How is the division of the nucleus? Types of Kernel Division

Each cell begins its life, when it separates from the maternal, and ends existence, giving the opportunity to appear to its daughter cells. Nature provides more than one way of dividing their core, depending on their structure.

Methods of cell division

The division of the nucleus depends on the type of cell :

- Binary division (occurs in prokaryotes).

- Amitosis (direct division method).

- Mitosis (found in eukaryotes).

- Meiosis (designed to divide sex cells).

The types of nuclear fission are determined by nature and correspond to the structure of the cell and the function it performs in the macroorganism or in itself.

Binary division

Most often this type occurs in prokaryotic cells. It consists in doubling the ring molecule of DNA. Binary division of the nucleus is called so because from the parent cell appear two identical in size daughter.

Once the genetic material (DNA or RNA molecule) is properly prepared, that is doubled, a transverse septum begins to form from the cell wall, which gradually narrows and divides the cytoplasm of the cell into two approximately equal parts.

The second process of division is called budding, or uneven binary division. In this case, a protrusion appears on the cell wall segment, which gradually grows. After the size of the "kidney" and the mother cell are equal, they will separate. A section of the cell wall is synthesized again.

Amitosis

This division of the nucleus is similar to that described above, with the difference that there is no duplication of the genetic material. This method was first described by the biologist Remak. This phenomenon occurs in pathologically altered cells (tumor degeneration), and is also a physiological norm for liver tissue, cartilage and cornea.

The process of nuclear division is called amitosis, because the cell retains its functions, and does not lose it, as during mitosis. This explains the pathological properties inherent in cells with this division method. In addition, the direct division of the nucleus passes without a fission spindle, so chromatin in the daughter cells is distributed unevenly. Subsequently, such cells can not use the mitotic cycle. Sometimes as a result of amitosis, multinucleate cells are formed.

Mitosis

This is an indirect division of the nucleus. It is most often found in eukaryotic cells. The main difference between this process is that the daughter cells and the maternal cell contain the same number of chromosomes. Thanks to this, the body maintains the required number of cells, and regeneration and growth processes are possible. The first mitosis in the animal cell was described by Flemming.

The process of nuclear division in this case is divided into an interphase and directly mitosis. Interphase is the state of cell rest in the interval between the divisions. There are several phases in it:

1. Presynthetic period - the cell grows, proteins and carbohydrates accumulate in it, ATP (adenosine triphosphate) is actively synthesized.

2. Synthetic period - the genetic material is doubled.

3. Post-synthetical period - cell elements are doubled, proteins appear, of which the fission spindle is composed.

Phases of mitosis

Division of the nucleus of a eukaryotic cell is a process for which the formation of an additional organelle - centrosome - is necessary. It is located next to the nucleus, and its main function is the formation of a new organelle - the spindle of division. This structure helps to evenly distribute chromosomes between daughter cells.

There are four phases of mitosis:

1. Prophase : chromatin in the nucleus condenses into chromatids, which near the centromere collect, pairwise forming chromosomes. The nucleoli disintegrate, the centrioles diverge to the poles of the cell. The spindle of division is formed.

2. Metaphase: the chromosomes are arranged in a line passing through the center of the cell, forming a metaphase plate.

3. Anaphase: the chromatids from the center of the cell diverge to the poles, and then the centromere divides in two. This movement is possible due to the fission spindle, whose filaments contract and stretch the chromosomes in different directions.

4. The body phase: the daughter nuclei are formed. Chromatids again turn into chromatin, the nucleus is formed, and in it - nucleoli. It ends with the separation of the cytoplasm and the formation of the cell wall.

Endomitosis

The increase in genetic material, which does not provide for the fission of the nucleus, is called endomitosis. It is found in plant and animal cells. In this case, there is no destruction of the cytoplasm and shell of the nucleus, but the chromatin is transformed into chromosomes, and then again despiralizes.

This process makes it possible to obtain polyploid nuclei in which the DNA content is increased. Similar occurs in the colony-forming cells of the red bone marrow. In addition, there are cases when DNA molecules are doubled, and the number of chromosomes remains the same. They are called polythene, and they can be found in insect cells.

