Final answer:
Low M-cyclin protein levels are maintained during the G1 phase through their degradation by cytoplasmic enzymes, preventing premature activation of the cell cycle.
Step-by-step explanation:
In most mammalian cells, low M-cyclin protein levels are maintained during the G1 phase by the degradation of cyclins by cytoplasmic enzymes. This degradation ensures that cyclin levels do not rise inappropriately, which is crucial for the cell to properly regulate its progression through the cell cycle. Cyclin-dependent kinases (Cdks) also play a vital role in this process. When coupled with cyclins, they form activated complexes that can push the cell through the various checkpoints of the cell cycle. However, without the specific concentration of fully activated cyclin/Cdk complexes, the cell cycle cannot proceed. Thus, maintaining low levels of M-cyclin during G1 phase serves to prevent premature activation of the cell cycle progression.