Meiosis is a set of chromosomes in each phase. Meiosis as the process of formation of haploid cells

13.02.2021

Definition

Meiosis (reduction cell division)- division during which one diploid (2n) cell produces 4 haploid (n) cells.

Since in daughter cells there is a decrease (reduction) in the number of chromosomes from 2n to n, this division is called reductionist.

Meiosis diagram

Meiosis in animals is observed during the formation of gametes (gametogenesis). Meiosis in plants and fungi usually occurs with the formation of haploid spores. In various unicellular eukaryotes, meiosis can be observed at different stages life cycle. To restore diploidity in a cycle, the fusion of haploid cells (fertilization) is always necessary.

Meiosis consists of two divisions. The first of them is actually reductional, that is, it is during the first division that the ploidy of the cell decreases. The reason for this is the divergence of homologous chromosomes (“maternal” and “paternal”) into two different daughter cells. The second division is similar to mitosis and is called equational(that is, “equal”). Ploidy does not change as a result of the second division. During this division, as during mitosis, sister chromatids (copies of DNA) separate. There is no DNA replication between two divisions of meiosis (since the “goal” of meiosis is to reduce the ploidy of the cell, there is no need to increase the amount of DNA here).

In prophase I of the meiotic division, the most important process related to genetic recombination occurs - crossing over, that is, the exchange of sections of homologous chromosomes. This process creates new combinations of genes in the offspring. Chromosomes as a whole are not passed on directly from grandparents to grandchildren, but are “reconstructed” in each generation through the process of crossing over.

The table below describes the phases of meiosis in a cell for which n=2, 2n=4. Each set has three chromosomes, varying in size. The maternal and paternal chromosome sets are highlighted in blue and red.

Phase	Process	Description
Prophase I		condensation (supercoiling) of chromosomes (visible under an electron microscope); conjugation (connection) of homologous chromosomes with the formation of bivalents; crossing over - exchange of sections between homologous chromosomes; homologous chromosomes remain connected to each other; the nuclear membrane dissolves; centrioles move towards the poles
Metaphase I		bivalents line up along the equator of the cell
Anaphase I 2n4c		spindle microtubules contract, bivalents divide; Whole chromosomes, consisting of two chromatids each, diverge to the poles
Telophase I		chromosomes despiral (“unwind”); the nuclear envelope is formed
The second division of meiosis follows immediately after the first, without interphase: DNA replication (doubling) does not occur. During meiosis I, 2 daughter cells were formed. Next, their division will be considered, so the chromosome set formula contains a coefficient of 2.
Prophase II		condensation (supercoiling) of chromosomes; cell center divides, centrioles diverge towards the poles of the nucleus; the nuclear membrane is destroyed; spindle is formed
Metaphase II		bichromatid chromosomes are arranged in a plane "equator" (metaphase plate)
Anaphase II		centromeres divide; single chromatid chromosomes move towards the poles
Telophase II		chromosomes despiral; the nuclear envelope is formed

The course of meiosis, as a rule, is disrupted in the cells of hybrid organisms, since in prophase I pairwise fusion (conjugation) must occur homologous chromosomes, and in hybrids the set of maternal genes is not homologous to the paternal one.

This mechanism underlies the sterility of interspecific hybrids. Since interspecific hybrids combine in the cell nucleus the chromosomes of parents belonging to various types, chromosomes usually cannot enter into conjugation. This leads to disturbances in the divergence of chromosomes during meiosis and ultimately to the non-viability of gametes, and consequently to the sterility (infertility) of hybrids.

In breeding, to overcome the sterility of hybrids, polyploidy (fold increase) of chromosome sets is artificially caused. In this case, each chromosome conjugates with the corresponding chromosome of its set.

The meaning of meiosis

Parents' sex cells , formed by meiosis, have a haploid set of (n) chromosomes. In a zygote, when two such sets are combined, the number of chromosomes becomes diploid (2n). The formation of a new organism occurs through mitotic divisions of the zygote, and each of its cells contains a diploid (2n) set of chromosomes. Each pair of homologous chromosomes contains one paternal and one maternal chromosome. Based on this:

Meiosis is the basis of combinative variability due to crossing over (prophase I) and independent divergence of homologous chromosomes (anaphase I and II).

