Gamete- a generative, reproductive cell, formed as a result (in spore plants - as a result) and containing in its nucleus a haploid (single) set of chromosomes. Ensures the transmission of hereditary information from parents to descendants.

Gametogenesis— the process of formation of germ cells is the basis for the continuation of life on Earth.

Organisms in which different individuals produce male and female gametes are dioecious.
Types of organisms in which the same individual produces both male and female gametes - hermaphrodites.

Organs in which sex cells, gametes are formed - gonads

As has already been shown in the topic, sex cells are haploid, i.e. have a single set of chromosomes. This is intended by nature so that, having united, two cells with a single set form a full-fledged organism with diploid - double set.

Let's look at the process of formation of these cells in more detail...

1. Reproduction

   Future germ cells are formed from “blanks” - special cells with double ( diploid) a set of chromosomes called Oogonia(female) and spermatogonia(male cells).
   And at first these cells divide vigorously, dividing to increase their number.
   It is interesting that in the male and female body this period occurs at different times.
   
   Ovogonia They reproduce when a person cannot yet be called a woman, it is still an embryo. Those. The female body is born with a certain number of oogonia. After 7 months of embryonic development, the cells begin to Spermatogonia reproduce throughout the entire reproductive period of the male body. This period is different for all organisms, but, of course, it is much longer than for women, and of course, much more germ cells are formed in the male body.

2. Height

   Growth, increase in size, accumulation of nutrients - all these are characteristics of the growth stage, preparation for division - to. It is at this stage that these cells are already called oocytes and spermatocytes of the first order.
   Important: at this stage the number of chromosomes remains the same, but the DNA doubles!

3. Maturation
   

   Tail- contains microtubules that ensure cell motility.

   • Meiosis 1 occurs - the number of chromosomes is halved. Formed second order spermatocyte.
   • Second division - meiosis 2 - four haploid cells are formed - spermatids. They move to the 4th stage of the process.

   4. Formation (spermiogenesis)

   The cells are “finished”. They have a long and difficult journey to reach the egg. There will be only one winner in this marathon, so you need to prepare: the nucleus becomes denser, the chromosomes spiral, the cytoplasm leaves; is being formed flagellum— it is due to this that the sperm carries out forward movement; it must contain a lot of proteins and mitochondria. The sprinter is ready.

These cells differ significantly between men and women. In men, germ cells or sperm have tail-like projections () and are relatively mobile. Female reproductive cells, called eggs, are immobile and much larger than male gametes. When these cells fuse in a process called fertilization, the resulting cell (zygote) contains a mixture of what is inherited from the father and mother. Human sex organs are produced by the organs of the reproductive system - the gonads. produce sex hormones necessary for the growth and development of primary and secondary reproductive organs and structures.

The structure of human germ cells

Male and female reproductive cells differ greatly in size and shape. Male sperm resemble long, mobile projectiles. These are small cells that consist of a head, middle and tail parts. The head contains a cap-like covering called an acrosome. The acrosome contains enzymes that help the sperm cell penetrate the outer membrane of the egg. located in the head of the sperm. The DNA in the nucleus is tightly packed and the cell does not contain much. The middle part contains several mitochondria that provide energy for. The tail consists of a long projection called a flagellum, which aids in cellular locomotion.

A woman's eggs are one of the largest cells in the body and have a round shape. They are produced in the female ovaries and consist of a nucleus, a large cytoplasmic region, a zona pellucida and a corona radiata. The zona pellucida is a membranous covering that surrounds the eggs. It binds sperm cells and helps in fertilization. The corona radiata is the outer protective layer of follicular cells surrounding the zona pellucida.

Formation of germ cells

Human germ cells are produced through a two-step process of cell division called. Through a series of sequential events, the replicated genetic material in the parent cell is distributed among the four daughter cells. Since these cells have half the number of the parent cell, they are . Human germ cells contain one set of 23 chromosomes.

There are two stages of meiosis: meiosis I and meiosis II. Before meiosis, chromosomes are replicated and exist in the form. At the end of meiosis I, two are formed. The sister chromatids of each chromosome in the daughter cells are still linked. At the end of meiosis II, sister chromatids and four daughter cells are formed. Each cell contains half the chromosomes of its parent cell.

Meiosis is similar to the process of division of non-sex cells known as mitosis. produces two daughter cells that are genetically identical and contain the same number of chromosomes as the parent cell. These cells are diploid because they contain two sets of chromosomes. Humans include 23 pairs or 46 chromosomes. When germ cells unite during fertilization, the haploid cell becomes a diploid cell.

