At how many main checkpoints can a eukaryotic cell halt its own cycle, and where do they sit?

1. A checkpoint is a point where progress can be stopped until conditions are right. 2. The first sits near the end of G1, before DNA replication begins. 3. The second sits at the transition from G2 into mitosis. 4. The third sits during metaphase, before the chromatids are separated. 5. That makes three internal checkpoints, not two or four. _Source: OpenStax Biology 1e (CC BY 4.0), Chs 10 and 11, section 10.3 Control of the Cell Cycle_