The cell goes through typical stages between two cell divisions. The successive phases can be visualized as the so-called cell cycle. The cell cycle comprises two main phases, namely division, also called mitosis or M-phase, and the phase of DNA and protein synthesis, also called S-phase. The two main phases are connected via gap phases, the G1- and G2-phases. Before a cell can enter and also before it can complete the M-phase as well as before it can enter the S-phase, several specific requirements have to be fulfilled. For example, before the cell can start mitosis, the DNA must be fully replicated, and before the cell can enter S-phase, there has to be an excess of ATP. If these requirements are not met, the cell cycle is stopped. The positions at which the cell cycle is stopped because a phase cannot be completed or the next phase cannot be started are called checkpoints.
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