Reproduction. What is reproduction in biology? Definition and examples of reproduction in nature What is reproduction definition

Between individuals - sexual process.

Asexual reproduction is the oldest and simplest method of reproduction and is widespread in unicellular organisms (bacteria, blue-green algae, chlorella, amoebas, ciliates). This method has its advantages: there is no need to find a partner, and beneficial hereditary changes are preserved almost forever. However, with this method of reproduction, the variability necessary for natural selection is achieved only through random mutations and therefore occurs very slowly. However, it should be noted that the ability of a species to reproduce asexually does not exclude the ability to undergo the sexual process, but then these events are separated in time.

The most common method of reproduction of single-celled organisms is by dividing into two parts, forming two separate individuals.

Alternation of generations in plants

The gametophyte develops from a spore, has a single set of chromosomes and has sexual reproductive organs - gametangia. In heterogametic organisms, male gametangia, that is, producing male gametes, are called antheridia, and female gametangia are called archegonia. Since the gametophyte, like the gametes it produces, has a single set of chromosomes, gametes are formed by simple mitotic division.

When gametes fuse, a zygote is formed, from which a sporophyte develops. The sporophyte has a double set of chromosomes and carries organs of asexual reproduction - sporangia. In heterosporous organisms, male gametophytes develop from microspores, bearing exclusively antheridia, and from megaspores, female gametophytes develop. Microspores develop in microsporangia, megaspores - in megasporangia. During sporulation, meiotic reduction of the genome occurs, and a single set of chromosomes characteristic of the gametophyte is restored in the spores.

Evolution of reproduction

The evolution of reproduction, as a rule, went in the direction from asexual forms to sexual ones, from isogamy to anisogamy, from the participation of all cells in reproduction to the division of cells into somatic and sexual ones, from external fertilization to internal fertilization with intrauterine development and care for the offspring.

The rate of reproduction, the number of offspring, the frequency of generational changes, along with other factors, determine the rate of adaptation of the species to environmental conditions. For example, high rates of reproduction and frequent changes of generations allow insects to quickly develop resistance to pesticides. In the evolution of vertebrates - from fish to warm-blooded animals - there is a tendency towards a decrease in the number of offspring and an increase in their survival.

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• Khachaturyan, Aram Ilyich
• Borrowed words in Russian

See what “Reproduction” is in other dictionaries:

REPRODUCTION- the property of reproducing their own kind inherent in all organisms, ensuring the continuity and continuity of life. R.'s methods are extremely varied. Usually there are three main ones. forms of R.: asexual (in protozoa, division into two, schizogony, in higher... ... Biological encyclopedic dictionary

REPRODUCTION- REPRODUCTION, reproduction, plural. no, cf. 1. Action under Ch. multiply multiply and state according to ch. multiply multiply. 2. The process of producing offspring (biol.). Sexual reproduction. Asexual reproduction. Reproduction by division. Reproduction... ... Ushakov's Explanatory Dictionary

reproduction- Cm … Synonym dictionary

REPRODUCTION- REPRODUCTION, the process by which living organisms create new organisms like themselves. Reproduction can be sexual or asexual; the first is the fusion of two special CELLS of different parents; and the second is the creation of new organisms from... ... Scientific and technical encyclopedic dictionary

Reproduction- the ability of organisms to produce their own kind, which ensures the preservation of their species and the continuity of stay in biocenoses. Reproduction is distinguished by asexual reproduction, by dividing individuals (for example, in unicellular plants), vegetative development... ... Ecological dictionary

REPRODUCTION- REPRODUCTION, or the ability of self-reproduction, is one of the main characteristics of living things, ensuring the preservation of the life of the species. Among the seemingly endless variety of methods of R., two main types can be outlined: R. using one cell, or... ... Great Medical Encyclopedia

reproduction- REPRODUCTION, reproduction REPRODUCE/MULTIPLY, reproduce, breed/divorce, obsolete. kept, outdated multiply, expand to be fruitful/to multiply and multiply... Dictionary-thesaurus of synonyms of Russian speech

Reproduction- * reproduction * reproduction is the ability of the body to produce its own kind, which ensures the preservation of life. R. is divided into: a) asexual, or vegetative, excluding meiosis (see); b) fertilization occurs by separating parts... ... Genetics. encyclopedic Dictionary

An important property of all organisms is reproduction, which ensures the maintenance of life.

Reproduction carried out without the participation of reproductive cells is called asexual reproduction.

Asexual reproduction

Asexual reproduction is characterized by the fact that the daughter cells are completely identical to the parent cells in terms of the content of hereditary information, morphological, anatomical and physiological characteristics. Asexual reproduction is carried out with the help of individual (asexual) cells (various methods of division, sporulation), from which daughter cells are formed or multicellular organisms develop.

Vegetative propagation is ensured by the separation of multicellular sections from the mother multicellular organism (root, leaf, shoot, cuttings, layering, as well as modified underground shoots - tubers, bulbs, rhizomes in plants and body parts, “buds” in animals).

The biological significance of asexual and vegetative reproduction is that in a short period the number of species can be significantly increased.

Sexual reproduction

Sexual reproduction is characterized by the exchange of genetic information between females and males through special haploid sex cells - gametes.

Gametogenesis is the process of formation of gametes.

Sexual reproduction exists in almost all plants and animals. Mature highly specialized germ cells - gametes: female - eggs, male - sperm - when fused, they form a zygote, from which a new daughter organism develops. Upon reaching sexual maturity, the new organism in turn produces gametes that give rise to subsequent offspring. This is how the continuity of generations is carried out.

Gametes are formed from diploid cells through a special type of cell division - meiosis.

The process of meiosis consists of two successive divisions - meiosis and meiosis.

The main feature of meiosis is the reduction in the number of chromosomes by 2 times.

Comparing mitosis and meiosis, we can note the following similarities and differences:

During the formation of germ cells in animals, a decrease in the number of chromosomes occurs at the last stage of oogenesis and spermatogenesis (the formation of female and male germ cells).

By merging, the gametes form a zygote (fertilized egg), which carries the makings of both parents, due to which the hereditary variability of the descendants sharply increases. This is the advantage of sexual reproduction over asexual reproduction.

Types of reproduction

A type of sexual reproduction is parthenogenesis (from the Latin “parthenos” - virgin + gr. “genesis” - birth), in which the development of a new organism occurs from an unfertilized egg (in bees). Conjugation - two individuals come together and exchange hereditary material (ciliates).

Copulation is the fusion of two cells of equal size into one (colonial flagella, etc.)

In higher plants, meiosis occurs not during the formation of gametes, but at an earlier stage of development - during the formation of spores (in angiosperms - during the formation of pollen and the embryo sac).

For angiosperms, the process of double fertilization, discovered by S. G. Navashin in 1898, is typical.

