Final answer:
Cyclin-cdk activity is regulated in cells through positive regulation. There are three major checkpoints in the cell cycle: the G1 checkpoint, the G2 checkpoint, and the M checkpoint. The triggers for these checkpoints can be external or internal signals, including growth factor proteins and fluctuations in cyclin concentration.
Step-by-step explanation:
In cells, cyclin-cdk activity is regulated through a mechanism called positive regulation. Two groups of proteins, cyclins and cyclin-dependent kinases (Cdks), play a key role in this regulation. Cyclins and Cdks bind together to form complexes that activate the cell cycle and progress through the various checkpoints.
The cell cycle checkpoints are crucial stages where the progression of the cell cycle is monitored and regulated. There are three major checkpoints: the G1 checkpoint, the G2 checkpoint, and the M checkpoint (metaphase checkpoint). Each checkpoint ensures that specific conditions are met before the cell can proceed to the next phase of the cell cycle.
The triggers for these checkpoints can vary. External signals, such as growth factor proteins, arriving at the cell's plasma membrane can trigger the cell to start dividing. Internal signals, such as fluctuations in the concentration of cyclins, play an important role in regulating the checkpoints. Cyclins are synthesized and degraded at specific stages of the cell cycle, and their levels fluctuate based on internal and external signals.