Is it the oldest?
2. Are all living things capable of reproduction?
Reproduction is a universal property of living organisms, consisting in the ability produce similar individuals of their own species. Thanks to reproduction, there is an endless change of generations of each species. During the process of reproduction, unique combinations of genetic material may arise, leading to the appearance of hereditary changes in the body. Thus, genetic diversity of individuals within one species arises and the foundations for variability and further evolution of the species are laid.
Reproduction - necessary condition existence of life on Earth.
Asexual reproduction.
The oldest form of reproduction on our planet is asexual reproduction. It involves the division of a unicellular organism (or one or more cells of a multicellular organism) and the formation of daughter individuals. This form of reproduction occurs most often in prokaryote, plants, fungi and protozoa, it is also observed in some animal species.
Kinds asexual reproduction.
Let's look at the main types of asexual reproduction.
Reproduction by division.
In prokaryotes, before division, the only ring doubles, a septum appears between the two daughter chromosomes, and the cell divides in two.
Many unicellular algae (for example, Chlamydomonas, Euglena green) and protozoa (amoeba) divide by mitosis, forming two cells.
Reproduction by spores.
Spores are specialized haploid cells of fungi and plants (not to be confused with spores bacteria), serving for reproduction and dispersal. In fungi and lower plants, spores are formed by mitosis, in higher plants- as a result of meiosis.
In seed plants, spores have lost their dispersal function, but are necessary step cycle playback
Vegetative propagation.
The methods of asexual reproduction presented above are united by the fact that new organism in all these cases it develops from a single cell of a unicellular or multicellular parent. However, very often during asexual reproduction of multicellular organisms, offspring develop from a group of parent cells. This method of asexual reproduction is called vegetative. There are several types of vegetative propagation. The first of them is the propagation of plants by parts of vegetative organs (part of the thallus, stem cutting, root cutting) or special modifications of shoots (rhizome, bulb, tuber).
Another type of vegetative propagation is fragmentation, a process based on regeneration.
For example, a body fragment earthworm gives rise to a whole individual. However, it should be taken into account that in natural conditions fragmentation is rare, in particular in polychaete worms, molds, and some algae (spirogyra).
The third type of vegetative propagation is budding.
In this case, a group of cells of the parent individual begins to divide in concert, giving rise to a daughter individual, which develops for some time as part of the maternal organism, and then separates from it (freshwater hydra) or forms colonies of many individuals (coral polyps).
The meaning of asexual reproduction.
Asexual reproduction allows a rapid increase in the number of individuals of a given species under favorable conditions. But with this method of reproduction, all descendants have a genotype identical to the parent. Consequently, with asexual reproduction there is practically no increase in genetic diversity, which could be very useful if it is necessary to adapt to changed environmental conditions. For this reason, the vast majority of living organisms periodically or continuously reproduce sexually.
Asexual reproduction. Vegetative propagation.
1. What kind of reproduction is called asexual?
2. What types of asexual reproduction are there?
3. What is the biological significance of asexual reproduction?
A special type of vegetative reproduction of organisms is polyembryony. In this case, the embryo (embryo) of higher animals, soon after formation, is divided into several fragments, each of which independently develops into a full-fledged individual. This division of embryos occurs, for example, in armadillos. Polyembryony also includes the formation of identical twins in humans. In this case, the zygote resulting from the usual fertilization, breaking up, forms an embryo, which, for reasons that are not yet entirely clear, is divided into several parts. Each of these parts goes through the normal path embryonic development, as a result of which two or more practically identical babies are born, necessarily of the same sex. The birth rate of identical twins does not exceed one in 250 normal births. But sometimes the separation of the developing embryo is incomplete. In this case, organisms arise that have common body parts or internal organs. Such identical twins are usually called Siamese, in honor of Chang and Eng Banker, born in Thailand (then Siam) (Fig. 50), Chang and Eng were connected in the chest area by a dense ligament about 9 cm thick. Even in those years, they were probably , could be separated surgically, but they did not agree to this. Having married two American sisters, they became prosperous farmers. Their wives had a total of 22 children. Sometimes nature makes more serious mistakes. In France, a teenage girl who suffered from anemia and spinal curvature was unexpectedly found to have... abdominal cavity baby fetus.
However, this embryo was not located in the uterus, but was connected by blood vessels to the vessels of the abdominal cavity, since the embryo grew quickly, it had to be removed surgically, otherwise its “mother” would have died. The embryo reached 30 cm in length. How could such a mistake of nature arise? Apparently, one of the cells in the girl’s abdominal cavity began to fragment in the same way as a zygote fragments after fertilization, and gave rise to a new one. to the human body. However, the embryo was doomed from the beginning, and it would never have been able to turn into a full-fledged one. healthy child, because it developed in the wrong place and was not supplied with the necessary hormones and nutrients. After the embryo was removed, the girl quickly recovered and her internal organs, compressed by the growing embryo, began to develop normally.
