It is known about living organisms that they breathe, feed, multiply and die, this is their biological function. But due to what does all this happen? Due to the bricks - cells that also breathe, feed, die and multiply. But how is this going?
About the structure of cells
The house consists of bricks, blocks or logs. So the body can be divided into elementary units - cells. The whole 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 - they differ so much in their purpose. But regardless of what functions this or that cell performs, they are all arranged approximately the same. 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 consist of eukaryotic, having 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, Golgi complex, centrioles, lysosomes and motor elements are isolated. With the help of them, all the processes that ensure the functioning of the body occur.
Cell activity
As already mentioned, all living things feed on, breathe, multiply and die. This statement is true for whole organisms, that is, people, animals, plants, etc., and for cells. This is surprising, but each "brick" has its own life. Due to his organelles, he receives and processes nutrients, oxygen, removes all unnecessary to the outside. The cytoplasm itself and the endoplasmic reticulum perform a transport function, mitochondria are also responsible for respiration, as well as providing energy. The Golgi complex is engaged in the accumulation and withdrawal of cell waste products. The remaining organelles 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 should be considered in more detail.
Cell division process
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 become ready for reproduction. Prokaryotic cells simply divide in two, lengthening, and then forming a septum. This process is simple and almost completely studied by the example of rod-shaped bacteria.
With eukaryotic cells, things are somewhat more complicated. They multiply in three different ways, which are called amitosis, mitosis and meiosis. Each of these paths has its own characteristics, it is inherent in a certain type of cell. Amitosis
considered the simplest, it is also called direct binary division. With it, a doubling of the DNA molecule occurs. However, a spindle of division is not formed, so this method is the most energy-efficient. Amitosis is observed in unicellular organisms, while multicellular tissues multiply by other mechanisms. However, it is sometimes observed where mitotic activity is reduced, for example, in mature tissues.
Direct division is sometimes distinguished as a form of mitosis, but some scientists consider this a separate mechanism. The course of this process, even in old cells, is quite rare. Next, meiosis and its phases, the process of mitosis, as well as the similarities and differences of these methods will be considered. Compared to simple division, they are more complex and perfect. This is especially true of reduction division, so that the characteristic of the phases of meiosis will be the most detailed.
An important role in cell division is played by centrioles - special organoids, usually located next to 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 later.
Mitosis
The lifespan of cells varies. Some live a couple of days, and some can be attributed to centenarians, since their complete change is very rare. And almost all of these cells multiply through mitosis. In most of them, between the periods of division, an average of 10-24 hours pass. Mitosis itself takes a short period of time - in animals about 0.5-1
hour, and in plants about 2-3. This mechanism ensures the growth of the cell population and the reproduction of units identical in their genetic content. Thus, the continuity of generations is observed at the elementary level. The number of chromosomes remains unchanged. This mechanism is the most common variant of 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 development of the whole organism occurs. In addition, it is mitosis that underlies asexual reproduction. And another function is the movement of cells and the replacement of already obsolete. Therefore, to assume that due to the fact that the stage of meiosis is more complicated, its role is much higher, is wrong. Both of these processes perform different functions and are important and indispensable in their own way.
Mitosis consists of several phases that differ 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 another 5 stages, which must be considered in more detail.
Mitosis phases
Being in interphase, the cell prepares for division: DNA and proteins are synthesized. This stage is divided into several more, during which the growth of the entire structure and the doubling of chromosomes occur. In this state, the cell is up to 90% of the entire life cycle.
The remaining 10% is occupied directly by division, which is divided into 5 stages. With 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 proposed place for future division.
In all other cells, the mitosis process proceeds as follows:
Table 1
| Stage name | Characteristic |
| Prophase | The nucleus grows in size, the chromosomes in it spiral, become visible through a microscope. A spindle of division is formed in the cytoplasm. Often nucleolus decays, but this does not always happen. The content of genetic material in the cell remains unchanged. |
| Prometaphase | The decay of the nuclear membrane occurs. Chromosomes begin an active but erratic 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 equatorial plane of the fission spindle at approximately equal distance from both poles. The number of microtubules holding the entire structure in a stable state reaches a maximum. Sister chromatids repel each other, retaining the compound only in 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. Subsequently, the fission poles themselves diverge. In the cells of some protozoa, the division spindle increases in length up to 15 times. And this sub-step is called anaphase B. The duration and sequence of processes at this stage are variable. |
| Telophase | After the divergence to the opposite poles ends, the chromatids stop. The decondensation of chromosomes occurs, that is, their increase in size. The reconstruction of the nuclear membranes of future daughter cells begins. Microtubules of the fission spindle disappear. Nuclei are formed, RNA synthesis resumes. |
After completion of the division of genetic information, cytokinesis or cytotomy occurs. By this term is meant the formation of bodies of daughter cells from the body of the mother. In this case, the organelles, as a rule, are divided in half, although exceptions are possible, a septum forms. Cytokinesis is not isolated in a separate phase, as a rule, considering it in the framework of telophase.
So, chromosomes that carry genetic information are involved in the most interesting processes. What is it and why are they so important?
About chromosomes
Even without a clue about genetics, people knew that many of the qualities of the offspring were dependent on their 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 transmitted to future generations.
At the end of the 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 during the division period. Most often, the discovery is attributed to the German scientist V. Fleming, who not only streamlined everything that was 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 "chromosome" was proposed a little later by another scientist - the German histologist G. Valdeyer.
The structure of chromosomes at the moment when they are clearly visible is quite simple - they are two chromatids connected in the middle by a centromere. It is a specific nucleotide sequence and plays an important role in the process of cell reproduction. Ultimately, the chromosome externally in prophase and metaphase, when it can be best seen, resembles the letter X.
