Main article: Asexual reproduction
Asexual reproduction is a form of reproduction that is not associated with the exchange of genetic information between individuals - the 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 division into two parts, forming two separate individuals.
Among multicellular organisms, almost all plants and fungi have the ability to reproduce asexually - the exception is, for example, Welwitschia. Asexual reproduction of these organisms occurs vegetatively or by spores.
Among animals, the ability to reproduce asexually is more common in lower forms, but is absent in more developed ones. The only way of asexual reproduction in animals is vegetative.
There is a widespread misconception that individuals resulting from asexual reproduction are always genetically identical to the parent organism (if mutations are not taken into account). The most striking counterexample is reproduction by spores in plants, since during sporulation a reductional division of cells occurs, as a result of which the spores contain only half of the genetic information available in the sporophyte cells (see Life cycle of plants).
Sexual reproduction
Sexual reproduction is associated with the sexual process (cell fusion), and also, in the canonical case, with the fact of the existence of two complementary sexual categories (male organisms and female organisms).
During sexual reproduction, gametes, or sex cells, are formed. These cells have a haploid (single) set of chromosomes. Animals are characterized by a double set of chromosomes in ordinary (somatic) cells, therefore gamete formation in animals occurs during the process of meiosis. In many algae and all higher plants, gametes develop in the gametophyte, which already has a single set of chromosomes, and are obtained by simple mitotic division.
Based on the similarities and differences between the resulting gametes, several types of gamete formation are distinguished:
isogamy - gametes of the same size and structure, with flagella
anisogamy - gametes of different sizes, but similar structure, with flagella
oogamy - gametes of different sizes and structures. Small male gametes with flagella are called sperm, and large female gametes without flagella are called eggs.
When two gametes merge (in the case of oogamy, the fusion of different types of gametes is necessary), a zygote is formed, which now has a diploid (double) set of chromosomes. From the zygote a daughter organism develops, the cells of which contain genetic information from both parents.
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 preserved only in some forms: sponges, coelenterates, 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.
Offspring and ensure the survival of the species. Natural selection, which selects which traits of organisms are favorable for a given environment and which are unfavorable. Those individuals that have undesirable traits will eventually disappear, while organisms with “good” traits will live long enough to reproduce and pass on those genes to the next generation.
There are two types of reproduction: sexual and asexual. Sexual reproduction involves the fusion of male and female reproductive cells during fertilization, which will ultimately create offspring that are partially similar to the parents. Asexual reproduction requires only one parent, who will pass all of its genes to the offspring. This means that there is no mixing of genes and the offspring is actually a clone of the parent (prohibiting any mutations).
Asexual reproduction is usually common in less complex species and is quite efficient. There is no need to find a breeding partner, and one parent is able to pass on all of its traits to the next generation. However, without variety natural selection cannot work, and unless there are mutations to create more favorable traits, species that reproduce in this way may not survive in an environment that is constantly changing.
There are several various types asexual reproduction. Let's look at some of the most common ones.
Binary fission
Almost everyone reproduces. This type of reproduction is very similar to the process of mitosis. However, since it is not, and prokaryotic DNA is usually found in only one ring, the process is not as complex as . Binary fission begins with one cell copying its DNA and then dividing into two identical cells.
It's very fast and effective method creating offspring for bacteria, and similar types of cells. However, if a DNA mutation were to occur during the breeding process, it could change the genetics of the offspring so that they would no longer be identical clones.
Budding
Another type of asexual reproduction is called budding. Budding occurs when a new organism or offspring grows from the parent's side through a part called a bud. The descendant remains attached to the ancestor until it reaches maturity and becomes an independent organism. One parent can have many buds and many offspring at the same time.
With the help of budding, both unicellular organisms such as yeast and multicellular ones such as hydra can reproduce. Again, the offspring is a clone of the parent unless some mutation occurs during DNA copying or cell reproduction.
Fragmentation
Some types of organisms have many viable parts that can live independently of a single individual. These species are capable of reproducing through an asexual method of reproduction known as fragmentation. It occurs when part of an individual is separated and a completely new organism is formed from it. The original organism also regenerates the body part that was separated. This part may come off naturally or during an injury or other life-threatening situation.
The most famous organism that undergoes fragmentation is the starfish. Starfish can detach any of their five arms from their body, which then become their offspring. This is mainly due to their radial symmetry. They have a central nerve ring in the middle that branches into five rays or arms. Each arm has all the elements necessary to create a completely new individual through fragmentation. , some flatworms and fungi can also reproduce by fragmentation.
Parthenogenesis
How more complex organisms, the more likely they are to undergo sexual rather than asexual reproduction. However, there are some complex animals and plants that are capable of reproducing through parthenogenesis when necessary. This is not the preferred method of reproduction for most of these species, but it may be the only way to leave offspring for various reasons.
Parthenogenesis is a type of reproduction in which offspring emerge from an unfertilized egg. A lack of available partners, an immediate threat to the female's life, or other similar situations may result in parthenogenesis being necessary for the persistence of the species. Of course, this will not be ideal, because the offspring will become a clone of the mother.