Kamensky A. A., Kriksunov E. V., Pasechnik V. V. Biology 10th grade
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Reproduction is a universal property of living things, ensuring material continuity in a series of generations. Evolution of methods of reproduction.
Reproduction – the ability of organisms to reproduce themselves. The properties of organisms to produce offspring. This is a condition for the existence of a species, which is based on the transfer of genetic material. The evolution of reproduction, as a rule, went in the direction from asexual to sexual reproduction, from isogamy to oogamy, from the participation of all cells in reproduction to the formation of germ cells, and from external to internal fertilization with intrauterine development and care for the offspring. In the course of evolution, different groups of organisms have developed different ways and strategies of reproduction, and the fact that these groups have survived and exist proves the effectiveness of different ways of carrying out this process. All the variety methods of reproduction can be divided into two main types: asexual and sexual reproduction.
Asexual reproduction, its types and biological significance.
At asexual reproduction one individual participates; individuals are formed that are genetically identical to the original parent; sex cells are not formed. Asexual reproduction enhances the role of stabilizing natural selection and ensures the preservation of fitness in changing environmental conditions.
There are two types of asexual reproduction: vegetative and sporulation (Table 10). A special case is polyembryony in vertebrates - asexual reproduction by early stages embryonic development. First described by I.I. Mechnikov using the example of the splitting of blastulae in a jellyfish and the development of cells of the whole organism from each aggregate. In humans, an example of polyembryony is the development of identical twins.
Table 10 - Types of asexual reproduction at the organismal level
|
Vegetative: |
Sporulation: |
|
Reproduction by a group of somatic cells. Simple division into two: in prokaryotes and unicellular eukaryotes. Schizogony (endogony): in unicellular flagellates and sporozoans. Budding: in unicellular yeast; in multicellular organisms - hydra. Fragmentation: in multicellular worms. Polyembryony. Vegetative organs: stem and root buds, bulbs, tubers. Ordered division: uniform, longitudinal and transverse amitosis in starfish and annelids. |
A spore is a specialized cell with a haploid set of chromosomes. It is formed by meiosis, less often by mitosis, on the mother plant sporophyte in sporangia. Found in protozoan eukaryotes, algae, fungi, mosses, ferns, horsetails and mosses. |
Sexual reproduction, its types and advantages over asexual reproduction.
Evolutionarily, sexual reproduction was preceded by a sexual process - conjugation. Conjugation ensures the exchange of genetic information without increasing the number of individuals. Found in protozoa, eukaryotes, algae and bacteria.
Sexual reproduction – the emergence and development of offspring from a fertilized egg - zygote (Table 11). In the course of historical development, sexual reproduction of organisms has become dominant in the plant and animal world. It has a number of advantages:
High reproduction rate.
Update of genetic material. Source of hereditary variability. Success in the struggle for existence.
Great adaptive abilities of daughter individuals.
Sexual reproduction is characterized by the following features:
Two individuals participate.
The source of the formation of new organisms are special cells - gametes, which have sexual differentiation.
To form a new organism, the fusion of two germ cells is necessary. One cell from each parent is enough.
Irregular types of sexual reproduction (Table 11):
1. Parthenogenesis -development of an embryo from an unfertilized egg. Found in lower crustaceans, rotifers, bees, and wasps. There are somatic or diploid and generative or haploid parthenogenesis. In somatic cases, the egg either does not undergo reduction division, or two haploid nuclei merge together, restoring the diploid set of chromosomes. In generative, the embryo develops from a haploid egg. Thus, in the honey bee, drones develop from unfertilized haploid eggs. In wasps and ants, during parthenogenesis, the diploid set is restored in somatic cells due to endomitosis.
Table 11 - Types of sexual reproduction in eukaryotes
2. Gynogenesis – a type of sexual reproduction in which sperm participate as stimulators of egg development, but fertilization (karyogamy) does not occur in this case. The development of the embryo is carried out at the expense of the female nucleus. It is observed in roundworms and in the viviparous fish Molinesia. The sperm nucleus is destroyed and loses the ability to perform karyogamy, but retains the ability to activate the egg. The offspring receives genetic information from the mother.
3. Androgenesis – a type of reproduction in which the development of an egg occurs due to the male nucleus and maternal cytoplasm. The haploid embryo is characterized by low viability, which is normalized when the diploid set of chromosomes is restored. With polyspermy, the fusion of two paternal pronuclei is possible and the formation of a diploid nucleus, like in the silkworm.
Gametogenesis. Features of oogenesis and spermatogenesis in humans, its hormonal regulation.
The process of formation of germ cells is called Gametogenesis . This process occurs in the gonads (testes and ovaries) and is divided into permatogenesis – sperm formation and oogenesis – oocyte formation.
