Meiosis, differences from mitosis

Mitosis (along with the stage of cytokinesis) is the process by which a eukaryotic somatic cell (or body cell) is divided into two identical diploid cells.

Meiosis is another type of cell division that starts with one cell having the correct number of chromosomes and ends with the formation of four cells with half the number of chromosomes (haploid cells).

In humans, almost all cells undergo mitosis. The only human cells that divide by meiosis are gametes or germ cells (ovum in women and sperm in men).

Gametes have only half the chromosomes relative to body cells, because when germ cells merge during fertilization, the resulting cell (called the zygote) has the correct number of chromosomes. This is why offspring is a mixture of mother and father genetics (father's gametes contain one half of the chromosomes, and mother's gametes contain the other).

Although mitosis and meiosis give very different results, these processes are quite similar and proceed with slight differences at the main stages. Let's look at the main differences between mitosis and meiosis in order to better understand how they work.

Both processes begin after the cell passes through the interphase and synthesizes DNA in the S-phase (or synthesis phase). At this point, each chromosome consists of sister chromatids that are held together by centromeres.

Sister chromatids are identical to each other. During mitosis, a cell passes the M phase (or mitotic phase) only once, forming a total of two identical diploid cells. In meiosis, two rounds of the M phase occur, so the end result is four haploid cells that are not identical.

Stages of mitosis and meiosis

There are four (some sources distinguish five) phases of mitosis and a total of eight phases of meiosis (or four repeating twice). Since meiosis passes through two stages, it is divided into meiosis I and meiosis II. At each stage of mitosis and meiosis, there are many changes in the cell, but they have very similar, if not identical, important events in each phase. It is fairly easy to compare mitosis and meiosis, given these most important changes.

The first stage is called prophase in mitosis and prophase I in meiosis I (or prophase II meiosis II). During prophase, the nucleus prepares for fission. This means that the nuclear membrane is destroyed and chromosomes begin to condense. In addition, a spindle of division forms in the centriole of the cell, which helps with chromosome separation at later stages. This is all that occurs in mitotic prophase, prophase I, and usually in prophase II. As a rule, at the beginning of prophase II, the nuclear membrane is absent, and chromosomes are already condensed from prophase I.

There are several differences between mitotic prophase and prophase I. During prophase I, homologous chromosomes combine. Each chromosome has a corresponding chromosome that carries the same genes, and also usually has the same size and shape. These pairs are called homologous pairs of chromosomes. During prophase I, homologous chromosomes fuse and sometimes intertwine.

A process called intersection can occur during prophase I. This occurs when homologous chromosomes overlap and exchange genetic material. The actual parts of one of the sister chromatids break down and reattach to another homolog. The purpose of the intersection is to further increase genetic diversity, since the alleles for these genes are now on different chromosomes and can be placed in different gametes at the end of meiosis II.

In the metaphase, the chromosomes are going to line up at the equator or in the middle of the cell, and the newly formed spindle division attaches to these chromosomes in order to prepare for their separation. In mitotic metaphase and metaphase II, a spindle is attached to each side of the centromeres, which together hold sister chromatids. However, in metaphase I, the spindle joins various homologous chromosomes in the centromere. Therefore, in the mitotic metaphase and metaphase II, spindle fibers of fission on each side of the cell are associated with the same chromosome.

Anaphase is the stage at which physical breakdown occurs. In mitotic anaphase and anaphase II, sister chromatids move apart and move to opposite sides of the cell by shortening the division spindle. Since spindle microtubules are attached to kinetochores in the centromere on both sides of the same chromosome during the metaphase, they break the chromosome into two separate chromatids.

Mitotic anaphase separates identical sister chromatids, so identical genetics will be in every cell. In anaphase I, sister chromatids are not identical, as they underwent a transition during prophase I. In anaphase I, sister chromatids remain together, but homologous pairs of chromosomes move apart and are transferred to opposite poles of the cell.