The production of sperm is known as spermatogenesis. This process occurs continuously inside the male testicles. Hundreds of millions of sperm must be released for this to happen. The vast majority of sperm do not reach the egg. During oogenesis, or egg development, daughter cells divide unevenly in meiosis. This asymmetric cytokinesis results in the formation of one large egg (oocyte) and smaller cells called polar bodies, which degrade and are not fertilized. After meiosis I, the egg is called a secondary oocyte. The secondary oocyte will complete the second stage of meiosis if the fertilization process begins. Once meiosis II is completed, the cell becomes an egg and can fuse with a sperm cell. When fertilization is complete, the combined sperm and egg become a zygote.

Sex chromosomes

Male sperm in humans and other mammals are heterogametic and contain one of two types of sex chromosomes: X or Y. However, female eggs contain only the X chromosome and are therefore homogametic. Sperm of an individual. If a sperm cell containing an X chromosome fertilizes an egg, the resulting zygote will be XX or female. If the sperm cell contains a Y chromosome, then the resulting zygote will be XY or male.

The ability of organisms to reproduce their own kind, which ensures the continuity of life, is called reproduction.

Sexual reproduction is called reproduction, in which the continuity of generations to increase the number of individuals is carried out with the help of specialized germ cells - gametes: female - eggs and male - sperm. When mature germ cells fuse, they form a zygote, from which a new daughter organism develops. Upon reaching sexual maturity, the new organism in turn produces gametes, which give rise to subsequent offspring. This is how the continuity of generations is carried out. Sexual reproduction has very great evolutionary advantages compared to asexual reproduction. This is due to the fact that the genotype of the offspring arises from a combination of genes belonging to both parents. As a result, the ability of organisms to adapt to environmental conditions increases. The essence of sexual reproduction is the combination in the hereditary material of a descendant of genetic information from two different sources - the parents. During parthenogenesis, when the development of a new organism occurs only from an egg, a new combination of genes arises in it as a result of crossing over and independent combination of chromosomes.

Development of germ cells (gametogenesis)

Sex cells - gametes - develop in the gonads. Male gametes mature in the male gonads - the testes; this process is called spermatogenesis. Female gametes mature in the ovaries during the process oogenesis. In the process of formation of germ cells - both sperm and eggs - a number of stages are distinguished: the reproduction zone, the growth zone and the maturation zone; In the maturation zone, gametes are finally formed by meiosis.

Breeding period

Primary germ cells divide by mitosis, resulting in an increase in their quantity.

Growth period

U in immature male gametes it is not clearly expressed. Their sizes increase slightly. Oocytes increase in size, sometimes hundreds, and more often thousands and even millions of times. At the end of the growth period - at interphase-1 - DNA reduplication occurs (2n4c).

