes occurs as a result of further fusion of gametes. Due to the decrease in the number of chromosomes, this method is also defined as reduction cell division.

Meiosis and its phases were studied by such well-known scientists as V. Fleming, E. Strasburgrer, V. I. Belyaev and others. The study of this process in the cells of both plants and animals continues to this day - it is so complicated. Initially, this process was considered a variant of mitosis, but almost immediately after the discovery, it was nevertheless isolated as a separate mechanism. The characterization of meiosis and its theoretical significance were first adequately described by August Weissmann as early as 1887. Since then, the study of the reduction fission process has advanced greatly, but the conclusions drawn have not yet been refuted.

Meiosis should not be confused with gametogenesis, although the two processes are closely related. Both mechanisms are involved in the formation of germ cells, but there are a number of serious differences between them. Meiosis occurs in two stages of division, each of which consists of 4 main phases, there is a short break between them. The duration of the entire process depends on the amount of DNA in the nucleus and the structure of the chromosome organization. In general, it is much longer than mitosis.

By the way, one of the main reasons for significant species diversity is meiosis. As a result of reduction division, the set of chromosomes is split in two, so that new combinations of genes appear, primarily potentially increasing the adaptability and adaptability of organisms, eventually receiving certain sets of traits and qualities.

Phases of meiosis

As already mentioned, reduction cell division is conventionally divided into two stages. Each of these stages is divided into 4 more. And the first phase of meiosis - prophase I, in turn, is divided into 5 separate stages. As this process continues to be studied, others may be identified in the future. The following phases of meiosis are now distinguished:

table 2

Stage name	Characteristic
First division (reduction)
Prophase I
leptotene	In another way, this stage is called the stage of thin threads. Chromosomes look like a tangled ball under a microscope. Sometimes a proleptotene is isolated when individual threads are still difficult to discern.
zygotene	The stage of merging threads. Homologous, that is, similar in morphology and genetically, pairs of chromosomes merge. In the process of fusion, that is, conjugation, bivalents, or tetrads, are formed. So called fairly stable complexes of pairs of chromosomes.
pachytene	Stage of thick threads. At this stage, the chromosomes spiralize and DNA replication is completed, chiasmata are formed - the points of contact of individual parts of the chromosomes - chromatids. The process of crossover takes place. Chromosomes cross over and exchange some pieces of genetic information.
diplotene	Also called the double strand stage. Homologous chromosomes in bivalents repel each other and remain connected only in chiasms.
diakinesis	At this stage, the bivalents diverge at the periphery of the nucleus.
Metaphase I	The shell of the nucleus is destroyed, a fission spindle is formed. Bivalents move to the center of the cell and line up along the equatorial plane.
Anaphase I	Bivalents break up, after which each chromosome from the pair moves to the nearest pole of the cell. Separation into chromatids does not occur.
Telophase I	The process of divergence of chromosomes is completed. Separate nuclei of daughter cells are formed, each with a haploid set. Chromosomes are despiralized and the nuclear envelope is formed. Sometimes there is cytokinesis, that is, the division of the cell body itself.
Second division (equational)
Prophase II	Chromosomes condense, the cell center divides. The nuclear envelope is destroyed. A division spindle is formed, perpendicular to the first.
Metaphase II	In each of the daughter cells, the chromosomes line up along the equator. Each of them consists of two chromatids.
Anaphase II	Each chromosome is divided into chromatids. These parts diverge towards opposite poles.
Telophase II	The resulting single chromatid chromosomes are despiralized. The nuclear envelope is formed.

So, it is obvious that the phases of meiosis division are much more complicated than the process of mitosis. But, as already mentioned, this does not detract from the biological role of indirect division, since they perform different functions.

By the way, meiosis and its phases are also observed in some protozoa. However, as a rule, it includes only one division. It is assumed that such a one-stage form later developed into a modern, two-stage one.

Differences and similarities of mitosis and meiosis

At first glance, it seems that the differences between these two processes are obvious, because they are completely different mechanisms. However, with a deeper analysis, it turns out that the differences between mitosis and meiosis are not so global, in the end they lead to the formation of new cells.