Due to a decrease in the number of chromosomes in gametes, a constant diploid (2n) set of chromosomes is maintained in new organisms.

prophase I of meiotic division

Prophase I of the meiotic division is unique, includes many processes and is divided into stages:

Leptotene

Zygotene

Pachytena

Diplotena

Diakinesis

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

What is meiosis

This method of division is characteristic of the formation of gametes in animals, plants and fungi. As a result of meiosis, cells are formed that have a haploid set of chromosomes, also called germ cells.

Unlike another variant of cell multiplication - mitosis, in which the number of chromosomes of daughter individuals is characteristic of the mother's, with meiosis the number of chromosomes is halved. This occurs in two stages - meiosis 1 and meiosis 2. The first part of the process is similar to mitosis - before it, DNA is doubled and the number of chromosomes increases. 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.

A chromosome is a structure in the cell nucleus that is of a nucleoprotein nature and concentrates most of the hereditary information.
Somatic and germ cells are cells of the body that have a different set of chromosomes. Normally (excluding polyploids), somatic cells are diploid (2n), and sexual cells are haploid (n). When two germ cells fuse, a full-fledged somatic cell is formed.
A centromere is a section of a chromosome responsible for gene expression and connecting chromatids to each other.
Telomeres are the ends of chromosomes and perform a protective function.
Mitosis is a method of dividing somatic cells, creating copies identical to them in the process.
Euchromatin and heterochromatin are areas of chromatin in the nucleus. The first one retains a despiralized state, the second one is spiralized.

Process stages

Meiosis of a cell consists of two successive 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 centromeres. Spiralization occurs and, as a consequence, shortening of the chromatids in the chromosome. Meiosis begins with metaphase 1. Homologous chromosomes are located in the equatorial plane of the cell. This is called the alignment of tetrads (bivalents) chromatid to chromatid. At this moment, the processes of conjugation and crossing over occur, they are described below. During these actions, telomeres often cross and overlap each other. The nuclear membrane begins to disintegrate, the nucleolus disappears and the filaments of the spindle become visible. During anaphase 1, entire chromosomes, consisting of two chromatids, move to the poles, and randomly.

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-lived because it does not require DNA doubling.

In the second division at the metaphase 2 stage, one chromosome (of two chromatids) moves to the equatorial part of the cell, forming a metaphase plate. The centromere of each chromosome divides and the chromatids move towards the poles. At the telophase stage of this division, two cells are formed, each containing a haploid set of chromosomes. Normal interphase begins.

Conjugation and crossing over

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

Gamete diversity

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

There are errors in replication and chromosome segregation. This leads to the formation of defective gametes. Normally, such cells would undergo apoptosis (cell death), but sometimes they merge with another germ cell to form a new organism. For example, this is how Down's disease is formed in humans, associated with one additional chromosome.

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

Meiosis: biological significance

It is clear why the number of chromosomes in a cell decreases during the process of meiosis: if this mechanism did not exist, then with the fusion of two germ cells there would be a constant increase in the chromosome set. Thanks to reduction division, during the process of reproduction, a full-fledged diploid cell emerges from the fusion of two gametes. Thus, the constancy of the species and the stability of its chromosome set are maintained.

Half of the daughter organism's DNA 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 types, 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, and there is a high risk of getting defective gametes. These plants are propagated vegetatively.

Thus, understanding what meiosis is is a fundamental question in biology. The processes of sexual reproduction, the accumulation of random mutations, as well as 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 protozoa, the mechanism looks somewhat different. During this process, one meiotic division occurs, and the crossing over phase accordingly also shifts. This mechanism is considered more primitive. It served as the basis for the division of haploid cells of modern animals, plants, and fungi, which occurs in two phases and ensures better recombination of genetic material.

Differences between meiosis and mitosis

To summarize the differences between these two types of division, it should be noted the ploidy of the daughter cells. If during mitosis the amount of DNA and 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 are formed. 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, i.e., the recombination of genetic material over generations, then the goal of mitosis is to maintain tissue stability and the integrity of the organism.

