What is the biological meaning of sexual reproduction? Abstract sexual reproduction and its biological significance. Biological significance of reproduction

Drilling

Sexual reproduction has existed on Earth for more than 3 billion years

Sexual reproduction- a process in most eukaryotes associated with the development of new organisms from germ cells (in unicellular eukaryotes, during conjugation, the functions of germ cells are performed by the sex nuclei).

PR forms

Unicellular:

- copulation- the merging of two individuals into one, the union and recombination of hereditary material. This individual then reproduces by division.

- conjugation- in the temporary connection of two individuals for the purpose of exchange (recombination) of hereditary material. As a result, individuals appear that are genetically different from the parent organisms. They subsequently reproduce asexually. Since the number of ciliates remains unchanged after conjugation, there is no reason to talk about reproduction in the literal sense

Scheme of conjugation in ciliates: 1 - micronucleus (mi) and macronucleus (ma); 2 - first division of micronuclei, 4 chromosomes are visible in each; 3 - second division, in which the number of chromosomes is reduced to 2; 4 - 3 of the formed micronuclei become compacted and die; 5 - third division of the micronucleus; 6 and 7 - exchange of nuclei (♂ - mobile nucleus, ♀ - nucleus remaining in the cell; when they merge, a double set of chromosomes is restored); 8 - 10 - formation of a new macronucleus due to division of the micronucleus.

Multicellular:

With fertilization

For external type fertilization occurs in water, and the development of the embryo also occurs in the aquatic environment (lancelet, fish, amphibians).

For internal type fertilization occurs in the female genital tract, and the development of the embryo can occur either in the external environment (reptiles, birds), or inside the mother’s body in a special organ - the uterus (placental mammals, humans).

Without fertilization

Parthenogenesis- the so-called “virgin reproduction”, one of the forms of sexual reproduction of organisms, in which female reproductive cells (eggs) develop into an adult organism without fertilization. Although parthenogenetic reproduction does not involve the fusion of male and female gametes, parthenogenesis is still considered sexual reproduction, since the organism develops from a germ cell. It is believed that parthenogenesis arose during the process of evolution in dioecious forms.

Androgenesis- the development of an egg with a male nucleus introduced into it by sperm during the process of fertilization.

Androgenesis is observed in certain species of animals (silkworm) and plants (tobacco, corn) in cases where the maternal nucleus dies before fertilization, which is false, that is, the female and male nuclei do not merge (Pseudogamy) and only the male one participates in fragmentation core.

Gynogenesis- a special case of parthenogenesis, a special form of sexual reproduction in which, after the sperm penetrates the egg, their nuclei do not merge, and only the nucleus of the egg participates in subsequent development, or fertilization does not occur. In this case, there is no unification of the hereditary material of the parents through the fusion of the nuclei of their germ cells (bees, ants)

Biological significance

It provides significant genetic diversity

Phenotypic variability of offspring.

Great evolutionary and ecological (dispersal into different environments) possibilities

The combination in the hereditary material of a descendant of genetic information from two different sources - the parents.

Emerging organisms can combine beneficial characteristics of the father and mother.

Such organisms are more viable

1. Give definitions of concepts.
Reproduction - the ability of living things to reproduce their own kind.
Asexual reproduction - This is a method of reproduction in which gametes are not formed and one maternal individual participates.
Sporulation - a method of asexual reproduction in which a new organism develops from specialized cells - spores formed in sporangia.
Vegetative propagation - a method of asexual reproduction in which a daughter organism is formed from several parent cells.
Sexual reproduction – the process of formation of a daughter organism with the participation of gametes.
Sexual dimorphism – external differences between individuals of the opposite sex.

2. Fill out the table.

3. Label the methods of vegetative propagation in the figure.

4. Can the offspring obtained as a result of vegetative reproduction differ from the maternal organism? Justify your answer.
Yes, they can, but only slightly. Even identical twins are different because there is non-hereditary (modification) variability. Also, if the descendants have somatic mutations.

5. Compare asexual and sexual reproduction. Highlight the advantages and disadvantages of both types of reproduction. Fill the table.

6. The meaning of sexual reproduction:
Ensures the emergence of unique combinations of genetic material in a new individual, which helps to survive in changing environmental conditions.

7. Importance of asexual reproduction:
Under constant environmental conditions, it has the advantage that the daughter individuals are identical, i.e., adapted specifically to certain conditions. The reproduction process goes very quickly.

8. Why in nature are there two forms of reproduction of organisms, and not one?
What is good in one situation may not be suitable in another, which is why many species alternate between different forms of reproduction.

9. What significance did the emergence of sexual reproduction have for the evolution of life on Earth?
It ensured the emergence of new, more complex organisms, adaptable to different conditions, and the emergence of genetic diversity of species.

10. In many higher plants, the main method of reproduction is asexual vegetative, and sexual reproduction plays an auxiliary role. In most animals the situation is the opposite. Why do you think?
Animals lead an active lifestyle, move around, and their living conditions are constantly changing. While there are more plants, they produce much more pollen than sex cells. Animals find it easier to find a partner to reproduce than plants. It is necessary for the offspring of animals to differ from their parents for their survival and evolution.

11. Establish a correspondence between methods of asexual reproduction and the organisms for which they are characteristic.
Reproduction methods
1. Simple division into two (not mitosis)
2. Mitotic division
3. Sporulation
4. Using specialized body parts
5. Fragmentation
6. Budding
Organisms
A. Aspergillus and penicillium
B. Vibrio cholerae
B. Earthworm
G. Dysenteric amoeba
D. Red coral
E. Tulip

12. Choose the correct answer.
Test 1.
Is not a hermaphrodite:
3) human roundworm;

Test 2.
The specialized parts of the plant body that provide vegetative propagation do not include:
3) gladiolus flower;

Test 3.
The widespread occurrence of asexual reproduction among higher plants is associated with:
4) the high speed of this type of reproduction.

