Final answer:
Cyclin-CDK complexes are vital for cell cycle regulation, with cyclins binding and activating CDKs, which phosphorylate other proteins to control cycle progression. CDK levels remain stable while cyclin levels fluctuate, ensuring cell cycle checkpoints are regulated. Different complexes trigger specific transitions in the cell cycle.
Step-by-step explanation:
Cyclin-CDK Complexes and Cell Cycle Regulation
Cyclin-dependent kinases (CDKs) are crucial protein kinases in cell cycle regulation. To be fully active, a CDK must not only bind to a cyclin but also get phosphorylated at specific sites. This phosphorylation causes activation of the CDK/cyclin complex by altering protein shapes, allowing them to advance the cell to the next phase of the cell cycle. The concentration of CDK proteins remains relatively stable, while the levels of different cyclins fluctuate during the cell cycle, signaling CDK/cyclin complex formation at precise points.
The number of types of cyclin-CDKs present in vertebrates is not explicitly mentioned, but these complexes are involved in different cell cycle transitions. For instance, the Cyclin D-CDK4/6 complex is pivotal for the progression from G1 to S phase, while the Cyclin E-CDK2 complex prepares the cell for DNA replication. The Cyclin A-CDK2 complex is important for the S phase and the Cyclin B-CDK1 complex for the transition from G2 to mitosis.
Cyclins are produced in response to various signals and are degraded once they have served their purpose in a given cell cycle phase. This cycle of synthesis and degradation of cyclins helps to regulate the cell cycle progression strictly.