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The Cell Cycle. G1 and G2 (gap 1 and gap 2) are characterized by protein and RNA synthesis, but no DNA synthesis. S (synthesis) is the period of DNA synthesis. M (mitosis) and D (division or cytokinesis) are the periods when the nucleus and then the rest of the cell divides. G0 is a "resting" stage when the cell is not actively dividing and may be omitted when cells are continuously cycling. The timing of the cell cycle and the relative lengths of the various stages depends on the specific type of cell and on the local conditions. At the completion of division (D) the daughter cells may re-enter the cell cycle or may undergo terminal differentiation. |