Final answer:
A cell enters the G0 phase if it fails the checks at the G1 checkpoint, such as insufficient reserves, size, or DNA damage, and awaits further signals or permanently for non-dividing cells.
Step-by-step explanation:
Factors that determine whether a cell enters the G0 phase include a review at the G1 checkpoint to assess if all conditions are favorable for cell division to proceed. If a cell does not satisfy necessary criteria such as adequate reserves, appropriate size, and integrity of genomic DNA, it may not proceed to the S phase.
Cells in G0 are in a quiescent state and can either be there temporarily, awaiting further external signals like growth factors and nutrient availability, or permanently, as in the case of mature cardiac or nerve cells. Regulatory proteins like cyclins and cyclin-dependent kinases (Cdks), as well as tumor suppressor proteins, such as Rb and p53, also play crucial roles in determining a cell's progression through the cycle.