Which factor contributes to the regulation of cell cycle progression via checkpoints?

Cyclin-dependent kinases (Cdks) play a pivotal role in the regulation of cell cycle progression, particularly at key checkpoints. These kinases are activated when they bind to specific regulatory proteins known as cyclins. The formation of this cyclin-Cdk complex is essential for the phosphorylation of target proteins that promote the progression of the cell cycle.

During the various checkpoints (G1, G2, and the M phase), Cdks assess whether the cell is ready to proceed to the next stage. For instance, at the G1 checkpoint, the activity of specific cyclin-Cdk complexes ensures that the conditions are favorable for DNA synthesis; this includes evaluating DNA integrity and the availability of nutrients and growth factors. If issues are found, such as DNA damage or insufficient resources, the Cdk activity may be inhibited, preventing the cell from continuing to replicate, thus safeguarding genomic integrity.

Other factors such as protein synthesis rates, cell size, and external nutrient availability can influence cell cycle progression indirectly but are not the primary regulators at the checkpoints. While they may affect whether a cell is prepared to progress, it is the cyclin-Cdk complexes that directly enforce the checkpoints and facilitate the necessary biochemical events that transition a cell through the cell cycle phases.