First of all, it is worth talking about what these mechanisms have in common. In fact, there are only two coincidences: in the same sequence of phases, and also in the fact that

before both types of division, DNA replication occurs. Although, with regard to meiosis, before the start of prophase I, this process is not completed completely, ending at one of the first substages. And the sequence of phases, although similar, but, in fact, the events occurring in them do not completely coincide. So the similarities between mitosis and meiosis are not so numerous.

There are much more differences. First of all, mitosis occurs in while meiosis is closely related to the formation of germ cells and sporogenesis. In the phases themselves, the processes do not completely coincide. For example, crossing over in mitosis occurs during interphase, and not always. In the second case, this process accounts for the anaphase of meiosis. Recombination of genes in indirect division is usually not carried out, which means that it does not play any role in the evolutionary development of the organism and the maintenance of intraspecific diversity. The number of cells resulting from mitosis is two, and they are genetically identical to the mother and have a diploid set of chromosomes. During reduction division, everything is different. The result of meiosis is 4 different from the mother. In addition, both mechanisms differ significantly in duration, and this is due not only to the difference in the number of fission steps, but also to the duration of each of the steps. For example, the first prophase of meiosis lasts much longer, because chromosome conjugation and crossing over occur at this time. That is why it is additionally divided into several stages.

In general, the similarities between mitosis and meiosis are rather insignificant compared to their differences from each other. It is almost impossible to confuse these processes. Therefore, it is now even somewhat surprising that the reduction division was previously considered a type of mitosis.

Consequences of meiosis

As already mentioned, after the end of the reduction division process, instead of the mother cell with a diploid set of chromosomes, four haploid ones are formed. And if we talk about the differences between mitosis and meiosis, this is the most significant. Restoration of the required amount, if we are talking about germ cells, occurs after fertilization. Thus, with each new generation there is no doubling of the number of chromosomes.

In addition, during meiosis occurs in the process of reproduction, this leads to the maintenance of intraspecific diversity. So the fact that even siblings are sometimes very different from each other is precisely the result of meiosis.

By the way, the sterility of some hybrids in the animal kingdom is also a problem of reduction division. The fact is that the chromosomes of parents belonging to different species cannot enter into conjugation, which means that the process of formation of full-fledged viable germ cells is impossible. Thus, it is meiosis that underlies the evolutionary development of animals, plants and other organisms.

Meiosis- this is a special way of dividing eukaryotic cells, as a result of which the transition of cells from a diploid state to a haploid one occurs. Meiosis consists of two consecutive divisions preceded by a single DNA replication.

First meiotic division (meiosis 1) called reduction, since it is during this division that the number of chromosomes decreases by half: from one diploid cell (2 n 4c) form two haploid (1 n 2c).

Interphase 1 (at the beginning - 2 n 2c, at the end - 2 n 4c) - synthesis and accumulation of substances and energy necessary for the implementation of both divisions, an increase in cell size and the number of organelles, doubling of centrioles, DNA replication, which ends in prophase 1.

Prophase 1 (2n 4c) - dismantling of nuclear membranes, divergence of centrioles to different poles of the cell, formation of fission spindle threads, "disappearance" of nucleoli, condensation of two-chromatid chromosomes, conjugation of homologous chromosomes and crossing over. Conjugation- the process of convergence and interlacing of homologous chromosomes. A pair of conjugating homologous chromosomes is called bivalent. Crossing over- the process of exchange of homologous regions between homologous chromosomes.

Prophase 1 is divided into stages:

leptotene(completion of DNA replication),
zygotene(conjugation of homologous chromosomes, formation of bivalents),
pachytene(crossing over, recombination of genes),
diplotene(detection of chiasmata, 1 block of human oogenesis),
diakinesis(terminalization of chiasma).

1 - leptotene; 2 - zygotene; 3 - pachytene; 4 - diplotene; 5 - diakinesis; 6 - metaphase 1; 7 - anaphase 1; 8 - telophase 1; 9 - prophase 2; 10 - metaphase 2; 11 - anaphase 2; 12 - telophase 2.

Metaphase 1 (2n 4c) - alignment of bivalents in the equatorial plane of the cell, attachment of the fission spindle threads at one end to the centrioles, the other - to the centromeres of the chromosomes.