The peculiarity of fertilization in flowering plants, in contrast to animals, is that it involves not one, but two spermatozoa, which is why it is called double fertilization. Its essence lies in the fact that one sperm fuses with the egg, and the second with the central diploid cell, from which the endosperm further develops.

In nature, reproduction with alternating sexual and asexual generations is widespread in plants and some animals (coelenterates). This type of reproduction is described in detail in the first part of the manual.

Reproduction

the inherent property of all organisms to reproduce their own kind, ensuring the continuity and continuity of life. All forms of R. in organisms with a cellular structure are based on cell division. Various classifications of forms of R. have been proposed. There are three main methods of R.: asexual, vegetative and sexual. In asexual R., the organism develops from a single cell that is not differentiated sexually. In vegetative R., the beginning of a new organism is given by multicellular rudiments, sometimes complexly differentiated. Sexual R. is preceded by the formation of gametes (See Gametes) (sex cells); R. itself comes down to their fusion into a zygote (See Zygote) - fertilization, accompanied by the union of not only the cytoplasm of the gametes, but also their nuclei. The beginning of the R. period in some cases coincides with the cessation of growth, in others it does not entail a stop in the growth of the individual and stops only with the onset of old age or continues until the death of the organism, in others it begins several years after the cessation of growth. R. can be single or multiple. For unicellular organisms that reproduce by division, as well as for annual and biennial flowering plants, R. is at the same time the completion of their life cycle. Some (so-called monocarpic) perennial plants, as well as a few species of fish, reproduce once in their life.

Much more often in the plant and animal world, multiple reactions are observed. Each species is characterized by a certain intensity of radiation, sometimes varying within quite a wide range depending on the conditions of existence.

Animal reproduction. Asexual reproduction of protozoa occurs by dividing into two (transversely or longitudinally). In some of them, the fission products are not separated and, as a result, colonies appear (See Colony). In addition to division in two, there are other forms of asexual R. of protozoa: multiple division, or schizogony, and a number of others.

Vegetative R. of multicellular organisms arose secondarily and independently in different groups of organisms and is carried out in a wide variety of forms. It is often combined with R. with the help of unicellular rudiments called asexual R. (in the broad sense of the word) on the basis of the absence of the sexual process, although by origin these are two different forms of R. Among multicellular animals, the ability for vegetative R. is possessed mainly by the lower ones - sponges, coelenterates, flatworms, bryozoans, some ringworms. Among chordates, vegetative growth is common in secondary simplified forms—tunicates. It is carried out more often by budding (external or internal), less often by dividing the body into equal sections. In coelenterates and bryozoans, incomplete vegetative growth leads to the formation of colonies.

In sexual reproduction, the main process is the fusion of gametes (see Fertilization). In this case, the zygote combines a chromosomal complex that carries hereditary information, originating from both parents. The emergence of the sexual process on the basis of a more primitive asexual R. was a progressive factor in evolution that increased hereditary variability and, accordingly, the rate of evolution. Gametes are always haploid - they carry a single set of chromosomes. The zygote is diploid - it has a paired set of chromosomes. The transformation of a diploid chromosome complex into a haploid one occurs as a result of Meiosis a. The latter in multicellular animals precedes the formation of gametes. In protozoa, its location during the life cycle may vary. Isogamy occurs in some protozoa - copulation of morphologically indistinguishable gametes. Others exhibit more or less pronounced anisogamy - the presence of various gametes, some of which are female, or macrogametes, are large and rich in cytoplasm and reserve substances, while others are male, or microgametes, are very small and mobile. An extreme form of anisogamy is Oogamy, in which the macrogamete is represented by a large, immobile egg cell rich in reserve substances, and the microgametes are represented by motile small sperm.

In some animals (many arthropods, especially insects), the development of the reproductive cell under certain conditions occurs without fertilization. This secondarily simplified form of sexual reproduction is called parthenogenesis, or virgin reproduction. Its special form is represented by pedogenesis - virgin reproduction at the larval stage (typical of some dipterans and beetles).

Many animals are characterized by a natural alternation of different forms of R., which can be combined with the alternation of morphologically different generations. There are primary and secondary alternation of generations. During primary, asexual and sexual R. alternate. This is observed in many protozoa (for example, in Sporozoa). The secondary form of alternation of generations includes Metagenesis and Heterogony. During metagenesis, sexual R. and vegetative R. alternate; Thus, in the class of hydroids (a type of coelenterates), polyps bud and form colonies on which jellyfish develop (sexual generation); the latter separate from the colonies, float freely in the water, and develop gonads. An example of heterogony is the alternation of generations in cladoceran crustaceans and rotifers. For most of the summer, these animals reproduce parthenogenetically, only in the fall do they develop males and females.

The onset of the period of R. and its intensity are greatly influenced by environmental conditions - temperature, length of daylight hours, lighting intensity, nutrition, etc. In higher animals, the activity of the reproductive organs is associated with the functions of the endocrine glands, which makes it possible to stimulate or delay puberty. For example, in fish, an additional transplantation of the pituitary gland or the introduction of its hormones causes the onset of maturity, which is used in the practice of breeding valuable fish, such as sturgeon.

Lit.: Myasoedov S.V., Phenomena of reproduction and sex in the organic world, Tomsk, 1935; Hartmann M., General biology, trans. from German, M. - L., 1936; Dogel V. A., Polyansky and Yu. I., Heisin E. M., General protozoology, M. - L., 1962; Willy K. and Dethier V., Biology. (Biological processes and laws), trans. from English, M., 1974; Meisenheimer J., Geschlecht und Geschlechter im Tierreiche, Jena, 1921; Hartmann M., Die Sexualität, Stutt., 1956.

Yu. I. Polyansky.

Plant propagation. Plants, along with sexual reproduction, are characterized by a variety of methods of asexual and vegetative growth. Vegetative growth is carried out through the development of new individuals from vegetative organs or parts thereof, sometimes from special formations that arise on stems, roots, or leaves and are specially designed for vegetative growth. As in In both lower and higher plants, the methods of vegetative growth are varied. In higher plants, it is based on the ability to regenerate (See Regeneration). Vegetative R. plays a very important role in nature and is widely used by humans. Many cultivated plants are propagated almost exclusively by vegetative means - only in this case their valuable varietal qualities are preserved.

