Meiosis and its phases. Characterization of the phases of meiosis. Reproduction of organisms. Similarities of mitosis and meiosis

About living organisms it is known that they breathe, feed, reproduce and perish, this is their biological function. But due to what is this all happening? Due to the bricks - cells that also breathe, feed, die and multiply. But how does this happen?

About the structure of cells

The house consists of bricks, blocks or logs. So the body can be divided into elementary units - cells. All variety of living beings consists precisely 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 are performed by this or that cell, they are all arranged approximately equally. First of all, any "brick" has a membrane and a cytoplasm with organoids located in it. Some cells do not have a nucleus, they are called prokaryotic, but all more or less developed organisms are composed of eukaryotic ones that have a nucleus in which genetic information is stored.

Organoids located in the cytoplasm are diverse and interesting, they perform important functions. In cells of animal origin, the endoplasmic reticulum, ribosomes, mitochondria, the Golgi complex, centrioles, lysosomes and motor elements are isolated. With the help of these, all the processes that ensure the functioning of the organism take place.

Cell vital activity

As already mentioned, all living things feed on, breathe, reproduce and die. This statement is true both for whole organisms, that is, humans, animals, plants, etc., and for cells. It's amazing, but every "brick" has its own life. At the expense of his organoids, he receives and processes nutrients, oxygen, removes all the excess out. The cytoplasm itself and the endoplasmic reticulum carry out a transport function, the mitochondria respond, including for breathing, and also with energy. The Golgi complex deals with the accumulation and excretion of the products of the cell's vital activity. The remaining organelles also participate in complex processes. And at a certain stage of its life cycle, the cell begins to divide, that is, the process of reproduction takes place. It should be considered in more detail.

The process of cell division

Reproduction is one of the stages in the development of a living organism. The same applies to cells. At a certain stage in the life cycle, they enter a state when they are ready to reproduce. Prokaryotic cells simply divide in two, extending, and then forming a septum. This process is simple and almost completely studied using the example of rod-shaped bacteria.

With eukaryotic cells, everything is somewhat more complicated. They multiply in three different ways, called amitosis, mitosis and meiosis. Each of these paths has its own peculiarities, it is inherent in a certain kind of cells. Amitosis It is also called the direct binary division. With it, the DNA molecule doubles. However, there is no fission spindle, so this method is the most energetically economical. Amytosis is observed in unicellular organisms, while multicellular tissues multiply through other mechanisms. However, it is sometimes observed also where mitotic activity is reduced, for example, in mature tissues.

Sometimes direct division is distinguished as a kind of mitosis, but some scientists consider it a separate mechanism. The process of this process even in old cells is quite rare. Further, meiosis and its phases, the process of mitosis, as well as the similarities and differences in these methods will be considered. Compared with simple division, they are more complex and perfect. This is especially true of the reduction division, so that the characteristics of the phases of meiosis will be the most detailed.

Important role in cell division has centrioles - special organelles, usually located near the Golgi complex. Each such structure consists of 27 microtubules grouped into three. The whole structure has a cylindrical shape. Centrioles are directly involved in the formation of the spindle of cell division in the process of indirect division, which will be discussed further.

Mitosis

The duration of existence of cells differs. Some live for a couple of days, and some can be attributed to long-livers, since their complete replacement is very rare. And practically all these cells multiply with the help of mitosis. Most of them have an average of 10-24 hours between the fission periods. Mitosis itself takes a short period of time - in animals approximately 0.5-1 Hour, and in plants about 2-3. This mechanism ensures the growth of the cell population and the reproduction of identical in its genetic content of units. This is how the continuity of generations is observed at the elementary level. The number of chromosomes remains unchanged. It is this mechanism that is the most common variant of the reproduction of eukaryotic cells.

The importance of this type of division is great - this process helps to grow and regenerate tissues, due to which the whole organism develops. In addition, it is mitosis that underlies asexual reproduction. And one more function - the movement of cells and the replacement of already obsolete ones. Therefore, it is wrong to assume that due to the fact that the stages of meiosis are more complicated, its role is also much higher. Both these processes perform different functions and are important and indispensable in their own way.

Mitosis consists of several phases, differing in their morphological features. The state in which the cell is located, being ready for indirect division, is called interphase, and the process itself is divided into 5 stages that need to be considered in more detail.

Phases of mitosis

Being in the interphase, the cell prepares for division: the synthesis of DNA and proteins occurs. This stage is subdivided into several more, during which the whole structure grows and the chromosome doubles. In this condition, the cell lives up to 90% of the entire life cycle.

The remaining 10% is directly divided, divided into 5 stages. At mitosis of plant cells, preprofase is also excreted, which is absent in all other cases. There is the formation of new structures, the core moves to the center. A pre-phase tape is formed that marks the prospective place of future fission.

In all other cells, the process of mitosis proceeds as follows:

Table 1

After the completion of the division of genetic information, cytokinesis or cytotomy occurs. By this term is meant the formation of the bodies of daughter cells from the maternal body. In this case, organoids, as a rule, are divided in half, although exceptions are possible, a septum is formed. Cytokinesis is not isolated into a separate phase, as a rule, considering it within the 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 the slightest idea of genetics, people knew that many qualities of the offspring depend on the parents. With the development of biology, it became apparent that information about an organism is stored in every cell, and some of it is transmitted to future generations.

