What is the significance of the cell cycle control system and checkpoints? Explain step by step in your own words. [please make it simple]

Can cell cycle checkpoints fail? If so what are some examples of the results the can occur if they fail?

You are examining a cell to determine the stage of the cell cycle it is currently in. What stage are you most likely to see it in (what stage do cells spend most of their time in)? What stage are you least likely to see it in (what stage do cells spend the least of their time in)?

How does p53 induce apoptosis?  How does the cell determine what "too much" damage is? Meaning, how can it tell if the damage to the DNA is too severe to fix in order for the cell to move on to S phase? Thank you.