Spermatogenesis takes place in the convoluted seminiferous tubules of the testes and includes four phases (Table 12):
maturation;
formation.
reproduction;
Breeding phase: multiple mitosis of spermatogonia.
Growth phase: cells lose their ability to undergo mitosis and increase in size. Now they are called first-order spermatocytes, which enter the long (about 3 weeks) prophase of the 1st division of meiosis.
Table 12 - Stages of spermatogenesis
|
Gonad zones |
Stages |
|
1. Reproduction |
Spermatogonia (2n4C) |
|
Spermatocytes I (2n4C) |
|
|
3. Maturation |
Spermatocytes II (1n2C) Spermatids (1n1C) |
|
4. Formations |
Sperm |
Maturation phase: It includes two successive divisions of meiosis: as a result of the 1st (reduction) division, haploid spermatocytes of the 2nd order are formed from spermatocytes of the 1st order (1n 2 chromatids 2c). They are smaller in size than first-order spermatocytes and are located closer to the lumen of the tubule. The second division of meiosis (equational) leads to the formation of four spermatids - relatively small cells with a haploid set of DNA (1n 1 chromatid 1c).
Formation phase: It involves the transformation of spermatids into spermatozoa. The chromatin in the nucleus becomes denser, and the size of the nucleus decreases. The Golgi complex is converted into an acrosome containing lytic enzymes necessary for the breakdown of egg membranes. The acrosome is adjacent to the nucleus and gradually spreads over it in the form of a cap. Centrioles move to the opposite pole of the cell. A flagellum is formed from the distal centriole, which then becomes the axial filament of the developing sperm. Excess cytoplasm is dumped into the lumen of the tubule and phagocytosed by Sertoli cells.
Spermatogenesis in humans occurs throughout the entire period of puberty in the convoluted seminiferous tubules. Sperm development lasts 72-75 days.
Oogenesis – a set of sequential processes in the development of the female reproductive cell. Oogenesis includes periods of reproduction, growth and maturation (Table 13). During the reproduction period, the number of diploid germ cells - oogonia - increases through mitosis; after the cessation of mitosis and DNA replication in the premeiotic interphase, they enter the prophase of meiosis, which coincides with the period of growth of cells called first-order oocytes. At the beginning of the growth period (slow growth phase), the oocyte increases slightly; conjugation of homologous chromosomes and crossing over occur in its nucleus. The number of organelles increases in the cytoplasm. This phase lasts for years in humans. In phase rapid growth the volume of oocytes increases hundreds of times or more, mainly due to the accumulation of ribosomes and yolk. During maturation, 2 meiotic divisions occur. As a result of the 1st division, a second order oocyte and a reduction body are formed. By the end of the maturation period, oocytes acquire the ability to be fertilized, and further division of their nuclei is blocked. Meiosis is completed during the process of fertilization with the formation of one egg and the release of 3 reduction bodies. The latter subsequently degenerate.
Table 13 - Stages of oogenesis
Differences between oogenesis and spermatogenesis:
The period of reproduction of oogonia ends at the time of birth.
The growth period during oogenesis is longer than during spermatogenesis and has a period of slow growth, when the size of the nucleus and cytoplasm increases, and a period of rapid growth - the accumulation of yolk inclusions.
During oogenesis, one full-fledged germ cell is formed from one oocyte I, and four are formed from spermatocyte I during spermatogenesis.
The formation phase is characteristic only of spermatogenesis. The formation of the egg occurs during the period of fertilization.
In humans, eggs and sperm develop from primordial germ cells, which are formed in the extraembryonic mesoderm. Primary germ cells subsequently migrate to their final location - the bisexual gonad. In many animals, the areas of the cytoplasm responsible for the release of primary germ cells are distinguished by pigmentation or granules. These are sex determinants. The reproductive cytoplasm is concentrated at the vegetative pole of the cell.
Specific signs of female sex (ovarian development) become noticeable at the end of the 8th week. By the end of the 3rd month of intrauterine development, oocytes are formed deep in the gonads (prophase 1). By the 7th month, ovarian differentiation will accelerate. By the 9th month there are 200-400 thousand oocytes in the ovary.
During oogenesis, the mitotic division of primary female germ cells (oogonium) ceases by the 5th month of intrauterine development. Their number reaches almost 7 million. Oogonia in the process of their development turn into first-order oocytes. Further intrauterine reproduction of oogonia ceases. Therefore, by the time of birth, a girl’s ovary already contains about 2 million oocytes in the primary follicles. However, an intensive process of atresia occurs among them. Therefore, by the beginning of puberty, about 400-500 thousand oocytes remain in a woman’s ovary, capable of further development.
The formation of primary follicles is completed by the end of the 3rd month of intrauterine development, when follicular cells completely cover the oocyte. By the time the formation of the primary follicle is completed, the oocytes are at the meiosis I stage, at the dictyotene stage (diplotene phase). From this moment there comes a long break in their further development. The arrest of oocyte division I persists until puberty.
