It is known about living organisms that they breathe,feed, multiply and die; this is their biological function. But at the expense of what is all this happening? At the expense of bricks - cells that also breathe, feed, die and multiply. But how does this happen?
About the structure of cells
A house consists of bricks, blocks or logs.So the body can be divided into elementary units - cells. All variety of living beings consists of them, the difference lies only in their number and types. They consist of muscles, bone tissue, skin, all internal organs - so much they differ in their purpose. But regardless of what functions a particular cell performs, they are all approximately the same. First of all, any "brick" has a shell and cytoplasm with organoids located in it. Some cells do not have a nucleus, they are called prokaryotic, but all more or less developed organisms consist of eukaryotic, having a nucleus in which genetic information is stored.
Organoids located in the cytoplasmdiverse and interesting, they perform important functions. The endoplasmic reticulum, ribosomes, mitochondria, Golgi complex, centrioles, lysosomes and motor elements are isolated in cells of animal origin. With the help of them and all the processes occur that ensure the functioning of the body.
Cell vital activity
As already mentioned, all living things feed, breathe,multiplies and dies. This statement is true for whole organisms, that is, people, animals, plants, etc., and for cells. It is surprising, but each "brick" has its own life. At the expense of his organoids, he receives and processes the nutrients, oxygen, removes all the excess to the outside. The cytoplasm itself and the endoplasmic reticulum perform the transport function, mitochondria are responsible, among other things, for respiration, as well as the provision of energy. Golgi complex is engaged in the accumulation and removal of waste products of the cell. The remaining organoids are also involved in complex processes. And at a certain stage of its life cycle, the cell begins to divide, that is, the process of reproduction occurs. It is worth considering in more detail.
Cell division process
Reproduction - one of the stages of development of livingorganism. The same applies to cells. At a certain stage of the life cycle, they enter a state when they are ready for reproduction. Prokaryotic cells simply divide in two, lengthening and then forming a septum. This process is simple and almost completely studied on the example of rod-shaped bacteria.
With eukaryotic cells everything is somewhatmore difficult. They multiply in three different ways, called amitosis, mitosis and meiosis. Each of these paths has its own characteristics, it is inherent in a certain type of cells. Amitoz
Sometimes direct division is isolated as a species.mitosis, however, some scientists consider it a separate mechanism. The course of this process, even in old cells, is quite rare. Next will be considered meiosis and its phases, the process of mitosis, as well as the similarities and differences of these methods. Compared to simple division, they are more complex and perfect. This is especially true for reduction division, so that the characteristics of the meiosis phases will be the most detailed.
Важную роль в делении клетки имеют центриоли - special organoids, usually located near the Golgi complex. Each such structure consists of 27 microtubules, grouped in three. The whole structure has a cylindrical shape. Centrioles are directly involved in the formation of the cell division spindle in the process of indirect division, which will be discussed further.
Mitosis
Продолжительность существования клеток varies. Some live a couple of days, and some can be attributed to long-livers, because their complete change is very rare. And almost all of these cells multiply with mitosis. Most of them have an average of 10-24 hours between periods of division. Mitosis itself takes a short period of time - in animals about 0.5-1.
Значение этого вида деления велико - этот процесс helps grow and regenerate tissues, due to which the development of the whole organism occurs. In addition, mitosis is the basis of asexual reproduction. And one more function - the movement of cells and the replacement of already outdated. Therefore, it is wrong to assume that due to the fact that the stages of meiosis are more complicated, its role is much higher, wrong. Both of these processes perform different functions and are important and irreplaceable in their own way.
Mitosis consists of several phases that differ inits morphological features. The state in which the cell is located, being ready for indirect division, is called interphase, and the process itself is further divided into 5 stages, which need to be considered in more detail.
Phases of mitosis
Being in interphase, the cell prepares for division:DNA and protein synthesis occurs. This stage is divided into several more, during which the growth of the entire structure occurs and the doubling of the chromosomes. In this state, the cell remains up to 90% of the entire life cycle.
The remaining 10% is taken directly by the division,divided into 5 stages. During mitosis of plant cells, a preprophase is also released, which is absent in all other cases. The formation of new structures occurs, the core moves to the center. A preprophase tape is formed, marking the expected place of the future division.