Meiosis- This is the division of diploid cells, which results in the formation of haploid cells. That is, from each pair of homologous chromosomes of the mother cell, only one chromosome ends up in the daughter cells. Meiosis underlies the formation of germ cells - gametes. As a result of the fusion of male and female gametes, the diploid set is restored. Thus, one of the important meanings of meiosis is to ensure the constancy of the number of chromosomes in a species during sexual reproduction.

In a cell that begins meiotic division, chromosome duplication (replication) has already occurred, just as it happens in the interphase of mitosis. So each chromosome consists of two chromatids, and the number of chromosomes is diploid. That is, in terms of the amount of genetic information, cells entering mitosis and meiosis are the same.

Unlike mitosis, meiosis occurs in two divisions. As a result of the first division, the homologous chromosomes of each pair separate into different daughter cells, and two cells are formed with a haploid number of chromosomes, but each chromosome consists of two chromatids. The second division also proceeds as mitotic, since the chromatids of each chromosome are separated, and one chromatid of each chromosome ends up in the daughter cells.

Thus, as a result of meiosis, four cells with a haploid set of chromosomes are formed. In males, all four become sperm. But in females, only one becomes an egg, the others die. This is due to the fact that the supply of nutrients is concentrated in only one cell.

Stages, or phases, of the first meiotic division:

Prophase I. Spiralization of chromosomes. Homologous chromosomes are located parallel to each other and exchange some homologous regions (chromosomal conjugation and crossing over, which results in gene recombination). The nuclear envelope is destroyed and the fission spindle begins to form.
Metaphase I. Pairs of homologous chromosomes are located in the equatorial plane of the cell. A spindle strand is attached to the centromere of each chromosome. Moreover, there is only one to each one in such a way that a thread is attached to one homologous chromosome from one pole of the cell, and to the other - from the other.
Anaphase I. Each chromosome from a pair of homologous ones goes to its own pole of the cell. In this case, each chromosome continues to consist of two chromatids.
Telophase I. Two cells are formed containing a haploid set of doubled chromosomes.

Stages, or phases, of the second meiotic division:

Prophase II. Destruction of nuclear membranes, formation of a fission spindle.
Metaphase II. The chromosomes are located in the equatorial plane, and the spindle threads are attached to them. Moreover, in such a way that two threads are attached to each centromere - one from one pole, the other from the other.
Anaphase II. The chromatids of each chromosome are separated at the centromere, and each of the pair of sister chromatids goes to its own pole.
Telophase II. Formation of nuclei, unwinding of chromosomes, division of cytoplasm.

The diagram shows the behavior during meiosis of only one pair of homologous chromosomes. There are more of them in real cells. Thus, human cells contain 23 pairs. The diagram shows that the daughter cells are genetically different from each other. This important difference meiosis from mitosis.

Another important significance of meiosis should be noted (the first, as already indicated, is the provision of a mechanism for sexual reproduction). As a result of crossing over, new combinations of genes are created. They are created as a result of chromosome divergence independent of each other during meiosis. Therefore, meiosis underlies the combinative variability of organisms, which in turn is one of the sources natural selection, i.e. evolution.

Accompanied by a halving of the number of chromosomes. It consists of two sequential divisions that have the same phases as mitosis. However, as shown in table “Comparison of mitosis and meiosis”, the duration of individual phases and the processes occurring in them differ significantly from the processes occurring during mitosis.

These differences are mainly as follows.

In meiosis prophase I longer lasting. What happens in it conjugation(connection of homologous chromosomes) and exchange of genetic information. In anaphase I centromeres, holding chromatids together, don't share, and one of the homologmeiosis of mitosis and egg chromosomes goes to the poles. Interphase before the second division very short, in it DNA is not synthesized. Cells ( halites), formed as a result of two meiotic divisions, contain a haploid (single) set of chromosomes. Diploidy is restored by the fusion of two cells - maternal and paternal. The fertilized egg is called zygote.