Shortly before ovulation, the first arrest at the diplotene stage of the first meiotic division is interrupted. The division quickly ends with the formation of a second-order oocyte and one so-called reduction body. An ovulated oocyte is called a second order oocyte. After ovulation, the second meiotic division begins in the oocyte, which lasts until metaphase II. If fertilization has occurred, then the second phase of meiosis is completed almost simultaneously with it. As a result, an egg is formed. If fertilization does not occur within 48 hours after ovulation, the ovulated egg (oocyte II) dies.
Each month, one follicle matures in the ovary, inside which there is a gamete capable of fertilization. Follicle maturation has several stages. Initially, first-order oocytes are surrounded by a layer of cells, and a primary follicle is formed. Further, in the period before puberty, the follicles increase in size due to the growth of the oocyte, the formation of the zona pellucida and the corona radiata. Then the secondary follicle grows, turns into a tertiary or mature one, containing a second-order oocyte. In total, 400-800 follicles mature in a woman during her childbearing period.
After the ovarian follicle matures, its walls rupture and oocyte II enters the body cavity. The oviduct funnel (fallopian tubes) are located near the ovary. Cilia ensure the movement of the egg through the oviduct, where fertilization occurs. After ovulation, the destroyed ovarian follicle contracts and, as a result of the division of follicular cells, a “corpus luteum” is formed that fills the cavity of the vesicle. If fertilization does not occur, it degenerates, and new follicles begin to grow in another part of the ovary. When pregnancy occurs, the “corpus luteum” remains, and new follicles are formed after childbirth. During the juvenile and mature periods of ontogenesis, oocytes in the ovaries are in prophase I (diplotene stage: chromosomes in them are in the form of lamp brushes, intense RNA synthesis on certain genes). Prophase block 1 is periodically removed from the oocytes, meiosis I is completed and meiosis II begins. During fertilization, after 24 hours, meiosis II is completed, and after another 10 hours, a synkaryon is formed and synkaryogamy occurs.
The blocking is adaptive. Conjugation and crossing over in meiosis are protected by the mother's body, which guarantees fewer abnormalities in the embryo. In the postembryonic period, the body is exposed to a variety of influences environment, which increases the frequency of formation of abnormal gametes.
The growth of follicles and their ovulation are hormonal-dependent processes that are regulated by three gonadotropic hormones of the pituitary gland: follicle-stimulating hormone (FSH), luteinizing hormone (LH), luteotropic hormone (LTG), ovarian hormones - estrogen and progesterone. Under the influence of FSH, follicles develop and mature in the ovary. With the combined action of FSH and LH, the mature follicle ruptures, ovulation, and the formation of the “corpus luteum.” After ovulation, LH promotes the production of the hormone progesterone in the ovary by the corpus luteum.
The secretion of LH and FSH by the pituitary gland is regulated by the neurohumoral activity of the hypothalamus, which produces neurohormones: vasopressin, oxytocin. These centers, in turn, are influenced by ovarian hormones - estrogens. They affect the development of secondary sexual characteristics, metabolism (increase protein dissimilation) and thermoregulation. In addition, the ovaries also produce androgens - male sex hormones. The latter are also formed in the adrenal cortex.
Specific signs of the male sex, the development of the testis are observed at the end of the 7th week of intrauterine development.
The male reproductive gland, the testis, consists of seminiferous tubules surrounded by connective and loose interstitial tissue that produces hormones.
Spermatogenesis - This is the process of transformation of primary germ cells - spermatogonia into spermatozoa in the testes. The process occurs in the seminiferous tubules of the male gonads. Spermatogonia are located at the outer wall of the seminiferous tubules. At a certain point, they begin to grow and move from the periphery to the center of the tubules, proceeding to mitotic division, resulting in the formation of spermatogonia. Spermatogonia grow and, after numerous mitotic divisions, form spermatocytes that proceed to meiosis, two successive divisions of which culminate in the formation of full-fledged cells - spermatids, which differentiate into spermatozoa. Two successive divisions of meiosis are often called maturation divisions.
In humans, the first meiotic division lasts several weeks, the second - 8 hours. During the second division, second-order spermatocytes produce four immature haploid (1n1c) germ cells - spermatids. In the formation zone they become spermatozoa.
Spermatogenesis occurs throughout the entire period of male puberty. Full cell maturation takes 72 days.
The functions of the testes are regulated by the endocrine glands and the pituitary gland. The main male sex hormone produced in the Leydig cells of the testes is testosterone. Under the influence of male sex hormones, the formation and breakdown of protein in the body increases, which leads to the development of muscles, bone tissue, and body size.
Morphofunctional characteristics of mature gametes in humans.