The final stage of the cell cycle is called telophase. In mitotic telophase and telophase II, most of what was done during prophase will be reversed. The spindle division breaks down and disappears, the nuclear membrane forms, the chromosomes unravel, and the cell prepares for separation during cytokinesis.

At this point, mitotic telophase passes into cytokinesis, the result of which will be two identical diploid cells. Telophase II has already passed one division at the end of meiosis I, so it will go into cytokinesis to make a total of four haploid cells. In telophase I, similar events are observed depending on the type of cell. The spindle is destroyed, but a new nuclear envelope is not formed, and the chromosomes can remain tightly entangled. In addition, some cells go directly to prophase II instead of being split into two cells by cytokinesis.

Table of the main differences between mitosis and meiosis

Mitosis and meiosis in evolution

Typically, mutations in the DNA of somatic cells that undergo mitosis are not transmitted to offspring and therefore are not applicable to natural selection and do not contribute to the evolution of the species. However, errors in meiosis and the random mixing of genes and chromosomes throughout the process do contribute to genetic diversity and lead to evolution. Intersection creates a new combination of genes that can encode favorable adaptation.

In addition, an independent assortment of chromosomes during metaphase I also leads to genetic diversity. Homologous pairs of chromosomes line up at this stage, so mixing and matching traits has many options, which contributes to diversity. Finally, accidental fertilization can also increase genetic diversity. Since at the end of meiosis II four genetically different gametes are formed, which are actually used during fertilization. As the existing characters are mixed and transmitted, natural selection affects them and selects the most favorable adaptations as the preferred phenotypes of individuals.

Differences of meiosis from mitosis according to the results

1. After mitosis, two cells are obtained, and after meiosis, four.

2. After mitosis, somatic cells (body cells) are obtained, and after meiosis, germ cells (gametes are sperm and eggs, spores are obtained in plants after meiosis).

3. After mitosis, identical cells (copies) are obtained, and after meiosis - different ones (hereditary information is recombined).

4. After mitosis, the number of chromosomes in daughter cells remains the same as in the mother, and after meiosis decreases by 2 times (there is a reduction in the number of chromosomes, if there weren’t, then after each fertilization the number of chromosomes would double), alternation reduction and fertilization ensures the constancy of the number of chromosomes).

Differences between meiosis and mitosis along the way

1. In mitosis, one division, and in meiosis - two (because of this, 4 cells are obtained).

2. In the prophase of the first division of meiosis, conjugation (close convergence of homologous chromosomes) and crossing-over (exchange of sites of homologous chromosomes) occur, this leads to recombination (recombination) of hereditary information.

3. In the anaphase of the first division of meiosis, an independent divergence of homologous chromosomes occurs (two-chromatid chromosomes diverge to the poles of the cell). This leads to recombination and reduction.

4. In the interphase between two divisions of meiosis, chromosome doubling does not occur, since they are already double.

The second division of meiosis is no different from mitosis. As in mitosis, in anaphase II of meiosis, single sister chromosomes (former chromatids) diverge to the poles of the cell.

Cell Life Cycle (Cell Cycle)

From the moment a cell appears until its death as a result of apoptosis (programmed cell death), the cell's life cycle continues uninterruptedly.

Here and in the future we will use the genetic formula of the cell, where "n" is the number of chromosomes, and "c" is the number of DNA (chromatid). Let me remind you that each chromosome can contain either one DNA molecule (one chromatid) (nc), or two (n2c).

The cell cycle includes several stages: division (mitosis), postmitotic (presynthetic), synthetic, postsynthetic (premitotic) period. The last three periods make up interphase - preparation for cell division.

We analyze the periods of interphase in more detail:

Postmitotic period G₁ - 2n2c

Ribosomes are intensively formed, ATP and all types of RNA, enzymes are synthesized, mitochondria divide, the cell grows.