Mitosis and its phases

Mitosis, or indirect division, most widely distributed in nature. Mitosis underlies the division of all non-reproductive cells (epithelial, muscle, nerve, bone, etc.). Mitosis consists of four consecutive phases (see table below). Thanks to mitosis uniform distribution of the genetic information of the parent cell between the daughter cells is ensured. The period of cell life between two mitoses is called interphase. It is ten times longer than mitosis. A number of very important processes take place in it prior to cell division: ATP and protein molecules are synthesized, each chromosome doubles, forming two sister chromatids, held together by a common centromere, the number of main organelles of the cytoplasm increases.

In prophase spiral and as a result chromosomes thicken, consisting of two sister chromatids held together by a centromere. By the end of prophase the nuclear membrane and nucleoli disappear and the chromosomes are dispersed throughout the cell, the centrioles move to the poles and form spindle. In metaphase, further spiralization of chromosomes occurs. During this phase they are most clearly visible. Their centromeres are located along the equator. The spindle threads are attached to them.

In anaphase Centromeres divide, sister chromatids separate from each other and, due to the contraction of spindle filaments, move to opposite poles of the cell.

In telophase The cytoplasm divides, chromosomes unwind, and nucleoli and nuclear membranes are formed again. In animal cells the cytoplasm is laced, in plant- a septum is formed in the center of the mother cell. So from one original cell (mother) two new daughter cells are formed.

Table - Comparison of mitosis and meiosis

Phase	Mitosis	Meiosis
Phase	Mitosis	1 division	2 division
Interphase	Chromosome set 2n. There is an intensive synthesis of proteins, ATP and other organic matter. The chromosomes double, each consisting of two sister chromatids held together by a common centromere.	Set of chromosomes 2n The same processes are observed as in mitosis, but longer, especially during the formation of eggs.	The set of chromosomes is haploid (n). There is no synthesis of organic substances.
Prophase	It is short-lived, spiralization of chromosomes occurs, the nuclear membrane and nucleolus disappear, and a fission spindle is formed.	Longer lasting. At the beginning of the phase, the same processes occur as in mitosis. In addition, chromosome conjugation occurs, in which homologous chromosomes come together along their entire length and become twisted. In this case, an exchange of genetic information can occur (crossing of chromosomes) - crossing over. The chromosomes then separate.	Short; the same processes as in mitosis, but with n chromosomes.
Metaphase	Further spiralization of chromosomes occurs, their centromeres are located along the equator.	Processes similar to those in mitosis occur.
Anaphase	The centromeres holding sister chromatids together divide, each of them becomes a new chromosome and moves to opposite poles.	Centromeres do not divide. One of the homologous chromosomes, consisting of two chromatids held together by a common centromere, departs to opposite poles.	The same thing happens as in mitosis, but with n chromosomes.
Telophase	The cytoplasm divides, two daughter cells are formed, each with a diploid set of chromosomes. The spindle disappears and nucleoli form.	Does not last long. Homologous chromosomes end up in different cells with a haploid set of chromosomes. Cytoplasm does not always divide.	The cytoplasm divides. After two meiotic divisions, 4 cells with a haploid set of chromosomes are formed.

Comparison table between mitosis and meiosis.

a) transcription;

b) reduction division;

c) denaturation;

d) crossing over;

e) conjugation;

e) broadcast.

5. As a result of reduction division in oogenesis, the following are formed:

a) one reduction body;

b) ovogogy;

c) first order oocyte;

d) two reduction bodies;

e) oocyte of the 1st order.

Option 5

1. As a result of the first meiotic division, the following are formed from one mother cell:

a) two daughter cells with a halved set of chromosomes;

b) four daughter cells with the number of chromosomes reduced by half;

c) two daughter cells with a doubled number of chromosomes;

d) four daughter cells with the number of chromosomes equal to the mother cell.

The first phase of meiosis is characterized by the process

a) conjugation;

b) broadcasts;

c) reduplication;

d) transcriptions.

Biological significance meiosis in animals consists of

a) preventing the doubling of the number of chromosomes in the new generation;

b) the formation of male and female reproductive gametes;

c) creation of new gene combinations;

d) creation of new chromosomal combinations;

e) increasing the number of cells in the body;

e) a multiple increase in the set of chromosomes.