In 1900, Mendel’s laws were discovered , describing the principles of transmission of hereditary traits. Then it became completely clear that chromosomes are exactly what genetic information is transmitted through. Later, scientists conducted a series of experiments proving this. And then the effect that cell division has on them has also become a subject of study.
Meiosis
Unlike mitosis, this mechanism ultimately leads 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 first of all
we are talking about nuclear fission, and already in the second - the whole cell. The restoration of a complete set of chromosomes occurs as a result of further fusion of gametes. Due to the decrease in the number of chromosomes, this method is also defined as reduction cell division.
Meiosis and its phases were studied by such famous scientists as 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 complicated. Initially, this process was considered a variant of mitosis, but almost immediately after the discovery, it was nevertheless isolated as a separate mechanism. The characterization of meiosis and its theoretical significance were first sufficiently described by August Weisman back in 1887. Since then, the study of the process of reduction fission has advanced greatly, but the conclusions reached have not yet been refuted.
Meiosis should not be confused with gametogenesis, although both of these processes are closely related. Both mechanisms are involved in the formation of germ cells, however, there are a number of serious differences between them. Meiosis occurs in two stages of division, each of which consists of 4 main phases, there is a short break between them. The duration of the whole process depends on the amount of DNA in the nucleus and the structure of the chromosome organization. In general, it is much longer in comparison with mitosis.
By the way, one of the main reasons for the significant species diversity is meiosis. As a result of reduction division, the set of chromosomes is split in two, so that new combinations of genes appear, first of all, potentially increasing the adaptability and adaptability of organisms, as a result of which they receive certain sets of characters and qualities.
Meiosis phases
As already mentioned, reduction cell division is conditionally divided into two stages. Each of these stages is further divided by 4. And the first phase of meiosis - prophase I, in turn, is divided into 5 separate stages. As the study of this process continues, others can be highlighted in the future. Now the following phases of meiosis are distinguished:
table 2
| Stage name | Characteristic |
| The first division (reduction) |
Prophase I | |
| leptotene | In another way, this stage is called the stage of thin threads. Chromosomes look like a tangled ball under a microscope. Sometimes, propleptotene is secreted when it is still difficult to discern individual strings. |
| zygotene | Stage of merging threads. Homologous, that is, similar in morphology and in genetic terms, pairs of chromosomes merge. In the process of fusion, that is, conjugation, bivalents, or tetrads, are formed. So called fairly stable complexes of pairs of chromosomes. |
| paquitena | Stage of thick threads. At this stage, the chromosomes spiral and DNA replication is completed, chiasms are formed - the contact points of the individual parts of the chromosomes - chromatids. A crossover process is taking place. Chromosomes intersect and exchange some areas of genetic information. |
| diplotena | Also called double strand stage. Homologous chromosomes in bivalents repel each other and remain connected only in chiasm. |
| diakinesis | At this stage, bivalents diverge on the periphery of the nucleus. |
| Metaphase I | The core shell is destroyed, a spindle of division is formed. Bivalents move to the center of the cell and line up along the equatorial plane. |
| Anaphase I | Bivalents break up, after which each chromosome from a pair moves to the nearest pole of the cell. Chromatid separation does not occur. |
| Telophase I | The process of chromosome divergence is nearing completion. The formation of individual nuclei of daughter cells occurs, each with a haploid set. Chromosomes despiralize, a nuclear membrane forms. Sometimes there is cytokinesis, that is, division of the cell body itself. |
| The second division (equational) |
| Prophase II | Condensation of chromosomes occurs, the cell center divides. The nuclear shell is destroyed. A spindle of division is formed, 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 to opposite poles. |
| Telophase II | The obtained monochromatic chromosomes are despiralized. A nuclear shell forms. |
So, it is obvious that the phases of meiosis division are much more complicated than the mitosis process. 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 also observed in some protozoa. However, as a rule, it includes only one division. It is assumed that such a single-stage form later developed into a modern, two-stage.
Differences and similarities of mitosis and meiosis
At first glance, it seems that the differences between the two processes are obvious, because these are completely different mechanisms. However, a deeper analysis reveals that the differences between mitosis and meiosis are not so global, in the end they lead to the formation of new cells.
First of all, it's worth talking 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 division, DNA replication occurs. Although, as for meiosis, before the start of prophase I, this process does not complete completely, ending at one of the first substages. And the sequence of phases, although similar, but, in fact, the events occurring in them do not completely coincide. 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 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 this is not always the case. In the second case, this process accounts for the anaphase of meiosis. 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 the organism and maintenance of intraspecific diversity. The number of cells resulting from mitosis is two, and in the genetic sense, they are identical to the mother and have a diploid set of chromosomes. During reduction division, everything is different. The result of meiosis is 4 haploid cells that differ from the mother.In addition, both mechanisms differ significantly 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 of the stages. For example, the first prophase of meiosis lasts much longer, because at this time conjugation of chromosomes and crossingover occurs. That is why it is further divided into several stages.
On the whole, the similarities between mitosis and meiosis are quite insignificant in comparison with their differences from each other. Confusing these processes is almost impossible. Therefore, it is now even somewhat surprising that reduction fission was previously considered a type of mitosis.
The effects of meiosis
As already mentioned, after the completion of the process of reduction division, instead of the mother cell with a diploid set of chromosomes four haploid cells are formed. And if we talk about the differences between mitosis and meiosis - this is the most significant. The restoration of the required amount, when it comes to germ 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 sometimes very different from each other is precisely the result of meiosis.
By the way, the sterility of some hybrids in the animal kingdom 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 process of formation of full-fledged viable germ cells is impossible. Thus, it is meiosis that underlies the evolutionary development of animals, plants, and other organisms.