Figure 2.

Comparing the circadian cycle to the cell cycle G1/S transition.

A critical point for both the cell cycle and the circadian cycle is the activation of key transcription factors. This is achieved in both cases by the relief of repression by a negative regulator. The mechanism for this is phosphorylation: of Rb by cyclin-dependent kinases for the cell cycle, and of PER by CK1δ/ε for the circadian cycle. Hence the de-repression of crucial activating transcription factors via kinase activity is a common network motif for the temporal regulation of cellular processes.