Test 4.
The great evolutionary progressivity of sexual reproduction is due to the fact that it:
2) ensures genetic diversity of the offspring;

13. Fill in the missing terms.
The most ancient type of reproduction of animals and plants is asexual reproduction.
Most algae reproduce by spores, and among higher plants - mosses, lycophytes, horsetails and pteridophytes, which are called Higher spore plants.
Vegetative propagation is based on the ability of plants to regenerate.
Relatively few plants, for example begonia, gloxinia, Uzumbara violet, can be restored from cut parts of the leaf.

14. Cognitive task (oral answer).
Some wild plants are capable of forming so-called brood buds, which, falling into water or onto favorable soil, give rise to a new plant. Explain why such plants are distributed mainly in polar, high mountain and steppe areas.
In such conditions, plants can easily lose flowers or fruits, so brood buds spread like seeds, fall off and take root.

15. Explain the origin and general meaning of the word (term), based on the meaning of the roots that make it up

16. Select a term and explain how its modern meaning matches the original meaning of its roots.
The term chosen is hermaphrodite.
Correspondence - corresponds in meaning, but the modern meaning is a term, and previously it was the name of a bisexual creature, considered a god and a monster.

17. Formulate and write down the main ideas of § 3.5.
Reproduction is the ability of living things to reproduce their own kind.
Methods: asexual and sexual.
Asexual - no gametes are formed and one maternal individual is involved. Happens:
1. Division (in protozoa, bacteria).
2. Sporulation (in plants, fungi).
3. Vegetative (in plants and some primitive animals).
Types of vegetative propagation: fragmentation, budding, using specialized parts of the body (rhizome, tubers, tendrils, etc.)
Sexual reproduction - with the help of gametes, 2 individuals participate.
Advantages of sexual reproduction - the unique combination of genetic material in the new individual helps to survive in changing environmental conditions.

The advantages of asexual reproduction are that daughter individuals are adapted to certain stable conditions, the process proceeds very quickly.

Reproduction is a necessary property of life. The continuity of life on earth, the duration of existence of any type of living organisms - plants and animals - is supported by the process of reproduction. Sexual reproduction is a biological process aimed at increasing the number of individuals and ensuring the continued existence of the species. Gives a sharp increase in variability. It is widespread in nature. All farm animals reproduce sexually. During sexual reproduction, the fusion of two sex cells of a male and a female is necessary - fertilization. The biological significance of the fertilization process lies in the fact that during sexual reproduction, a new organism that has arisen is more adapted to changing living conditions, since as a result of the fusion of two germ cells, an organism with double heredity arises - a male and a female, having different origins. During fertilization, the gametes are mutually enriched and, at the same time, the metabolism is balanced, which gives increased vitality to the new generation.

4. Natural insemination of animals. Manual and cook mating

Natural insemination in the genitals is a complex of conditioned and unconditioned reflexes that ensure the release of sperm from the male organ into the female genital organs. For sexual intercourse to occur, direct contact between the male and female is necessary. EO- is divided into in domestic animals with EO with the vaginal-uterine type of insemination in animals, sexual intercourse is not large, the volume of ejaculate is small, the concentration of sperm is high, the accessory sex glands produce a small amount of sperm, animals with the uterine type of insemination, the sperm enters the female’s uterus, The duration of sexual intercourse is significant, the volume of ejaculate is large, and the sperm concentration is low. Occurs during sexual intercourse, that is, during contact between a female and a male, when the entire complex of sexual reflexes characteristic of mating appears. The main methods of natural insemination are manual, cooking, and free mating. Manual mating- mating takes place under the supervision of service personnel. It makes it possible to select queens and sires according to a planned plan and within a certain time frame; regulate the sexual load on the manufacturer; prevent the occurrence of sexually transmitted diseases, since females and males are examined by a veterinarian before mating; accurately take into account the offspring of parents. Cooking case It consists in the fact that the female in heat and the male are placed in a separate room (boil, base, pen, cage) and left for some time. The disadvantage of this method is that the producer inseminates the same female several times.

5. Joint, free and harem (cool) mating

Free mating– the producer is constantly in the herd (a specific irritant to the sexual function of females). The onset of heat (stage of arousal) is accelerated, proceeds brightly, missing heat is eliminated, and a high percentage of fertilization is achieved. Disadvantage: – recording of inseminations is difficult; – breeding work cannot be carried out in large herds; – used in beef cattle breeding; on small farms School mating– used in herd horse breeding. The stud stallion is kept around the clock along with the mares in the pasture under the supervision of a herdmaster who registers the inseminated queens. Cool mating - used in sheep farms. Sheep are divided into groups (classes) and certain rams are assigned to them. Harem mating– a certain number of ewes of appropriate quality are attached to each ram. The stud ram grazes with them and inseminates them.

13. Reproduction is the main property of living things. Asexual and sexual reproduction. Forms of asexual reproduction. Definition, essence, biological significance.

Reproduction is an integral part of living beings. The existence of an organism is a preparation for the fulfillment of the main biological task - participation in reproduction.

Reproduction (reproduction, self-reproduction) refers to the ability of organisms to reproduce their own kind.

Biological role of reproduction: ensures a change of generations; with its help, biological species and life as such are preserved over time; intraspecific variability is maintained; the problems of increasing the number of individuals are solved.

