Introduction of cell division
Cell division is the process by which a living cell proliferates from one cell to two cells. The cells before division are called mother cells, and the new cells formed after division are called daughter cells. Generally, it includes two steps: nuclear division and cytokinesis. During mitosis, the mother cells pass the genetic material to the daughter cells. In single-celled organisms, cell division is the reproduction of individuals. In multicellular organisms, cell division is the basis of individual growth, development, and reproduction. Little is known about the division of prokaryotic cells, and only a few specific understandings are about the division of a few bacteria. Prokaryotic cells have neither a nuclear membrane nor a nucleolus. Only a circular DNA molecule constitutes a nuclear region, also known as a pseudonuclei, which has a nuclear-like function. The nuclear molecule of the quasi-nucleus is either attached to the plasma membrane or connected to the plasma membrane formed by the intrusion of the plasma membrane. The plasma membrane is also called the inter-body. The replicated DNA was also copied into two. Later, the two interstitials gradually leave due to the growth of the plasma membrane therebetween, and the two DNA molecules connected to them have then pulled apart, and each DNA loop is connected to an inter-body. The cell membrane grows in the center between the two DNA loops that are pulled apart, forming a membrane that finally divides one cell into two cells. Eukaryotic cells can be divided into mitosis, meiosis, and amitosis according to the condition of nuclear division. Mitosis is the basic form of eukaryotic cell division. Meiosis is the process of dividing the chromosomes into germ cells.
Process of different kind of cell division and regulation of cell division
The following a brief introduction to the common method of division. Since the chromosomes are not regularly distributed, there is a problem that genetic material cannot be uniformly distributed. It is an abnormal way of division. Amitosis is the earliest method of cell division. In amitosis, the nucleolus and nuclear membrane do not disappear, no chromosomes appear, and no spindle is formed in the cytoplasm. Of course, the process of chromosome replication and even distribution into daughter cells is not seen. However, cells undergo amitosis, the chromosomes are also replicated, and the cells are enlarged. When the nucleus volume doubles, the cells divide. As for how the DNA of the genetic material in the nucleus is distributed into the daughter cells, further research is needed. Amitosis is the simplest way to divide. In the past, amitosis was thought to be mainly found in aging or diseased cells in lower organisms and higher organisms but was later found to be more prevalent in normal tissues of animals and plants. Amitosis has been observed in animal epithelial loose connective tissue, muscle tissue and liver tissue, and parenchyma epidermal growth point and endosperm cells in plant. The asexual division is also a common form of division, and this type of reproduction is common in single-celled organisms, but for different single-celled organisms, the way in which nuclear divides during reproduction is different and can be grouped into the following ways: amitosis, also known as direct division, is one of the simplest ways of cell division. The whole process of division does not undergo changes in spindle and chromosome. This type of division is most common in the division and reproduction of prokaryotes such as bacteria and cyanobacteria. The division of prokaryotic cells involves two aspects: the distribution of cellular DNA, enabling the divided daughter cells to obtain a complete set of genetic material from the parental cells; and cytokinesis dividing the cells into two equal parts. The two DNA molecules that are duplicated are connected to the plasma membrane. As the cells grow, the two DNA molecules are pulled apart. When the cells divide, the cell wall and the plasma membrane are pleated, and the mother cells are finally divided into two equal daughter cells. The process of mitosis is much more complicated than amitosis and is the main way of multicellular biological cell division. On the upper part, the nuclear membrane shrinks inward in the middle to form a concave groove. The cytoplasm in the trough appears like a spindle-shaped structure arranged in the same direction by the microtubules, regulates the nuclear membrane and chromosomes, separates into daughter nuclei, and finally splits into two parts. When the eye worm camp divides reproduction, the nucleus undergoes mitosis. During the division process, the nuclear membrane does not disappear, and the nucleus shrinks into two daughter nuclei in the middle of the nucleus. The original one is flagella, and the other grows a new flagellum, forming two eye worms. When the amoeba grows to a certain size, it undergoes division and reproduction. As the middle part of the nucleus contracts, the chromosome is distributed into the daughter nucleus, and then the cytoplasm is divided into two, dividing the cell into two progeny individuals. The most typical representative of asexual division