Final answer:
Cells not ready to proceed beyond the G1 checkpoint enter an alternate resting phase called Go phase, where they may remain temporarily or permanently.
Step-by-step explanation:
Cells may leave the cell division cycle before reaching the restriction point late in G1 and enter an alternate phase called Go phase. The G1 checkpoint plays a critical role in determining whether cells proceed to the S phase, where DNA synthesis occurs, or enter this resting phase. At the G1 checkpoint, cells are assessed for size, nutrient status, growth factors, and DNA integrity. If conditions are not favorable, the cell may either attempt to fix the problems before proceeding or enter Go to wait for more suitable conditions. This phase can be temporary, as in the case of limited nutrients, or permanent for cells like nerve and muscle cells that have exited the cell cycle indefinitely.