explain the regulation of retinoblastoma protein Rb?
in resting cells of G1, Rb is active (hypophosphorylated)
--> binds E2F and is an inhibitor of cell cycle progression
in G1 -> S, Rb is inactive (hyperphosphorylation)
--> due to increased cyclin D, Cdk 4/6 activity, Rb dissociates from E2F (which is now free to activate gene expression)