Some animals that can reproduce through parthenogenesis include insects (bees and grasshoppers), lizards (Komodo dragon), and very rarely birds.
Reproduction by spores
Many plants and fungi use spores as a form of asexual reproduction. These types of organisms are subject to life cycle, called at which they pass different phases of their lives, characterized by the presence of or. During the diploid phase, they are called sporophytes and produce diploid spores, which are used for asexual reproduction. Species that form spores do not require a partner or fertilization to produce offspring. Just like all other types of asexual reproduction, the offspring of organisms that reproduce are clones of the parent. Examples of spore-producing organisms include fungi and ferns.
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 variability are random hereditary changes that may 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
The circular DNA molecule attaches to the cell membrane and replicates. 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 a small amount of 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 vegetative propagation 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, under 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. An artificial reproduction method that 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, then when certain conditions it can be forced to divide, resulting in 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 put forward 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 ultraviolet rays and transplanted intestinal epithelial nuclei 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 various groups population. However, using this method it is intended to reproduce only individual organs and tissue for subsequent transplantation into the donor’s body, rather than creating separate 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 diploid set chromosomes are 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 body more competitive and adapted to changing conditions environment, 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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1) Division of single-celled organisms(amoeba). At schizogony(malarial plasmodium) it turns out not two, but many cells.
2) Sporulation
- Spores of fungi and plants serve for reproduction.
- Bacterial spores do not serve for reproduction, because One spore is formed from one bacterium. They serve to survive unfavorable conditions and dispersal (by wind).
3) Budding: daughter individuals are formed from outgrowths of the body of the mother organism (buds) - in coelenterates (hydra), yeast.
4) Fragmentation: The mother organism is divided into parts, each part turns into a daughter organism. (Spirogyra, coelenterates, starfish.)
5) Vegetative propagation of plants: reproduction with vegetative organs:
- roots - raspberries
- leaves - violet
- specialized modified shoots:
- bulbs (onions)
- rhizome (wheatgrass)
- tuber (potato)
- mustache (strawberry)
Methods of sexual reproduction
1) With the help of gametes, sperm and eggs. Hermaphrodite- this is an organism that produces both female and male gametes (most higher plants, coelenterates, flat and some annelids, shellfish).
2) Conjugation green algae Spirogyra: two filaments of spirogyra come together, copulation bridges are formed, the contents of one filament flow into the other, one filament is formed from zygotes, the second from empty shells.
3) Conjugation in ciliates: two ciliates approach each other, exchange reproductive nuclei, and then separate. The number of ciliates remains the same, but recombination occurs.
4) Parthenogenesis: a child develops from an unfertilized egg (in aphids, daphnia, bee drones).
Establish a correspondence between the features of sexual and vegetative reproduction and the method of reproduction: 1) asexual, 2) sexual. Write numbers 1 and 2 in the correct order.
A) forms new combinations of genes
B) forms combinative variability
C) produces offspring identical to the mother’s
D) occurs without gametogenesis
D) due to mitosis
Answer
Choose three correct answers out of six and write down the numbers under which they are indicated. Bacterial spores, unlike fungal spores,
1) serve as an adaptation to endure unfavorable conditions
2) perform the function of nutrition and breathing
3) NOT used for reproduction
4) ensure distribution (settlement)
5) are formed by meiosis
6) are formed from the mother cell by loss of water
Answer
Choose three options. Asexual reproduction is characterized by the fact that
1) the offspring has genes only from the mother’s body
2) the offspring is genetically different from the maternal organism
3) one individual participates in the formation of offspring
4) in the offspring there is a splitting of characteristics
5) offspring develop from an unfertilized egg
6) a new individual develops from somatic cells
Answer
Establish a correspondence between the characteristics and the method of plant propagation: 1) vegetative, 2) sexual
A) is carried out by modified shoots
B) is carried out with the participation of gametes
C) daughter plants retain great resemblance with maternal
D) is used by humans to preserve valuable characteristics of maternal plants in offspring
D) a new organism develops from a zygote
E) the offspring combines the characteristics of the maternal and paternal organisms
Answer
Establish a correspondence between the peculiarity of reproduction and its type: 1) vegetative, 2) sexual. Write numbers 1 and 2 in the correct order.
A) due to the combination of gametes
B) individuals are formed by budding
C) ensures the genetic similarity of individuals
D) occurs without meiosis and crossing over
D) due to mitosis
Answer
1. Establish a correspondence between an example of reproduction and its method: 1) sexual, 2) asexual. Write numbers 1 and 2 in the correct order.
A) sporulation in sphagnum
B) seed propagation of spruce
B) parthenogenesis in bees
D) propagation by bulbs in tulips
D) laying eggs by birds
E) spawning of fish
Answer
2. Match concrete example and method of reproduction: 1) asexual, 2) sexual. Write numbers 1 and 2 in the correct order.
A) fern sporulation
B) formation of chlamydomonas gametes
B) formation of spores in sphagnum
D) yeast budding
D) fish spawning
Answer
3. Establish a correspondence between a specific example and the method of reproduction: 1) asexual, 2) sexual. Write numbers 1 and 2 in the correct order.