Anaphase 1 (2n 4c) - random independent divergence of two-chromatid chromosomes to opposite poles of the cell (from each pair of homologous chromosomes, one chromosome moves to one pole, the other to the other), recombination of chromosomes.

Telophase 1 (1n 2c in each cell) - the formation of nuclear membranes around groups of two-chromatid chromosomes, the division of the cytoplasm. In many plants, a cell from anaphase 1 immediately transitions to prophase 2.

Second meiotic division (meiosis 2) called equational.

Interphase 2 , or interkinesis (1n 2c), is a short break between the first and second meiotic divisions during which DNA replication does not occur. characteristic of animal cells.

Prophase 2 (1n 2c) - dismantling of nuclear membranes, divergence of centrioles to different poles of the cell, the formation of fission spindle filaments.

Metaphase 2 (1n 2c) - alignment of two-chromatid chromosomes in the equatorial plane of the cell (metaphase plate), attachment of the spindle fibers with one end to the centrioles, the other - to the centromeres of the chromosomes; 2 block of oogenesis in humans.

Anaphase 2 (2n 2with) - the division of two-chromatid chromosomes into chromatids and the divergence of these sister chromatids to opposite poles of the cell (in this case, the chromatids become independent single-chromatid chromosomes), recombination of chromosomes.

Telophase 2 (1n 1c in each cell) - decondensation of chromosomes, the formation of nuclear membranes around each group of chromosomes, the disintegration of the fission spindle threads, the appearance of the nucleolus, the division of the cytoplasm (cytotomy) with the formation of four haploid cells as a result.

The biological significance of meiosis . Meiosis is the central event of gametogenesis in animals and sporogenesis in plants. Being the basis of combinative variability, meiosis ensures the genetic diversity of gametes.

With the number reduced by two relative to the parent cell. 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.

This is an important process in evolutionary terms, which allows organisms to create diverse populations in response to environmental changes. Without understanding the significance of meiosis, further study of such sections of biology as selection, genetics, and ecology is impossible.

What is meiosis

This method of division is characteristic for the formation of gametes in animals, plants and fungi. Meiosis produces cells that have a haploid set of chromosomes, also called sex cells.

Unlike another variant of cell multiplication - mitosis, in which the number of chromosomes of daughter individuals is characteristic of the mother, during meiosis, the number of chromosomes is halved. This happens in two stages - meiosis 1 and meiosis 2. The first part of the process is similar to mitosis - DNA doubling occurs before it, an increase in the number of chromosomes. Next comes the reduction division. As a result, cells with a haploid (rather than diploid) set of chromosomes are formed.

Basic concepts

In order to understand what meiosis is, it is necessary to remember the definitions of some concepts so as not to return to them later.

Chromosome - a structure in the nucleus of a cell, which has a nucleoprotein nature and has concentrated most of the hereditary information.
Somatic and germ cells - cells of the body that have a different set of chromosomes. Normally (excluding polyploids) somatic cells are diploid (2n) and sex haploid (n). When two germ cells merge, a complete somatic cell is formed.
Centromere is a section of the chromosome responsible for gene expression and connecting chromatids to each other.
Telomeres - end sections of chromosomes, perform a protective function.
Mitosis is a way of dividing somatic cells, creating copies identical to them in the process.
Euchromatin and heterochromatin are sections of chromatin in the nucleus. The first retains the despiralized state, the second is spiralized.

Process steps

Meiosis of a cell consists of two consecutive divisions.

First division. During prophase 1, chromosomes can be seen even with a light microscope. The structure of a double chromosome consists of two chromatids and a centromere. Spiralization occurs and, as a result, shortening of the chromatids in the chromosome. Meiosis begins at metaphase 1. Homologous chromosomes are located in the equatorial plane of the cell. This is called the alignment of tetrads (bivalents) of chromatid to chromatid. At this point, the processes of conjugation and crossing over occur, they are described below. During these actions, telomeres often cross over and overlap each other. The shell of the nucleus begins to disintegrate, the nucleolus disappears and the fission spindle threads become visible. During anaphase 1, whole chromosomes, consisting of two chromatids, move to the poles, and in a random way.

As a result of the first division in the telophase 1 stage, two cells with a single set of DNA are formed (in contrast to mitosis, the daughter cells of which are diploid). Interphase is short because it does not require DNA duplication.