The egg, unlike the sperm, is characterized by

a) haploid set of chromosomes;

b) diploid set of chromosomes;

c) a large supply of nutrients;

d) larger sizes;

5) immobility;

d) active movement.

5 The chromosome set of metaphase 1 of meiosis is equal to:

b) 2n4s 4 xp;

c) 4n4c 4хр;

d) 1nb4s4хр.

ANSWERS TO INPUT TEST CONTROL

1 var. 1-a,b, 2-a,d; 2-in; 3-g; 4-a; 5-a.

2 var. 1- 1-b,c,d,e,f 2-a,g,h. 2-a, 3-a, 4-a, 5-a.

3 var. 1-a,b,c,d, 2-a,b,c; 2-c, 3-a, 4-a,c,d; 5-g

4 var. 1-a,d,e, 2-b,c,f; 2-a; 3-b,4-b,d,e. 5-a,c.

5 var. 1-a, 2-a, 3-a, b, c. 4-c, d, d, 5-d

APPENDIX No. 3 SITUATIONAL TASKS.

LEARNING OBJECTIVES:

1.2. Sequencing the human genome within international program“The Human Genome” laid the foundation for a new direction - predictive medicine (genetic testing of predisposition genes). It makes it possible not only to reliably make a diagnosis, but, if possible, modern technologies carry out treatment and prevention of hereditary diseases. This is especially true in the pre-embryonic period of ontogenesis, when young people are examined even before the birth of children.

For example, testing the CFT gene, a mutation in which leads to the development of cystic fibrosis. The gene includes 1245 triplets; as a result of one of the missense mutations in the 455th triplet, C is replaced by A. Determine the sequence of amino acids in normal conditions (in the area 451-461) and in pathology.

DNA is normal in the region of triplets 451-461

DNA: CCT GTC AAC AAC CGC TsAA CGA CCT AGG TGA

ala- val-ala-gli-ser-tre

altered DNA: CCT GTC AAC AAC CGC CAA CGA CCT AGG TGA

mRNA: GGA TsAG UUG UUG GCG GUU GCU GGA UCC ACU

polypeptide gly - gln- leu - leu- gray- val-ala - gli- ser-tre

TRAINING OBJECTIVES

1.3. A married couple contacted the pregnancy planning center “Marriage and Family” regarding infertility. They lived in marriage for 5 years. What objective reasons can cause infertility?

SOLUTION ALGORITHM.

The causes of infertility may be the following:

1) disturbance of spermatogenesis;

2) violation of oogenesis;

3) disruption of the structure and function of the uterus and fallopian tubes;

4) endocrine disorders(hypothyroidism, diabetes), disorders of the structure and function of the adrenal glands and pituitary gland;

5) acute infections (mumps);

6) chronic infections(tuberculosis);

7) deficiency of vitamins A, B, C;

8) chronic renal failure;

9) exposure to salts of heavy metals and radioactive substances that disrupt spermatogenesis;

10) medicinal drugs used to treat leukemia and psoriasis (Mileran, metatrexate).

1.4. A 21-year-old pregnant woman, being examined at a consultation, asked about the possibility of giving birth to twins. Her question was related to the fact that twins were born to her mother, grandmother and even great-grandmother. How would you answer this question? Do you consider it appropriate, in the form of additional information, to find out whether identical identical or fraternal twins were born in her family? Does information about the birth of twins from paternal relatives matter?

SOLUTION ALGORITHM.

There is no doubt that heredity influences the birth of polyzygotic twins. There is no certainty that the frequency of monozygotic twins depends on heredity. In the case of polyzygotic twins, children differ in both their physical and mental abilities. Children of monozygotic twins have identical physical and mental characteristics. It has been established that the father's genotype is not able to change the frequency of twin births.

CONTROL TASKS

1.5. The microphotograph shows an egg cell whose cytoplasm contains a small amount of evenly spaced yolk inclusions. The egg is surrounded by two structures: the zona pellucida and the corona radiata. Name the type of egg and who is it typical for? What is the corona radiata and the zona pellucida formed by? What functions do they perform? How do they differ in chemical composition parts of the egg? What is the significance of ooplasmic segregation for embryo development?