There are 2 methods of reproduction: asexual and sexual.

Asexual - the beginning of a new organism is given by one parent organism, the offspring is an exact genetic copy of the parent; There is no sexual process, therefore there is no exchange of genetic information. There are no special germ cells; the cellular material for the development of offspring is: a) several somatic cells of a multicellular parent; b) a whole organism, if it is a protozoan. The cellular mechanism for the formation of a descendant is mitosis. From the 1st cell, identical offspring are formed - a clone. The source of clone variability is random mutations. In evolutionary terms, such reproduction increases the influence of stabilizing selection and helps maintain the greatest adaptability to little changing environmental conditions.

Forms: 1) in unicellular eukaryotes: a) binary (division by 2); b) schizogony – a single division into many parts; c) budding - a descendant is formed on the body of the parent, as an outgrowth with subsequent splitting off; d) sporogony – repeated division into many parts. 2) in multicellular organisms: a) vegetative – parts of the body or groups of somatic cells; b) budding - the formation of buds; c) sporulation – the formation of spores in special structures; d) fragmentation - the disintegration of a multicellular body into parts that turn into independent individuals.

14. Sexual reproduction in unicellular and multicellular organisms. The sexual process as a mechanism for the exchange of hereditary information within a species. Morphophysiological characteristics of germ cells.

The basis of sexual reproduction is the sexual process, the essence of which boils down to the combination in the hereditary material for the development of the descendant of genetic information from two different sources - the parents. An idea of the sexual process is given by the processes of conjugation of ciliates. It consists in the temporary connection of two individuals for the purpose of exchanging (recombining) hereditary materials, resulting in the appearance of individuals that are genetically distinct from the parent organizations. They subsequently reproduce asexually.

At a certain stage of evolution in multiclerical organizations, the sexual process as a way of exchanging gene information among individuals within a species turned out to be associated with reproduction.

To carry out sexual reproduction, the parental individuals produce gametes - cells specialized to ensure the generative function. The fusion of maternal and paternal gametes leads to the emergence of a zygote - a cell that is a daughter individual at the earliest stage of development.

Features of gametes: haploid; low level of metabolic processes; Only the egg enters mitosis in case of fertilization; only the egg has a protective protein shell; only the sperm has a cell center that is transferred to the egg; the sperm is mobile; the egg develops (in case of fertilization); The sperm transports genetic material.

18. Plant propagation. The meaning of sexual and asexual reproduction. Types of sexual process.

Plant propagation- a set of processes leading to an increase in the number of individuals of a certain species; occurs in plants asexual, sexual And vegetative(asexual and sexual reproduction are combined into the concept generative reproduction). The study of various aspects of reproduction is the subject reproductive biology.

Asexual reproduction differs from vegetative reproduction in that during vegetative reproduction a daughter individual is genetically identical to the maternal one ( clone), necessarily receives a fragment of the maternal organism, since it is formed from it; With asexual reproduction, this does not happen.

Generative reproduction is based on the alternation of two nuclear phases - haploid and diploid. This alternation is due to two alternative processes - fertilization and reduction division (meiosis). In plants, the haploid phase that produces haploid gametes is called gametophyte, and the diploid phase, which forms haploid spores from which gametophytes develop, is sporophyte. Sporophyte and gametophyte may differ from each other morphologically ( heteromorphic life cycle), and be of the same structure ( isomorphic life cycle).

The difference between sexual reproduction and sexual reproduction is that in the first case a single sporophyte embryo is formed on the gametophyte, and in the second - several. Most plants undergo sexual reproduction

Asexual reproduction plants are carried out by haploid spores - aplanomeiospores. They are formed in special bodies - sporangia. In algae, in most cases, sporangia are unicellular (only in some algae sporangia are multicellular, but not differentiated into tissues).

In higher plants, sporangia are multicellular, their cells are differentiated. Fertile cells make up archesporium- sporogenic tissue, outer sterile cells form a protective wall. The lining layer is formed from the outer cells of the archesporium - tapetum, which, spreading out, forms periplasmodium. The nutrients it contains are used to form spores.

Archesporium cells, dividing by mitosis, give rise to sporocytes, which, dividing by meiosis, form tetrads of spores.

The spores are covered with a two- or three-layer shell - sporoderm. The spores are light, rich in cytoplasm, have a large nucleus and proplastids; reserve substances are often represented by fats.

Gametophytes (thickles) develop from spores. When homosporous In plants, all spores are equal in size. This phenomenon is called isosporia. At heterosporia spores of different sizes are formed. Larger spores (megaspores) give rise to female gametophytes, and smaller ones (microspores) give rise to male gametophytes; such plants are called heterosporous.

Sexual process in the plant world it is extremely diverse and often very complex, but essentially comes down to the fusion of two sex cells (gametes) - male and female.

Gametes arise in certain cells or organs of plants. In some cases, the gametes are identical in size and shape and both have mobility due to the presence of flagella (isogamy); sometimes they are slightly different from each other in size (heterogamy). But more often - with the so-called oogamy - the sizes of the gametes are sharply different: the male gamete, called the sperm, is small and mobile, and the female - the egg - is immobile and large. The process of fusion of gametes is called fertilization. Gametes have one set of chromosomes in their nucleus, and in the cell formed after the fusion of gametes, which is called a zygote, the number of chromosomes doubles. The zygote germinates and gives rise to a new individual plant.

The sexual process occurs in plants at a certain time and at a certain stage of its development, during which the plant can also reproduce asexually (with the formation of spores) and vegetatively.