Egg – oval, large, sedentary or immobile. Most animals lack a centrosome and are not capable of independent division. Based on the content and distribution of the yolk, several types of oocytes are distinguished (Table 14).
Table 14 - Types of eggs
The distribution of the yolk determines the spatial organization of the embryo. Isolecithal ovules are characterized by a small amount of evenly distributed yolk, such as in the lancelet. Polylecithales – with moderate (amphibians) and excessive yolk content (reptiles, birds). Telolecithal eggs are characterized by uneven distribution of the yolk and the formation of poles: animal , which has no yolk, vegetative – with yolk. Centrolecithal – characterized by a large amount of evenly distributed yolk in the center of the egg and are characteristic of arthropods.
The egg forms 3 types of protective shells:
Primary – vitelline, a waste product of the oocyte or egg, is in contact with the cytoplasm. In humans, it is part of the dense membrane, forming it inner part. Its outer zone is formed by follicular cells and is secondary (corona radiata).
Secondary – is formed as a derivative of follicular cells (their secretion) surrounding the oocyte (cells of the granular layer). In insects there is a chorion, in humans there is a corona radiata. The dense shell is penetrated by microvilli of the egg from the inside, and from the outside by microvilli of follicular cells. Thus, a person develops a corona radiata and a zona pellucida.
Tertiary – is formed after fertilization due to the secretion of glands or mucous epithelium of the genital tract as it passes through the female’s oviduct. These are the gelatinous membranes of amphibian eggs, the albumen, subshell and shell membranes of birds.
During fertilization, the sperm overcomes the secondary and primary membranes.
Sperm. The gamete is small and mobile. It has parts: head, neck, middle part and tail. The head consists of an acrosome and a nucleus. The acrosome is formed from elements of the Golgi complex of the spermatid. The acrosome ensures the penetration of sperm into the egg and activation of the latter using the enzyme hyaluronidase.
The sperm nucleus contains compactly packed deoxynucleoproteins. This packaging of the haploid set of chromosomes is associated with protamine proteins. Its meaning is the almost complete inactivation of genetic material.
The neck has proximal and distal centrioles located at right angles. The proximal one is involved in the formation of the spindle of the fertilized egg, and from the distal one the axial filament of the tail is formed.
Mitochondria are concentrated in the middle part, forming a compact cluster - the mitochondrial helix. This part ensures the energetic and metabolic activity of the sperm.
The base of the tail is an axial filament, surrounded by a small amount of cytoplasm and a cell membrane.
The viability of sperm depends on the concentration of sperm (thick suspension), the concentration of hydrogen ions (the greatest activity in an alkaline environment) and temperature.
Fertilization, its phases, biological essence.
The process of fertilization (fusion of the nuclei of male and female gametes) is preceded by insemination. Insemination – processes that cause the meeting of sperm and egg. The interaction of gametes is ensured by the release of special substances – gamons (gynogamons and androgamonics). Gynogamon I stimulates sperm motility. Gynogamon II blocks the motor activity of sperm and promotes their fixation on the egg shell. Androgamon I inhibits the movement of sperm, which protects them from premature waste of energy. Androgamon II promotes the dissolution of the egg membrane.
There are two methods of insemination: external and internal. Some animals experience skin insemination, which is a transitional form. This is typical for nemerteans and leeches.
Stages of fertilization:
Gamete approximation, acrosomal reaction and sperm penetration;
Activation of the egg and its synthetic processes;
Fusion of gametes (syngamy).
External phase. Rapprochement gametes belong to the external phase. Female and male gametes secrete specific compounds called gamones. Eggs produce gynogamones I and II, and sperm produce androgamones I and II. Gynogamones I activate the movement of sperm and provide contact with the egg, and androgamones II dissolve the egg shell.
The viability period of eggs in mammals ranges from a few minutes to 24 hours or more. It depends on internal and external conditions. Sperm viability is 96 hours. The ability to fertilize lasts 24-48 hours.
The moment the sperm comes into contact with the outer shell of the egg, the acrosomal reaction begins. The enzyme hyaluronidase is released from the acrosome. At the point of contact of the sperm with the plasma membrane of the egg, a protrusion or fertilization tubercle is formed. The fertilization tubercle helps draw the sperm into the egg. The membranes of the gametes fuse. The fusion of male and female reproductive cells is called syngamy. In some cases (in mammals), the sperm penetrates the egg without the active participation of the fertilization tubercle. The nucleus and centriole of the sperm pass into the cytoplasm of the egg, which contributes to the completion of meiosis II in the oocyte.
Internal phase. Characterized by a cortical reaction on the part of the egg. The vitelline membrane is detached, which hardens and is called the fertilization membrane. At the moment of completion of meiosis, the male and female pronuclei are formed. Both pronuclei merge. Fusion of gamete nuclei - synkaryogamy constitutes the essence of the fertilization process, resulting in the formation of a zygote.
Modern human reproductive strategy.
