How is the cell cycle controlled? Normal cells halt their progress towards cell division at a series of three checkpoints, which occur during G1 , G2 , and metaphase. At these checkpoints, proteins survey the cell to ensure that conditions for a favorable cellular division have been met. At the G1 checkpoint, proteins in the cell determine whether cell division is called for. To do this, the proteins survey the cell's environment for the presence of other proteins called growth factors, which stimulate cell division. When growth factors are limited in number, cell division does not occur. If enough growth factors are present to trigger cell division, then other proteins check to see if the cell is large enough and if sufficient nutrients are available for the cell to synthesize its DNA and divide. At the G2 checkpoint, proteins ensure that the DNA has replicated correctly and again make sure that the cell is large enough to divide. The third and final checkpoint occurs during metaphase of mitosis. Proteins present at metaphase verify that all the chromosomes have attached to the microtubules of the spindle so that cell division can proceed properly. If the systems surveying the cell at any of these three checkpoints determine that conditions for cell division are not met, the progression towards division is halted. When this happens, the cell may die.