Sexual reproduction arose in the plant world during the process of evolution. Bacteria and blue-green algae do not yet have it. In most algae and fungi, as well as in all higher land plants, the sexual process is clearly expressed.

Sexual reproduction is very important for the body in that due to the fusion of the paternal and maternal cells, a new organism is created. It has greater variability and is better adapted to environmental conditions.

The simplest process of sexual reproduction can be observed in unicellular algae, for example in Chlamydomonas.

Meaning of asexual: the first and most important thing is speed: asexual reproduction requires significantly less energy, which means it provides more opportunities, roughly speaking, per 1 J of energy expended. The consequence of this first is that there are more chances for dispersal, but provided that the genotype of the plant that reproduces asexually is sufficiently optimal for local conditions. In this case, the descendants of the plant undergo a kind of expansion. Finally, the preservation of the genotype: sexual reproduction is a launching pad for the possibility of speciation, and asexual reproduction is a kind of conservation of the existing genotype.

Meaning of sexual: With sexual reproduction, compared to vegetative reproduction, the following is achieved: 1) a higher reproduction rate, i.e., a much larger number of rudiments of new individuals; 2) the possibility of settlement over much longer distances and, consequently, the settlement of a larger territory; 3) transfer of seeds to other conditions, which makes it possible for various changes to occur under the influence of new conditions and, therefore, provides new material for natural selection. It is even more important that during vegetative (or asexual) propagation, the new plant completely inherits all the properties of the maternal plant, including age-related senile changes that occur in most people sooner or later; in addition, it does not receive any new properties and is able to live only within the same boundaries of external conditions as the mother plant.

During sexual reproduction, a complete renewal occurs, life begins in the full sense all over again, and all age-related changes of the parents are not transmitted to the offspring. In addition, and this is very important, during sexual reproduction there is a combination of more or less different paternal and maternal hereditary inclinations, the offspring are more diverse, with new combinations of paternal and maternal properties, and sometimes with completely new characteristics. Such genetically more heterogeneous offspring have a wider range of adaptability to external conditions; individual representatives of it can get along in conditions where their parents would have died, and the entire species (the complex of forms closest to each other) will be more persistent in the struggle for existence. Such sexually reproducing species were the winners in the struggle of life.

The great importance of sexual reproduction for evolution is that

Vlad ustelyomov

Sexual reproduction is a more progressive form of reproduction, very widespread in nature, both among plants and among animals. Organisms formed during sexual reproduction differ from each other genetically, as well as in the nature of their adaptability to living conditions.

During sexual reproduction, maternal and paternal organisms produce specialized sex cells - gametes. Female non-motile gametes are called eggs, male non-motile gametes are called sperm, and mobile gametes are called sperm. These germ cells fuse to form a zygote, i.e. fertilization occurs. Sex cells, as a rule, have a half set of chromosomes (haploid), so that when they merge, a double (diploid) set is restored, and a new individual develops from the zygote. In sexual reproduction, offspring are formed by the fusion of haploid nuclei. Haploid nuclei are formed as a result of meiotic division.

Meiosis leads to a reduction in genetic material by half, due to which the amount of genetic material in individuals of a given species remains constant over a number of generations. During meiosis, several important processes occur: random chromosome segregation (independent segregation), exchange of genetic material between homologous chromosomes (crossing over). As a result of these processes, new combinations of genes arise. Since the zygote nucleus after fertilization contains the genetic material of the two parents, this increases genetic diversity within the species. If the essence and biological significance of the sexual process is the same for all organisms, then its forms are very diverse and depend on the level of evolutionary development, habitat, lifestyle and some other features.
Sexual reproduction has very great evolutionary advantages compared to asexual reproduction. The essence of sexual reproduction is the combination in the hereditary material of a descendant of genetic information from two different sources - the parents. Fertilization in animals can be external or internal. Fusion produces a zygote with a double set of chromosomes.

In the nucleus of the zygote, all chromosomes become paired: in each pair, one of the chromosomes is paternal, the other is maternal. The daughter organism that develops from such a zygote is equally equipped with hereditary information from both parents.

The biological meaning of sexual reproduction is that emerging organisms can combine useful characteristics of the father and mother. Such organisms are more viable. Sexual reproduction plays an important role in the evolution of organisms.

What is the biological significance of sexual reproduction?

Alex

In genetic recombination. You pose the question incorrectly about the meaning of the sexual process. This is a source of combinative variability for selection and at the same time a mechanism for comparing the genotypes of two organisms to maintain the genetic unity of the population and species.

Alexander mashtakov

I think you need to turn to biology textbooks. The meaning of sexual reproduction is the restoration of working genes and blocking of genes damaged by mutation. That is, there is some mechanism that, at the molecular level, is still capable of “correcting” a harmful mutation, which, with same-sex reproduction, would require much more time to correct through Darwinian natural selection. Thus, sexual reproduction allows, first of all, to cut off random and harmful mutations of genes, nevertheless allowing the carriers of these mutations to survive.

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Group No. 153

Topic: Sexual reproduction and its biological significance

Student: A.E. Petrenko

Teacher: M.A. Varochkin

1. Sexual reproduction

In sexual reproduction, offspring are produced by the fusion of genetic material from haploid nuclei. Usually these nuclei are contained in specialized germ cells - gametes; During fertilization, the gametes fuse to form a diploid zygote, which during development produces a mature organism. Gametes are haploid - they contain one set of chromosomes resulting from meiosis; they serve as a link between this generation and the next (during sexual reproduction of flowering plants, not cells, but nuclei, merge, but usually these nuclei are also called gametes.)

