Final answer:
When Rb is hypophosphorylated, it binds to the transcription factor E2F, blocking the production of proteins necessary for the G1/S transition. As Rb is phosphorylated and becomes inactive, E2F is released and can turn on the gene that produces the transition protein, advancing the cell cycle.
Step-by-step explanation:
When Rb is hypophosphorylated, it is in the active, dephosphorylated state. In this state, Rb binds to the transcription factor E2F, blocking the production of proteins necessary for the G1/S transition.
As the cell increases in size, Rb is slowly phosphorylated, becoming inactive and releasing E2F. When E2F is released, it can turn on the gene that produces the transition protein, allowing the cell cycle to progress.