Meaning of mitosis

Mitotic division of the nucleus is a method of maintaining a constant set of chromosomes. The daughter cells have the same set of genes as the mother, and all the characteristics inherent in it. Mitosis is necessary for:

- growth and development of a multicellular organism (from the fusion of sex cells);

- Move the cells from the lower layers to the upper ones, and replace blood cells (red blood cells, leukocytes, platelets);

- restoration of damaged tissues (in some animals, the ability to regenerate is a prerequisite for survival, for example, in marine stars or lizards);

- asexual reproduction of plants and some animals (invertebrates).

Meiosis

The mechanism of fission of the nuclei of germ cells differs somewhat from the somatic. As a result, it produces cells that have half the genetic information than their predecessors. This is necessary in order to maintain a constant number of chromosomes in each cell of the body.

Meiosis takes place in two stages:

- reduction stage;

- an equational stage.

The correct course of this process is possible only in cells with an even set of chromosomes (diploid, tetraploid, hexaproic, etc.). Of course, there remains the possibility of passage of meiosis in cells with an odd set of chromosomes, but then the offspring may not be viable.

It is this mechanism that ensures sterility in inter-species marriages. Since in the sex cells there are different sets of chromosomes, this makes it difficult for them to merge and the appearance of a viable or fertile offspring.

The first division of meiosis

The name of phases repeats those in mitosis: prophase, metaphase, anaphase, telophase. But there are a number of significant differences.

1. Prophase : a doubled set of chromosomes undergoes a series of transformations, passing through five stages (leptotene, zygotene, pachytene, diplotene, diakinesis). All this is due to conjugation and crossing-over.

Conjugation is the convergence of homologous chromosomes. In the leptotene, thin filaments form between them, then in the zygotene the chromosomes are joined in pairs and as a result, structures of four chromatids are obtained.

Crossingover is the process of cross-sharing chromatid sites between sister or homologous chromosomes. This occurs at the pachytene stage. Chromosome intersections (chiasms) are formed. In a person such exchanges can be from thirty-five to sixty-six. The result of this process is the genetic heterogeneity of the resulting material, or the variability of the sex cells.

When the stage of diplotenes begins, the complexes of the four chromatids are destroyed and the sister chromosomes are interrelated. Diakinesis completes the transition from prophase to metaphase.

2. Metaphase : the chromosomes line up near the equator of the cell.

3. Anaphase : the chromosomes, still consisting of two chromatids, diverge to the poles of the cell.

4. The body phase : the fission spindle is destroyed, resulting in the formation of two cells with a haploid set of chromosomes, having a doubled amount of DNA.

Second division of meiosis

This process is called "mitosis of meiosis". At the time between the two phases, DNA doubling does not occur, and in the second prophase the cell enters with the same set of chromosomes that it has left after telophase.

1. Prophase : the chromosomes condense, the cell center separates (its residues diverge to the poles of the cell), the core shell breaks down and a fission spindle located perpendicular to the spindle from the first division is formed.

2. Metaphase : the chromosomes are located on the equator, a metaphase plate is formed.

3. Anaphase : the chromosomes are divided into chromatids, which diverge in different directions.

4. The body phase : in the daughter cells a nucleus is formed, the chromatids are despiralized into chromatin.

At the end of the second phase, from one parent cell, we have four children with a half-set of chromosomes. If meiosis occurs in conjunction with gametogenesis (i.e., formation of sex cells), then the division passes abruptly, unevenly, and one cell is formed with a haploid set of chromosomes and three reduction corpuscles that do not carry the necessary genetic information. They are necessary in order to keep only half the genetic material of the parent cell in the egg cell and sperm cell. In addition, this form of nuclear fission provides the appearance of new gene combinations, as well as the inheritance of pure alleles.

In the protozoans, there is a variant of meiosis, when only one division occurs in the first phase, and the second one is crossing-over. Scientists suggest that this form is an evolutionary precursor of the usual meiosis of multicellular organisms. Perhaps there are other ways of dividing the nucleus, which scientists do not yet know.