Question

what are checkpoints and why are they important in the cell cycle?

Answer 1

there are two checkpoints g1-s and g2-m
the importance of these checkpoints stems from:
1. the commitment to replicate is costly to the cells in terms of energy and nutrients. in order to complete it succesfully, a cell "must know" that it is possible. g1-s checkpoint is in charge of this (mainly).
2. fatihful dna replication is a must for successful proliferation. as this process is inherently imperfect (due to the fidelity of the replication enzymes) and since different insults (such as lack of nucleosides) or radiation, or whatever for that matter: it is imperative to verify that dna synthesis was completed fatihfully . g2-m mainly does that