Synthetic period S - 2n4c

Lasts 6-10 hours. The most important event of this period is the doubling of DNA, due to which, at the end of the synthetic period, each chromosome consists of two chromatids. Structural DNA proteins, histones, are actively synthesized.

Premitotic period G₂ - 2n4c

Short, lasts 2-6 hours. This time the cell spends preparing for the next process - cell division, proteins and ATP are synthesized, centrioles are doubled.

Mitosis (Greek μίτος - thread)

Mitosis is an indirect method of cell division, the most common among eukaryotic organisms. The duration takes about 1 hour. A cell is prepared for mitosis during the interphase period by synthesizing proteins, ATP and doubling the DNA molecule in the synthetic period.

Mitosis consists of 4 phases, which we will consider in more detail below: prophase, metaphase, anaphase, telophase. Let me remind you that a cell enters mitosis with an already doubled (in the synthetic period) amount of DNA. We will consider mitosis as an example of a cell with a set of chromosomes and 2n4c DNA.

• The shapeless chromatin in the nucleus begins to assemble into clearly formed structures - chromosomes - this happens due to DNA helixing (remember my example of the association of a chromosome with a skein of thread)
• The shell of the nucleus breaks up, the chromosomes appear in the cytoplasm of the cell
• Centrioles move to the poles of the cell, fission spindle centers form

DNA is maximally spiralized into the chromosomes that are located at the equator of the cell. Each chromosome consists of two chromatids connected by a centromere (kinetochore). The threads of the division spindle are attached to the centromeres of the chromosomes (more precisely, they are attached to the centromere kinetochore).

The shortest phase of mitosis. Chromosomes consisting of two chromatids break down into separate chromatids. The threads of the fission spindle pull chromatids (synonymous with daughter chromosomes) to the poles of the cell.

In this phase, chromatids (daughter chromosomes) reach the poles of the cell.

• The process of DNA despiralization begins, chromosomes disappear and become chromatin (remember the association about the untwisted skein of thread)
• A nuclear shell appears, a core is formed
• The threads of the spindle are broken

In the telophase, cytoplasm is divided - cytokinesis (cytotomy), as a result of which two daughter cells with a set of 2n2c are formed. In animal cells, cytokinesis is carried out by constriction of the cytoplasm, in plant cells - by the formation of a dense cell wall (which grows from the inside outwards).

The daughter cells 2n2c formed in telophase enter the postmitotic period. Then, in the synthetic period, where DNA is doubled, after which each chromosome consists of two chromatids - 2n4c. A cell with a set of 2n4c and enters the prophase of mitosis. So the cell cycle closes.

The biological significance of mitosis is very significant:

• The number of chromosomes in daughter and mother cells is the same; the number of chromosomes in generations is constant.
• As a result of mitosis, daughter cells are formed - genetic copies (clones) of the mother.
• Mitosis is a universal method of asexual reproduction, regeneration and proceeds identically in all eukaryotes (nuclear organisms).
• The universality of mitosis is yet another proof of the unity of the whole organic world.

Try to recall the phases of mitosis on your own and describe the events that occur in them. Pay particular attention to the state of chromosomes, emphasize how many DNA molecules (chromatids) they contain.

Meiosis (from the Greek. Μείωσις - reduction), or reduction cell division - a way of cell division, in which the hereditary material in them (the number of chromosomes) is halved. Meiosis occurs during the formation of germ cells (gametes) in animals and spores in plants.

As a result of meiosis, diploid cells (2n) produce haploid (n) cells. Meiosis consists of two consecutive divisions, between which there is practically no pause. DNA doubling before meiosis occurs in the synthetic period of the interphase (as in mitosis).

As already mentioned, meiosis consists of two divisions: meiosis I (reduction) and meiosis II (equational). The first division is called reduction (lat. Reductio - decrease), since by the end of it the number of chromosomes is halved. The second division - equational (Latin aequatio - equalization) is very similar to mitosis.