Meiosis is an important stage in life cycles involving sexual reproduction, as it leads to a reduction in the amount of genetic material by half. Thanks to this, in a series of generations that reproduce sexually, this number remains constant, although during fertilization it doubles each time. During meiosis, as a result of random divergence of chromosomes (independent distribution) and the exchange of genetic material between homologous chromosomes (crossing over), new combinations of genes appear in one gamete, and such shuffling increases genetic diversity. The fusion of haploid nuclei contained in gametes is called fertilization or syngamy; it leads to the formation of a diploid zygote, that is, a cell containing one chromosome set from each parent. This combination of two sets of chromosomes in the zygote (genetic recombination) represents the genetic basis of intraspecific variation. The zygote grows and develops into a mature organism of the next generation. Thus, during sexual reproduction in the life cycle, an alternation of diploid and haploid phases occurs, and in different organisms these phases take different forms.

Gametes usually come in two types, male and female, but some primitive organisms produce only one type of gamete. In organisms that produce two types of gametes, they can be produced by male and female parents, respectively, or it may be that the same individual has both male and female reproductive organs. Species in which there are separate male and female individuals are called dioecious; such are most animals and humans. Among flowering plants there are also dioecious species; If in monoecious species male and female flowers are formed on the same plant, as, for example, in cucumber and hazel, then in dioecious species some plants bear only male, and others only female flowers, as in holly or yew.

Hermaphroditism.

Parthenogenesis.

Parthenogenesis is one of the modifications of sexual reproduction in which the female gamete develops into a new individual without fertilization by the male gamete. Parthenogenetic reproduction occurs in both the animal and plant kingdoms and has the advantage of increasing the rate of reproduction in some cases.

There are two types of parthenogenesis - haploid and diploid, depending on the number of chromosomes in the female gamete. In many insects, including ants, bees and wasps, various castes of organisms arise within a given community as a result of haploid parthenogenesis. In these species, meiosis occurs and haploid gametes are formed. Some eggs are fertilized and develop into diploid females, while unfertilized eggs develop into fertile haploid males. For example, in the honey bee, the queen lays fertilized eggs (2n = 32), which develop into females (queens or workers), and unfertilized eggs (n = 16), which produce males (drones) that produce sperm by mitosis, and not meiosis. The development of individuals of these three types in the honey bee is schematically presented in. This mechanism of reproduction in social insects has adaptive significance, since it makes it possible to regulate the number of descendants of each type.

In aphids, diploid parthenogenesis occurs, in which the female oocytes undergo a special form of meiosis without chromosome segregation - all chromosomes pass into the egg, and the polar bodies do not receive a single chromosome. The eggs develop in the mother's body, so that young females are born fully formed, rather than hatching from eggs. This process is called viviparity. It can continue for several generations, especially in the summer, until almost complete nondisjunction occurs in one of the cells, resulting in a cell containing all pairs of autosomes and one X chromosome. From this cell the male develops parthenogenetically. These autumn males and parthenogenetic females produce haploid gametes through meiosis that participate in sexual reproduction. Fertilized females lay diploid eggs, which overwinter, and in the spring they hatch into females that reproduce parthenogenetically and give birth to living offspring. Several parthenogenetic generations are followed by a generation resulting from normal sexual reproduction, which introduces genetic diversity into the population through recombination. The main advantage that parthenogenesis gives to aphids is the rapid growth of the population, since all its mature members are capable of laying eggs. This is especially important during periods when environmental conditions are favorable for the existence of a large population, i.e. during the summer months.

Parthenogenesis is widespread in plants, where it takes various forms. One of them, apomixis, is parthenogenesis, simulating sexual reproduction. Apomixis is observed in some flowering plants in which the diploid ovule cell, either a nucellus cell or a megaspore, develops into a functional embryo without the participation of a male gamete. The rest of the ovule forms the seed, and the ovary develops into the fruit. In other cases, the presence of a pollen grain is required, which stimulates parthenogenesis, although it does not germinate; the pollen grain induces hormonal changes necessary for the development of the embryo, and in practice such cases are difficult to distinguish from true sexual reproduction.

2. Sexual reproduction in humans

Male reproductive system.

sexual haploid sperm

The male reproductive system consists of paired testes (testes), vas deferens, a number of accessory glands and the penis (penis). The testis is a complex tubular gland of an ovoid shape; it is enclosed in a capsule - the tunica albuginea - and consists of approximately a thousand highly convoluted seminiferous tubules, immersed in connective tissue that contains interstitial (Leydig) cells. The seminiferous tubules produce gametes - sperm (spermatozoa), and interstitial cells produce the male sex hormone testosterone. The testes are located outside the abdominal cavity, in the scrotum, and therefore sperm develop at a temperature that is 2-3°C lower than the temperature of the internal areas of the body. The lower temperature of the scrotum is partly determined by its position, and partly by the choroid plexus formed by the artery and vein of the testis and acting as a countercurrent heat exchanger. Contractions of special muscles move the testes closer or further from the body, depending on the air temperature, to maintain the temperature in the scrotum at a level optimal for sperm production. If a man has reached puberty and the testes have not descended into the scrotum (a condition called cryptorchidism), he remains sterile forever, and in men who wear too tight underpants or take very hot baths, sperm production may be so reduced that it leads to infertility. Only a few mammals, including whales and elephants, have their testes in the abdominal cavity their entire lives.