Modern human reproductive strategy includes:
Prenatal diagnosis of hereditary diseases;
Using methods to overcome infertility:
artificial insemination;
fertilization of an egg in vitro;
embryo transplantation using “surrogacy”.
donation of eggs and embryos.
Reproduction- the ability of living organisms to reproduce their own kind. There are two main reproduction method- asexual and sexual.
Asexual reproduction occurs with the participation of only one parent and occurs without the formation of gametes. The daughter generation in some species arises from one or a group of cells of the mother’s body, in other species - in specialized organs. The following are distinguished: methods of asexual reproduction: division, budding, fragmentation, polyembryony, sporulation, vegetative propagation.
Division- a method of asexual reproduction, characteristic of unicellular organisms, in which the maternal individual is divided into two or large quantity daughter cells. We can distinguish: a) simple binary fission (prokaryotes), b) mitotic binary fission (protozoa, unicellular algae), c) multiple fission, or schizogony (malarial plasmodium, trypanosomes). During the division of the paramecium (1), the micronucleus is divided by mitosis, the macronucleus by amitosis. During schizogony (2), the nucleus is first divided repeatedly by mitosis, then each of the daughter nuclei is surrounded by cytoplasm, and several independent organisms are formed.
Budding- a method of asexual reproduction in which new individuals are formed in the form of outgrowths on the body of the parent individual (3). Daughter individuals can separate from the mother and move to independent image life (hydra, yeast), can remain attached to it, in this case forming colonies (coral polyps).
Fragmentation(4) - a method of asexual reproduction, in which new individuals are formed from fragments (parts) into which the maternal individual breaks up ( annelids, starfish, spirogyra, elodea). Fragmentation is based on the ability of organisms to regenerate.
Polyembryony- a method of asexual reproduction in which new individuals are formed from fragments (parts) into which the embryo breaks up (monozygotic twins).
Vegetative propagation- a method of asexual reproduction in which new individuals are formed either from parts of the vegetative body of the mother individual, or from special structures (rhizome, tuber, etc.) specifically designed for this form of reproduction. Vegetative propagation is typical for many groups of plants and is used in gardening, vegetable gardening, and plant breeding (artificial vegetative propagation).
| Vegetative organ | Method of vegetative propagation | Examples |
|---|---|---|
| Root | Root cuttings | Rosehip, raspberry, aspen, willow, dandelion |
| Root suckers | Cherry, plum, sow thistle, thistle, lilac | |
| Aboveground parts of shoots | Dividing bushes | Phlox, daisy, primrose, rhubarb |
| Stem cuttings | Grapes, currants, gooseberries | |
| Layerings | Gooseberries, grapes, bird cherry | |
| Underground parts of shoots | Rhizome | Asparagus, bamboo, iris, lily of the valley |
| Tuber | Potatoes, sunflower, Jerusalem artichoke | |
| Bulb | Onion, garlic, tulip, hyacinth | |
| Corm | Gladiolus, crocus | |
| Sheet | Leaf cuttings | Begonia, gloxinia, coleus |
Sporulation(6) - reproduction through spores. Controversy- specialized cells, in most species they are formed in special organs - sporangia. In higher plants, spore formation is preceded by meiosis.
Cloning- a set of methods used by humans to obtain genetically identical copies of cells or individuals. Clone- a collection of cells or individuals descended from a common ancestor through asexual reproduction. The basis for obtaining a clone is mitosis (in bacteria - simple division).
Sexual reproduction is carried out with the participation of two parent individuals (male and female), in which specialized cells are formed in special organs - gametes. The process of gamete formation is called gametogenesis, the main stage of gametogenesis is meiosis. The daughter generation develops from zygotes- a cell formed as a result of the fusion of male and female gametes. The process of fusion of male and female gametes is called fertilization. An obligatory consequence of sexual reproduction is the recombination of genetic material in the daughter generation.
Depending on the structural features of the gametes, the following can be distinguished: forms of sexual reproduction: isogamy, heterogamy and oogamy.
Isogamy(1) - a form of sexual reproduction in which gametes (conditionally female and conditionally male) are mobile and have the same morphology and size.
Heterogamy(2) - a form of sexual reproduction in which female and male gametes are motile, but female gametes are larger than male ones and less mobile.
Ovogamy(3) - a form of sexual reproduction in which female gametes are immobile and larger than male gametes. In this case, female gametes are called eggs, male gametes, if they have flagella, - spermatozoa, if they don’t have it, - sperm.
Oogamy is characteristic of most species of animals and plants. Isogamy and heterogamy occur in some primitive organisms (algae). In addition to the above, some algae and fungi have forms of reproduction in which sex cells are not formed: hologamy and conjugation. At hologamia single-celled haploid organisms merge with each other, which in this case act as gametes. The resulting diploid zygote then divides by meiosis to produce four haploid organisms. At conjugation(4) the contents of individual haploid cells of filamentous thalli merge. Through specially formed channels, the contents of one cell flow into another, a diploid zygote is formed, which usually, after a period of rest, also divides by meiosis.