SOLUTION ALGORITHM.

This type of egg is alecithal, characteristic of mammals and humans. The zona pellucida is a product of both the oocyte itself and the follicular cells that feed it. Her important feature is the presence of special proteins - glycoproteins ZP1, ZP2 and ZP3, responsible for the species specificity of fertilization. In addition, it plays a significant role in protecting the egg and transporting nutrients.

The corona radiata, or secondary membrane of the egg, consists of several layers of follicular cells located around the egg. It contacts the egg with its thin cytoplasmic processes, penetrating through holes in the zona pellucida. Follicular cells forming the corona radiata play important role in the directional movement of the egg through the fallopian tubes.

Ooplasmic segregation, leading to the fact that the composition of the cytoplasm in different parts of the egg becomes different. Thus, glycogen and RNA are concentrated at one of the poles, vitamin C is located at the equator.

1.6. An 18-year-old man was diagnosed with bilateral cryptorchidism (both testicles are not descended into the scrotum). What is the meaning for young man Could this congenital anomaly have? What advice should be given to the patient?

SOLUTION ALGORITHM

The doctor should explain to the patient that both testicles must be surgically lowered into the scrotum. This operation is necessary for the following reasons:

1) in the child’s testicles, located in the inguinal canal or peritoneal cavity, after 5 years, degenerative changes develop in the seminiferous tubules. since the temperature in the scrotum is 2-3 degrees lower than the intraperitoneal temperature, due to this, spermatogenesis is irreversibly disrupted and there is a threat of infertility;

2) if the testicles are not located in the scrotum before puberty, sperm are not formed. although Leydig cells actively synthesize testosterone;

3) if the testicles remain intraperitoneal until 30-35 years of age, fibrous connective tissue replaces interstitial cells - glandulocytes, which explains the decrease in the synthesis of the male sex hormone;

4) cellular elements of undescended testicles can often be a source of malignant tumors.

1.7. A man, aged 36 years, consulted an andrologist. The patient was concerned about the question: “Can viral mumps (mumps), which he suffered from in childhood and which was complicated by acute inflammation of the testicle (orchitis), cause infertility?”

SOLUTION ALGORITHM.

Inflammatory changes in the testicles cause the development of atrophy of the convoluted tubules of the testicle and regression of spermatogenesis. Mumps can rarely be the cause of sterility, since this infection most often affects only one of the glands.

1.8 . Sequencing of the two smallest human chromosomes, 21 and 22, determined their size, the number of genes and their location. The DNA size in chromosome 21 is 33.8 Mb, it contains 225 genes, the DNA size of chromosome 22 is 33.4 Mb, it contains 545 genes. Given this fact, explain why trisomy 22 is often incompatible with life. What disease develops with trisomy 21? Indicate the possible causes and mechanisms leading to the development of this pathological condition.

SOLUTION ALGORITHM.

It is obvious that chromosome 22, despite its small size, contains 2 times more genes than chromosome 21. Trisomy on chromosome 22 will lead to the development of anomalies incompatible with life. Trisomy 21 leads to the formation of Down syndrome. Among possible reasons, leading to incorrect segregation of chromosomes in meiosis may be the age of the mother. Perhaps as the body ages, the pool of oocytes and chromosomes in “overripe” oocytes is depleted older women more susceptible to nondisjunction. It is assumed that age-related hormonal changes may accelerate the process of meiotic maturation of oocytes and cause abnormal chromosome segregation. It is also possible that as a woman ages, the formation of the spindle is disrupted or the duration of the cell cycle changes.

Glossary.

Acrosome- sperm organelle located at the anterior end of the sperm head, developing from the Golgi complex by condensation of acrosomal granules.

Egg activation- inducing an egg to develop, which occurs when it is fertilized by a sperm or under the influence of other stimuli.

Animal pole- part of the telolecithal egg, which contains active cytoplasm, not overloaded with yolk inclusions. The latter are concentrated on the opposite – vegetative pole.

Bivalent a pair of homologous chromosomes that join (conjugate) with each other in meiosis.

Vegetative pole- part of the cytoplasm of the egg in which is concentrated a large number of yolk.

