Both mitotic and meiotic cell division must occur only at certain times and places. Mitotic cell division, for example, occurs during growth of a multicellular organism, wound healing, or in the maintenance of actively dividing tissues such as the skin or lining of the intestine. Similarly, meiotic cell division occurs only at certain times during development. Even for unicellular organisms, cell division must be regulated so that it takes place only when conditions are favorable—
Even when a cell receives a signal to divide, it does not divide until it is ready. Has all of the DNA been replicated during S phase? Has the cell grown to a size sufficient to support division into viable daughter cells? If these and other preparations have not been accomplished, the cell halts its progression through the cell cycle.
So, cells have regulatory mechanisms that initiate cell division, as well as mechanisms for spotting faulty or incomplete preparations and arresting cell division. When these mechanisms fail—