Asexual reproduction in many plants occurs through the formation of motile or immobile spores (See Spores). In lower plants, special spores of asexual R. are formed, which arise endogenously - usually inside special sporangia (See Sporangium). (in algae and lower fungi) or exogenously - on the surface of the branches of the thallus - conidiophores (in higher fungi). In plants associated in their development with the aquatic environment, these spores are mobile. Sporulation in higher plants (except seed plants) is an obligatory phase of their life cycle, regularly alternating with sexual reproduction (see Alternation of generations). Sexual R. is present in most plants; It is absent in blue-green algae, many imperfect fungi, and lichens. In blue-green algae, sexual reproduction apparently never existed; in imperfect fungi and lichens it was probably lost during the process of evolution. In other lower plants, sexual reproduction is expressed extremely diversely. As a result of the sexual process (conjugation, isogamy, heterogamy, oogamy, gametangiogamy), they form a zygote, which goes into a resting state (in most green algae, some brown algae and lower fungi) or immediately germinates, giving either a diploid vegetative thallus (in most brown algae), or spores of sexual R. (carpospores of red algae). In marsupials and basidiomycetes, the sexual process is unique: a typical zygote is not formed; the initial stage of growth (fusion of protoplasm) is separated by a certain period of time from the final stage (fusion of nuclei), followed by the formation of ascospores or basidiospores. Fungi are characterized by the formation of a binuclear mycelium, which in basidiomycetes forms the basis of both the vegetative body (mycelium) and fruiting bodies. Lower plants, which produce many spores of asexual R., usually have low energy of sexual R. In mosses, the organs of sexual R. arise on the plant itself - the gametophyte (sexual generation). In some mosses, male reproductive organs (antheridia (See Antheridium)) and female (archegonia (See Archegonium)) develop on the same plant, in others - on different ones. The archegonium contains one large egg. Many motile spermatozoa develop in the antheridium. In drops of dew or rain, sperm released from the antheridium reach the archegonium, penetrate into it and merge with the egg. A sporogonium develops from a fertilized egg, within which spores for asexual reproduction develop through meiosis. In ferns, horsetails, mosses, and selaginella, the organs of sexual reproduction are similar to those of mosses, but are simplified and are formed on a small prothallus (gametophyte), developing from a spore and living in most of them, regardless of the sporophyte. The prothallae are usually unisexual, but in some species they are bisexual. Fertilization is the same as in mosses.

Seed plants are characterized by a special type of regeneration - seminal, in which seeds are formed - rudiments that ensure the most effective dispersal of the species. In gymnosperms, seeds develop from ovules (See Ovule), mostly on special modified leaves - sporophylls (sporolists). In the ovule, which is homologous to the megasporangium (See Megasporangium), 4 megaspores arise, 3 of them die, and the remaining one, through division, gives rise to a prothallus consisting of a complex of thin-walled cells - the Endosperm and 2 or several primitive archegonia. From the fertilized eggs of the archegonia, embryos develop, and from the ovule, a seed containing 1 embryo (the rest die). In angiosperms, seeds develop from ovules enclosed within the ovary of a flower. Megaspores are also formed inside the ovule. In most plants, 3 of them usually die off, and the remaining one gives rise to an embryo sac, usually consisting of 7 cells, one of which, the egg, develops into an embryo after fertilization. A seed is formed from the ovule, and the entire ovary turns into a fruit. In some flowering plants, seeds are formed without fertilization (see Apomixis).

Lit.: Meyer K.I., Plant propagation, M., 1937; Kursanov L.I., Mycology, 2nd ed., M., 1940; Mageshwari P., Embryology of angiosperms, trans. from English, M., 1954; Poddubnaya-Arnoldi V. A., General embryology of angiosperms, M., 1964; Botany, 7th ed., vol. 1, M., 1966; Schnarf K., Embryologie der Angiospermen, B 1 B., 1927; his, Embryologie der Gymnospermen, B., 1933; Chamberlain Chi. J., Gymnosperms. Structure and evolution, Chi., .

D. A. Trankovsky.

Great Soviet Encyclopedia. - M.: Soviet Encyclopedia. 1969-1978 .

See what “Reproduction” is in other dictionaries:

The property of reproducing their own kind inherent in all organisms, ensuring the continuity and continuity of life. R.'s methods are extremely varied. Usually there are three main ones. forms of R.: asexual (in protozoa, division into two, schizogony, in higher... ... Biological encyclopedic dictionary

REPRODUCTION, reproduction, many. no, cf. 1. Action under Ch. multiply multiply and state according to ch. multiply multiply. 2. The process of producing offspring (biol.). Sexual reproduction. Asexual reproduction. Reproduction by division. Reproduction... ... Ushakov's Explanatory Dictionary

Cm … Synonym dictionary

REPRODUCTION, the process by which living organisms create new organisms like themselves. Reproduction can be sexual or asexual; the first is the fusion of two special CELLS of different parents; and the second is the creation of new organisms from... ... Scientific and technical encyclopedic dictionary

The ability of organisms to produce their own kind, which ensures the preservation of their species and the continuity of their presence in biocenoses. Reproduction is distinguished by asexual reproduction, by dividing individuals (for example, in unicellular plants), vegetative development... ... Ecological dictionary

REPRODUCTION- REPRODUCTION, or the ability of self-reproduction, is one of the main characteristics of living things, ensuring the preservation of the life of the species. Among the seemingly endless variety of methods of R., two main types can be outlined: R. using one cell, or... ... Great Medical Encyclopedia

reproduction- REPRODUCTION, reproduction REPRODUCE/MULTIPLY, reproduce, breed/divorce, obsolete. kept, outdated multiply, expand to be fruitful/to multiply and multiply... Dictionary-thesaurus of synonyms of Russian speech

Our daily experience sadly demonstrates that all living things are subject to death.* Creatures get sick, grow old and finally die. Many have an even shorter life - they are eaten by predators. To ensure that life on Earth does not cease, all creatures are endowed with a universal property - the ability to reproduce.

With all the diversity of living organisms inhabiting the planet, with all the differences in structure and lifestyle, the methods of their reproduction in nature come down to two forms: asexual and sexual. Some plants combine these two forms, reproducing by tubers, cuttings or layering (asexual reproduction) and at the same time by seeds (sexual reproduction).

In the case of asexual reproduction, offspring develop from cells of the original organism. During sexual reproduction, the development of a new creature begins with a single cell formed from the fusion of two parent cells - male and female.

The essence of reproduction is the preservation not only of life as a whole, but also of each individual species of animals and plants, in the organization of continuity between offspring and parental beings. The molecular basis of the reproduction processes of all organisms is the ability of DNA to self-duplicate. As a result, the genetic material is reproduced in the structure and functioning of daughter organisms.

* The Holy Scriptures and the works of the holy fathers are permeated with the idea that death and corruption were not created initially, but entered the world as a result of the fall of the first man (Wis. 1:13 and 2:23, Rom. 5:12, etc. ).

Cell division. Mitosis

Cell life cycle. The process of division and interphase are closely interrelated; their totality constitutes the life cycle of the cell. Its duration in plant and animal cells averages 10-20 hours.

In the chemically active environment of the food tract, intestinal epithelial cells quickly wear out and therefore continuously divide - twice a day, corneal cells begin dividing once every three days, and skin epithelial cells - once a month. The cell spends an average of 1 to 3 hours on the division process, depending on external conditions (lighting, temperature, etc.).

