Each cell begins its life when it is separated from the mother, and ends its existence, allowing its daughter cells to appear. Nature provides for more than one way of dividing their nucleus, depending on their structure.
Cell division methods
Nuclear fission depends on the type of cell :
- Binary division (found in prokaryotes).
- Amitosis (direct method of division).
- Mitosis (found in eukaryotes).
- Meiosis (intended for the division of germ cells).
The types of nuclear fission are determined by nature and correspond to the structure of the cell and the function that it performs in a macroorganism or on its own.
Binary division
Most often, this type occurs in prokaryotic cells. It consists in doubling the ring DNA molecule. Binary fission of the nucleus is called so because two identical daughter-size cells appear from the mother cell.
After the genetic material (DNA or RNA molecule) is prepared appropriately, that is, doubled, a transverse septum begins to form from the cell wall, which gradually narrows and divides the cell cytoplasm into two approximately identical parts.
The second division process is called budding, or uneven binary division. In this case, a protrusion appears on the cell wall site, 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 nuclear division is similar to that described above, with the difference that there is no doubling 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 them, as during mitosis. This explains the pathological properties inherent in cells with this method of division. In addition, direct fission of the nucleus takes place without a fission spindle; therefore, chromatin in the daughter cells is unevenly distributed. Subsequently, such cells cannot use the mitotic cycle. Sometimes, as a result of amitosis, multinucleated cells are formed.
Mitosis
This is an indirect fission 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 necessary number of cells is maintained in the body, and regeneration and growth processes are possible. The first mitosis in an animal cell was described by Fleming.
The process of nuclear fission in this case is divided into interphase and directly mitosis. Interphase is a state of rest of the cell in the interval between divisions. It can distinguish several phases:
1. The presynthetic period - the cell grows, proteins and carbohydrates accumulate in it, ATP (adenosine triphosphate) is actively synthesized.
2. Synthetic period - genetic material is doubled.
3. The postsynthetic period - the cellular elements double, proteins appear, of which the spindle of division consists.
Mitosis phases
The division of the nucleus of a eukaryotic cell is a 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 chromosomes between daughter cells.
Four phases of mitosis are distinguished:
1. Prophase : the chromatin in the nucleus condenses into chromatids, which are collected near the centromere, forming chromosomes in pairs. The nuclei break up, centrioles diverge towards the poles of the cell. A spindle of division is formed.
2. Metaphase: chromosomes are arranged in a line passing through the center of the cell, forming a metaphase plate.
3. Anaphase: chromatids from the center of the cell diverge towards the poles, and then the centromere is divided in two. Such a movement is possible due to the division spindle, the filaments of which contract and stretch the chromosomes in different directions.
4. Telophase: daughter nuclei are formed. Chromatides are again converted to chromatin, a nucleus is formed, and nucleoli are formed in it. It ends with the separation of the cytoplasm and the formation of the cell wall.
Endomitosis
An increase in genetic material that does not involve nuclear fission is called endomitosis. It is found in the cells of plants and animals. In this case, there is no destruction of the cytoplasm and membrane of the nucleus, but chromatin turns into chromosomes, and then despiralizes again.
This process allows to obtain polyploid nuclei in which the DNA content is increased. This is found in colony-forming cells of the red bone marrow. In addition, there are cases when DNA molecules double, and the number of chromosomes remains the same. They are called polytene and can be found in insect cells.
The value of mitosis
Mitotic nuclear fission is a way to maintain a constant set of chromosomes. The daughter cells have the same set of genes as the mother, and all the characteristics inherent to it. Mitosis is necessary for:
- the growth and development of a multicellular organism (from the fusion of germ cells);
- the movement of cells from the lower layers to the upper, as well as the replacement of blood cells (red blood cells, white blood cells, platelets);
- restoration of damaged tissues (in some animals, the ability to regenerate is a prerequisite 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 somewhat different from somatic. As a result of it, cells are obtained that have half as much genetic information as 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;
- equational stage.
The correct course of this process is possible only in cells with an even set of chromosomes (diploid, tetraploid, hexaproid, etc.). Of course, there remains the possibility of passing 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 interspecific marriages. Since different sets of chromosomes are located in germ cells, this makes it difficult to merge and the emergence of viable or fertile offspring.
The first division of meiosis
The name of the 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, and diakinesis). All this happens thanks to conjugation and crossing over.
Conjugation is the convergence of homologous chromosomes. In the leptotene, thin filaments are formed between them, then in the zygotene the chromosomes are connected in pairs and as a result structures of four chromatids are obtained.
Crossover is a process of cross-exchange of chromatid regions between sister or homologous chromosomes. This occurs at the stage of pachytene. Crossroads (chiasms) 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 variation of germ cells.
When the diplotene stage begins, complexes of four chromatids are destroyed and sister chromosomes repel each other. Diakinesis completes the transition from prophase to metaphase.
2. Metaphase : chromosomes line up near the equator of a cell.
3. Anaphase : chromosomes still consisting of two chromatids diverge towards the poles of the cell.
4. Telophase : the fission spindle is destroyed, resulting in the formation of two cells with a haploid set of chromosomes that have double the amount of DNA.
The second division of meiosis
This process is also called “meiosis mitosis”. DNA does not double at the moment between the two phases, and the cell enters the second prophase with the same set of chromosomes that it left after telophase 1.
1. Prophase : chromosomes condense, undergoes separation of the cell center (its residues diverge to the poles of the cell), the shell of the nucleus is destroyed, and a fission spindle is formed 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 is formed in daughter cells, chromatids despiralize into chromatin.
At the end of the second phase, from one mother cell, we have four daughter cells with a half set of chromosomes. If meiosis occurs in conjunction with gametogenesis (i.e., the formation of germ cells), then the division takes place sharply, unevenly, and one cell is formed with a haploid set of chromosomes and three reduction bodies 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 and sperm. In addition, this form of nuclear division provides the emergence of new combinations of genes, as well as the inheritance of pure alleles.
In protozoa, there is a variant of meiosis, when only one division occurs in the first phase, and a crossingover is observed in the second. Scientists suggest that this form is the evolutionary precursor of ordinary meiosis of multicellular organisms. Perhaps there are other methods of nuclear fission, which scientists are not yet aware of.