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.

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

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.

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 whole 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 in 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 gamete maturation, 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 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 homologs - 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. Why are chemical and radiation exposure dangerous 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. 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 probability 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, she 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 cells), unlike spermatozoa, 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?

The ability of living beings to reproduce their own kind is called reproduction. In this case, the genetic material is transmitted to the offspring, and the parental characteristics, to one degree or another, will be inherent in the resulting daughter organisms.

Types of reproduction

Scientists distinguish two main forms of reproduction of organisms. It can be sexual or asexual. In the first case, 2 individuals are needed to reproduce offspring, and in the second, only one is enough.

In asexual reproduction, a new organism emerges from somatic cells. In nature, there are several ways to reproduce offspring without the participation of genitals. These include vegetative propagation, budding, fragmentation, sporulation, division, cloning.

In sexual reproduction, new organisms are produced by the fusion of specialized sex cells called gametes and the subsequent formation of a zygote. This method is more progressive compared to the asexual one.

Comparison of benefits

It is worth noting that both methods of reproduction have their advantages. For example, biologists highlight the following advantages of asexual reproduction:
the ability to reproduce a significant number of individuals;

the offspring is similar to the parent organism in all respects.

But sexual reproduction is characteristic of the vast majority of living beings. It is able to guarantee the genetic diversity of the resulting daughter individuals. This is what allows them to quickly adapt to changing living conditions. After all, during the formation of a new organism, a combination of genes from the parents occurs.

Types of asexual reproduction of offspring

There are several ways to obtain daughter organisms without the participation of germ cells. Biology studies them all. Reproduction, in which the types of daughter organisms do not change in any way, can be carried out on the basis of the division of one or several cells.

In the first case, the following forms are distinguished:

single or multiple (schizogony) cell division;
sporulation;
budding of unicellular organisms.

When dividing a group of cells, classification is carried out as follows:

fragmentation;
budding of multicellular organisms (for example, hydra).

Each of these types of asexual reproduction has its own characteristics.

Forms of reproduction

The simplest option is regular division. It is characteristic of many protozoa. Examples of asexual reproduction by binary fission: amoeba, slipper ciliates,

Sporulation is considered widespread. It is characteristic of almost all plants, fungi, some protozoa and prokaryotic organisms (for example, bacteria or blue-green algae).

But other examples of asexual reproduction of organisms can be given. So, don't forget about fragmentation. During this process, the mother is divided into several parts. From each of them a new organism is formed. For example, the filamentous alga spirogyra can be torn anywhere. The two parts will produce two new organisms in the future.

Plants are characterized by vegetative propagation. According to the principle of the processes, it does not differ from budding or fragmentation. The plant can form special structures necessary for reproduction. Also, the appearance of a daughter organism is possible from a part of the mother’s organism.

Sexual reproduction

Most living things reproduce similar organisms by mixing the genetic material of two individuals. To do this, two gametes fuse, resulting in a diploid zygote. In the process of development, it turns into a full-fledged new organism. Sexual forms of reproduction of organisms are characteristic of some flowering plants, most animals and, of course, humans.

There are two types of gametes - male and female. If a species is dioecious, then each cell type is produced by male and female individuals, respectively. Some organisms are capable of producing both types of gametes independently. In this case, they are called hermaphrodites.

A variant of sexual reproduction is also possible, in which gametes are not involved. These are types such as conjugation, gametangiogamy, apogamy, hologamy.

Reproduction process

All organisms are made up of cells. Their growth and development are possible due to the fact that they are constantly reproducing. During life, some cells age and die. They are being replaced by others. The only way to produce new cells is by dividing their precursors. This is a vital process for every living being. For example, in the human body several million of these structural units are divided every second.

Biologists have described three ways of cell reproduction. Direct division is called amitosis, indirect division is called mitosis, and reduction division is called meiosis. Regardless of the form of reproduction of organisms, these processes occur in each of them.

Amitosis and mitosis

The least common and poorly studied method of cell division is amitosis. In this process, the core is separated by a constriction. At the same time, it is impossible to ensure an even distribution of genetic material. A cell that has divided by amitosis, in most cases, cannot continue to enter the normal mitotic cycle. Therefore, she is considered doomed to death.

The universal method of reproduction of eukaryotic cells is mitosis. In animal cells, it usually takes place within an hour. The biological importance of reproduction cannot be underestimated, because it is thanks to it that the development and growth of all organisms is ensured.

Stages of mitosis

The sequence of all processes that occur during the formation of new cells is called the cell cycle. It consists of three stages: interphase, mitosis, cytokinesis. The duration of the cycle depends both on the types of cells and on external factors. Temperature, availability of nutrients, and oxygen influence. For example, in the intestinal epithelium, such formation of new cells occurs every 8-10 minutes, in bacteria - every 20 minutes.

The process begins with interphase. At this time, processes of intensive growth take place. Substances are produced that help the cell grow and perform all its assigned functions. During interphase, DNA replication occurs.