In the liver of animals there are so-called resting cells, which divide only in crisis situations. For example, when part of the liver is removed, these cells begin to multiply intensively, quickly replenishing the number necessary for the normal functioning of the organ.

Some highly specialized cells (neurons, some leukocytes) in adult creatures never divide. Their cell cycle ends with apoptosis (Greek apo from  ptosis fall) - programmed death. In some cases, other cells in the body undergo apoptosis. This happens as follows. First, the cell receives a certain chemical signal to carry out self-destruction. Then, in its Golgi complex and lysosomes, enzymes are activated that destroy (lyse) the main components of the cytoplasm and nucleus. After this, the cell breaks up into membrane vesicles, which are absorbed by phagocyte cells that process foreign components. There is no inflammatory process during apoptosis.

Through apoptosis, tadpoles lose their tails, and insect larvae lose excess tissue as they mature into adults. The fingers of a human embryo are connected by tissue membranes. During embryogenesis, membranes are programmed to be destroyed.

Apoptosis helps the body get rid of cells that have accumulated genetic damage, as well as diseased and aged cells. Many viruses, penetrating a cell, first of all try to disrupt its apoptosis mechanism, so as not to be destroyed along with the diseased cell.

When apoptosis is disrupted, such serious diseases as systemic lupus erythematosus, Parkinson's disease develop, and viral infections progress.

Apoptosis can be triggered by external factors: chemical exposure or radiation. This is the basis for the action of some drugs and special emitters that cause apoptosis of cancer cells. Provoked apoptosis sometimes leads to dangerous consequences. Thus, prolonged disruption of the blood circulation of the heart muscle leads to the destruction of only a small part of its cells, but their death causes apoptosis of many neighboring cells and, as a result, extensive myocardial infarction.

In addition to apoptosis, there are other mechanisms that limit the vital activity of cells. Thus, as a result of each act of division, the terminal sections of the DNA of the chromosomes are shortened. When the loss of genetic material becomes critical, the cell stops dividing. Some groups of cells of multicellular creatures, like unicellular organisms, have the ability to produce an unlimited number of generations. These are so-called stem cells. In humans, stem cells are red bone marrow cells, from which red blood cells, white blood cells and platelets are formed. In stem cells, as in unicellular organisms, a special enzyme is synthesized that lengthens the terminal sections of DNA - telomerase.

Ciliates, unlike amoebas and bacteria, cannot divide indefinitely. After a certain, sufficiently large number of divisions, they show signs of aging (degeneration). Then two aged ciliates “stick together” and conjugate - they exchange part of the nuclear DNA, i.e. genetic information. After conjugation, the viability of each ciliate is restored: the metabolic rate increases, the rate of division increases, etc.

Cell division forms the basis of the processes of reproduction and development of organisms. Division occurs in two stages. First, the nucleus divides, and then cytokinesis occurs - the division of the cell itself.

Mitosis. The main method of nuclear division in eukaryotic cells is called mitosis. There are four phases of mitosis: prophase, metaphase, anaphase and telophase.

Prophase. In prophase, preparations for division are completed. The chromosomes thicken greatly and become visible under a light microscope. Now they are two spiralized DNA (chromatids), formed during the duplication process and connected to each other by centromeres.

Reading of information from DNA stops, RNA synthesis ends. The ribosomal subunits are released into the cytoplasm and the nucleoli disappear. Microtubules of the cytoskeleton disintegrate. From the proteins that make them up, a division spindle begins to form on the centrioles. Centrioles diverge to opposite poles of the cell. Outer microtubules attach to the outer membrane and fix the position of the centrioles. Finally, the nuclear membrane breaks down into fragments, and the chromosomes end up in the cytoplasm. The edges of the shell fragments close together, forming small vesicles-vacuoles, which merge with the membranes of the endoplasmic reticulum.

Metaphase. This stage of division is characterized by rearrangement of chromosomes in the cytoplasm. When microtubules from the nearest centriole grow to the chromosome, it begins to move towards the center of the cell as the microtubule grows until it connects at its centromeric region with microtubules from another centriole. Contacts of chromosomes with microtubules occur randomly; through a microscope one can see how chromosomes vigorously rotate and move back and forth until they are “caught” by microtubules coming from two opposite sides. By the end of metaphase, all chromosomes are assembled in the equatorial zone of the cell. They are as compact as possible and clearly visible. Metaphase chromosomes are used to determine the number and structure of an organism's chromosomes - its karyotype.

The centromeric regions of the chromosomes are separated and they become independent. Each of them turns out to be attached by the centromere to its division pole.

Anaphase. The onset of the stage is characterized by the divergence of the chromatids of each chromosome to opposite poles. Contractile proteins are located in the centromeric regions. The movement occurs as a result of their active work using the energy of ATP (20 molecules are split to move each chromosome). The chromosome arms passively follow the centromere. The released sections of microtubules are immediately destroyed. It seems that it is not the chromosomes that move along the microtubules, but the microtubules themselves, contracting, pulling the chromosomes.

When the chromosomes reach the division poles, anaphase ends.

Obviously, in the absence of a spindle, cell reproduction does not occur. Chemical exposure that destroys microtubules is one way to suppress tumor growth.

Telophase. At this last stage of mitosis, a new nuclear envelope is formed by the fusion of endoplasmic reticulum vesicles. Chromosomes despiral into long thin filaments on which nucleoli are formed. The fission spindle is destroyed. Microtubules of a new cytoskeleton begin to grow from the proteins that make it up from the centrioles.

Cytokinesis. The final division into two in animal cells is carried out by constriction. In plant cells, a membrane grows from the middle to the edges, on which a dense cell wall then appears. Organelles (mitochondria, ribosomes, Golgi complex, etc.) are distributed between daughter cells in approximately equal quantities.

During mitosis of some cells of the heart muscle and liver, a constriction is not formed, therefore some of the cells of these organs are binucleate.

Let us pay attention to the fact that all processes of mitosis are determined by chromosome transformations. Having doubled in interphase, the chromosomes begin to spiral and enter the cytoplasm in prophase. In metaphase they gather in the equatorial zone and separate to disperse to different poles in anaphase. At the final stage of telophase, chromosomes take on their original form of thin despiralized threads characteristic of interphase.

Number of chromosomes. Through mitotic division, daughter cells receive a set of chromosomes from the mother cell, so that cells throughout the body have the same chromosomes.

The cells that form all the tissues and organs of the body are called somatic. Specialized germ cells are involved in reproduction. Somatic cells contain a diploid (double) set of chromosomes. In this set, each gene is encoded on two similar (homologous) chromosomes. The set of germ cells is haploid (single). The chromosomes of germ cells do not have homologs; each gene in their set is unique. The number of chromosomes of the haploid and diploid sets is species-specific, that is, constant for each type of organism.