The seminiferous tubules reach 50 cm in length and 200 µm in diameter and are located in areas called lobules of the testis. Both ends of the tubules are connected to the central region of the testis - the rete testis - by short, straight seminiferous tubules. Here the sperm is collected in 10-20 efferent tubules; along them it is transferred to the head of the epididymis, where it is concentrated as a result of the reabsorption of fluid secreted by the seminiferous tubules. In the head of the epididymis, sperm mature, after which they pass along a convoluted 5-meter efferent tubule to the base of the epididymis; here they remain for a short time before entering the vas deferens. The vas deferens is a straight tube about 40 cm long, which, together with the artery and vein of the testis, forms the spermatic cord and carries sperm to the urethra (urethra), which passes inside the penis. The relationship between these structures, the male accessory glands and the penis is shown in.

Sperm development (spermatogenesis).

Spermatozoa (sperm) are formed through a series of successive cell divisions, collectively called spermatogenesis, followed by a complex process of differentiation called spermiogenesis. The process of sperm formation takes approximately 70 days; per 1 g of testicle weight, 107 sperm are produced per day. The epithelium of the seminiferous tubule consists of an outer layer of germinal epithelial cells and approximately six layers of cells formed as a result of repeated cell divisions of this layer; these layers correspond to successive stages of sperm development. First, the division of germinal epithelial cells gives rise to numerous spermatogonia, which increase in size and become first-order spermatocytes. These spermatocytes, as a result of the first meiotic division, form second-order haploid spermatocytes, after which they undergo the second meiotic division and turn into spermatids. Between the “strands” of developing cells are large Sertoli cells, or trophic cells, located throughout the entire space from the outer layer of the tubule to its lumen.

Spermatocytes are located in numerous invaginations on the lateral surfaces of Sertoli cells; here they turn into spermatids, and then move to that edge of the Sertoli cell, which faces the lumen of the seminiferous tubule, where they mature, forming sperm. Apparently, Sertoli cells provide mechanical support, protection and nutrition to maturing sperm. All nutrients and oxygen delivered to developing gametes through the blood vessels surrounding the seminiferous tubules, and metabolic waste released into the blood, pass through the Sertoli cells. These cells also secrete fluid with which sperm travel through the tubules.

Sperm.

Spermatozoa, or sperm, are very small, motile male gametes produced by the male gonads-testes; their number is in the millions. The shape of sperm varies in different animals, but their structure is the same. Each sperm can be divided into five sections. In the head of the sperm there is a nucleus containing the haploid number of chromosomes and covered by an acrosome. The acrosome, a special structure bounded by a membrane, contains hydrolytic enzymes that facilitate the penetration of sperm into the oocyte immediately before fertilization; thus, functionally it can be considered as an enlarged lysosome. The short neck of the sperm contains a pair of centrioles lying at right angles to each other. The microtubules of one of the centrioles elongate, forming an axial filament of the flagellum, running along the rest of the sperm. The middle part is expanded due to the numerous mitochondria it contains, assembled into a spiral around the flagellum. These mitochondria provide energy for the contractile mechanisms that provide movement of the flagellum. The main and tail parts of the sperm have a structure characteristic of flagella: a cross section shows a typical “9 + 2” structure - 9 pairs of peripheral microtubules surrounding a pair of central microtubules.

If you look at the head of a human sperm from above, it appears round, and when viewed from the side, it appears flattened. Flagellar movement alone is not enough for the sperm to travel the distance from the vagina to the site where fertilization occurs. The main locomotor task of sperm is to swarm around the oocyte and orient themselves in a specific way before penetrating the oocyte's membranes.

Female reproductive system.

The woman's role in the reproductive process is much larger than the man's and involves interactions between the pituitary gland, ovaries, uterus and fetus. The female reproductive system consists of paired ovaries and fallopian tubes, the uterus, vagina and external genitalia.

The ovaries are attached to the wall of the abdominal cavity by a fold of peritoneum and perform two functions: they produce female gametes and secrete female sex hormones. The ovary is almond-shaped, consists of an outer cortex and an inner medulla, and is enclosed in a connective tissue membrane called the tunica albuginea. The outer layer of the cortex consists of germinal epithelial cells from which gametes are formed. The cortex is formed by developing follicles, and the medulla is composed of stroma containing connective tissue, blood vessels and mature follicles.

The fallopian tube is a muscular tube about 12 cm long through which female gametes leave the ovary and enter the uterus. The opening of the fallopian tube ends in an extension, the edge of which forms a fimbria, approaching the ovary during ovulation. The lumen of the fallopian tube is lined with ciliated epithelium; The movement of female gametes to the uterus is facilitated by peristaltic movements of the muscular wall of the fallopian tube.

The uterus is a thick-walled sac approximately 7.5 cm long and 5 cm wide, consisting of three layers. The outer layer is called the serosa. Below it is the thickest middle layer - the myometrium; it is formed by bundles of smooth muscle cells that are sensitive to oxytocin during childbirth. The inner layer - the endometrium - is soft and smooth; it consists of epithelial cells, simple tubular glands and spiral arterioles that supply the cells with blood. During pregnancy, the uterine cavity can increase 500 times - from 10 cm3 to 5000 cm3. The lower entrance to the uterus is the cervix, which connects the uterus to the vagina. The entrance to the vagina, the external opening of the urethra and the clitoris are covered by two folds of skin - the labia majora and minora, forming the vulva. The clitoris is a small formation capable of erection, homologous to the male penis. In the walls of the vulva there are Bartholin's glands, which secrete mucus during sexual arousal, which moisturizes the vagina during intercourse.

Menstrual cycle.

In men, the formation and release of gametes is a continuous process, beginning with the onset of puberty and continuing throughout life. In women, this is a cyclical process, repeating approximately every 28 days and associated with changes in the structure and functions of the entire reproductive system. This process is called the menstrual cycle, and it can be divided into four stages. The events that occur during the menstrual cycle involve the ovaries (ovarian cycle) and the uterus (uterine cycle) and are regulated by ovarian hormones, the secretion of which is in turn regulated by pituitary gonadotropins.