Go to lectures No. 13“Methods of division of eukaryotic cells: mitosis, meiosis, amitosis”
Go to lectures No. 15"Sexual reproduction in angiosperms"
Asexual reproduction is the reproduction of organisms in which there is no participation of another individual, and the reproduction of their own kind occurs by separating several or one cell from the maternal organism. A single parent individual takes part in this process. the cells completely correspond to the original mother.
Asexual reproduction is extremely simple. This is due to the fact that the organization of the structure of unicellular organisms is also relatively simple. Organisms with this method of reproduction reproduce their own kind very quickly. Under favorable conditions, the number of such cells doubles every hour. This process can continue indefinitely until a random change occurs, the so-called mutation.
In nature, such reproduction occurs in both plants and
Asexual reproduction of organisms
Simple division is observed in animals, for example, in ciliates, amoebas and some algae. First, the nucleus in the cell is divided in half through mitosis, and then a constriction is formed, and the parent is divided into two parts, which are daughter organisms.
In animals, asexual reproduction has been preserved only in some forms: sponges, coelenterates, and tunicates. In these organisms, a new individual is obtained as a result of budding or division, after which the part separated from the parent organism is completed to form the whole. In some cases, parts of the body have the ability to develop into a separate organism in animals. A whole hydra, for example, can develop from a two-hundredth part. With asexual reproduction, newly created individuals arise from several cells or one through mitotic divisions, receiving the same hereditary information that the cell of the mother’s body possessed.
Asexual plant reproduction
This method of reproduction is widespread in flora. There are a number of plants that reproduce well by tubers, layering, cuttings and even leaves, which makes it possible to use the vegetative organs of the parent plant to grow new organisms. This type of asexual reproduction is called vegetative, and it is characteristic of highly organized plants. An example of such reproduction can be considered one that occurs by whiskers, for example, in strawberries.
Sporulation is asexual reproduction that occurs in many plants, for example, algae, ferns, mosses, and fungi at some stage of development. In this case, special cells take part in the reproduction mechanism, often covered with a dense membrane, which protects them from adverse effects external environment: overheating, cold, drying out. As soon as favorable conditions arise, the spore shell bursts, the cell begins to divide repeatedly, giving life to a new organism.
Budding is a method of reproduction when a small portion of the body is separated from the parent individual, from which a daughter organism is later formed.
A set of individuals that descended from one common ancestor using this type of reproduction is called clones in biology.
Asexual reproduction is widely used in agriculture in order to obtain plants with a set of necessary characteristics useful for human life. Strawberries are spread with long “mustaches” and shoots, and trees are spread with cuttings. Scientists are studying the mechanisms of reproduction to learn how to control them and manage their development. The necessary hereditary information is first multiplied, and then the necessary whole plant is grown from them.
4. Forms of reproduction of organisms
The succession of generations of organisms in nature is carried out through reproduction. Reproduction- This is the ability of an organism to reproduce its own kind. In nature, there are two types of reproduction: asexual and sexual.
Types of asexual reproduction
Asexual reproduction- the formation of a new organism from one cell or group of cells of the original maternal organism. In this case, only one parent individual participates in reproduction, which passes on its hereditary information to its daughter individuals. Asexual reproduction produces identical offspring. The only source of variability is random hereditary changes that can arise during the process of individual development.
Asexual reproduction is based on mitosis. There are several types of asexual reproduction.
Asexual reproduction in bacteria is interesting (Fig. 7).
Rice. 7. Asexual reproduction of bacteria: A - general scheme reproduction; B - cell division diagram
A circular DNA molecule is attached to cell membrane and is replicated. A transverse partition begins to form in the cell on the side where DNA molecules attach. The transverse septum then bifurcates, moving the anchored DNA to different parts of the cell. Ribosomes are evenly distributed between the two daughter cells, and a constriction is formed that divides the cell into two daughter cells.
Budding - This is a form of asexual reproduction in which a small outgrowth (bud) is separated from the parent individual and a daughter organism is formed. A new organism develops from a group of cells of the original organism. This type of asexual reproduction is characteristic of coelenterates (hydra) and some other animals and plants. Single-celled fungi - yeasts - also reproduce by budding. In contrast to simple division, during budding the mother cell is divided into unequal parts, budding a constantly smaller daughter cell (Fig. 8, B).
