Cell biology

                                                         Cell cycle check point

            The cell cycle checkpoints determine if a cell is ready to progress to the next stage.

            Late in the G1 phase, the G1 checkpoint determines if the cell will enter the following S phase.

            In animals, the G1 checkpoint or restriction point, is largely controlled by growth factors.

The G2 checkpoint determines if the cell will enter the M phase and requires the proper completion of DNA synthesis.

         The third cell cycle checkpoint is the spindle assembly checkpoint which occurs between metaphase and anaphase and requires the proper attachment of all the chromosomes to the spindle apparatus.

          The fusion of cells in different stages of the cell cycle (to form a heterokaryon) demonstrated that lat    ter stages possess factor trigger progression. 

G1 + S _____ G1 nucleus enters S phase immediately. 
S + G2 _____  G2 nucleus does not enter S phase before mitosis. 
M+ G1, S or G2  ____non-M phase nuclei enters mitosis, whether or not the DNA is duplicated.