Ovarian cycle.

In an adult woman, the ovarian cycle begins with the development of several primary follicles (containing first-order oocytes) under the influence of follicle-stimulating hormone (FSH) secreted by the anterior pituitary gland. Of these follicles, only one continues to grow, while the rest are destroyed as a result of a degenerative process (follicular atresia). The cells of the granulosa membrane of the growing follicle proliferate, forming an outer fibrous layer several cells thick, called theca extema, and an inner layer rich in blood vessels, theca interna. Granulosa cells secrete follicular fluid, which accumulates in the follicle cavity. Luteinizing hormone (LH), secreted by the pituitary gland, stimulates cells, causing them to produce steroids, mainly estradiol. The increase in estradiol levels during the follicular phase acts on the pituitary gland according to the principle of negative feedback, causing a decrease in FSH levels in the blood (days 4-11); LH levels remain unchanged. Estrogen levels peak approximately three days before ovulation and act at this time on the pituitary gland according to the principle of positive feedback, stimulating the release of both FSH and LH. FSH is believed to be necessary to stimulate follicular growth, but further follicular development is controlled primarily by LH. Granulosa cells are located at the periphery, the egg is shifted to one side of the follicle, but is still surrounded by a layer of granulosa cells. The mature follicle, called the Graafian vesicle, reaches approximately 1 cm in diameter and protrudes above the surface of the ovary in the form of a tubercle. The exact mechanism of ovulation is unknown, but it is believed that LH, FSH and prostaglandins are involved.

During ovulation, a second-order oocyte separates from the wall of the ruptured follicle, exits into the abdominal cavity and enters the fallopian tube.

Typically, each month only one oocyte is released by one of the ovaries, so that ovulation occurs alternately in one or the other ovary. An ovulated oocyte is a cell whose nucleus is in metaphase 1 of meiosis; it is surrounded by a layer of cells called the zona pellucida and a layer of granulosa cells (corona radiata), which protects the oocyte until fertilization. After ovulation, the LH level drops to the level characteristic of the follicular phase, and under the influence of another gonadotropin - prolactin - the cells of the ruptured follicle change, forming the corpus luteum. The corpus luteum begins to secrete another female hormone - progesterone - and small amounts of estrogen. These two hormones maintain the normal structure of the endometrium lining the uterus and inhibit the release of FSH and LH, acting on the principle of negative feedback on the hypothalamus. If fertilization does not occur, then under the influence of factors that will be described later, the corpus luteum undergoes involution and only a small scar remains - corpus albicans; this is accompanied by a decrease in progesterone and estrogen levels, as a result of which the inhibition of FSH secretion ceases, its level rises and a new cycle of follicle development begins.

Uterine cycle.

The uterine cycle is divided into three phases associated with certain structural and functional changes in the endometrium.

1. MENSTRUAL PHASE. In this phase, the epithelial layer of the endometrium is rejected. Just before menstruation, the blood supply to this area decreases as a result of narrowing of the spiral arterioles in the uterine wall, caused by a drop in the level of progesterone in the blood after involution of the corpus luteum. Insufficient blood supply leads to the death of epithelial cells. Then the narrowing of the spiral arterioles is replaced by their expansion, and under the influence of increased blood flow, the epithelium is rejected and its remains are excreted along with the blood in the form of menstrual flow.

2. PROLIFERATIVE PHASE. This phase coincides with the follicular phase of the ovarian cycle and consists of rapid proliferation of endometrial cells, leading to its thickening under the control of estrogen secreted by the developing follicle.

H. SECRETORY PHASE. During this phase, progesterone secreted by the corpus luteum stimulates the secretion of mucus by the tubular glands; this maintains the lining of the uterus in a condition in which a fertilized egg can be implanted into it.

3. What happens if fertilization does not take place?

If fertilization does not occur within 24 hours after ovulation, then the second-order oocyte undergoes autolysis in the fallopian tube; the same thing happens with sperm remaining in the woman’s genital tract. The corpus luteum persists for 10-14 days after ovulation (usually until the 26th day of the cycle), but then stops secreting progesterone and estrogen due to insufficient levels of LH in the blood and undergoes autolysis. As recent studies have shown, in some species the uterine wall, which does not contain a fertilized egg, appears to secrete a factor called luteoliasis; this factor is a prostaglandin (prostaglandin F2?). It is believed that luteolysin enters the blood into the ovary, where it causes involution of the corpus luteum, destroying lysosomes in the granulosa cells of the latter, which leads to their autolysis.

Fertilization results.

If fertilization occurs, a blastocyst develops from the resulting zygote, which 8 days after ovulation sinks into the wall of the uterus. The outer cells of the blastocyst, forming the trophoblast, then begin to secrete a hormone, hornonic gonadotropium, which is similar in function to luteinizing hormone. This function includes preventing autolysis of the corpus luteum and inducing it to secrete large quantities of progesterone and estrogens, which cause increased endometrial growth. The rejection of the epithelial lining of the endometrium is suppressed, and the next menstruation does not occur, which is the earliest sign of pregnancy. (Chorionic gonadotropin also stimulates the interstitial cells of the testes in the male embryo and causes them to produce testosterone, which induces the growth of male genital organs.) The role of the placenta increases at about the 10th week of pregnancy, when it begins to secrete most of the progesterone and estrogen necessary for normal course of pregnancy. Premature cessation of the activity of the corpus luteum (before the placenta fully demonstrates its secretory ability) is a common cause of miscarriage in the 10th-12th weeks of pregnancy.

