THE CELL CYCLE AND CELL DIVISION

The life of a cell is called the cell cycle. It is usually divided into five phases or gaps: G₀, G₁, S, G₂, and M. Some cells may not have a G1 phase, and others may not have a G₂ stage. However, all cells must grow, replicate their genetic material if they are to divide, and undergo the process of mitosis if they are to replicate. G₀ is the stage during which the cell may leave the cell cycle and either remain in a state of inactivity or reenter the cell cycle at another time. G1 is the stage during which the cell is starting to prepare for mitosis through DNA and protein synthesis and an in­crease in organelle and cytoskeletal elements. The S phase is the synthesis phase, during which DNA replication occurs and the centrioles are beginning to replicate. G₂ is the premitotic phase and is similar to G1 as for RNA and pro­tein synthesis. The M phase is the phase during which cell mitosis occurs. Nondividing cells, such as mature nerve cells and cells not preparing for mitosis, are said to be in the G₀ phase of the cell cycle (Fig. 4-12).