The chromosome set of human somatic cells includes 46 chromosomes: 22 homologous pairs and two unpaired chromosomes that determine sex. Human germ cells contain only 23 single chromosomes. In a chicken, the diploid set includes 78 chromosomes, and the haploid set includes 39. Examples of other sets are given in the table.

Analysis of chromosome sets shows that the complexity and perfection of various organisms is not determined only by the number of chromosomes.

Biological significance of mitosis. In addition to building up the body, mitosis has another, more important purpose. During the process of mitosis, genetic material is reproduced. Thanks to this, it is possible to preserve the structure and functioning of organs and tissues for countless generations. The identity of the genetic material is especially important for multicellular organisms, whose cells are in close and coordinated interaction. The accurate reproduction and transmission of genetic information is the main biological significance of mitosis.

Mitotic division ensures the most important life processes: embryonic development and growth, regeneration of organs and tissues after damage, maintaining the structure and functioning of the body with the constant loss of working cells. Skin cells exfoliate, intestinal epithelial cells are destroyed by the active environment, red blood cells function intensively and quickly die, they are completely replaced in the body every four months (2.5 million cells per second).

1. Why is DNA duplication called the molecular basis of reproduction?
2. What processes make up the life cycle of a cell?
3. Describe the main phases of mitosis, what is its main biological significance?
4. As is known, the set of chromosomes of germ cells is half that of somatic cells. Can we say that some minor proteins in the sex chromosomes are not encoded?

Methods of reproduction of organisms

All known methods of reproduction of organisms in nature come down to two main forms: asexual and sexual.

Asexual reproduction. In the asexual form, reproduction is carried out by the parent individual independently, without exchanging hereditary information with other individuals. A daughter organism is formed by separating one or more somatic (body) cells from the parent and their further reproduction through mitosis. The offspring inherits the characteristics of the parent, being genetically its exact copy. There are several types of asexual reproduction.

Simple division. Asexual reproduction is especially common in bacteria and blue-green algae. The single cell of these nuclear-free organisms is divided in half or into several parts at once. Each part is a complete functional organism.

Amoebas, ciliates, euglena and other protozoa reproduce by simple division. Division occurs through mitosis, so daughter organisms receive the same set of chromosomes from their parents.

Budding. This type of reproduction is used by both unicellular and some multicellular organisms: yeast (lower fungi), ciliates, coral polyps.

Budding in freshwater hydras occurs as follows. First, a growth forms on the wall of the hydra, which gradually lengthens. Tentacles and a mouth opening appear at its end. A small hydra grows from the bud, which separates and becomes an independent organism. In other creatures, the kidneys may remain on the parent's body.

Fragmentation. A number of flat and annelid worms, echinoderms (sea stars) can reproduce by dismembering the body into several fragments, which are then built up into a complete organism. Fragmentation is based on the ability of many simple creatures to regenerate lost organs. So, if a ray is separated from a starfish, then a starfish will develop from it again. Hydra is able to recover from 1/200 of its body. Typically, reproduction by fragmentation occurs when damaged. Spontaneous fragmentation is carried out only by molds and some marine annelids.

Sporulation. The ancestor of a new organism can be a specialized cell of the parent creature - a spore. This method of reproduction is typical for plants and fungi. Multicellular algae, mosses, ferns, horsetails and mosses reproduce by spores.

Spores are cells covered with a durable membrane that protects them from excessive loss of moisture and is resistant to temperature and chemical influences. Spores of terrestrial plants are passively transported by wind, water, and living creatures. Finding itself in favorable conditions, the spore opens its shell and begins mitosis, forming a new organism. Algae and some fungi that live in water reproduce by zoospores equipped with flagella for active movement.

A single-celled animal, Plasmodium falciparum (the causative agent of malaria), reproduces through schizogony - multiple divisions. First, a large number of nuclei are formed in his cell by division, then the cell breaks up into many daughter cells.

Vegetative propagation. This type of asexual reproduction is widespread in plants. Unlike sporulation, vegetative reproduction is carried out not by special specialized cells, but by almost any part of the vegetative organs.

Perennial wild herbs reproduce by rhizomes (sow thistle produces up to 1800 individuals/m2 of soil), strawberries by tendrils, and grapes, currants and plums by layering. Potatoes and dahlias use tubers for propagation - modified underground sections of the root. Tulips and onions reproduce from bulbs. In trees and shrubs, shoots - cuttings - take root to form a new plant, and in begonias the role of cuttings can be played by leaves. Raspberries, plums, cherries and roses are propagated by cuttings. Shoots form on the roots and stumps of trees, which then turn into independent plants.

Sexual reproduction. In contrast to asexual reproduction, sexual reproduction involves a pair of individuals. Their sex cells (gametes) carry haploid sets of chromosomes. During the process of fertilization, gametes fuse and form a diploid fertilized egg (zygote), which gives rise to a new organism.

One of the homologous chromosomes of a somatic cell comes from the “mom”, and the other from the “dad”. As a result, parts of the genetic material of the parents are combined, and new combinations of genes appear in the offspring. The diversity of genetic material allows the offspring to more successfully adapt to changing external conditions. The main advantage of sexual reproduction, its main biological significance, is the enrichment of hereditary information.

Bisexual plants have a number of features that exclude self-fertilization. The stamens and pistils of bisexual flowers do not mature at the same time, so cross-pollination of different individuals occurs. Hemp has separate male pistillate and female staminate flowers on different individuals.

Development of germ cells. The formation of germ cells (gametogenesis) occurs in the gonads. The development of female gametes (eggs) occurs in the ovaries and is called oogenesis (lat. ovum egg + genesis origin). Male gametes (sperm) are formed in the testes during the process of spermatogenesis. The gonads of almost all creatures have a tubular structure. Gametogenesis occurs sequentially in three zones: reproduction, growth and maturation. Accordingly, three periods of gamete development are distinguished.

During the initial period of reproduction, sex cells have a diploid set of chromosomes and divide through mitosis. Male gametes reproduce especially intensively. In males, reproductive cells are formed almost throughout their lives. The formation of mammalian eggs occurs only during the embryonic period, after which they remain dormant.

Once in the growth zone, the germ cells no longer divide, but only grow. Male gametes do not grow too much, but eggs increase their size hundreds, thousands and millions of times (remember a chicken egg). The outer shells of the egg reliably protect the developing fetus; bacteria and viruses do not penetrate through them, especially through the shells of bird eggs, and air passes freely.