A) hydra budding
B) division of a bacterial cell into two
B) formation of spores in fungi
D) parthenogenesis of bees
D) formation of strawberry whiskers
Answer
4. Establish a correspondence between the examples and methods of reproduction: 1) asexual, 2) sexual. Write numbers 1 and 2 in the correct order.
A) live birth in a shark
B) division in two of the ciliate-slipper
B) parthenogenesis of bees
D) propagation of violets by leaves
D) fish spawning
E) hydra budding
Answer
5. Establish a correspondence between the processes and methods of reproduction of organisms: 1) sexual, 2) asexual. Write numbers 1 and 2 in the order corresponding to the letters.
A) laying eggs by lizards
B) penicillium sporulation
C) propagation of wheatgrass by rhizomes
D) parthenogenesis of daphnia
D) division of euglena
E) propagation of cherries by seeds
Answer
6. Establish a correspondence between the examples and methods of reproduction: 1) asexual, 2) sexual. Write numbers 1 and 2 in the correct order.
A) raspberry cuttings
B) formation of spores in horsetail
B) sporulation in cuckoo flax
D) fragmentation of lichen
D) parthenogenesis of aphids
E) budding in a coral polyp
FORMED 7. Establish a correspondence between the examples and methods of reproduction: 1) asexual, 2) sexual. Write numbers 1 and 2 in the correct order.
A) formation of gametes in Chlorella
B) sturgeon spawning
B) sporulation in mosses
D) division of amoeba vulgaris
Choose the one that suits you best correct option. Reproduction in which a daughter organism appears without fertilization from the body cells of the mother organism is called
1) parthenogenesis
2) sexual
3) asexual
4) seed
Answer
All but two of the terms below are used to describe the sexual reproduction of organisms. Define two terms that “fall out” from general list, and write down the numbers under which they are indicated.
1) gonad
2) dispute
3) fertilization
4) ovogenesis
5) budding
Answer
Write down the numbers that indicate what happens during sexual reproduction of animals.
1) usually two individuals participate
2) germ cells are formed by mitosis
3) the initial ones are somatic cells
4) gametes have a haploid set of chromosomes
5) the genotype of the offspring is a copy of the genotype of one of the parents
6) the genotype of the offspring combines the genetic information of both parents
Answer
Select three characteristics characteristic of sexual reproduction of seed plants and write down the numbers under which they are indicated.
1) Sperm and eggs are involved in reproduction
2) As a result of fertilization, a zygote is formed
3) During the process of reproduction, the cell divides in half
4) The offspring retains all the hereditary characteristics of the parent
5) As a result of reproduction, new characteristics appear in the offspring
6) Vegetative parts of the plant participate in reproduction
Answer
Select two differences between sexual and asexual reproduction.
1) sexual reproduction is energetically more profitable than asexual reproduction
2) two organisms participate in sexual reproduction, and one organism in asexual reproduction
3) during sexual reproduction, offspring are exact copies of parents
4) somatic cells participate in asexual reproduction
5) sexual reproduction is possible only in water
Answer
1. All but two of the terms listed below are used to describe asexual reproduction. Identify two terms that “drop out” from the general list and write down the numbers under which they are indicated.
1) schizogony
2) parthenogenesis
3) fragmentation
4) budding
5) copulation
Answer
2. All but two of the terms below are used to describe the asexual method of reproduction in living organisms. Identify two terms that “drop out” from the general list and write down the numbers under which they are indicated.
1) fragmentation
2) seed propagation
3) sporulation
4) parthenogenesis
5) budding
Answer
Establish a correspondence between the characteristics and the method of plant reproduction: 1) sexual, 2) vegetative. Write numbers 1 and 2 in the order corresponding to the letters.
A) is carried out with the participation of gametes
B) a new organism develops from a zygote
B) carried out by modified shoots
D) the offspring has characteristics of the paternal and maternal organisms
D) the offspring has characteristics of the maternal organism
E) is used by humans to preserve valuable characteristics of the mother plant in offspring
Answer
All but two of the examples below refer to asexual reproduction of organisms. Identify two examples that “fall out” from the general list and write down the numbers under which they are indicated.
1) reproduction by fern spores
2) reproduction of earthworms by fragmentation
3) ciliate-slipper conjugation
4) budding of freshwater hydra
5) parthenogenesis of bees
Answer
Establish a correspondence between the characteristics and methods of reproduction: 1) asexual, 2) sexual. Write numbers 1 and 2 in the order corresponding to the letters.
A) Haploid nuclei fuse.
B) A zygote is formed.
B) Occurs with the help of spores or zoospores.
D) Combinative variability appears.
D) Offspring are produced that are identical to the original individual.
E) The genotype of the parent individual is preserved over a number of generations.
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