Cell cycle, immunity, cell membrane

In the existence of a cell the period of time from the day of its formation by dividing the maternal cell to its own division or death is called the term "cell cycle". Different cells have different duration. 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 rapidly enough. Short life cycles of the cell last about 30 minutes, they occur during the rapid fragmentation of eggs in amphibians, echinoderms and other animals. A large number of species of cell cultures under experimental conditions has a short cell cycle lasting about 20 minutes. In the prevailing number of actively dividing cells, the duration 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 (the period of synthesis of proteins and DNA) and mitosis (the period of cell division). Interphase includes several periods:

1. G1-phase is a period of growth during which the synthesis of proteins, RNA and other constituents of the cell occurs.

2. S-phase - during this interphase interval, the synthesis of the daughter molecule of the DNA of the cell nucleus takes place and the doubling of intracellular organelles (centrioles) occurs;

3. 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 a rest phase (G0).

The process of cell division (mitosis) has two stages:

1. Cell division of the cell - karyokinesis.

2. Division of the cell cytoplasm - cytokinesis.

Regulation of cell activity. The change in the periods of the cell cycle occurs when proteins - cyclins and cyclin-dependent kinases interact. Cells in phase G0 are able to enter the cycle under the influence of various growth factors. Epidermal, platelet and nerve growth factors, in constant interaction with the receptors, drive the intracellular signaling system, which leads to transcription of the protein genes. In this case, kinases can be active only when interacting with certain cyclins whose content is constantly changing throughout the cycle.

Violation of the normal regulation of the cell cycle leads to the formation of solid tumors. The formation of malignant tumors is the responsibility of the p53 protein: it provokes the synthesis of the p21 protein, which in turn inhibits the CDK cyclin complex, which inevitably leads to the cessation of the cell cycle in the G1 and G2 phases. A cell with damaged DNA does not enter the S phase. During the occurrence of mutations leading to the loss or alteration of the genes of the p53 protein, there is no blockage of the cycle and the cells enter into mitosis, generating mutant cells, some of which die, and the other leads 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 a complex of complex proteins present in the blood (complement system) do not participate, is called the term "cellular immunity". It is directed primarily against microorganisms that survive in phagocytes, and against microorganisms that attack other cells. Especially it is effective in combating viruses, fungi, protozoa, bacteria and tumor cells. The system of cellular immunity is very important in tissue rejection.

Cell sheath. Rigid cell wall placed From the outside of the cytoplasmic membrane, performing protective, structural and transport functions is the cell membrane. It is also called a 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.