Sperm are much smaller than eggs. In mammals they have the shape of a long filament with a head, neck and flagellum. The head contains chromosomes, and on its front part there is a Golgi complex with enzymes that dissolve the egg membrane and ensure the penetration of the sperm nucleus (the membrane remains outside). Male gametes not only contribute genetic information, but also initiate the development of the egg. The centriole is located in the neck, forming the flagellum of the sperm, allowing it to move intensively. The source of energy for the movements of the flagellum are ATP molecules stored in the neck. To replenish ATP, mitochondria are located in the neck.

After the gametes grow to the size of adult germ cells, they enter the maturation zone.

The basis for the maturation of gametes is the specific process of dividing each germ cell into four new ones. The maturation of eggs and sperm proceeds in basically the same way; differences arise only at the last stage for the following reason. A sufficiently large number of sperm are required for successful fertilization. Therefore, all four resulting male cells are functional and viable. The main task of the egg is not only fertilization, but also the successful maturation of the fetus. For this purpose, the division process occurs unequally: the entire yolk goes into one egg, and it turns out to be the only viable one. The remaining three fully functional eggs do not receive nutrients during maturation and soon die. They are called directional or polar bodies.

The period of maturation of gametes, accompanied by the specific division of each of them into four new ones, is called meiosis. In the next paragraph we will look at the processes occurring in meiosis in more detail.

1. What is the difference between asexual reproduction and sexual reproduction? Name the main advantage of sexual reproduction.
2. List the five main types of asexual reproduction. Give examples.
3. Where does a pair of homologous chromosomes appear in a daughter organism during asexual and sexual reproduction?
4. Describe the three periods of gamete maturation; which one is called meiosis?
5. Why and why do you think the germinal disc in a chicken egg always ends up in the upper part of the yolk?

The development of an organism begins with a single cell - a zygote, which is formed from the fusion of specialized germ cells - male and female gametes. During the process of fusion, their nuclei combine, and the zygote contains twice as many chromosomes as each gamete. If germ cells were diploid, then in each next generation the number of chromosomes in the cells of the body would double. Therefore, germ cells carry half the number of chromosomes. Thus, somatic (body) cells of organisms have a diploid (double) set of chromosomes and maintain its species constancy through mitotic division, and sex cells have a haploid set, which is restored to diploid during the process of fertilization. Let's look at the main phases of meiosis.

The maturation of gametes includes two successive divisions: the first is typical meiosis, the second is similar to mitotic. Both divisions, like mitosis, go through four stages: prophase, metaphase, anaphase and telophase. Before the first division, as well as before mitosis, DNA replication occurs with chromosome doubling, each chromosome enters into the process of double division.

First meiotic division

In prophase, homologous chromosomes come very close to each other. Using special protein threads with thickenings at the ends, they seem to be fastened to each other like a zipper. They remain in this state, called conjugation, for quite a long time (in humans, about a week). Fastening occurs in those places of DNA where replication has not yet completed and the double helix is ​​somewhat unwound.

Conjugating chromosomes are tightly adjacent to each other and can exchange sections. Such an exchange is called a crossover, or crossing over. After the crossover, each chromosome combines genes that were located on different homologous chromosomes before the crossover.

At the end of prophase, a division spindle is attached to the centromeres of the chromosomes, and they begin to diverge in centromeric sections to different division poles, remaining linked at the crossing over sites.

Unlike mitosis, in the metaphase of meiosis, the duplicated chromosomes are not separated at the centromeres; each pair interacts with one spindle. If in the metaphase of mitosis individual chromatids diverge to different poles, then in the metaphase of the first division of meiosis - conjugated chromosomes. During telophase, a nuclear envelope is formed for a short period.

Second meiotic division. Since the chromosomes remain connected at centromeres, that is, duplicated, DNA replication does not occur before the second division. The second meiotic division occurs in a manner similar to mitosis. As a result, four haploid germ cells are formed from two diploid cells. Due to the lack of conjugation, the second division occurs much faster.

Somatic cells contain two homologous chromosomes (identical in shape and size, carrying the same groups of genes): one from the paternal organism, the other from the maternal. In germ cells, out of two homologous chromosomes, only one remains; their chromosomes do not have homologues - they are single, and therefore the set is haploid. If during mitosis the amount of genetic information is preserved, then during meiosis it is halved.

The formation of germ cells with a haploid set of chromosomes reduced by half is the biological essence of meiosis.

Due to the random divergence of pairs to the poles in the metaphase of the first division, the chromosome sets of mature germ cells contain the most diverse combinations of parental chromosomes. A gamete may have, for example, 5 paternal and 18 maternal chromosomes (humans have 23 chromosomes in total), 20 paternal and 3 maternal, etc. Each of the 23 chromosomes is different from the other and can be one of two homologous parental ones - a total of 223 (8.6 million) gamete variants. In the daughter organism, the number of possible combinations of chromosomes is 423, this number is thousands of times greater than the population of the globe. Crossing over, combining the genes of parental individuals in the chromosomes, increases the diversity of traits in the offspring by many orders of magnitude. Such a variety of possible genotypes makes each creature unique, genetically unique.

During meiosis, the genetic material is very vulnerable. If, for example, as a result of irradiation or exposure to chemical compounds, a DNA break occurs at the time of chromosome divergence, then part of the hereditary material will be lost. The loss of a section of DNA in a somatic cell during mitosis will only cause damage in its daughter cells, which make up a small part of the creature. If part of the chromatid of a maturing germ cell is lost, then the offspring will suffer: its hereditary information will be incomplete, some vital processes will not be able to be carried out. In this case, the female embryo is exposed to greater danger, since the entire supply of female gametes (about 300 in humans) is formed during the embryonic period throughout life, while male gametes are formed for almost the entire period of life. Minor doses of radiation, not at all dangerous to the body itself, can disrupt the chromosomes of the embryo's eggs and lead to genetic abnormalities in the next generation.

Parthenogenesis. Some animals (daphnia, rock lizards, some fish, aphids) and plants (dandelions) are capable of reproducing at certain periods without the fusion of male and female gametes. Development occurs from an unfertilized egg. Diploidy, for example, in rock lizards is achieved by the fusion of the egg with the polar body. In this case, as a rule, only female individuals are formed. This type of sexual reproduction is called parthenogenesis.

The queen bee lays two types of eggs: fertilized diploid and unfertilized haploid. From unfertilized eggs, drones develop, and from fertilized eggs, females develop, from which, with good feeding, queens grow, and when a lack of nutrition is created, worker bees are obtained.

Sometimes parthenogenesis can be induced artificially by exposure to light, acids, high temperature and other agents. If, for example, you prick an unfertilized egg of a frog with a needle, then this egg may, without being fertilized, begin division and develop into an adult. Parthenogenesis does not occur spontaneously in frogs. The division of the egg of some fish can begin after surface contact with the sperm of related fish species. Fertilization does not occur, but the egg begins to divide.

The main method of breeding silkworms is to stimulate parthenogenesis by briefly heating the eggs to 46°C. From unfertilized eggs, genetically complete female silkworms develop.