Rice. 8. Types of asexual reproduction: A - simple division in two of green euglena (longitudinal); B - budding of yeast and hydra; B - sporulation of mosses; G - vegetative propagation by begonia leaves
Reproduction by spores (sporulation) is typical for spore-bearing plants (algae, mosses, ferns). Reproduction occurs with the help of special cells - spores formed in the mother's body (Fig. 8, B). A spore is a small cell consisting of a nucleus and small quantity cytoplasm. They are formed in large quantities in the original maternal organism. Each spore, germinating, gives rise to a new organism. Since they are microscopically small, they are easily transported by wind, water or other organisms, which facilitates the spread of these plants. Fungi, such as penicillum and cap mushrooms, also reproduce by spores.
Vegetative propagation- reproduction by individual organs, parts of organs or the body. Vegetative propagation most often occurs in plants that can reproduce by roots, shoots and parts of shoots (stems, leaves), modified shoots. Methods of vegetative propagation of plants are very diverse. This is propagation by bulbs (tulip), underground stolons - tubers (potatoes), rhizomes (wheatgrass), root cones (dahlia), layering (currants), root suckers (raspberries), leaves (begonia, violet), aboveground stolons - tendrils (strawberries ) etc. (Fig. 8, D).
Fragmentation- this is the division of an individual into two or more parts, each of which can give rise to a new organism. This method is based on regeneration- the ability of organisms to restore missing parts of the body. It is characteristic of lower invertebrate animals (coelenterates, flatworms, starfish, etc.). The animal's body, divided into separate parts, completes the missing fragments. For example, when unfavorable conditions The planarian flatworm breaks up into separate parts, each of which, when favorable conditions occur, can give rise to a new organism.
Fragmentation also occurs in plants; for example, multicellular algae can reproduce in parts of the thallus.
Cloning. Artificial method reproduction, which appeared relatively recently, in the early 60s. XX century It is based on obtaining a new organism from one cell of the original one. Since the cell nucleus contains the entire set of chromosomes, and therefore genes, under certain conditions it can be forced to divide, which will lead to the formation of a new organism. The formation of a clone is based on mitosis. To clone plants, the cells of the educational tissue are separated and grown on special nutrient media. A plant cell, dividing successively, gives rise to a whole organism. This method is currently widely used to obtain valuable plant varieties.
There is experience in animal cloning. It was first introduced by the English biologist D. Gurdon and gave positive results in experiments with the South American toad. Tadpole intestinal cells were used as a nuclear donor. The nuclei of the recipient eggs were destroyed by ultraviolet rays and the intestinal epithelial nuclei were transplanted into these cells. As a result of the experiment, it was possible to obtain several cloned toad individuals, completely identical to each other. In 1995, English scientists managed to obtain a clone of sheep that were similar to the original maternal individual. However, the lambs died in early age, before reaching nine months.
In 1997, Dolly the sheep was obtained by cloning. To do this, the nuclei of mammary gland cells from a sheep of one breed (nucleus donor) were taken and transplanted into eggs with previously destroyed nuclei from a sheep of another breed (recipient). The cloned sheep was no different from the nuclear donor, but very different from the recipient.
The use of the cloning method will make it possible not only to preserve economically valuable animals, but also to reproduce them without limit. Currently, work is underway on human cloning, which causes heated debate not only among scientists, but also among various groups of the population. However, using this method, it is intended to reproduce only individual organs and tissues for subsequent transplantation into the donor’s body, and not to create individual individuals. This method will solve the problem of incompatibility of tissues of different organisms.
Features of sexual reproduction
Sexual reproduction - This is the formation of a new organism with the participation of two parent individuals. The new organism carries hereditary information from two parents, and the resulting offspring differ genetically from each other and from their parents. This process is characteristic of all groups of organisms; in its simplest form, it occurs even in prokaryotes.
During sexual reproduction, special cells are formed in the body sex cells - gametes masculine and female type, which are capable of merging. Male gametes - spermatozoa, or sperm(if they are motionless). Female gamete - egg. Gametes are different from all other cells in the body, which are called somatic(from lat. soma - body). They always have haploid set of chromosomes (n).
As a result of the fusion of two gametes, the diploid set of chromosomes is restored again. In this case, half of all chromosomes are paternal, and the other half are maternal. For example, a person has 46 chromosomes, of which 23 are received from the mother and 23 from the father.
Sexual reproduction has a number of advantages. As a result of this process, a change in hereditary information occurs, and new individuals combine the characteristics of two parents. This leads to the emergence of new combinations of traits and genes. Sexual reproduction makes the organism more competitive and adapted to changing environmental conditions, as it increases the chances of survival. In the process of evolution, sexual reproduction turned out to be more preferable and progressive.
Questions for self-control
1. What types of reproduction occur in organisms? How are they different from each other?
2. What type of cell division underlies asexual reproduction?
3. Compare reproduction by spores and vegetative reproduction in plants. What are their similarities and differences?
4. What advantage does spore reproduction provide to the body?
5. Describe the features of each type of asexual reproduction.
6. What are the features of sexual reproduction? What advantages does this type of reproduction provide?
7. What cells are called gametes? What makes them special?
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