What are the Checkpoints of the cell cycle*?  
 


       Loss of cell cycle controls can initiate uncontrolled division (tumors)

             a model of tumor suppresor control of the cell cycle*    







A checkpoint is a stage in the eukaryotic cell cycle at which the cell examines internal and external cues and "decides" whether or not to move forward with division




 


 


 

 

 

 
















             
 
   
             the transition from G1 to S Phase -    the Retinoblastoma tumor suppressor Model:

  Rb-E2F
       MPF phosphorylates Rb releasing E2F that acts as a transcription factor to synthesize S-phase proteins
       and thereby stimulating S-phase protein that initiate cell division. The normal function of Rb-E2F is "OFF"

       Retinoblastoma is a rare form of cancer that rapidly develops from the immature cells of a retina, the
       light-detecting tissue of the eye when the retinoblastoma protein has mutated and loss normal function.
       A mutated Rb protein is always active thus E2F promotes tumor growth. 

       Retinoblastoma protein (Rb-E2F) is classified as a tumor suppressor protein* because it normally
       blocks the initiation of cell division from occurring, resulting in a normal retinal tissues, but a mutant
       form keeps E2F active promoting rapid cell division and growth.  
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            The eukaryotic cell cycle has many Cyclin Dependent Kinases (Cdk's) that serve as on/off
            swithches at staged of the cell cycle, checking if the previous step is completed properly.
  
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Rb-E2F

   

 

 

 

 

 

 

 

 

 

 

 

 
                                     

               



         CDK-cyclins cascades
Retinoblastoma is a rare form of cancer that rapidly develops from the immature cells of a retina, the light-detecting tissue of the eye when the retinoblastoma protein  has mutated and loss function. Retinoblastoma protein (Rb-E2F) is a  tumor suppressor protein* that blocks the initiation of cell division. MPF phosphorylates  Rb releasing E2F that acts as a transcription factor to synthesize S-phase proteins and   thereby stimulate cell division. A mutated Rb protein is always active thus E2F promotes tumor growth.   

      back		 Rb-E2F

     




                    
                                                     

                                   

             

 






    

    

                                                    a specific G1 to S-phase checkpoint protein*