Probably, there is not one concept more often studied in the framework of the school curriculum in biology than a cell. They get acquainted with it in the 5th grade at natural science, then in 6th they consider the species and how the cell reproduces, its methods of division. In grades 7 and 8, it is studied in terms of plant, animal and human affiliation. Grade 9 involves the consideration of internal processes occurring in it, that is, the molecular structure. At 10 and 11, this is cellular theory, discovery and evolution.
The program is built this way because it is these small structures, the "building blocks of life", that are the most important elements of any organism. All vital functions, processes, growth and development, formation - everything that is connected with life is carried out by them and in them. Therefore, in this article we will consider the main points of reproduction, development of cells and the history of their discovery.
Cell opening
These structural particles are extremely small in size. Therefore, for their discovery, it took a lot of time and the creation of a certain technique. So, for the first time, Robert Hook saw the cellular structure of living plant tissue . That was in 1665. In order to consider them, he invented the world's first microscope. This device did not resemble modern magnifying devices. Rather, it looked like a few loops arranged together, giving an increase.
Using this device, the scientist examined a section of a cork tree. What he saw laid the foundation for the development of a number of related sciences and biology as a whole. Many tightly adjacent cells of approximately the same shape and size. Hook called them cella, which means "cell."
Subsequently, a number of discoveries were made that allowed knowledge to grow, accumulate and result in several sciences involved in their study.
- 1675 - the scientist Malpigi studied the variety of cells in shape and came to the conclusion that these are most often round or oval vesicles filled with vital juice.
- 1682 - N. Gru confirmed the conclusions of Malpighi, and also studied the structure of the cell membrane.
- 1674 - Antonio van Levenguk discovers bacterial cells, as well as blood structures and sperm.
- 1802-1809 - S. Brissot-Mirbe and J. B. Lamarck suggest the existence of tissues and the similarity of animal and plant cells.
- 1825 - Purkinje opens the nucleus in the reproductive cage of birds.
- 1831-1833 - Robert Brown reveals the presence of a nucleus in plant cells and introduces the concept of the significance of the internal composition, rather than the cell membrane, as previously thought.
- 1839 - Theodor Schwann concludes that all living organisms are composed of cells, as well as the similarity of the latter among themselves (future cell theory).
- 1874-1875 - Chistyakov and Strasbourg discover ways of cell reproduction - mitosis, meiosis.
All further discoveries in the field of cell structure, their functions, diversity and role in the life of organisms were made quite quickly thanks to the intensive development of special magnifying and lighting equipment.
Cell reproduction
Each cell during its life completes a whole cell cycle - this is the time of its life from the moment of its birth to death (or division). Moreover, it does not matter if it is animal or vegetable. The life cycle is the same for all of them, and most often, at the end of its cells multiply by division.
Of course, this process is not identical for all organisms. For eukaryotes and prokaryotes, it is fundamentally different; there are also some differences in the reproduction of plant and animal cells.
How does a cell multiply? There are several basic ways to do this.
- Mitosis.
- Meiosis.
- Amitosis.
Each of them represents a number of processes, phases. Moreover, all these processes are characteristic of multicellular organisms, both plant and animal origin. In unicellular, reproduction occurs by simple division into two. That is, the methods of cell reproduction are not the same. There is even such a thing as cell suicide. This is self-destruction of cells instead of division processes.
How does a cell multiply, for example, bacteria, blue-green algae, some protozoa? Asexually, in the simplest way: the contents of their cell doubles, a transverse or longitudinal constriction forms in the cell wall and one cell is divided into two completely new, identical to the mother, organism.
This process is called direct cell division. They reproduce unicellular and bacteria, but it has nothing to do with mitotic or meiotic processes. They occur only in the body of multicellular living organisms.
Mitosis
Multicellular beings contain billions of cells. And each of them seeks to complete its life cycle, namely leaving offspring, and not dying. Cells multiply by division, but this process is not the same for all of them.
Somatic structures (these include all cells of the body except the reproductive) use mitosis or amitosis as their method of reproduction. This is a very interesting, capacious and complex process, as a result of which two identical daughter cells with the same diploid composition are formed from one mother diploid cell (that is, with a double set of chromosomes).
The whole process includes two main points:
- Karyokinesis - fission of the nucleus and its entire contents.
- Cytokinesis - division of protoplasm (cytoplasm and all cellular organelles).
These processes take place simultaneously, leading to the formation of full-fledged maternal copies of a reduced size.
Mitosis consists of four phases (prophase, metaphase, anaphase, telophase) and the state preceding division - interphase. Let's consider each in detail.
