In addition, it remains possible that changes in the catalytic rate of an enzyme, such as a kinase or phosphatase, play an important role. It remains unclear whether changes in number or concentration of cell cycle regulators trigger cell cycle entry. Conversely, if the number of protein molecules stays constant as growth occurs the concentration of the molecule will decrease.

In this case, if the number of molecules per cell increases faster than growth, the concentration and activity of the protein should also increase. Alternatively, cell cycle entry could require changes in the concentration of cell cycle regulatory proteins. In this case, an increase in the number of protein molecules per cell could be sufficient to trigger cell cycle entry if the activity of the protein is titrated against an activity that does not scale with growth. If the number of molecules of a protein increases at the same rate as growth, the number of molecules per cell will increase but the concentration of the protein will stay constant. Most models suggest that growth-dependent changes in the number or concentration of key cell cycle regulatory proteins trigger cell cycle entry.

Transcription of the late G1 cyclins is initiated only when sufficient growth has occurred.Ī key question concerns how growth triggers cell cycle entry. The key molecular event that marks cell cycle entry is transcription of late G1 phase cyclins, which bind and activate cyclin-dependent kinases (CDKs) to initiate cell cycle events. A particularly important point at which cell growth controls cell cycle progression is at the end of G1 phase, when cells decide whether to commit to cell cycle entry. Thus, key cell cycle transitions occur only when sufficient growth has occurred. Cell cycle progression is subservient to cell growth.

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