We proceed to the study of the first division of meiosis. As a basis, we take a cell with two chromosomes and doubled (in the synthetic period of interphase) amount of DNA - 2n4c.

Prophase of meiosis I

In addition to the processes typical of prophase (DNA spiraling into chromosomes, destruction of the nuclear membrane, movement of centrioles to the poles of the cell), two major processes occur in meiosis I prophase: conjugation and crossing over.

Conjugation (lat. Conjugatio - connection) - the convergence of homologous chromosomes with each other. Homologous chromosomes are those that correspond to each other in size, shape and structure. As a result of conjugation, complexes are formed consisting of two chromosomes - bivalents (lat. Bi - double and valens - strong).

После конъюгации становится возможен следующий процесс - кроссинговер (от англ. crossing over — пересечение), в ходе которого происходит обмен участками между гомологичными хромосомами.

Кроссинговер является важнейшим процессом, в ходе которого возникают рекомбинации генов, что создает уникальный материал для эволюции, последующего естественного отбора. Кроссинговер приводит к генетическому разнообразию потомства.

Биваленты (комплексы из двух хромосом) выстраиваются по экватору клетки. Формируется веретено деления, нити которого крепятся к центромере (кинетохору) каждой хромосомы, составляющей бивалент.

The threads of the division spindle are reduced, as a result of which the bivalents break up into individual chromosomes, which are attracted to the poles of the cell. As a result, a haploid set of the future cell, n2c, is formed at each pole, due to which meiosis I is called reduction division.

Cytokinesis occurs - division of the cytoplasm. Two cells are formed with a haploid set of chromosomes. A very short interphase after meiosis I is replaced by a new division - meiosis II.

Meiosis II is very similar to mitosis in all phases, so if you have forgotten something: look in the topic about mitosis. The main difference between meiosis II and meiosis I is that in the anaphase of meiosis II, chromatids (daughter chromosomes) do not diverge towards the poles of the cell.

As a result of meiosis I and meiosis II, we obtained from a diploid cell 2n4c a haploid cell - nc. This is the essence of meiosis - the formation of haploid (sex) cells. We still have to recall the set of chromosomes and DNA in various phases of meiosis when we study gametogenesis, which results in the formation of sperm and egg cells - germ cells (gametes).

Now we take a cell in which 4 chromosomes. Try to independently describe the phases and stages through which it will pass during meiosis. Speak and comprehend the set of chromosomes in each phase.

Remember that before meiosis, DNA is doubled in the synthetic period. Because of this, already at the beginning of meiosis, you see their increased number - 2n4c (4 chromosomes, 8 DNA molecules). I understand that I want to write 4n8c, but this is a wrong entry!) After all, our original cell is diploid (2n), not tetraploid (4n),)

So, it's time to discuss the biological significance of meiosis:

• Maintains a constant number of chromosomes in all generations, prevents the doubling of the number of chromosomes
• Thanks to crossing over, new combinations of genes arise, and the genetic diversity of gamete composition is ensured.
• Offspring with new traits - material for evolution that undergoes natural selection

Tests and Tasks

All of the terms below are used to describe meiosis. Define two terms that “fall out” of the general list and write down in the numbers under which they are indicated.
1) bivalents
2) reduction division
3) cloning
4) fertilization
5) crossingover

1. Establish a correspondence between the methods of cell division and their features: 1) mitosis, 2) meiosis. Write down the numbers 1 and 2 in the correct order.
A) reduction division
B) provides growth, regeneration
C) daughter cells are identical to the parent
D) four haploid cells are formed
D) increases genetic diversity
E) indirect division

2. Establish a correspondence between the processes occurring during cell division and the methods of division: 1) mitosis, 2) meiosis. Write down the numbers 1 and 2 in the correct order.
A) provides the growth and development of the body
B) as a result of division, somatic cells are formed
C) maintains a constant number of chromosomes in the cells of individuals of the same species during sexual reproduction
D) is the basis of combinational variability
D) is the basis of vegetative propagation
E) bivalents are formed in the process of division