Gametogenesis- development of germ cells (sperm and eggs).

Gametes- male and female reproductive cells with a haploid set of chromosomes.

Gonads- gonads - organs that form sex cells and sex hormones in animals and humans.

Reduction division (meiosis 1)– the process of division of maturing germ cells, as a result of which the number of chromosomes is halved (reduced).

Zygote- a cell that arises from the fusion of two gametes. This is a fertilized egg.

Cortical reaction- a chain of changes in the cortical layer of the cytoplasm of the egg during its fertilization (destruction of cortical granules, thickening of the vitelline membrane and its transformation into the fertilization membrane, change in membrane potential, blocking polyspermy).

Cryptorchidism- undescended testicle into the scrotum. With this developmental anomaly, the testicles remain sterile, because due to the high temperature in abdominal cavity spermatogenesis is suspended.

Crossing over– mutual exchange of homologous regions of conjugating chromosomes.

Meiosis– the process of division of maturing germ cells, as a result of which a decrease (reduction) in the number of chromosomes occurs.

Monosomy– the absence of one of the homologous chromosomes in the chromosome set of cells of a diploid organism.

Fertilization membrane- thickened and, as it were, hardened primary shell of the egg.

Primary egg membrane- the vitelline membrane produced by the egg itself. It looks like a thin film associated with the cytoplasm of the egg.

Oogenesis- development of the female reproductive cell.

Ovulation- the process of ejection (exit) of an egg from the Graafian vesicle of the ovary, after which it enters the oviduct.

Fertilization- the process of fusion of male and female reproductive cells to form a zygote.

Oogonia– immature female germ cells with the ability to reproduce mitotically.

Oocyte- an immature female reproductive cell of animals during periods of growth and maturation of oogenesis.

Pronucleus- the nuclear substance of a spermatozoon or the nucleus of an egg, which, during the process of fertilization before the formation of a synkaryon, passes from a dense to a looser state, acquiring at the same time a resemblance to a regular cell nucleus.

Polyploidy– a hereditary change consisting in a multiple increase in the number of sets of chromosomes in the cells of the body.

Reproduction- the property of reproducing their own kind inherent in all organisms, ensuring the continuity and continuity of life.

Reproduction is asexual- reproduction of living organisms, in which one parent individual gives rise to two or more offspring individuals identical in hereditary characteristics to the parent individual.

Sexual reproduction- methods of reproduction in which a new organism usually develops from a zygote, formed as a result of the fusion of female and male reproductive cells - gametes.

Gray sickle- part of the egg in the form of a gray crescent on the side opposite to the point of penetration of the sperm.

Sinkarion– 1) the nucleus of the zygote, formed during the fusion of the male and female pronuclei.

Spermatids– haploid male germ cells formed during the 4th (last) period of spermatogenesis.

Spermatogenesis– transformation of diploid primary cells in animals and many plant organisms into haploid differentiated male germ cells – sperm.

Spermatogonia– diploid male germ cells of the first period of spermatogenesis.

Spermatozoon - sperm– a mature haploid male reproductive cell of animals and many plant organisms.

Spermatocyte – male reproductive cell during the period of growth and maturation (2nd and 3rd periods of spermatogenesis).

Chiasmus – point of connection of conjugating homologous chromosomes in prophase of the first meiotic division.

Chromosomes– self-reproducing structures of the cell nucleus, which are carriers of genes that determine the hereditary properties of cells and organisms.

Testicles– external organs of the male reproductive system are oval or bean-shaped.

Ovaries– female gonads that perform generative (formation of egg cells) and endocrine (production of ovarian hormones) functions).

Egg- a female reproductive cell specialized to perform a generative function.

Using a high magnification microscope, examine a section of a rat testis. Find cells in different zones of development in the seminiferous tubules. Draw a segment of the seminiferous tubule and label spermatogonia, first order spermatocytes, second order spermatids. Label the chromosome complex of each cell.

PRACTICAL TASK 2.

At high magnification of the microscope, examine a permanent preparation of spermatozoa guinea pig. Pay attention to the size of the sperm. Examine the head, find the acrosome, nucleus in it. Sketch 1-2 spermatozoa and make a designation.