In all the other cells, the process of mitosis is as follows:
Table 1
| Stage name | Characteristic |
| Prophase | The nucleus grows in size, the chromosomes in itspiraling, becoming visible in the microscope. A division spindle forms in the cytoplasm. Often there is a decay of the nucleolus, but this does not always happen. The content of genetic material in the cell remains unchanged. |
| Prometaphase | The disintegration of the nuclear membrane occurs. Chromosomes begin active, but disorderly movement. Ultimately, they all come to the plane of the metaphase plate. This stage lasts up to 20 minutes. |
| Metaphase | Chromosomes line up along the equatorialthe plane of the spindle division approximately equal distance from both poles. The number of microtubules, keeping the whole structure in a stable state, reaches a maximum. Sister chromatids repel each other, keeping the compound only at the centromere. |
| Anaphase | The shortest stage.Chromatids are separated and repelled from each other in the direction of the nearest poles. This process is sometimes isolated separately and is called anaphase A. Further, the division poles themselves diverge. In the cells of some of the simplest spindle divisions, this increases in length up to 15 times. And this sub-step is called anaphase B. The duration and sequence of processes at this stage is variable. |
| Telophase | After the end of the divergence to the oppositepoles chromatids stop. Decondensation of chromosomes occurs, that is, their increase in size. The reconstruction of the nuclear membranes of the future daughter cells begins. Microtubules of spindle division disappear. Nuclei are formed, RNA synthesis is resumed. |
После завершения деления генетической информации cytokinesis or cytotomy occurs. This term refers to the formation of the bodies of daughter cells from the body of the parent. In this case, organelles, as a rule, are divided in half, although exceptions are possible, a partition is formed. Cytokinesis is not isolated in a separate phase, as a rule, considering it in the framework of telophase.
So, in the most interesting processes involved chromosomes that carry genetic information. What is it and why are they so important?
About chromosomes
Still not having a clue about genetics, peopleknew that many qualities of the offspring depended on the parents. With the development of biology, it became obvious that information about a particular organism is stored in each cell, and part of it is passed on to future generations.
In the late 19th century, chromosomes were discovered - structures consisting of a long
Структура хромосом в момент, когда они четко visible, quite simple - they are two chromatids connected in the middle of the centromere. It is a specific nucleotide sequence and plays an important role in the process of cell multiplication. Ultimately, the chromosome externally in prophase and metaphase, when it can best be seen, is reminiscent of the letter X.
In 1900, the laws of Mendel were discovered,describing the principles of transmission of hereditary traits. Then it became finally clear that the chromosomes - this is exactly what genetic information is transmitted through. In the future, scientists conducted a series of experiments to prove this. And then the subject of study was the impact that cell division has on them.
Meiosis
Unlike mitosis, this mechanism eventuallyleads to the formation of two cells with a set of chromosomes 2 times smaller than the original. Thus, the process of meiosis serves as a transition from the diploid phase to the haploid phase, and in the first place
Meiosis and its phases were studied by such famous scientistsas V. Fleming, E. Strasburgrer, V. I. Belyaev and others. The study of this process in the cells of both plants and animals continues to this day — it is so complex. Initially, this process was considered a variant of mitosis, but almost immediately after the discovery, it was nevertheless identified as a separate mechanism. The characteristic of meiosis and its theoretical significance were first described sufficiently by Augustus Weismann as far back as 1887. Since then, the study of the process of reduction division has greatly advanced, but the conclusions made have not yet been refuted.
Meiosis should not be confused with gametogenesis, although boththese processes are closely related. Both mechanisms are involved in the formation of germ cells, but there are a number of serious differences between them. Meiosis occurs in two stages of division, each of which consists of 4 main phases, between them there is a short break. The duration of the whole process depends on the amount of DNA in the nucleus and the structure of the chromosomal organization. In general, it is much longer in comparison with mitosis.
By the way, one of the main reasons for significantspecies diversity - it is meiosis. The set of chromosomes as a result of reduction division is split in two, so that new combinations of genes appear, primarily potentially enhancing the adaptability and adaptability of organisms, as a result of which receive certain sets of traits and qualities.
