Describe kinases and cyclins. How do they interact to cause cells to move through the cell cycle?

Kinases are enzymes that catalyze the transfer of a phosphate group from ATP to a specific protein substrate. The most relevant kinases in the cell cycle are the cyclin-dependent kinases (CDKs), which are activated by the binding of cyclins, a family of proteins. Cyclins are synthesized and degraded in a cyclic manner, hence their name, and their levels rise and fall during the cell cycle.

CDKs are key regulators of the cell cycle. There are different CDKs that act at different stages of the cell cycle, and their activation and inactivation regulate the orderly progression through the different phases of the cycle. CDKs are only activated when bound to a cyclin partner, which determines their substrate specificity.

Cyclins regulate the progression through the cell cycle by associating with CDKs, forming cyclin-CDK complexes. These complexes are responsible for phosphorylating target proteins involved in cell cycle regulation. There are different types of cyclins, with specific roles at distinct phases of the cell cycle. Cyclins are synthesized in response to external cues and are quickly degraded to prevent the accumulation of active cyclin-CDK complexes.

Cyclin-CDK complexes act by phosphorylating various substrates, regulating the cell's metabolic pathways and allowing a controlled progression through the cell cycle. The pairs of cyclins and CDKs that form these complexes are specific to different stages of the cell cycle. For instance, cyclin D-CDK4 and cyclin E-CDK2 complexes are essential for progression from G1 to S phase, whereas cyclin A-CDK2 and cyclin B-CDK1 complexes are required for S phase and G2 to M phase transitions, respectively. To summarize, the interactions between cyclin and CDK molecules are crucial for moving cells through the cell cycle. The formation of specific cyclin-CDK complexes and their phosphorylation activities on target proteins regulate the different stages of the cell cycle and ensure the correct timing and progression of cellular division processes.