At low microscope magnification, examine a section of a cat's ovary. Find follicles at different stages of maturity. Sketch the specimen and label the primary follicle, medium-mature follicle (growing), mature follicle (Graafian vesicle). In the Graafian vesicle, examine and label the follicular layer, follicular cavity, oviductal tubercle, and first-order oocyte.

PRACTICAL TASK 7.

Study the structure of the sperm and egg of mammals using the table and transfer it to the album. Draw a diagram of the structure of a spermatozoon, label the head, nucleus, acrosome, neck, proximal, distal centrioles, tail. Draw a diagram of the structure of an egg. Label its coat pellucida, nucleus, nucleolus, and yolk grains.

Input test control

3 Reducing the number of chromosomes by half, the formation of cells with a haploid set of chromosomes occurs in the process

2) crushing

3) fertilization

4 The significance of mitosis is to increase the number

1) chromosomes in daughter cells compared to the mother’s

2) cells with a set of chromosomes equal to the mother cell

3)DNA molecules in daughter cells compared to the mother’s

4) cells with a halved set of chromosomes

5 At the end of interphase, each chromosome consists of DNA molecules

4) four

6 Conjugation and exchange of sections of homologous chromosomes occurs in

1) prophase I of meiosis

2) prophase of mitosis

3)metaphase II of meiosis

4) prophase II of meiosis

7 The dissolution of the nuclear membrane and nucleoli during mitosis occurs in

1) prophase

2) interphase

3)telophase

4)metaphase

8 in meiosis, DNA duplication and formation of two chromatids occurs in

1) prophase of the first division

2) prophase of the second division

3) interphase before the first division

4) interphase before the second division

10 The divergence of homologous chromosomes occurs in

1) anaphase of meiosis 1

2) metaphase of meiosis 1

3) metaphase of meiosis 2

4) anaphase of meiosis 2

11 The divergence of chromatids to the poles of the cell occurs in

1) telophase

2) anaphase

3) prophase

4) metaphase

12 During the process of meiosis in animals, gametes with a set of chromosomes are formed

1) diploid

2) haploid

3) equal to maternal

4) doubled

14 in animals, in the process of mitosis, in contrast to meiosis, cells are formed

1) somatic

2) with half the set of chromosomes

3) sexual

4) spore

Answer:_____________________

18 Animal germ cells as opposed to somatic cells

Answer:_____________________

20 Choose the correct answer. As a result of the second division of spermatogenesis maturation, the cells are called:

1). Spermatogonia

2). Spermatocytes of the first order

3). Spermatids

4). Spermatocytes of the first order

21. Choose the correct answers. The transparent shell consists of:

1). Glycosaminoglycans

2). Proteoglycans

3). Follicular cells

4). Pigment inclusions

5). Yolk granules

23. Choose the correct answer. Acrosome contains:

1). Hormones

2). Enzymes

3. Llipids

25 Choose the correct answer. The egg does not contain:

1). Mitochondria

2). Endoplasmic reticulum

3). Golgi complex

4). Centrioles

26. Choose the correct answer. The primary shell of the egg is a derivative of:

1). Follicular cells

2). Oocyte

3). Shiny shell

4). Products of the oviduct glands

5). Connective tissue

27. Choose the correct answers. Oogenesis consists of stages:

1). Reproduction

3). Maturation

4). Formations.

92. During spermatogenesis, cells called:

a) spermatogonia;

b) 1st order spermatocytes;

c) 2nd order spermatocytes;

d) spermatids.

97. Pairs of chromosomes line up in the equatorial plane of the cell during the first meiotic division:

a) in prophase 1;

b) to metaphase 1;

c) in anaphase 1;

d) in telophase 1.

98. Of all the phases of meiosis, the longest is:

a) prophase 1;

b) anaphase 1;

c) prophase 2;

d) telophase 2.

99. Conjugation and exchange of sections of homologous chromosomes occurs:

a) in prophase of mitosis;

b) in prophase 1 of meiosis;

c) in anaphase 2;

d) in interphase 1 of meiosis.