Meiosis phases
Как уже было упомянуто, редукционное клеточное division conditionally divided into two stages. Each of these stages is divided by another 4. And the first phase of meiosis - prophase I, in turn, is subdivided into another 5 separate stages. As the study of this process continues, others may be highlighted further. Now distinguish the following phases of meiosis:
table 2
| Stage name | Characteristic |
| First division (reduction) | |
Prophase I | |
| leptotena | In another way, this stage is called the stage of thin threads. Chromosomes appear in the microscope as a tangled tangle. Sometimes proleptoten is isolated, when individual strings are still difficult to make out. |
| zygotene | Stage of merging threads.Homologous, that is, similar to each other in morphology and genetically, pairs of chromosomes merge. In the process of merging, that is, conjugation, bivalents, or tetrads are formed. So called quite stable complexes of pairs of chromosomes. |
| pahitena | Stage of thick filaments.At this stage, the chromosomes are spiraling and DNA replication is completed, and chiasms are formed - the contact points of individual parts of the chromosomes - chromatid. There is a process of crossing over. Chromosomes intersect and exchange some pieces of genetic information. |
| diplotena | Also called the double strand stage. Homologous chromosomes in bivalents repel each other and remain connected only in chiasmas. |
| diakinesis | At this stage, the bivalents diverge at the periphery of the nucleus. |
| Metaphase I | The shell of the nucleus is destroyed, the spindle is formed. Bivalents move to the center of the cell and line up along the equatorial plane. |
| Anafaz I | The bivalents break up, after which each chromosome from the pair moves to the nearest pole of the cell. Separation into chromatids does not occur. |
| Telophase I | Завершается процесс расхождения хромосом.The formation of individual nuclei of daughter cells, each with a haploid set. Chromosomes despiralized, a nuclear envelope is formed. Cytokinesis is sometimes observed, that is, division of the cell body itself. |
| Second division (equational) | |
| Prophase II | Chromosome condensation occurs, the cell center divides. The nuclear envelope is destroyed. Formed spindle division, perpendicular to the first. |
| Metaphase II | In each of the daughter cells, the chromosomes line up along the equator. Each of them consists of two chromatids. |
| Anaphase II | Each chromosome is divided into chromatids. These parts diverge towards the opposite poles. |
| Telophase II | The resulting single-chromatid chromosomes are despiralized. Formed nuclear envelope. |
So, it is obvious that the phases of meiosis division are much more complicated than the process of mitosis. But, as already mentioned, this does not detract from the biological role of indirect division, since they perform different functions.
By the way, meiosis and its phases are observed insome of the simplest. However, as a rule, it includes only one division. It is assumed that such a single-stage form later evolved into a modern, two-stage one.
Differences and similarities of mitosis and meiosis
At first glance it seems that the differences between these twoprocesses are obvious, because they are completely different mechanisms. However, with a deeper analysis, it turns out that the differences in mitosis and meiosis are not so global; in the end, they lead to the formation of new cells.
First of all, we should talk about what these mechanisms have in common. In fact, there are only two coincidences: in the same phase sequence, and also in that
The differences are much greater.First of all, mitosis occurs in somatic cells, while meiosis is closely associated with the formation of germ cells and sporogenesis. In the phases themselves, the processes do not completely coincide. For example, crossing over in mitosis occurs during interphase, and that is not always the case. In the second case, this process accounts for anaphase of meiosis. The recombination of genes in indirect division is usually not carried out, which means that it does not play any role in the evolutionary development of an organism and in maintaining intraspecific diversity. The number of cells resulting from mitosis is two, and they are in a genetic sense identical to the maternal and possess a diploid set of chromosomes. During the reduction division everything is different. The result of meiosis is 4 haploid cells, different from the maternal one. In addition, both mechanisms differ considerably in duration, and this is due not only to the difference in the number of division stages, but also to the duration of each of the stages. For example, the first prophase of meiosis lasts much longer, because at this time chromosome conjugation and crossing-over occur. That is why it is further divided into several stages.
In general, the similarities of mitosis and meiosisquite insignificant compared to their differences from each other. To confuse these processes is almost impossible. So now it is even somewhat surprising that the reduction division was previously considered a type of mitosis.
Consequences of meiosis
As already mentioned, after the end of the processreduction division, instead of a maternal cell with a diploid set of chromosomes, four haploids are formed. And speaking of the differences in mitosis and meiosis, this is the most significant. Restoration of the required amount, in the case of germ cells, occurs after fertilization. Thus, with each new generation there is no doubling of the number of chromosomes.
In addition, during meiosis occursgene recombination. In the process of reproduction, this leads to the maintenance of intraspecific diversity. So the fact that even siblings sometimes differ greatly from each other is the result of meiosis.
Кстати, стерильность некоторых гибридов в the animal world is also a problem of reduction division. The fact is that the chromosomes of parents belonging to different species cannot enter into conjugation, which means that the formation of viable germ cells of full value is impossible. Thus, it is precisely meiosis that underlies the evolutionary development of animals, plants, and other organisms.