Maturation period or meiosis.
Meiosis occurs as a result of two successive divisions of the parent diploid cell. Each of them includes four phases. All phases of the first meiotic division are designated by the number I, and all phases of the second division by the number II. Before prophase I in cells, DNA doubles and cells enter meiosis with the chromosome set 2n4c.
IN prophase I chromosomes initially look like thin threads and then thicken. Homologous chromosomes come closer together, at points of contact they cross and exchange homologous regions - this process is called crossing over(and represents one of the sources of genotypic combinative variability). Each chromosome, as a result of self-duplication, consists of two chromatids and is called univalenta, and after the convergence of two homologous chromosomes (two univalents), tetrads (bivalents) are formed. As in the prophase of mitosis, a spindle is formed in the cell during this period, the centrioles move to the poles, the nuclear envelope disintegrates, and the tetrads move towards the center of the cell.
In metaphase I tetrads line up in the equatorial plane, homologous chromosomes in the centromere region move away from each other, remaining connected in the shoulder region. The spindle strands are attached to the centromeres of homologous chromosomes. The cell enters the third phase - anaphase I, during which the spindle threads carry the univalents to opposite poles. In this case, one of the two homologous chromosomes accidentally ends up at one pole, the second at the other. It is during this period that the number of chromosomes is halved (reduced) and their random redistribution in future gametes occurs. In the final phase, the cell enters telophase I. Thus, as a result of meiosis, two cells are formed, containing only one of two homologous chromosomes, each of which consists of two chromatids. As a result of crossing over, chromosomes exchange their sections and thus carry recombined hereditary material. Telophase I does not last long, and the cell enters interphase (short in time), after which the second meiotic division occurs. During interphase, unlike mitosis, DNA synthesis does not occur in cells.
In prophase II, thread-like chromosomes - univalents - are located along the periphery of the nucleus, a spindle of division is formed, the chromosomes approach the equatorial plane and the cell enters metaphase II. In anaphase II chromatids diverge and are carried away by the spindle threads from the equatorial plane to the opposite poles. Following this comes telophase II, during which the chromosomes thin out, forming threads, and the nuclei of daughter cells form at the poles. As a result, from two cells of meiosis I in the telophase of meiosis II, four daughter mature gametes are formed, one of which carries the haploid number of chromosomes. The described process is typical for the formation of male gametes. The formation of female germ cells proceeds in a similar way, but during oogenesis only one mature egg develops, and three small reduction bodies subsequently die.
Formation period
Formation is characteristic only of spermatogenesis and consists of the formation of a spermatozoon . The essence of sperm formation is that cells acquire a certain shape and size corresponding to their specific function.
The function of sperm is to deliver genetic information to the egg and stimulate its development. IN connection with thus, after completion of meiosis, the germ cell undergoes deep restructuring. The Golgi apparatus is located at the transmitting end of the head, transforming into a terminal body - an acrosome. secreting enzymes that dissolve the egg membrane.
Mitochondria are compactly packed around the emerging flagellum, forming a neck. The formed sperm also contains a centriole.

The processes of spermatogenesis and oogenesis are similar in principle, but there are also differences between them. As a result of spermatogenesis, four sperm are formed, aovogenesis ends with the formation of one egg. This is due to the fact that during the first and second divisions of maturation, the eggs do not divide in half, but separate small guiding, or reducing, bodies. The guide bodies carry complete sets of chromosomes, but are practically devoid of cytoplasm and soon die. The biological meaning of the formation of these bodies lies in the need to preserve in the cytoplasm of the egg the maximum amount of yolk required for the development of the future embryo.
Eggshell has a membrane and cytoplasm with a supply of nutrients - the yolk. Its quantity depends on where the zygote will develop in the future. In mammals and humans, the embryo develops in the uterus, and it receives nutrients from the mother's body through the placenta, so their eggs are poor in yolk. In fish, amphibians, reptiles, birds and mollusks, the embryo develops outside the mother’s body, so there is a lot of yolk in the egg, and it is concentrated at one of the poles of the egg - the vegetative one. The egg does not have organelles of movement.
Sperm They are small in size and consist of a head, neck, and tail. The nucleus and a small layer of cytoplasm are concentrated in the head; the cytoplasm of the neck contains the aentrosome and ATP. The tail part is used for movement. Male reproductive cells lacking a tail are called sperm. They are characteristic only of angiospermic plants.

Table Scheme of formation of germ cells ( T.L. Bogdanov. Biology. Assignments and exercises. A guide for applicants to universities. M., 1991)

20. Formation of germ cells. Meiosis

Remember!

Where in the human body do germ cells form?

What set of chromosomes do gametes contain? Why?

To carry out sexual reproduction, specialized cells are needed - gametes containing a single (haploid) set of chromosomes. When they merge (fertilization), a diploid set is formed, in which each chromosome has a pair - a homologous chromosome. In each pair of homologous chromosomes, one chromosome is received from the father and the second from the mother.

In animals, the process of formation of germ cells is gametogenesis– occurs in special organs – the gonads (gonads). In most animals, male gametes (spermatozoa) are formed in the testes, female gametes (eggs) in the ovaries. The development of eggs is called oogenesis or oogenesis, and spermatozoa – spermatogenesis.

The structure of germ cells.

Ovules- These are relatively large, stationary, rounded cells. In some fish, reptiles and birds, they contain a large supply of nutrients in the form of yolk and have sizes from 10 mm to 15 cm. The eggs of mammals, including humans, are much smaller (0.1–0.3 mm) and the yolk is almost do not contain.

Sperm – small motile cells, in humans their length is only about 60 microns. In different organisms they differ in shape and size, but, as a rule, all sperm have a head, neck and tail, which ensure their mobility. In the head of the sperm there is a nucleus containing chromosomes, and an acrosome - a special vesicle with enzymes necessary for dissolving the egg shell. Mitochondria are concentrated in the neck, which provide the moving sperm with energy (Fig. 63).

