Different cyclin–CDK complexes regulate each stage of the cell cycle.
In mammals, there are several different cyclins and CDKs that act at specific steps of the cell cycle. Three steps in particular are subject to cyclin–CDK regulation in all eukaryotes (Fig. 11.16).
FIG. 11.16 Three cyclin–CDK complexes. Each one is active at different times in the cell cycle.
The G1/S cyclin–CDK complex, which is active at the end of the G1 phase, is necessary for the cell to enter S phase. For example, this cyclin–CDK complex activates a protein that promotes the expression of histone proteins needed for packaging the newly replicated DNA strands.
The S cyclin–CDK complex is necessary for the cell to initiate DNA synthesis. It activates enzymes and other proteins necessary for DNA replication. Once replication has begun at a particular place on the DNA, S cyclin–CDK activity prevents the replication proteins from reassembling at the same place and re-replicating the same DNA sequence. Synthesizing the same sequence repeatedly would be dangerous because cells with too much DNA could die or become cancerous.
The M cyclin–CDK complex initiates multiple events associated with mitosis. For example, M cyclin–CDK phosphorylates structural proteins in the nucleus, triggering the breakdown of the nuclear envelope in prometaphase. M cyclin–CDK also phosphorylates proteins that regulate the assembly of tubulin into microtubules, promoting the formation of the mitotic spindle.