Each cell begins its life when it separates from the parent cell, and ends its existence, allowing its daughter cells to appear. Nature provides more than one way to divide their nucleus, depending on their structure.
Methods of cell division
Nuclear division depends on cell type:
- Binary fission (found in prokaryotes).
- Amitosis (direct division).
- Mitosis (found in eukaryotes).
- Meiosis (designed for the division of germ cells).
Types of nuclear division are determined by nature and correspond to the structure of the cell and the function it performs in the macroorganism or by itself.
Binary fission
This type is most common in prokaryotic cells. It consists in doubling the circular DNA molecule. Binary fission of the nucleus is called so because two identical-sized daughter cells appear from the mother cell.
After the genetic material (DNA or RNA molecule) is prepared in an appropriate way, that is, doubled, from the cell wall beginsa transverse septum is formed, which gradually narrows and divides the cytoplasm of the cell into two approximately identical parts.
The second fission process is called budding, or uneven binary fission. In this case, a protrusion appears on the site of the cell wall, which gradually grows. After the size of the "kidney" and the mother cell are equal, they will separate. And a section of the cell wall is synthesized again.
Amitosis
This nuclear division is similar to the one described above, with the difference that there is no duplication of 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 nuclear division takes place without a fission spindle, so the chromatin in the daughter cells is unevenly distributed. Subsequently, such cells cannot use the mitotic cycle. Sometimes, amitosis results in the formation of multinucleated cells.
Mitosis
This is an indirect nuclear fission. It is most commonly found in eukaryotic cells. The main difference between this process is that the daughter cells and the mother cell contain the same number of chromosomes. Therebythe required number of cells is maintained in the body, and the processes of regeneration and growth are also possible. Flemming was the first to describe mitosis in an animal cell.
The process of nuclear division in this case is divided into interphase and directly mitosis. Interphase is the resting state of the cell 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 - Genetic material is doubled.
3. Post-synthetic period - cellular elements double, proteins appear that make up the division spindle.
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 division spindle. This structure helps distribute chromosomes evenly between daughter cells.
There are four phases of mitosis:
1. Prophase: Chromatin in the nucleus condenses into chromatids, which gather near the centromere to form chromosomes in pairs. The nucleoli disintegrate and the centrioles move to the poles of the cell. A fission spindle is formed.
2. Metaphase: Chromosomes line up in a line through the center of the cell, forming the metaphase plate.
3. Anaphase: Chromatids move from the center of the cell to the poles, and then the centromere splits in two. Suchmovement is possible due to the division spindle, the threads of which contract and stretch the chromosomes in different directions.
4. Telophase: Daughter nuclei are formed. Chromatids turn into chromatin again, the nucleus is formed, and in it - the nucleoli. It all ends with the division of the cytoplasm and the formation of a cell wall.
Endomitosis
The increase in genetic material that does not involve nuclear division is called endomitosis. It is found in plant and animal cells. In this case, there is no destruction of the cytoplasm and the shell of the nucleus, but the chromatin turns into chromosomes, and then despiralizes again.
This process produces polyploid nuclei with increased DNA content. Similar occurs in colony-forming cells of the red bone marrow. In addition, there are cases when DNA molecules double in size, while the number of chromosomes remains the same. They are called polytene and can be found in insect cells.
Meaning of mitosis
Mitotic nuclear division 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 required for:
- growth and development of a multicellular organism (from the fusion of germ cells);
- moving cells from the lower layers to the upper ones, as well as replacing blood cells (erythrocytes, leukocytes, platelets);
- restoration of damaged tissues (in some animals, the ability to regenerate isa necessary condition for survival, such as starfish or lizards);
- asexual reproduction of plants and some animals (invertebrates).
Meiosis
The mechanism of nuclear division of germ cells is somewhat different from somatic. As a result, 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, it remains possible to undergo 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 interspecies marriages. Since the sex cells contain different sets of chromosomes, this makes it difficult for them to merge and produce viable or fertile offspring.
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 double set of chromosomes performs a series of transformations, passing through five stages (leptotene, zygotene, pachytene, diplotene, diakinesis). All this happens thanks to conjugation and crossing over.
Conjugation is the bringing together of homologous chromosomes. In leptoten between them are formedthin threads, then in the zygoten, the chromosomes are connected in pairs and as a result, structures of four chromatids are obtained.
Crossing-over is the process of cross-exchange of chromatid segments between sister or homologous chromosomes. This occurs at the stage of pachytene. Crossings (chiasmata) of chromosomes are formed. A person can have from thirty-five to sixty-six such exchanges. The result of this process is the genetic heterogeneity of the resulting material, or the variability of germ cells.
When the diplotene stage comes, the complexes of four chromatids break down and the sister chromosomes repel each other. Diakinesis completes the transition from prophase to metaphase.
2. Metaphase: Chromosomes line up near the cell's equator.
3. Anaphase: Chromosomes, still consisting of two chromatids, move apart towards the poles of the cell.
4. Telophase: The spindle breaks down, resulting in two haploid cells with twice the amount of DNA.
Second division of meiosis
This process is also called "mitosis of meiosis". At the moment between two phases, DNA duplication does not occur, and the cell enters the second prophase with the same set of chromosomes that it had left after telophase 1.
1. Prophase: chromosomes condense, the cell center separates (its remnants diverge towards the poles of the cell), the nuclear envelope is destroyed and a division spindle is formed, located perpendicular to the spindle from the first division.
2. Metaphase: chromosomes are located at the equator, formedmetaphase plate.
3. Anaphase: Chromosomes divide into chromatids, which move apart.
4. Telophase: a nucleus is formed in daughter cells, chromatids despiralize into chromatin.
At the end of the second phase, from one parent cell, we have four daughter cells with a half set of chromosomes. If meiosis occurs in conjunction with gametogenesis (that is, the formation of germ cells), then the division is abrupt, uneven, 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 ensures 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 in the second there is a crossing over. Scientists suggest that this form is an evolutionary precursor to normal meiosis in multicellular organisms. There may be other ways of nuclear fission that scientists don't yet know about.
