External factors that regulate cell cycle:
1) Growth factors : Amount of growth factors decides whether the cell will enter into the mitosis or not.
2) Cell cycle inhibitor like TGF- β also regulates cell cycle. It induces cell cycle arrest and apoptosis in the animal cell.
Internal factors that regulate cell cycle:
1) Cyclin - CDK dependent kinases are required for cell cycle regulation and progression. Cyclins are positive regulators of CDK.
2) Cell cycle check points like spindle assembly check points check the proper alignment of chromosomes on the equitorial plate.