1. Why is the haploid set necessary for germ cells?
2. Describe the main phases of meiosis.
3. What is the difference between the metaphases of mitosis and meiosis?
4. What two processes of meiosis provide a variety of characteristics in the offspring?
5. What are the dangers of chemical and radiation exposure when carrying girls?
6. What is called parthenogenesis? Give examples.

Fertilization

The essence of the fertilization process is the fusion of male and female gametes - specialized germ cells that have a haploid (single) set of chromosomes. As a result, a diploid fertilized egg is formed - a zygote. Thus, during fertilization, the double set characteristic of somatic cells is restored. The chromosomes in the zygote nucleus are contained in homologous pairs, that is, any trait (for example, the color of a person’s eyes or the hair of a dog) is written in DNA twice - by the genes of the father and the genes of the mother.

After fertilization, the zygote duplicates its chromosomes through DNA replication and begins mitotic division - the development of a new organism begins.

Fertilization, like gametogenesis, has similar features in plants and animals.

Fertilization in animals. The living organisms inhabiting the planet differ in structure, lifestyle, and habitat. Some of them produce a lot of germ cells, others - relatively few. There is a reasonable pattern: the less likely the male and female gametes are to meet, the greater the number of germ cells the organisms produce. Fish and amphibians are characterized by external insemination. Their gametes enter the water, where fertilization occurs. Many gametes die or are eaten by other creatures, so the effectiveness of external insemination is very low. To preserve the species, fish and amphibians need to produce a huge number of gametes (cod lays about 10 million eggs).

Higher animals and plants use internal insemination. In this case, the fertilization process and the resulting zygote are protected by the mother’s body. The likelihood of fertilization increases significantly, which is why, as a rule, only a few eggs are produced. But a lot of sperm are still produced; their excess quantity is necessary to create a certain chemical environment around the egg, without which fertilization is impossible. The egg has mechanisms that prevent the penetration of excess sperm. After the first one has penetrated, it secretes a substance that suppresses the mobility of male gametes. Even if several of them manage to penetrate the egg, only one merges with the egg, the rest die.

Fertilization usually occurs immediately after insemination, but in some animals there are mechanisms to delay fertilization until the spring and summer season. In bats, fertilization does not occur during late autumn mating. The egg matures only in the spring, and the sperm safely overwinter in the female’s genitals. In other organisms, the zygote that has begun to develop is preserved until the onset of a favorable season for offspring; with the onset of spring, its development continues. Thanks to this ability, the total gestation period in an ermine can last up to 300-320 days, in a sable - up to 230-280 days.

Fertilization in plants. The process of fertilization in plants, while generally similar to the fertilization of animals, has some peculiarities. In angiosperms, male gametes (sperm), unlike sperm, are inactive. Their development begins with the formation of microspores - pollen grains - in the anther of the flower. A mature pollen grain contains a vegetative cell and two sperm cells.

Once on the stigma of the pistil, the vegetative cell forms a pollen tube that grows towards the ovule. The sperm travel through this tube into the flower, and when the tip of the tube ruptures, they enter the embryo sac. One of them fuses with the egg and forms a zygote - the embryo of the future plant. The second sperm fuses with two nuclei of haploid cells located in the center of the embryo sac. As a result, a triploid cell is formed - endosperm. Through repeated mitoses, the endosperm forms a nutrient medium around the embryo.

The second fertilization with the formation and development of endosperm occurs only after the egg is fertilized. This sexual process, universal for all angiosperms, is called double fertilization. It was discovered in 1898 by the famous Russian botanist S. G. Navashin.

1. What is the genetic essence of fertilization?
2. How to explain at the molecular level the presence of characteristics of the father and mother in the offspring?
3. What is the relationship between the probability of meeting gametes and their number?
4. How does fertilization occur in animals?
5. Describe the sequence of fertilization in plants. How do the processes of fertilization differ between animals and plants?
6. Why is fertilization of angiosperms called double?

MINISTRY OF EDUCATION AND SCIENCE OF THE RF

Federal State Budgetary Educational Institution

higher professional education

"ULYANOVSK STATE UNIVERSITY"

O.V. Stolbovskaya, N.A. Kurnosova, E.P. Drozhdina, S.M. Slesarev, E.V. Slesareva

Biology Reproduction and development

Part 1 gender determination

Tutorial

UDC 57.017.64 (075.8)

BBK 28.073.8 ya73+28.03 ya73

Published by decision of the Academic Council

Institute of Medicine, Ecology and Physical Culture

Ulyanovsk State University

Reviewers:

Doctor of Medical Sciences,

Head of the Department of Anatomy

Institute of Medicine, Ecology and Physical Culture

Ulyanovsk State University ;

The manual contains in a concentrated form the main theoretical material, selected according to program issues. A large amount of information on the main topics of the section “Reproduction and Development” has been analyzed and systematized. The manual reflects a relatively small number of fundamental topics that are extremely important for the knowledge of living nature. One of the main objectives of the manual is to present the material in a concise and easy-to-understand form.

The manual is intended for undergraduate students in “Biology” studying the discipline “Biology of Reproduction and Development”.

Reproduction as a property of living organisms

The ability to reproduce is an integral property of living beings and consists in the ability of a living organism to reproduce its own kind. With its help, biological species and life as such are preserved over time. In the process of biological reproduction, along with the change of generations and maintaining species variability, the problems of increasing the number of individuals, preserving the structural and physiological organization, and transferring genetic material over a series of generations are solved.

Reproduction of living organisms is carried out in two ways depending on their evolutionary position: asexual and sexual.

In asexual reproduction, a single parent gives rise to a new organism. In this case, the descendants are exact genetic copies of the parent organism. The descendants of one parent are usually called a clone. Asexual reproduction is based on cell division - mitosis. The biological significance of asexual reproduction is: a rapid increase in the number of offspring; maintaining the genetic stability of the species; maintaining the adaptability of the species to constant environmental conditions.

Sexual reproduction is observed in multicellular organisms, which contain two types of cells: somatic and reproductive. During sexual reproduction, two parent individuals give rise to a new organism: male and female. Descendants are genetically different from their parents due to the phenomena of crossing over, independent divergence of homologous chromosomes in anaphase I, chromatids in anaphase II of meiosis, and the phenomenon of random fertilization.

The biological role of sexual reproduction is : increasing the genetic diversity of offspring, which increases survival in changing environmental conditions and contributes to the success of the evolution of the species as a whole.

Sexual differentiation

Sex is a set of morphological, physiological, biochemical and other characteristics of an organism that determine reproduction. Sex characteristics are inherent in all living organisms. Sexual differentiation is a sequential process that begins at fertilization with the establishment of chromosomal sex, continues with the determination of gonadal sex, and ends with the development of secondary sexual characteristics, including male and female phenotypes.

