Final answer:
In cell division, mitosis occurs when mitotic cyclin concentration is high enough to activate Cdk, which forms a complex essential for proceeding past cell cycle checkpoints.
Step-by-step explanation:
In cell division, mitosis is initiated when the mitotic cyclin concentration is high enough to activate Cdk (cyclin-dependent kinase). Cyclins and Cdks are crucial for the regulation of the cell cycle, as they control the transition through various checkpoints. Specifically, cyclins bind to Cdks, and together they must be phosphorylated to form an active complex that can phosphorylate other proteins, pushing the cell cycle past a checkpoint. It's the fluctuations in cyclin concentrations that determine when these complexes are formed, and a high concentration of mitotic cyclins is necessary to activate Cdks and allow the cell cycle to proceed.
Cdks' levels remain relatively constant, but the levels of cyclin proteins vary during the cell cycle, peaking at times when they are needed to move the cell through different stages. A high concentration of these cyclin/Cdk complexes is a key signal for the cell to progress through the checkpoints, specifically the G2/M transition that precedes mitosis.