Rice. 63. Spermatozoon of a mammal: A – electronic photograph; B – structure diagram

Spermatozoa were first described by the Dutch naturalist A. Leeuwenhoek in 1677. He also coined the term spermatozoon (from the Greek. sperma– seed and zoon– living being), i.e. living seed. The mammalian egg was discovered in 1827 by the Russian scientist K. M. Baer.

Formation of germ cells. The development of germ cells is divided into several stages: reproduction, growth, maturation, and in the process of spermatogenesis, the formation stage is also distinguished (Fig. 64).

Rice. 64. Gametogenesis in humans

Rice. 65. Phases of meiosis

Reproduction stage. At this stage, the cells that form the walls of the gonads actively divide by mitosis, forming immature germ cells. This stage in men begins with the onset of puberty and continues almost throughout life. In women, the formation of primary germ cells is completed in the embryonic period, i.e., the total number of eggs that a woman will mature during her reproductive period is determined already at an early stage of development of the female body. At the stage of reproduction, primordial germ cells, like all other cells of the body, are diploid.

Growth stage. At the growth stage, which is much better expressed in oogenesis, there is an increase in the cytoplasm of cells, accumulation of necessary substances and DNA reduplication (doubling of chromosomes).

Maturation stage. The third stage is meiosis. Meiosis- this is a special method of cell division, leading to a reduction in the number of chromosomes by half and to the transition of the cell from a diploid state to a haploid one.

Future gametes at the maturation stage are divided twice. Cells that begin meiosis contain a diploid set of already doubled chromosomes. During two meiotic divisions, one diploid cell produces four haploid cells.

Meiosis consists of two successive divisions, preceded by a single duplication of DNA, carried out during the growth stage. In each division of meiosis, there are four phases, which are also characteristic of mitosis (prophase, metaphase, anaphase, telophase), but they differ in some features (Fig. 65).

Prophase of the first meiotic division ( prophase I) is significantly longer than the prophase of mitosis. At this time, the doubled chromosomes, each of which already consists of two sister chromatids, spiral and acquire compact sizes. Then the homologous chromosomes are arranged parallel to each other, forming so-called bivalents or tetrads, consisting of two chromosomes (four chromatids). An exchange of corresponding homologous regions (crossing over) can occur between homologous chromosomes, which will lead to recombination of hereditary information and the formation of new combinations of paternal and maternal genes in the chromosomes of future gametes (Fig. 66).

By the end of prophase I, the nuclear envelope is destroyed.

IN metaphase I homologous chromosomes are located in pairs in the form of bivalents, or tetrads, located in the equatorial plane of the cell, and spindle threads are attached to their centromeres.

IN anaphase I homologous chromosomes from the bivalent (tetrad) diverge to the poles. Consequently, only one of each pair of homologous chromosomes ends up in each of the two resulting cells - the number of chromosomes is halved, and the chromosome set becomes haploid. However, each chromosome still consists of two sister chromatids.

Rice. 66. Crossing of chromosomes and exchange of homologous regions

IN telophase I Cells are formed with a haploid set of chromosomes and double the amount of DNA.

After a short period of time, the cells begin the second meiotic division, which proceeds as a typical mitosis, but differs in that the cells participating in it are haploid.

IN prophase II the nuclear membrane is destroyed. IN metaphase II chromosomes line up in the equatorial plane of the cell, spindle threads connect to the centromeres of the chromosomes. IN anaphase II The centromeres connecting sister chromatids divide, the chromatids become independent daughter chromosomes and move to different poles of the cell. Telophase II completes the second division of meiosis.

As a result of meiosis, from one original diploid cell containing doubled DNA molecules, four haploid cells are formed, each chromosome of which consists of a single DNA molecule.

During spermatogenesis at the maturation stage, as a result of meiosis, four identical cells are formed - sperm precursors, which at the formation stage acquire the characteristic appearance of a mature sperm and become motile.

Meiotic divisions in oogenesis are characterized by a number of features. Prophase I ends in the embryonic period, i.e. by the time a girl is born, her body already has a full set of future eggs. The remaining events of meiosis continue only after the woman reaches puberty. Every month, in one of the woman’s ovaries, one of the cells that have stopped dividing continues to develop. As a result of the first division of meiosis, a large cell is formed - the precursor of the egg and a small, so-called polar body, which enter the second division of meiosis. At the metaphase II stage, the precursor of the egg ovulates, that is, leaves the ovary into the abdominal cavity, from where it enters the oviduct. If fertilization occurs, the second meiotic division is completed - a mature egg and a second polar body are formed. If fusion with the sperm does not occur, the cell that has not completed division dies and is excreted from the body.

