Each cell begins its life whenit is separated from the maternal one, and ends its existence, giving its daughter cells the opportunity to appear. Nature provides more than one way of dividing their core, depending on their structure.
Cell division methods
Nuclear division depends on the type of cell:
- Binary division (found in prokaryotes).
- Amitoz (direct method of division).
- Mitosis (occurs in eukaryotes).
- Meiosis (intended for division of germ cells).
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 by itself.
Binary division
Most often this type is found inprokaryotic cells. It consists in doubling the circular DNA molecule. Binary division of the nucleus is called so because of the mother cell appear two identical in size daughter.
After the genetic material (DNA moleculeor RNA) is prepared accordingly, i.e., 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 division process is called budding,or uneven binary division. In this case, a protrusion appears on the section of the cell wall, which gradually grows. After the size of the "kidney" and the maternal cell become equal, they will separate. And the section of the cell wall is synthesized again.
Amitoz
This division of the nucleus is similar to that described above, withthe difference is that there is no doubling of the genetic material. This method was first described by biologist Remak. This phenomenon is found in pathologically altered cells (tumor degeneration), and is also the physiological norm for liver tissue, cartilage and cornea.
The process of nuclear fission is called amitosis, becausethat the cell retains its functions, and does not lose them, as during mitosis. This explains the pathological properties of cells with this method of division. In addition, direct division of the nucleus takes place without a spindle of division; therefore, chromatin in the daughter cells is unevenly distributed. Subsequently, these cells can not use the mitotic cycle. Sometimes as a result of amitosis, multinucleated cells are formed.
Mitosis
This is an indirect division of the nucleus.Most often found in eukaryotic cells. The main difference between this process is that the daughter cells and the mother contain the same number of chromosomes. Due to this, the body maintains the required number of cells, as well as the possible regeneration and growth processes. The first mitosis in the animal cell was described by Fleming.
The process of nuclear fission in this case is divided into an interphase and directly mitosis. The interphase is the state of rest of the cell in the interval between divisions. It can be divided into several phases:
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. The postsynthetic period - the cellular elements are doubled, the proteins that make up the division spindle appear.
Phases of mitosis
The division of the nucleus of a eukaryotic cell isa process that requires the formation of an additional organelle, the centrosome. 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 the chromosomes between the daughter cells.
There are four phases of mitosis:
1. Prophase: chromatin in the nucleus is condensed into chromatids, which are assembled near the centromere, forming chromosomes in pairs. The nucleoli disintegrate, the centrioles diverge towards the poles of the cell. Formed spindle division.
2. Metaphase: chromosomes are located in a line passing through the center of the cell, forming a metaphase plate.
3. Anaphase: chromatids from the center of the cell diverge to the poles,and then the centromere is split in two. Such a movement is possible thanks to the division spindle, whose threads shorten and stretch the chromosomes in different directions.
4. Body Phase: child kernels are formed. Chromatids are again transformed into chromatin, a nucleus is formed, and in it are nucleoli. Ends with the separation of the cytoplasm and the formation of the cell wall.
Endomitosis
An increase in genetic material that is notinvolves the division of the nucleus, called endomitosis. It is found in the cells of plants and animals. In this case, there is no destruction of the cytoplasm and the envelope of the nucleus, but chromatin turns into chromosomes, and then despiralized again.
This process allows you to get polyploidnuclei in which the increased DNA content. This occurs in the colony-forming cells of the red bone marrow. In addition, there are cases when the DNA molecules are doubled, and the number of chromosomes remains the same. They are called polythene, and they can be found in insect cells.
Value mitosis
Mitotic division of the nucleus is a way to maintain a constant set of chromosomes. Daughter cells have the same set of genes as the mother, and all the characteristics inherent in it. Mitosis is necessary for:
- the growth and development of a multicellular organism (from the fusion of germ cells);
- transfer of cells from the lower layers to the upper ones, as well as replacement of blood cells (erythrocytes, leukocytes, platelets);
- restoration of damaged tissues (in some animals, the ability to regenerate is a necessary condition for survival, for example, in starfish or lizards);
- asexual reproduction of plants and some animals (invertebrates).
Meiosis
The mechanism of division of the nuclei of germ cells is somewhatdifferent from somatic. As a result, it produces cells that have two times less genetic information than their predecessors. This is necessary in order to maintain a constant number of chromosomes in each cell of the body.
Meiosis occurs in two stages:
- reduction stage;
- Equation stage.
The correct course of this process is possible.only in cells with an even set of chromosomes (diploid, tetraploid, hexaproid, etc.). Of course, the possibility of passing meiosis in cells with an odd set of chromosomes remains, but then the offspring may be unviable.
It is this mechanism that ensures sterility ininter-specific marriages. Since there are different sets of chromosomes in the germ cells, this makes it difficult for them to merge and produce viable or fertile progeny.
First division of meiosis
The name of the phases follows those in mitosis: prophase, metaphase, anaphase, telophase. But there are a number of significant differences.
1. Prophase: a double set of chromosomes performs a series of transformations, going through five stages (leptothen, zygoten, pachiten, diplotene, diakinesis). All this happens due to conjugation and crossing over.
Conjugation - this is the convergence of homologous chromosomes. Thin filaments are formed between them in leptotene, then chromosomes in pairs in the zygotene are joined in pairs, resulting in structures of four chromatids.
Crossing over - process of cross exchange of chromatid sitesbetween sister or homologous chromosomes. This happens at the pahitena stage. Intersections (chiasmas) of chromosomes are formed. A person can have such exchanges from thirty five to sixty six. The result of this process is the genetic heterogeneity of the material obtained, or the variability of germ cells.
When the diplotena stage comes, the complexes of four chromatids are destroyed and sister chromosomes mutually repel each other. Diakinesis completes the transition from prophase to metaphase.
2. Metaphase: chromosomes line up near the equator of the cell.
3. Anaphase: chromosomes, still consisting of two chromatids, diverge towards the poles of the cell.
4. Telophase: the division spindle is destroyed, resulting in the formation of two cells with a haploid set of chromosomes, having a double amount of DNA.
Second division of meiosis
This process is also called “mitosis of meiosis”.At the moment between the two phases, DNA doubling does not occur, and the second prophase of the cell enters with the same set of chromosomes that it has left after telophase 1.
1. Prophase: chromosomes condense, undergo separationcell center (its remains diverge towards the poles of the cell), the nuclear envelope is destroyed and the division spindle is formed, which is perpendicular to the spindle from the first division.
2. Metaphase: chromosomes are located at the equator, a metaphase plate is formed.
3. Anaphase: chromosomes are divided into chromatids, which diverge in different directions.
4. Telophase: a nucleus forms in the daughter cells, chromatids are despiralized into chromatin.
At the end of the second phase from one mother cellwe have four children with half a set of chromosomes. If meiosis occurs in conjunction with gametogenesis (that is, the formation of germ cells), then the division takes place sharply, unevenly, and one cell with a haploid set of chromosomes and three reduction bodies are formed that do not carry the necessary genetic information. They are necessary so that only half of the genetic material of the parent cell is preserved in the egg cell and the spermatozoon. In addition, this form of nuclear division ensures the emergence of new combinations of genes, as well as the inheritance of pure alleles.
At the simplest there is a variant of meiosis, whenthere is only one division in the first phase, and in the second there is crossover. 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, about which scientists do not yet know.
