Final answer:
Kinases drive the cell cycle, but they must be activated by attachment of a cyclin. Cyclins bind to cyclin-dependent kinases (CDK), and the Cdk/cyclin complexes must then be phosphorylated at specific locations for the regulation of the cell cycle.
Step-by-step explanation:
Kinases require the attachment of a cyclin for activation. Cyclins bind to cyclin-dependent kinases (CDK) and this binding is necessary for the activation of the kinases. Cyclins are proteins that regulate the cell cycle by becoming tightly bound to CDK.
Once activated by a cyclin, CDK enzymes can phosphorylate other target molecules, which in turn activates these molecules to progress the cell cycle through various checkpoints. The levels of CDK proteins remain relatively stable, whereas the concentration of cyclins fluctuates throughout the cell cycle, determining the timing for the formation of Cdk/cyclin complexes.
These complexes must also be phosphorylated at specific locations for full activation to assist in advancing the cell to the next phase. Therefore, the correct answer to what must attach to kinases for their activation is a cyclin.