All the necessary substances for these processes are stored during the preliminary stage - interphase. Each stage of division consists of four periods: prophase, metaphase, anaphase and telophase. The same phases occur during mitosis, but each process has its own characteristics.

The first meiosis is a cell division in which the number of chromosomes is reduced by half. From one diploid formation two haploid ones appear. At this time, DNA helicalization processes occur and a fission spindle is formed. In addition, conjugation occurs in prophase. The resulting pairs form a bivalent. In some places the chromatids intersect. This process is called crossing over.

The final stage is the so-called second meiosis. This is a division that produces cells with a haploid set of chromosomes consisting of one chromatid. As a result of the described processes, 4 cells emerge from one diploid formation (oogonium or spermatogonia).

The biological significance of meiosis is the formation of cells that provide sexual reproduction in animals or sporulation in higher animals. It is this method of reproduction that guarantees the maintenance of the genetic constancy of the species.

Features of sexual and asexual reproduction of organisms

Depending on how cells divide to produce offspring, different types of this process are distinguished. Separately, it should be noted that the survival of many organisms in changing conditions is due precisely to the fact that they can combine different methods of reproduction.

Of course, the sexual and asexual reproduction of similar organisms is significantly different. The table of breeding types will help you understand what the fundamental difference is.

Key points	Asexual way	Sexual method
Number of parents
Reproduction process	There is no meiosis stage, gametes are not formed	Meiosis is an obligatory stage that prevents chromosomes from doubling in future generations. The result is haploid gametes, the nuclei of which fuse to form a diploid zygote.
Resulting offspring	Daughter individuals are identical to their parents; genetic variability is possible only through random mutations	Offspring differ from parents and there is genetic variability. It appears due to gene recombination.
Organisms that have a characteristic method of reproduction	Lower animals, microorganisms	Most plants and animals

It is clear that sexual forms of reproduction of organisms are more advanced. But the asexual method guarantees the rapid reproduction of a large number of offspring. During sexual reproduction, the number of daughter organisms does not grow so intensively.

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 as 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, sex is determined by one X chromosome (X0) in males, and by two X chromosomes in females.

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 heterozygous 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.

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 mother is divided into two or more 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 on to an independent lifestyle (hydra, yeast), or they 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 (anneli, 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	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.

Oogamy(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"

Which is associated with processes such as fertilization, division and direct reproduction, reproduction of their own kind. This concept is also used in painting, but the topic of the article does not concern this aspect.

What is reproduction in biology: definition

Self-reproduction is one of the most important concepts in biology. The process of creating their own kind ensures the continued existence of species. Reproduction, or reproduction, is often considered solely in terms of the production of offspring in animals and plants. This is one of the important characteristics of all living organisms. At the lowest level, this is called chemical replication.

In unicellular organisms, the ability of one cell to reproduce means the appearance of a new individual. However, it means growth and regeneration. Reproduction occurs in a variety of ways, accompanied by the participation of a complex system of organs and the work of specific hormonal mechanisms.

Reproduction levels

Reproduction means reproduction and reproduction of one's own kind. The following levels are distinguished:

molecular copying;
cell reproduction;
reproduction of organisms.

Let's take a closer look at the latter.

Sexual and asexual reproduction

Reproduction is an integral part of the existence of all life on the planet in biology. In multicellular organisms, asexual and sexual reproduction are distinguished.

Vegetative propagation can take a wide variety of forms. Many multicellular lower plants secrete asexual spores, which can be either mononuclear or multinuclear. Often entire fragments of the vegetative part of an organism can reproduce a new organism, which occurs in most plants.

In many cases, asexual reproduction occurs through roots and shoots. Sometimes other parts of plants have the ability to reproduce themselves, such as buds. Asexual reproduction is also characteristic of some animals, including numerous species of invertebrates (sponges, hydras, worms). Vertebrates have lost the ability to reproduce vegetatively; their only form of organismic reproduction is sexual intercourse.

Reproduction and natural selection

The significance of biological reproduction can be explained solely by natural selection. When developing his theory, Charles Darwin came to the conclusion that in order to evolve, living organisms must be able not only to reproduce themselves, but also to undergo certain changes. Thus, more successful generations will contribute more to the subsequent development of the descendant species. Moreover, the magnitude of these changes and genetic transformations is particularly important. There shouldn't be too few or too many of them.

Examples and methods of reproduction in nature

What does reproduction look like in biology? Examples, as well as methods, are quite numerous. Sexual reproduction, which involves the combination of parental genes, is a way to obtain a new individual organism. During fertilization, the genomes of the sperm and egg combine to form a zygote, which, after numerous transformations, becomes an embryo. This type of reproduction is widespread in almost all groups of multicellular organisms. Pollination is quite interesting from a biological point of view.

Reproduction is a feature in biology that is inherent in all living organisms. Reproduction ensures continuity and continuity of the entire life cycle. There are many methods of reproduction, but there are two main ones. These are sexual and asexual reproduction. Since all organisms have a cellular structure, cell division is the basis of all forms and methods of reproduction.