10.7: Positive Regulator Molecules
The cell cycle is positively regulated—promoting progress through the stages—via the interaction of two classes of proteins found in the cytoplasm, cyclins and cyclin-dependent kinases, or Cdks.
Upon binding together, Cdk is phosphorylated by another protein, and the complex is then ready to donate the phosphate to a target protein, which initiates the cell to advance to the next phase. For instance, during G1, when one type of cyclin—named D—is synthesized, the cell transitions into S phase, as another cyclin, E, peaks to promote DNA replication. E is then degraded by cytoplasmic enzymes, and Cyclin A concentrations increase throughout the S phase and remain high into G2 to promote entry into the M phase.
After A is degraded, concentrations of Cyclin B peak in M phase to promote the different stages of mitosis. When B levels drop, the cell exits mitosis and divides. Thus, levels of the four different cyclins vary in a predictable pattern and combine with consistent Cdks at specific points to achieve forward momentum.