Key elements of the Cell Cycle Apparatus

The activity of the cell cycle machinery is controlled by the following proteins in particular  

An oscillating system is formed by the interplay of the three protein classes and the activity of this system makes up the specific biochemical functions of the individual phases of the cycle. The activity of the cychn-dependent protein kinases (CDKs) is central to the oscillating system. These create a signal that initiates downstream biochemical processes and thus determines the individual phases of the cycle. CDK activity is also the starting point for intrinsic and external control mechanisms. 

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