Interphase
The growth and reproduction of cells is carried out throughout the life of the body. However, not all cells have the same lifespan. Some of them die in two to three days (blood cells), some remain functioning throughout their lives (nerves).
But in the life of each cell, most of the time, a state called interphase is preserved. This is the period of preparation for the division of a mature mature cell, which takes up to 90% of the entire process.
The biological meaning of this stage is the accumulation of nutrients, RNA and proteins, the synthesis of DNA molecules. After all, after division, exactly as many organoids, substances and genetic material should be in each daughter cell as there was in the mother. For this, a doubling of all available structures, including DNA strands, should occur.
In general, interphase occurs in three stages:
- presynthetic;
- synthetic;
- postsynthetic.
The result: the accumulation of nutrients, energy and DNA molecules for further fission processes. Thus, this stage is only the beginning of how the cell multiplies in the future.
Prophase
At this stage, the following basic processes occur:
- the nuclear membrane dissolves;
- nucleoli disappear (dissolve);
- chromosomes become visible under the microscope due to twisting (spiraling) of the structure;
- centrioles diverge towards the poles of the cell, stretching and forming a spindle of division.
At this stage, the reproduction of animal cells is no different from that of all others.
Metaphase
This phase is quite short, only about 10 minutes. Its basis is that chromatids line up at the equator of the cell. The spindle threads of one division cling to the centriole at the pole of the cell, and the other to the centromere of each chromatid. Genetic structures are almost unrelated to each other and are therefore easily ready for disconnection.
Anaphase
The shortest stage of the entire mitotic cycle. Duration is about 3 minutes. During this period, each chromatid goes to its cell pole and completes the missing half, turning into a normal chromosome structure.
However, for this formation, a special enzyme is required - telomerase. It was his accumulation that occurred in interphase.
Telophase
Each cell pole has its own genetic material, which is worn in the nuclear membrane to form the nucleus. Nucleoli appear. The whole process takes about 30 minutes. That is a fairly long time. This is because the formation of nucleoli and the nuclear membrane requires large energy costs, as well as the availability of building material - nutrients (proteins, carbohydrates, enzymes, fats, amino acids).
Cytokinesis
This process completes the entire mitotic cycle. Protoplasm is divided together with organoids strictly in half, and each daughter gets exactly the same amount as her sister. Then, a protein constriction (of an actin nature) is formed across the cell, which squeezes the structure across and divides it into two equal, but smaller in size, compared with the mother cells.
At this stage, there are some differences between the animal cell and how the plant cell reproduces . The fact is that there is less protein in plant structures, but there is no actin at all. Therefore, not a constriction is formed in the middle, but a partition, on both sides of which cellulose is deposited. This gives the plant cell rigidity, forms a framework in the form of a cell wall.
The growth and reproduction of cells further follows the path of the usual life cycle: specialization, the formation of tissues, then organs, active work and division, or death.
Sex cells and their reproduction
To the question of how the cell multiplies, the answer can be given when specifying which one. After all, the mitosis processes considered by us are characteristic only for somatic structures. While germ cells multiply in a slightly different way, or rather, meiosis.
This process is the basis of such vital functions in animals as gametogenesis, that is, sexual reproduction. The development of germ cells occurs in stages. Therefore, meiosis is an even more complex and capacious division than mitosis.
For plant cells, meiosis is the basis of sporogenesis, that is, the formation of germ cells. The main biological role of meiosis for all organisms is that as a result of it, four haploid (with half or single set of chromosomes) germ cells are formed. What for? In order for fertilization (the fusion of male and female germ cells) to occur, diploidy is restored in a new zygote (future fetus). This gives genetic diversity to organisms, leads to a combination of genes, the emergence and consolidation of new traits.
The structure of the meiosis process
There are two main divisions in meiosis: reduction and equational. Each of them includes all the same phases as mitosis: prophase, metaphase, anaphase and telophase. Let's consider a little more in detail each of them.
Reduction division
The bottom line: two haploid cells with a half set of chromosomes are formed from one diploid cell. Phase:
- prophase I;
- metaphase I;
- anaphase I;
- telophase I.
At each phase, all the same transformations are repeated as at the corresponding stages in mitosis. However, there is one difference: in the interphase, DNA does not double, it only divides in half, and that’s all. Therefore, only half of the genetic information gets into each daughter cell. This is the initial reproduction of animal cells, as well as plant, related to sexual.
Equational division
The second division of meiosis, as a result of which two more cells from each previous one are formed. Now there are already four identical haploid analogues, which become sex cells of animals or plants. The stages of equational division: prophase II, metaphase II, anaphase II, telophase II.
Thus, the question of how the cell reproduces has a rather complex and capacious answer. After all, these processes, like all others occurring in living beings, are very subtle and consist of many stages.