Final answer:
Cyclins and cyclin-dependent kinases (Cdks) are the key molecules that control the on/off switch of cellular machinery by signaling the cell to move through checkpoints. They ensure the cell is appropriately prepared for each phase of the cell cycle and their levels fluctuate predictably with internal and external signals.
Step-by-step explanation:
The elements that switch cellular machinery on and off are primarily cyclins and cyclin-dependent kinases (Cdks). These molecules act as signals regulating the cell's progression through various checkpoints in the cell cycle. Cyclins bind to Cdks, forming a complex that can activate other proteins to advance the cell past checkpoints.
The concentration of different cyclins varies predictably throughout the cell cycle, influenced by internal and external stimuli. External signals such as growth factors can prompt a cell to divide, while internal signals ensure the cell is properly prepared for each phase of the cell cycle. For example, at the G1 checkpoint, the cell must be ready for DNA synthesis, whereas at the G2 checkpoint, it must be prepared for mitosis.
Cyclins and Cdks play a crucial role in ensuring that the cell only progresses when it is appropriate, with the metaphase checkpoint being particularly important for ensuring correct chromosome alignment before separation during anaphase. Errors in this regulation can lead to diseases such as cancer, characterized by uncontrolled cell division.
Therefore the correct Option is A. Cyclins.