3. Establish a correspondence between the characteristics of the processes and the method of cell division: 1) mitosis, 2) meiosis. Write down the numbers 1 and 2 in the correct order.
A) the formation of germ cells in mammals
B) body growth
C) zygote division
D) conjugation and crossingover
E) halving the number of chromosomes

4. Establish a correspondence between the processes and the method of cell division: 1) mitosis, 2) meiosis. Write down the numbers 1 and 2 in the correct order.
A) there is a division of somatic cells
B) the chromosome set is halved
C) a new combination of genes is formed
D) conjugation and crossingover occur
D) bivalents are located at the equator of the cell

5. Establish a correspondence between the processes and methods of division: 1) meiosis, 2) mitosis. Write down the numbers 1 and 2 in the correct order.
A) there is the formation of bivalents
B) the formation of diploid cells
C) the number of chromosomes varies
D) crossover occurs
D) the content of genetic material does not change
E) there is a divergence of two chromatid chromosomes to the poles of the cell

6. Set the correspondence between the features of cell division and its type: 1) Mitosis, 2) Meiosis. Write down the numbers 1 and 2 in the correct order.
A) occurs in two stages
B) after division, diploid cells are formed
C) the resulting cells have a set of chromosomes and 2n2c DNA
D) is accompanied by conjugation of chromosomes
D) the resulting cells have a set of chromosomes and DNA nc
E) crossing over occurs

7. Establish a correspondence between the type of cell division and the biological value: 1) mitosis, 2) meiosis. Write down the numbers 1 and 2 in the order corresponding to the letters.
A) genetic stability
B) combinational variability
B) regeneration
D) body growth
D) asexual reproduction
E) sexual reproduction

8. Set the correspondence between the characteristics of the process and the methods of cell division: 1) mitosis, 2) meiosis. Write down the numbers 1 and 2 in the order corresponding to the letters.
1) pairs of homologous chromosomes are formed
2) homologous chromosomes diverge to the poles
3) conjugation and crossingover occur
4) there is a reduction in the number of chromosomes
5) at the end of the process, two daughter cells are formed
6) the identity of the hereditary information of new cells to the mother cell is respected

9. Establish a correspondence between the characteristics of the process and the methods of cell division: 1) mitosis, 2) meiosis. Write down the numbers 1 and 2 in the order corresponding to the letters.
A) cells are formed with the chromosome set nc
B) two-chromatid chromosomes diverge to the poles
C) conjugation and crossingover occurs
D) the number of chromosomes remains unchanged
D) at the end of the process, four daughter cells are formed
E) reduction of the number of chromosomes

10. Set the correspondence between the characteristics and methods of cell division: 1) mitosis, 2) meiosis. Write down the numbers 1 and 2 in the order corresponding to the letters.
A) a decrease in the number of chromosomes in the cell
B) the formation of cells identical to the maternal
C) the formation of somatic cells
D) the formation of gametes in animals
D) ensuring the growth of organisms
E) the formation of spores in plants

GATHER 11:
A) preserves the karyotype of the original cell

Choose one, the most correct option. Chromosome chromosomes during meiosis move to the poles of the cell in
1) anaphase I division
2) anaphase II division
3) prophase I division
4) prophase II division

Choose one, the most correct option. The first division of meiosis is different from the second division of meiosis
1) the divergence of daughter chromatids in the resulting cells
2) the divergence of homologous chromosomes and the formation of two haploid cells
3) dividing into two parts the primary constriction of chromosomes
4) the formation of two diploid cells

All of the signs below, except for two, can be used to characterize the processes and biological significance of meiosis. Identify two signs that “fall out” of the general list and write down the numbers under which they are indicated.
1) the formation of cells with double the number of chromosomes
2) the formation of haploid cells
3) the formation of bivalents
4) the emergence of new gene combinations
5) the appearance of a larger number of somatic cells