During pregnancy, human chorionic gonadotropin can be detected in the urine, and pregnancy tests are based on this. Currently, a test based on agglutination suppression is used: latex particles coated with human chorionic gonadotropin are added to a mixture of urine with an antiserum that agglutinates this hormone. If present in urine, it will react with the agglutinating antiserum and not with the latex particles. The absence of latex agglutination is an indicator of pregnancy; this test can be used as early as 14 days after missed menstruation.

Copulation.

Internal fertilization is an important part of the reproductive cycle in terrestrial organisms, and in many of them, including humans, it is facilitated by the copulatory organ, the male penis, which is inserted into the vagina and carries gametes as deep as possible into the female genital tract. An erection of the penis occurs due to a local increase in blood pressure in its erectile tissue as a result of narrowing of the veins and dilation of the arteries. This reaction is due to the activation of the parasympathetic nervous system during sexual arousal. In this state, the penis can be inserted into the vagina, where the friction created by the rhythmic movements during sexual intercourse produces tactile stimulation of the sensory cells in the glans penis. This activates sympathetic neurons that cause contraction of the internal bladder sphincter, epididymal smooth muscle, vas deferens, and accessory glands (seminal vesicle, prostate, and bulbourethral glands). As a result of all this, the future components of the seminal fluid are pushed into the proximal part of the urethra, where they mix. Increased pressure in the proximal urethra causes reflex activity of motor neurons innervating the muscles at the base of the penis. Rhythmic wave-like contractions of these muscles push seminal fluid through the distal part of the urethra, and ejaculation occurs (the release of sperm) - the climax of sexual intercourse. The sensations associated with this moment in both men and women are collectively called orgasm. Lubrication during sexual intercourse is provided mainly by the secretions of the glands of the vagina and vulva, and partly also by the clear mucus secreted by the male bulbourethral glands after an erection. The secretions of the accessory male glands are alkaline and contain mucus, fructose, vitamin C, citric acid, prostaglandins and various enzymes; they increase the pH of the environment in the vagina, which is usually acidic, to 6-6.5, which is optimal for sperm motility after ejaculation. The amount of seminal fluid ejaculated during ejaculation is about 3 ml, of which only 10% is sperm. Despite this low percentage, semen contains approximately 108 sperm per 1 ml.

Fertilization.

Sperm is released deep into the vagina, near the cervix. Studies have shown that sperm pass from the vagina into the uterus and reach the upper end of the fallopian tubes in five minutes due to contractions of the uterus and tubes. These contractions are believed to be initiated by oxytocin released during sexual intercourse and the local action of prostaglandins contained in seminal fluid on the uterus and fallopian tubes. Sperm remain viable in the female genital tract for 24-72 hours, but remain highly fertile for only 12-24 hours. A spermatozoon is able to fertilize an oocyte only after it has spent several hours in the genital tract, undergoing a process called capacitation; in this case, the properties of the membrane covering the acrosome are changed, which makes possible fertilization, which usually occurs in the upper part of the fallopian tube.

When the sperm approaches the oocyte, its outer membrane covering the acrosome area and the membranes of the acrosome itself are torn, and the enzymes located in the acrosome - hyaluronidase and protease - “digest” the cell layers surrounding the oocyte. These changes in the sperm head are called the acrosomal reaction. As a result of further changes in the sperm head, the inner membrane of the acrosome is turned inside out, which allows the sperm to penetrate the plasma membrane into the oocyte; In humans, the sperm enters the oocyte entirely. After one sperm has entered the egg, the cortical granules located under its plasma membrane rupture, starting from the point of entry of the sperm, and release a substance, under the influence of which the zona pellucida thickens and separates from the plasma membrane of the egg. This so-called cortical reaction spreads throughout the cell, as a result of which the zona pellucida forms an impenetrable fertilization membrane barrier, which prevents other sperm from penetrating the oocyte, i.e., the phenomenon of polyspermy.

Penetration of the sperm serves as a stimulus for the completion of the second meiotic division, and the second-order oocyte becomes a mature egg. In this case, a second polar body is also formed, which immediately degenerates, and the tail of the sperm is resorbed in the cytoplasm of the egg. The nuclei of both gametes turn into pronuclei and move closer together. The membranes of the pronuclei are destroyed, and the paternal and maternal chromosomes are attached to the resulting spindle threads. By this time, both haploid sets, containing 23 chromosomes in humans, have already replicated, and the resulting 46 pairs of chromatids are aligned along the equator of the spindle, as in metaphase of mitosis. The fusion of pronuclei is called karyogamy. At this stage, the diploid number of chromosomes is restored, and the fertilized egg is called a zygote.

The zygote goes through the stages of anaphase and telophase and completes its first mitotic division. The ensuing cytokinesis results in the formation of two diploid daughter cells.

Conclusion

Genetic variability is beneficial to the species, since it supplies “raw materials” for natural selection, and therefore for evolution. The offspring that are most adapted to their environment will have an advantage in competition with other members of the same species and will have a greater chance of surviving and passing on their genes to the next generation. Thanks to this species they are able to change, i.e. speciation process is possible. Increased variation can be achieved by shuffling the genes of two different individuals, a process called genetic recombination, which is an important feature of sexual reproduction; In a primitive form, genetic advice is already found in some bacteria.

Literature

1. Slyusarev A.A. - Biology with general genetics. 1978.

2. Bogen G. -Modern biology. - M.: Mir, 1970.

3. Willie K. - Biology (biological laws and processes). 1974.

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