Cell Biology

                                                               MPF regulation

                       Cyclin-CDK (cyclin-dependent protein kinase) complexes play a central role in cell cycle progression . The function of cyclin-CDKs is to run the cell cycle smoothly, and these are therefore called “cell cycle engines.” Cyclins are proteins that vary in quantity throughout the cell cycle. The cyclins are expressed between the G1 and S phases, during the S phase, and between the G2 and M phases, known as the G1/S-phase cyclin, the S-phase cyclin, and the G2/M-phase cyclin, respectively. Each cyclin is rapidly synthesized during a specific phase of the cell cycle and is again promptly broken down after it serves its purpose. They are broken down not only because they are no longer needed, but because the breakdown is required for the cell cycle to transition to the next step. Meanwhile, the CDKs need to be synthesized de novo when cell proliferation starts from the G0 phase. Once the cell cycle starts, some types of CDK are broken down during a specific phase of the cell cycle, whereas others are not. When the cyclins and CDKs that are expressed in a specific phase are bonded and activated, they phosphorylate the specific serine and threonine residues of a target protein. The phosphorylated target protein executes the events occurring in the respective phases of the cell cycle.