Final answer:
Cyclins are tagged for degradation with ubiquitin, not phosphate groups or GTP molecules. This process allows cells to regulate the cell cycle and ensure proper cell division.
Step-by-step explanation:
The cyclin is tagged for breakdown by the addition of ubiquitin molecules, which target the protein for degradation by the proteasome. This process is not directly related to ATP or its breakdown, and therefore the options 'phosphate group' and 'GTP molecule' mentioned in the question are not applicable.
Cyclins regulate the cell cycle by interacting with cyclin-dependent kinases (Cdks). The ubiquitin-proteasome pathway plays a crucial role in controlling the cell cycle by degrading cyclins at the appropriate time, which ensures the cell cycle progresses orderly. Degradation of cyclins is vital for the cell to pass certain checkpoints and to regulate the activity of Cdks, ultimately ensuring proper cell division and function.