In the existence of a cell, the period of time from the day of its formation by dividing the mother cell to its own division or death is called the term "cell cycle". Different cells have different durations. For example, basal and hematopoietic cells of the small intestine and epidermis are able to enter the cell cycle every 12-16 hours; in adult organisms, they multiply quite quickly. Short life cycles of the cell last about 30 minutes, they occur during the rapid crushing of eggs in amphibians, echinoderms and other animals. A large number of cell culture species under experimental conditions has a short cell cycle lasting about 20 minutes. In the predominant number of actively dividing cells, the length of the interval between mitoses is from 10 to 24 hours.
Phases and periods of the cell cycle. The cell cycle of animals and plant organisms consists of two periods: interphase (period of protein and DNA synthesis) and mitosis (period of cell division). Interphase includes several periods:
1. G1-phase is a growth period during which the synthesis of proteins, RNA and other components of the cell occurs.
2. S-phase - at this interphase gap, the process of synthesis of a daughter DNA molecule of the cell nucleus and the doubling of intracellular organoids (centrioles) takes place;
3. The G2 phase is the period during which preparation for mitosis occurs.
In cells that no longer divide, the G1 phase may be absent, during this period they are in the resting phase (G0).
The process of cell division (mitosis) has two stages:
1. Cell division - karyokinesis.
2. Division of cell cytoplasm - cytokinesis.
Regulation of cell activity. A change in the periods of the cell cycle occurs during the interaction of proteins - cyclins and cyclin-dependent kinases. Cells in the G0 phase are able to enter the cycle under the influence of various growth factors. Epidermal, platelet factors and nerve growth factor, being in constant interaction with receptors, set in motion the intracellular signaling system, which leads to transcription of protein genes. Moreover, kinases can be active only when interacting with certain cyclins, the content of which is constantly changing throughout the cycle.
Violation of the normal regulation of the cell cycle leads to the formation of solid tumors. The p53 protein is responsible for the formation of malignant tumors: it provokes the synthesis of p21 protein, which, in turn, inhibits the CDK cyclin complex, which inevitably leads to the termination of the cell cycle in phase G1 and G2. A cell with damaged DNA does not enter phase S. During mutations leading to the loss or alteration of p53 protein genes, the cycle does not block and the cells enter mitosis, causing mutant cells, some of which die, and the other part lead to the formation of malignant tumors.
Cellular immunity. The reaction of the body’s immune system to any stimulus called the immune response, in which antibodies and the complex of complex proteins present in the blood (complement system) are not involved, is called the term “cellular immunity”. It is directed primarily against microorganisms surviving in phagocytes, and against microorganisms that infect other cells. It is especially effective in combating viruses, fungi, protozoa, bacteria and tumor cells. The cellular immunity system is very important in tissue rejection.
The shell of the cell. Rigid cell wall placed on the outside of the cytoplasmic membrane, which performs protective, structural and transport functions - this is the cell membrane. It is also called the cell wall; it is present in most bacteria, fungi, plants, and archaea. As for animals and many protozoa, they do not have a cell membrane.