Final answer:
M-Cdk activity in cell division is contingent on the accumulation of cyclin, a regulatory protein that binds with Cdk to form a complex necessary for the cell to proceed through cell cycle checkpoints.
Step-by-step explanation:
In cell division, M-Cdk activity depends on cyclin accumulation. M-Cdk, or mitotic cyclin-dependent kinase, is a key regulator of the cell cycle, especially in triggering the cell's entry into mitosis. Cyclins are regulatory proteins whose concentrations fluctuate throughout the cell cycle, with their levels increasing due to both internal and external signals, peaking to enable progression through the cell cycle checkpoints, and then being degraded.
Cyclin binds to Cdk forming the cyclin/Cdk complex that must be fully activated by phosphorylation for the cell cycle to proceed. The levels of Cdk proteins remain relatively stable during the cell cycle, therefore cyclin levels are critical for the complex formation. Once the cyclin/Cdk complexes reach a certain threshold, they push the cell past the G2 checkpoint into mitosis. This action is essential for the proper progression and regulation of the cell cycle.