The chromosomal sex of an embryo genetically corresponds to its phenotypic sex. However, if sexual differentiation goes wrong, then individuals with abnormal sexual differentiation arise. Clinically detectable disorders of sexual development occur at many levels, ranging from relatively common disturbances of the final stages of male differentiation (eg, testicular descent, penile growth) to fundamental abnormalities that lead to varying degrees of phenotypic sex uncertainty. Most of these abnormalities impair reproduction but are usually not life-threatening.

Sex is characterized by primary and secondary characteristics:

primary sexual characteristics are represented by organs that are directly involved in the processes of reproduction and are formed during embryogenesis;

Secondary sexual characteristics do not directly participate in reproduction, but contribute to the meeting of individuals of different sexes. They depend on primary sexual characteristics, develop under the influence of sex hormones and appear during puberty (in humans at 12-15 years of age).

Sex determines the development of somatic characteristics of individuals, which are divided into three categories:

Limited by gender;

Floor controlled;

Linked to sex chromosomes.

The development of sex-limited traits is determined by genes located in the autosomes of both sexes, but are manifested in individuals of the same sex (egg production in chickens, milk production in cows).

The development of sex-controlled traits is determined by genes also located in the autosomes of both sexes, but the degree and frequency of manifestation is different in individuals of different sexes (baldness and normal hair growth in humans).

The development of traits that are controlled by genes located on the sex chromosomes is called gonosomal inheritance (linked to the sex chromosomes).

Traits whose development is determined by genes located in a non-homologous region of the X chromosome are called X-linked (sex-linked) (color blindness, hemophilia, etc.). Traits whose development is determined by genes located in a non-homologous region of the Y chromosome are called holandric, and appear only in men (ichthyosis, webbing between the toes, etc.).

MOLECULAR GENETIC BASIS OF SEX DETERMINATION

Sex in most animals and plants is determined genetically at the time of fertilization. The decisive genetic determinant of sex is the presence or absence of the Y chromosome; the normal female phenotype is 46,XX, and the normal male phenotype is 46,X Y(Figure 1). Meiosis in germ cells reduces their chromosome complement to a haploid state, so that oocytes have 23,X, and sperm have either 23,X or 23,Y. Fertilization restores the diploid set of chromosomes and, depending on the presence or absence of the Y chromosome, determines the genetic sex as either 46, XX (female) or 46, X Y (male).

Fig.1. Karyotypes of men and women

The most important function of the Y chromosome is sex determination. Analysis of people whose phenotypic sex does not correlate with genetic sex led to the identification of a gene called SRY (sex-determining region of the Y chromosome). S ex-determining R egion, Y-chromosome), which is necessary and sufficient for the determination of male sex. The SRY gene encodes a putative transcriptional regulator that likely triggers a cascade of events leading to testis development and subsequently to male sexual differentiation. The Y chromosome contains about 50 genes that influence the development of the gonads, spermatogenesis, skeletal growth, etc. (Figure 2).

Rice. 2. Scheme of the Y chromosome

It is believed that the Y chromosome arose from the original homolog of the X chromosome. Regions of homology at its ends, called pseudoautosomal regions, allow it to conjugate to the X chromosome during meiosis. Between these pseudoautosomal regions lie discontinuous regions of X-Y homology interspersed with regions that are unique to the Y chromosome. SRY, a crucial mediator of male sex determination, is located on the short arm of the Y chromosome within the pseudoautosomal region in which X-Y recombination usually occurs, SRY is sometimes transferred from the Y to the X chromosome in either males 46.XX or females 46.X Y .

When studying the karyotypes of many animals, it was found that the female organism has paired sex X chromosomes, the male has unpaired ones: the same with the female X chromosome, and a smaller one, available only in male organisms - the Y chromosome.

However, in nature there are deviations from this definition of sex in living organisms.

Determination of sex depends on the number and composition of sex chromosomes. In the water bug Protenor, in some butterflies and worms, in males sex is determined by one X chromosome (X0), and in females by two X chromosomes.

In birds, some butterflies, fish, amphibians and flowering plants, the heterogametic (i.e. with different sex chromosomes) sex is female, and females have a set of sex chromosomes XY or XO, while males have XX.

In some cases, the appearance of male or female sex is determined not by hereditary differences, but by the influence of environmental conditions. A classic example is the sea worm Bonellia viridis. Males, several millimeters in size, live in the female’s uterus, where they perform their task - fertilize eggs. The male is a typical parasite, living inside the female's body, which is approximately the size of a plum.

The larvae that develop after fertilization of the eggs lead a free lifestyle for some time, and then attach to the trunk of a mature female or settle and attach to the bottom. The larvae of these two types are no different from each other. The larvae attached to the female's trunk develop into males. They penetrate the female genital organs and live there as parasites. The larvae attached to the bottom become females.

Sex determination in reptiles is regulated by changes in external temperature.

Gynandromorphs, intersexes, hermaphrodites and other sexual deviations

In Drosophila and other organisms, cases of gynandromorphism are known, when different parts of the body, according to their characteristics, belong to different sexes (Fig. 3). The body looks like a mosaic, in which one part is male and the other is female. In this case, the zygote has two X chromosomes and should develop into a female. She is a heterozygote for the genes for white eyes and small wings located on the X chromosome. During the first cleavage divisions, the chromosome is lost, and if the equator of the mitotic division is located along the line of symmetry from the head to the tail of the embryo, one half of the fly’s body consists of cells with only one X chromosome, which corresponds to the male genotype. The other side has two X chromosomes and develops into a female.

Rice. 3. Drosophila gynandomorph (the right side of the body is male, the left is female).

The gypsy moth is characterized by sharp differences between females and males. The crossing of different geographical races of this butterfly (European and Japanese) led to the emergence of forms that are transitional in their characteristics between males and females, i.e. to the emergence of intersexuality. Intersex individuals have also been found in Drosophila.

Intersex people differ from gynandromorphs in that they do not have different sex-determined sectors.

Intersex people retain their genetically determined sex up to a certain point in development, but then development continues in the direction of the opposite sex.

As a result, intersex people differ from normal individuals in that their primary and secondary sexual characteristics are intermediate in nature, forming a continuous series of transitions from a normal male to a normal female (Fig. 4). As described by K. Bridges, intersex flies in Drosophila were easily distinguishable from males and females, were large flies with coarse bristles, large, rough eyes and jagged edges of the wings. Genital combs (a sign of a male) were present. The abdomen was intermediate in character between a male and a female. The external genitalia were formed predominantly according to the female type. The gonads were represented by rudimentary ovaries. Spermathecae were also present. Often one gonad was an ovary, the other a testis. Or the same gonad could be an ovary with a testis budding on it.

Along with heterosexuality, hermaphroditism occurs in many plants and lower animals, when the male and female sexes are combined in one organism.