In the late 19th century, chromosomes were discovered - structures consisting of a long DNA molecules. This became possible with the improvement of microscopes, and even now they can be considered only in the period of division. Most often, the discovery is attributed to the German scientist V. Fleming, who not only ordered everything that had been studied before him, but also contributed: he was one of the first to study the cellular structure, meiosis and its phases, and also introduced the term "mitosis". The very concept of the "chromosome" was suggested a little later by another scientist - the German histologist G. Valdeier.

The structure of the chromosomes at a time when they are clearly visible is quite simple - they are two chromatids, connected in the middle by a centromere. It is a specific sequence of nucleotides and plays an important role in the process of cell multiplication. In the final analysis, the chromosome externally in prophase and metaphase, when it can be best seen, is reminiscent of the letter X.

In 1900 the laws of Mendel were discovered , describing the principles of the transfer of hereditary traits. Then it became finally clear that the chromosomes - this is exactly what the genetic information is transmitted with. Later, scientists carried out a series of experiments proving this. And then the subject of studying was the influence that the division of cells on them makes.

Meiosis

In contrast to mitosis, this mechanism eventually leads to the formation of two cells with a set of chromosomes 2 times less 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 We are talking about the fission of the nucleus, and even the second - of the whole cell. Restoration of the full set of chromosomes occurs as a result of further fusion of gametes. In connection with the reduction in the number of chromosomes, this method is also defined as the reduction division of the cell.

Meiosis and its phases have been studied by such well-known scientists as W. Fleming, E. Strasburgrehrer, VI 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 allotted as a separate mechanism. Characteristics of meiosis and its theoretical significance were first described to a sufficient extent by August Vaisman in 1887. Since then, the study of the process of reduction division has greatly advanced, but the conclusions drawn have not yet been refuted.

Meiosis should not be confused with gametogenesis, although both of these processes are closely related. Both mechanisms participate in the formation of sex 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 entire process depends on the amount of DNA in the nucleus and the structure of the chromosomal organization. In general, it is much more prolonged in comparison with mitosis.

By the way, one of the main reasons for the significant species diversity is meiosis. The set of chromosomes as a result of the reduction division is split in two, so that new combinations of genes appear, first of all potentially increasing the adaptability and adaptability of organisms, which in the end receive certain sets of attributes and qualities.

Phases of meiosis

As already mentioned, the reduction cell division is 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 5 separate stages. As the study of this process continues, others can be identified later. Now the following phases of meiosis are distinguished:

table 2

So, it is obvious that the phases of division of meiosis are much more complicated than the process of mitosis. But, as already mentioned, this does not detract from the biological role of indirect division, because they perform different functions.

By the way, meiosis and its phases are also observed in some protozoa. However, as a rule, it includes only one division. It is assumed that this single-stage form later developed into a modern, two-stage form.

Differences and similarities of mitosis and meiosis

At first glance it seems that the differences between these two processes are obvious, because these are completely different mechanisms. However, in a deeper analysis, it turns out that the differences between mitosis and meiosis are not so global, eventually they lead to the formation of new cells.

First of all, it is worthwhile to talk about what these mechanisms have in common. In fact, there are only two coincidences: in the same sequence of phases, and also in the fact that Before both types of fission, DNA replication takes place. Although, with regard to meiosis, before the initiation of prophase I, this process does not complete completely, ending in one of the first sub-stages. A sequence of phases, albeit similar, but, in fact, the events occurring in them coincide not completely. So the similarities between mitosis and meiosis are not so numerous.

The differences are much greater. First of all, mitosis occurs in somatic cells, while meiosis is closely related to the formation of sex 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, anaphase of meiosis occurs in this process. Recombination of genes in indirect division is usually not carried out, which means that it plays no role in the evolutionary development of the organism and maintenance of intraspecific diversity. The number of cells resulting from mitosis is two, and they are genetically identical to the maternal cells and possess a diploid set of chromosomes. During the reduction division, everything is different. The result of meiosis is 4 haploid cells, which differ from the maternal cell . In addition, both mechanisms vary considerably in duration, and this is due not only to the difference in the number of stages of division, but also to the duration of each stage. For example, the first meiosis prophase lasts much longer, because at this time, chromosome conjugation and crossing-over occurs. That is why it is further divided into several stages.

In general, the similarities between mitosis and meiosis are rather insignificant in comparison with their differences from each other. It is almost impossible to confuse these processes. Therefore, now it is even somewhat surprising that the reduction division was previously considered a kind of mitosis.

Consequences of meiosis

As already mentioned, after the process of reduction division, instead of a mother cell with a diploid set of chromosomes, four haploid ones are formed. And if we talk about the differences between mitosis and meiosis - this is the most significant. The recovery of the required amount, when it comes to the sex cells, occurs after fertilization. Thus, with each new generation, the number of chromosomes does not double.

In addition, during meiosis, recombination of genes occurs. In the process of reproduction this leads to the maintenance of intraspecific diversity. So the fact that even siblings are very different from each other is the result of meiosis.

By the way, the sterility of some hybrids in the animal kingdom is also a problem of reduction division. The point is that the chromosomes of parents belonging to different species can not enter into conjugation, which means that the formation of viable sex cells is impossible. Thus, it is meiosis that underlies the evolutionary development of animals, plants and other organisms.