Consider the picture depicting cell division and determine (A) its type, (B) the set of chromosomes in the cell shown on the left, and (C) which specific cells are formed in animals as a result of such division. For each letter, select the appropriate term from the list provided.
1) mitosis
2) transcription
3) diploid
4) meiosis
5) direct
6) haploid
7) gamete
8) somatic

Choose three options. What signs characterize meiosis?
1) the presence of two successive divisions
2) the formation of two cells with the same hereditary information
3) the divergence of homologous chromosomes in different cells
4) the formation of diploid daughter cells
5) lack of interphase before the first division
6) conjugation and crossing over of chromosomes

1. Establish a sequence of processes occurring during meiosis
1) the location of pairs of homologous chromosomes in the equatorial plane
2) conjugation, crossing over homologous chromosomes
3) the location in the equatorial plane and the divergence of sister chromosomes
4) the formation of four haploid nuclei
5) divergence of homologous chromosomes

2. Establish the sequence of processes of the first division of meiosis. Write down the appropriate sequence of numbers.
1) conjugation of chromosomes
2) crossingover
3) the location of pairs (bivalents) of homologous chromosomes at the equator of the cell
4) the divergence of homologous chromosomes consisting of two chromatids to the opposite poles of the cell
5) spiralization of chromosomes with the formation of bivalents
6) the formation of nuclei, division of the cytoplasm - the formation of two daughter cells

3. Establish a sequence of processes occurring in meiosis.
1) the divergence of homologous chromosomes to the poles of the cell
2) the divergence of sister chromosomes (chromatids) to the poles of the cell
3) gene exchange between homologous chromosomes
4) the formation of four cells with a haploid set of chromosomes
5) conjugation of homologous chromosomes

4. Set the sequence of meiosis processes. Write down the appropriate sequence of numbers.
1) the location of pairs of chromosomes at the equator of the cell
2) the divergence of sister chromatids to the opposite poles of the cell
3) conjugation and crossingover
4) nucleation with a set of chromosomes and nc DNA
5) the divergence of two chromatid chromosomes to opposite poles of the cell

5. Establish a sequence of processes that occur during meiotic cell division of the animal. Write down the appropriate sequence of numbers.
1) the formation of two cells with a haploid set of chromosomes
2) divergence of homologous chromosomes
3) conjugation with possible crossing over homologous chromosomes
4) the location in the equatorial plane and the divergence of sister chromosomes
5) the location of pairs of homologous chromosomes in the plane of the equator of the cell
6) the formation of four haploid nuclei

Consider the picture depicting cell division and determine A) the type of division, B) the set of chromosomes in the original cell, C) which specific cells are formed. Write down the three digits (term numbers from the proposed list) in the correct order.
1) mitosis
2) transcription
3) diploid
4) meiosis
5) direct
6) haploid
7) gamete
8) somatic

Choose one, the most correct option. Spores in flowering plants, in contrast to bacterial spores, are formed during
1) adaptation to life in adverse conditions
2) mitosis of haploid cells
3) meiosis of diploid cells
4) sexual reproduction

Choose one, the most correct option. DNA doubling and the formation of two chromatids during meiosis occurs in
1) prophase of the first division of meiosis
2) prophase of the second division of meiosis
3) interphase before the first division
4) interphase before the second division

Consider the picture of cell division and determine (A) its phase, (B) the set of chromosomes in daughter cells, and (C) which specific cells are formed as a result of such division in plants.
1) prophase, metaphase, telophase
2) somatic
3) diploid
4) prophase 2, metaphase 2, anaphase 2, telophase 2
5) prophase 1, metaphase 1, anaphase 1, telophase 1
6) haploid
7) dispute
8) the first meiotic division

