Final answer:
In cell division, cyclins are the regulatory subunits that ensure the cell cycle progresses, while the enzymes known as cyclin-dependent kinases (cdks) are the catalytic subunits that push the cell through various checkpoints.
Step-by-step explanation:
In cell division, cyclins are regulatory subunits while cyclin-dependent kinases (cdks) are catalytic subunits. Cell division is a tightly regulated process that ensures cells replicate accurately. Regulatory molecules known as cyclins function as positive regulators, and their concentration varies with the cell cycle. Cyclin-dependent kinases (cdks), on the other hand, are catalytic subunits that remain constant in concentration, but require the binding of cyclins to be activated; together they push the cell past critical checkpoints. When it comes to negative regulators, proteins such as retinoblastoma protein (Rb), p53, and p21 play crucial roles. These molecules can halt the cell cycle, particularly at the G1 checkpoint, where they ensure that the cell does not enter S phase with damaged DNA, preventing the proliferation of potentially cancerous cells.