Polar bodies serve to remove excess genetic material and redistribute nutrients in favor of the egg. Some time after division they die.

The meaning of gametogenesis. As a result of gametogenesis, germ cells are formed containing a haploid set of chromosomes, which allows the number of chromosomes characteristic of the species to be restored during fertilization. In the absence of meiosis, gamete fusion would result in a doubling of the number of chromosomes in each successive generation resulting from sexual reproduction. This does not happen due to the existence of a special process - meiosis, during which the diploid number of chromosomes ( 2n) is reduced to haploid (1 n). Thus, the biological role of meiosis is to maintain a constant number of chromosomes over the course of generations of a species.

Review questions and assignments

1. Compare the structure of male and female reproductive cells. What are their similarities and differences?

2. What determines the size of eggs? Explain why mammalian eggs are among the smallest.

3. What periods are distinguished in the process of development of germ cells?

4. Explain how the maturation period (meiosis) occurs during spermatogenesis; oogenesis.

5. List the differences between meiosis and mitosis.

6. What is the biological meaning and significance of meiosis?

Think! Do it!

1. The organism developed from an unfertilized egg. Are its hereditary characteristics an exact copy of the characteristics of the maternal organism?

2. Explain why there are two terms for male reproductive cells: sperm (for example, in angiosperms) and sperm.

Work with computer

Refer to the electronic application. Study the material and complete the assignments.

Repeat and remember!

Human

Sex cells. The formation of sperm in men begins at puberty. The duration of all four phases of spermatogenesis is about 80 days. Over a lifetime, a man’s body produces a huge number of sperm – up to 10 10 .

Despite the fact that a lot of eggs are laid in a female embryo, only a few of them mature. During the reproductive period, that is, when a woman is capable of bearing children, about 400 eggs are finally formed.

The development of germ cells (ovogenesis and spermatogenesis) determines the health of the future generation. Smoking, consumption of alcoholic beverages, and drugs can have an irreversible effect on developing germ cells, which will subsequently lead to infertility or the birth of a child with hereditary or congenital disorders.

Formation of germ cells. Fertilization.

Performed by a group of biologists: Anastasia Martemyanova

Sokolova Ekaterina Tereshchenko Maria students of class 10 "A"

Checked by: Matrosova Evgenia Semenovna biology teacher

Gametes(from the Greek γᾰμετή - wife, γᾰμέτης - husband) - reproductive cells that have a haploid (single) set of chromosomes and participate in gametic, in particular, sexual reproduction. When two gametes fuse during the sexual process, a zygote is formed, which develops into an individual (or group of individuals) with the hereditary characteristics of both parental organisms that produced the gametes.

Sperm.

A sperm is a male reproductive cell, a male gamete, which serves to fertilize a female gamete, an egg.

Egg.

Egg(scientific oocyte, less commonly oocyte) - the female gamete of animals, higher plants, as well as many algae and other protists, which are characterized by oogamy. As a rule, eggs are haploid cells, but may have a different ploidy in polyploid organisms.

External fertilization

With external fertilization sperm live from 0.5 to 5 minutes. Productivity aquarium fish range from tens to several thousand pieces. The amount of developing eggs is influenced by the age, condition and size of the females, hereditary factors, habitat and adequate nutrition. Less fecundity is typical for species that care about their offspring

Internal fertilization.

During internal fertilization, the fusion of germ cells occurs inside the body. In this case, the probability of fertilization increases, and the possibility of death of gametes from the influence of unfavorable environmental conditions decreases.

Double fertilization

Two sperm penetrate the ovary of angiosperms, one of them fuses with the egg, giving rise to a diploid embryo. The other connects to the central diploid cell. A triploid cell is formed, from which the endosperm will emerge - the nutritional material for the developing embryo.

Double fertilization

This process, characteristic of all angiosperms, was discovered at the end of the last century by S.G. Navashin and received the name double fertilization. The significance of double fertilization is that it ensures the active development of nutritional tissue after fertilization. Therefore, the ovule in angiosperms does not store nutrients for future use and, therefore, develops much faster than in many other plants, for example, gymnosperms.