Consider the picture depicting cell division and determine: A) what phases of division are depicted, B) a set of chromosomes of cells in each phase, C) which specific cells are formed in plants as a result of such division. Write down the three digits (term numbers from the proposed list) in the correct order.
1) prophase, metaphase, telophase
2) interphase
3) diploid
4) prophase 2, metaphase 2, anaphase 2
5) prophase 1, metaphase 1, anaphase 1
6) haploid
7) dispute
8) somatic

All the signs listed below, except for two, are used to describe the cells depicted in the figure. Identify the two signs that “drop out” from the general list and write down the numbers under which they are indicated.
1) homologous chromosomes are present
2) each chromosome contains one DNA molecule
3) there is no cell center in the cell
4) the formation of mitotic spindle division
5) metaphase plate formed

All the signs below, except for two, can be used to describe the processes of the first division of meiosis. Identify two signs that “fall out” of the general list and write down the numbers under which they are indicated.
1) the formation of two haploid nuclei
2) the divergence of monochromatic chromosomes to the opposite poles of the cell
3) the formation of four cells with a set of nc
4) exchange of regions of homologous chromosomes
5) chromosome spiralization

Choose one, the most correct option. In the first division, meiosis forms
1) polyploid cells
2) diploid cells
3) gametes
4) haploid cells

Choose one, the most correct option. During sexual reproduction, maintaining the constancy of the chromosome set in a series of generations of the species is provided
1) recombination of genes in chromosomes
2) the formation of identical daughter cells
3) divergence of sister chromosomes
4) a decrease in the number of chromosomes in gametes

How is the prophase of the first division of meiosis different from the prophase of mitosis? In response, write down the numbers of the two correct options out of the five proposed.
1) the nuclear shell disappears
2) chromosomes spiral
3) conjugation of chromosomes occurs
4) chromosomes are randomly arranged
5) crossover occurs

All the signs listed below, except for two, are used to describe the phase of meiosis depicted in the figure. Identify two signs that “fall out” of the general list and write down the numbers under which they are indicated.
1) bivalents of chromosomes are located at the equator of the cell
2) homologous chromosomes, consisting of two chromatids, diverge to opposite poles
3) daughter chromatids diverge to opposite poles of the cell
4) there is a reduction in the number of chromosomes
5) chromosome set in n2c cell at each pole of the cell

Consider the pattern and determine (A) the type of division, (B) the phase of division, (C) the amount of genetic material in the cell. For each cell marked with letters, select the appropriate term from the list provided. Record the selected numbers in the order corresponding to the letters.
1) anaphase II
2) n2c (at each pole of the cell)
3) metaphase
4) meiosis
5) 2n2c
6) mitosis
7) anaphase I

How many spermatozoa are formed as a result of spermatogenesis from one diploid primary germ cell? In the answer write down only the corresponding number.

All but two of the symptoms listed below can be used to describe meiosis. Identify two signs that “fall out” of the general list and write down the numbers under which they are indicated.
1) two diploid cells are formed
2) four haploid cells are formed
3) there is one division, consisting of four phases
4) there are two divisions, each of which consists of four phases
5) homologous chromosomes containing two chromatids each diverge to the poles of the cell

All of the signs below, except for two, can be used to describe the processes that occur in the prophase of the first division of meiosis. Identify two signs that “fall out” of the general list and write down the numbers under which they are indicated in response.
1) the formation of two cores
2) divergence of homologous chromosomes
3) the convergence of homologous chromosomes
4) exchange of regions of homologous chromosomes
5) chromosome spiralization

Choose three features of mitotic cell division.
1) two-chromatid chromosomes diverge to the poles
2) sister chromatids diverge to the poles
3) doubled chromosomes appear in daughter cells
4) as a result, two diploid cells are formed
5) the process takes place in one division
6) as a result, haploid cells are formed

Choose three differences between the first division of meiosis from the second
1) pairs of homologous chromosomes are located at the equator of the cell
2) there is no telophase
3) conjugation and crossing over of chromosomes occurs
4) there is no conjugation and crossing over of chromosomes
5) sister chromatids diverge to the poles of the cell
6) homologous chromosomes diverge to the poles of the cell

What processes occur during meiosis?
1) transcription
2) reduction
3) denaturation
4) crossingover
5) conjugation
6) broadcast

The biological essence of meiosis is:
1) the emergence of a new nucleotide sequence,
2) the formation of cells with double the number of chromosomes,
3) the formation of haploid cells,
4) recombination of sections of non-homologous chromosomes,
5) new gene combinations,
6) the appearance of a larger number of somatic cells.