and reproduction in the form of nuclear amitosis and mitosis is the paramecium, the protozoan ciliate of the genus Paramecium. There are two types of the nucleus in the cell, namely the large nucleus and the small nucleus. The small nucleus is the reproductive nucleus, and the large nucleus is the nutrient nucleus. When the paramecia are asexually propagated, the small nucleus undergoes nuclear mitosis, while the large nucleus is filamentless split, and then the paramecia are divided into two new individuals from the middle. Mitosis, also known as indirect division, is one of the most common ways of the division. Mitosis is a continuous division that is generally divided into nuclear division and cytokinesis. The process of mitosis includes nuclear fission (long time), and nuclear fission is a continuous process. For the convenience of narrative, the nuclear fission is artificially divided into four periods: pre-, mid-, post-, and end. The characteristics of each stage of mitosis are as follows (taking plant cells as an example). Interval: divided into G1, S, G2, mainly for DNA replication and related protein synthesis, nuclear membrane nucleoli gradually disappear. Early stage: the chromatin in the nucleus condenses into a chromosome, the disintegration of the nucleolus disappears completely, the nuclear membrane ruptures and the spindle begins to form. Mid-term: The medium-term is the period in which the chromosomes are arranged on the equatorial plate and the spindle is fully formed. Late stage: The late stage is the period in which the two chromatids of each chromosome are separated and moved from the equator to the two poles of the cell under the traction of the spindle. The final stage: the period of formation of the two sub-nucleus and cytoplasmic division. Chromosome decomposition, nuclear membrane appears, the equatorial plate position forms a cell plate, and a new cell wall will be formed in the future. The spindle yarn accumulated on the equatorial plate is called a film forming body. Animal cells are like plant cells, except that the animal cells have a central body that emits a star-ray to form a spindle, and the plant cells directly emit a spindle from two stages. At the end of mitosis, the cell membrane of the animal cell is inwardly recessed to form two daughter cells. The plant cell forms a cell plate at the equatorial plate (virtual imagination) and divides the cell into two daughter cells. Cytoplasmic division (short time): In the late stage of nuclear fission, cytoplasmic division begins when the chromosome approaches the pole. Several short spindles are added to the continuous filament between the two daughter nuclei to form a barrel-like region densely packed with a spindle, referred to as a film former. The number of microtubules increases and there are vesicles (including polysaccharides) from the Golgi apparatus and the endoplasmic reticulum in the film-forming body, which gather, fuse, and release multinuclear substances along the direction of the microtubules to form a cell plate. From the middle to the periphery, until it is connected to the mother cell wall, it becomes the intercellular layer of the primary wall, and the new plasma membrane is formed by the capsule of the vesicle. After the formation of the new cell wall, the two newly formed nuclei and the cytoplasm surrounding them are separated into two daughter cells. Mitosis can divide each mother cell into two substantially identical daughter cells by cell division. The number, shape, and size of the daughter cells are the same. The genetic information contained in each chromatid is basically the same as that of the mother cells, so that the daughter cells are acquire approximately the same genetic information from the mother cell. The species maintain a relatively stable karyotype and genetic stability. Sexual reproduction requires the combination of amphoteric germ cells to form a zygote, which is then developed into a new individual. The number of chromosomes in germ cells is half of that in somatic cells. Since the number of chromosomes is reduced by half when forming germ cells, sperm or egg cells, the original cells must undergo meiosis.
Function of cell division
In addition to cell proliferation, cell division can also form specific cells. In the testis, many primordial germ cells, namely spermatogonia, are produced by mitosis. According to the characteristics of mitosis, it is known that the number of chromosomes of spermatogonia is the same as the number of chromosomes of somatic cells. In the spermatogonia phase, chromosome replication was performed. When the male animal is sexually mature, a part of the spermatogonia in the testis begins to undergo meiosis. After meiosis, sperm cells are formed, and the sperm cells are denatured to form male germ cells, sperm. Egg cells are formed in the ovary, and the process is basically the same as the sperm formation process, but there are also differences. The egg cell, which has a chromosome number that is also reduced by half compared to the egg cell. The cells are large in shape, spherical, and unable to swim; they contain many yolks and are rich in nutrients, ensuring the development of new individuals after fertilization.