In eukaryotic cells, the timing of the cell cycle is regulated by cyclin-dependent kinases, or CDKs. Cyclin-dependent kinases trigger the transition between cell cycle phases by phosphorylating substrates within the cell.
Phosphorylation is the process by which the CDK modifies proteins within the eukaryotic cell. The modification of the proteins changes the way these proteins interact with one another. CDK acts as signal for certain protein pathways to be turned on or off. Different CDKs function at different phases of the cell cycle.
In addition to CDKs, other protein kinases such as polo-like kinases and aurora kinases help control when and where the phosphorylation process takes place within the cell.