Choose three correct answers from six and write down the numbers under which they are indicated. Meiosis occurs
1) the formation of germ cells
2) the formation of prokaryotic cells
3) halving the number of chromosomes
4) preservation of the diploid set of chromosomes
5) the formation of two daughter cells
6) the development of four haploid cells

Establish a correspondence between the characteristics and phases of cell division: 1) metaphase of mitosis, 2) anaphase of mitosis, 3) prophase I of meiosis. Write down the numbers 1-3 in the order corresponding to the letters.
A) the exchange of chromosome regions
B) alignment of chromosomes at the equator of the cell
C) the formation of the spindle division
D) a set of chromosomes and the number of DNA molecules in a cell - 4n4c
D) division of centromeres of chromosomes

Establish a correspondence between the process feature and the meiosis phase, for which it is characteristic: 1) anaphase I, 2) anaphase II, 3) telophase II. Write down the numbers 1-3 in the order corresponding to the letters.
A) the divergence of sister chromosomes to different poles of the cell
B) the formation of four haploid nuclei
В) расхождение двухроматидных хромосом к противоположным полюсам
Г) увеличение вдвое числа хромосом в клетке при расхождении сестринских хроматид
Д) независимое расхождение хромосом из каждой гомологичной пары

Установите соответствие между характеристиками и фазами мейоза: 1) профаза первого деления, 2) анафаза второго деления. Write down the numbers 1 and 2 in the order corresponding to the letters.
A) conjugation of homologous chromosomes
B) the formation of bivalents
B) chromatid discrepancy
D) reduction of microtubules spindle division
D) dissolution of karyolemma

Determine the phase and type of division shown in the figure. Write down two numbers in the order specified in the task, without separators (spaces, commas, etc.).
1) anaphase
2) metaphase
3) prophase
4) telophase
5) mitosis
6) meiosis I
7) meiosis II

Determine the phase and type of division shown in the figure. Write down two numbers in the order specified in the task, without separators (spaces, commas, etc.).
1) anaphase
2) metaphase
3) prophase
4) telophase
5) mitosis
6) meiosis I
7) meiosis II

Determine the phase and type of division shown in the figure. Write down two numbers in the order specified in the task, without separators (spaces, commas, etc.).
1) anaphase
2) metaphase
3) prophase
4) telophase
5) mitosis
6) meiosis I
7) meiosis II

Determine the phase and type of division shown in the figure. Write down two numbers in the order specified in the task, without separators (spaces, commas, etc.).
1) anaphase
2) metaphase
3) prophase
4) telophase
5) mitosis
6) meiosis I
7) meiosis II

Binary halving

Mitosis and meiosis are possible only in eukaryotes, but what about prokaryotes - bacteria? They invented a slightly different method and share binary division in two. It is found not only in bacteria, but also in a number of nuclear organisms: amoeba, ciliates, green euglena.

Under favorable conditions, bacteria divide every 20 minutes. If the conditions are not so favorable, then more time is spent on growth and development, the accumulation of nutrients. Intervals between divisions become longer.

Amitosis (from the Greek. Ἀ - a particle of negation and μίτος - a thread)

The method of direct cell division, in which there is no formation of a spindle of division and uniform distribution of chromosomes. Cells are divided directly by constriction, the hereditary material is distributed "as anyone is lucky" - randomly.

Amitosis occurs in cancerous (tumor) cells, inflammatory changes in old cells.

© Bellevich Yuri Sergeevich

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