Cell Cycle

In 1953, Howard and Pek pioneered the concept of cell cycle, which formed an important part of cytology. When the cell ends from the end of one division to the end of the next division, it undergoes the same stage of change (ie, G1→S→G2→M) and recursively. The growth and division cycle of the cell is called the cell cycle (cell cyc1e). . One cell cycle includes the mitosis phase (M) and the interphase (G1, S, G2). Although the time in each phase varies from cell to cell, the M phase is the shortest and the S phase is relatively long. Figure 1 shows that the cell cycle is a set of ordered events that ultimately lead to cell growth and division. The eukaryotic cell cycle can be divided into interval and cell division, and the nutrients and replicating DNA required for mitosis are accumulated during the interval, which is called the mitosis (M) stage. Through the molecular mechanism of cell research, the division phase is divided into three phases, G1, S and G2. The cell cycle therefore consists of four phases: G1, S, G2 and M. Cells are released from mitosis into the G1 phase, during which RNA and protein are synthesized, but there is no DNA replication. The initiation of DNA replication marks the transition from the G1 phase to the S phase. The S phase extends until all DNA is copied. In the S phase, the entire composition of DNA is copied from diploid 2n to 4n. From the end of S period until mitosis is called G2 phase, during which cells have two complete diploid chromosomes. The increase in nuclear volume is mainly in the S phase, at which time protein accumulation corresponds to DNA replication. The chromatin remained concentrated and there was no visible change in morphology. The cell cycle regulatory protein binds to the protein encoding the cell differentiation cycle and activates the corresponding protein kinase, thereby promoting cell division. Less than 11 different cyclins are found, which are A, B1, B2, C, D1, D2, D3, E. , F, G, and H. Eight of the major cyclins have been isolated. According to the different phase of cell cycle regulation by cyclin, it can be divided into two major categories: G1 phase and M phase. All types of cyclins contain a conserved sequence of about 100 amino acids, called the cyclin box, which mediates the binding of cyclins to CDK. The animal cell cycle is controlled by many cdk-cyclin complexes. The activated CDK1 phosphorylates the target protein to produce corresponding physiological effects. For example, phosphorylation of laminin leads to disintegration of the nuclear fibril, disappearance of the nuclear membrane, and H1 phosphoric acid. It leads to condensation of chromosomes and so on. The end result of these effects is the constant operation of the cell cycle. The time at which various forms of Cdc2 and cdk cyclins are activated indicates a model in which the function of the cdk2-G1 cyclin dimer is regulated by the G1 and S phases, whereas the Cdcc2-cyclin A, B regulates the mitosis process (Orlando DA, 2008). Many in vivo factors have been found to stimulate or inhibit cell proliferation, such as multiple hormones, serum factors, polyamines, proteolytic enzymes, neuraminidase, cAMP, cGMP, and diglycerides (DG), inositol triphosphate (IP3). And Ca messenger systems and more. Increased intracellular cAMP concentration has an inhibitory effect on cell proliferation. Any factor that can increase intracellular cAMP can inhibit cell proliferation and decrease cell growth rate. Conversely, any factor that can decrease intracellular cAMP content can promote DNA. Synthesis and proliferation of cells. The cAMP content in each phase of the cell cycle is also different (see table). In the Chinese hamster ovary cell line, the M-phase cAMP content was the lowest, and the M-class cAMP level was increased three-fold. From the early G1 to the late G1, the cAMP level was reduced to a medium level until the S stage remained low. There are also many experiments that indicate that cGMP also regulates cell proliferation. For example, when cGMP or dibutyryl cGMP is added to 3T3 cells resting in G1 phase, it can induce an increase in DNA content and promote cell division. Such as increased cell cGMP levels, can promote cell mitosis, in turn, drugs that promote mitosis can also increase the concentration of cGMP. cAMP can inhibit cell division and promote cell differentiation. cGMP can inhibit cell differentiation and promote cell proliferation. In normal growth cells, cAMP and cGMP are maintained at appropriate levels to regulate cell cycle function (Browne G, 2010). ).