In the second division at the stage of metaphase 2, already one chromosome (from two chromatids) departs to the equatorial part of the cell, forming a metaphase plate. The centromere of each chromosome divides, the chromatids diverge towards the poles. At the telophase stage of this division, two cells are formed containing each haploid set of chromosomes. There is already a normal interphase.

conjugation and crossing over

Conjugation is the process of fusion of homologous chromosomes, and crossing over is the exchange of the corresponding sections of homologous chromosomes (begins in the prophase of the first division, ends in metaphase 1 or in anaphase 1 when the chromosomes diverge). These are two related processes that are involved in the additional recombination of genetic material. Thus, the chromosomes in haploid cells are not similar to those in the mother, but already exist with substitutions.

Variety of gametes

Gametes formed during meiosis are not homologous to each other. Chromosomes diverge into daughter cells independently of each other, so they can bring a variety of alleles to future offspring. Consider the simplest classical problem: determine the types of gametes formed in the parent organism according to two simple traits. Let us have a dark-eyed and dark-haired parent, heterozygous for these traits. The allele formula that characterizes it will look like AaBb. Sex cells will look like this: AB, Ab, aB, ab. That is four types. Naturally, the number of alleles in a living organism with many traits will be many times higher, which means that there will be many times more options for the diversity of gametes. These processes are enhanced by conjugation and crossing over occurring in the process of fission.

There are errors in replication and divergence of chromosomes. This leads to the formation of defective gametes. Normally, such cells should undergo apoptosis (cell death), but sometimes they merge with another germ cell, forming a new organism. For example, Down's disease is formed in a person in this way, associated with one extra chromosome.

It should be mentioned that the formed germ cells in different organisms undergo further development. For example, in a person, four equivalent spermatozoa are formed from one parental cell - as in classical meiosis, what an egg is - it is somewhat more difficult to find out. From four potentially identical cells, one egg and three reduction bodies are formed.

Meiosis: biological significance

Why in the process of meiosis the number of chromosomes in a cell decreases is understandable: if this mechanism did not exist, then when two germ cells merge, there would be a constant increase in the chromosome set. Due to reduction division, in the process of reproduction, a full-fledged diploid cell emerges from the fusion of two gametes. Thus, the constancy of the species, the stability of its chromosome set, is preserved.

Half of the DNA of the daughter organism will contain maternal and half paternal genetic information.

The mechanisms of meiosis underlie the sterility of interspecific hybrids. Due to the fact that the cells of such organisms contain chromosomes from two species, during metaphase 1 they cannot enter into conjugation and the process of formation of germ cells is disrupted. Fertile hybrids are possible only between closely related species. In the case of polyploid organisms (for example, many agricultural plants), in cells with an even set of chromosomes (octoploids, tetraploids), the chromosomes diverge as in classical meiosis. In the case of triploids, chromatids are formed unevenly, there is a high risk of getting defective gametes. These plants propagate vegetatively.

Thus, understanding what meiosis is is a fundamental question of biology. The processes of sexual reproduction, the accumulation of random mutations, and their transmission to offspring underlie hereditary variability and indefinite selection. Modern selection is formed on the basis of these mechanisms.

Meiosis variants

The considered variant of division in meiosis is characteristic mainly of multicellular organisms. In the simplest, the mechanism looks somewhat different. In the process of it, one meiotic division proceeds, the crossing-over phase, respectively, also shifts. Such a mechanism is considered more primitive. It served as the basis for the division of haploid cells of modern animals, plants, fungi, which proceeds in two phases and provides the best recombination of genetic material.

Differences between meiosis and mitosis

Summing up the differences between these two types of division, it is necessary to note the ploidy of the daughter cells. If during mitosis the amount of DNA, chromosomes in both generations is the same - diploid, then in meiosis haploid cells are formed. In this case, as a result of the first process, two are formed, and as a result of the second - four cells. There is no crossing over in mitosis. The biological significance of these divisions also varies. If the goal of meiosis is the formation of germ cells and their subsequent fusion in different organisms, that is, the recombination of genetic material in generations, then the goal of mitosis is to maintain tissue stability and the integrity of the body.