Final answer:
The signals registered at the checkpoints come from the nucleus, where DNA is transcribed into RNA molecules that are used by ribosomes to synthesize proteins.
Step-by-step explanation:
The signals registered at the checkpoints come from the nucleus of the cell. In the nucleus, DNA is transcribed into RNA molecules, which carry the genetic information to the cytoplasm. These RNA molecules are then used by the ribosomes on the rough endoplasmic reticulum to synthesize proteins needed by the cell.