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—for example, when enough nutrients are present in the environment. Thus, a cell may have to receive a signal before it will divide. In Chapter 9, we saw how cells respond to signals. Growth factors, for example, bind to cell-surface receptors and activate intracellular signaling pathways that lead to cell division.

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—for example, dividing in the absence of a signal or when the cell is not ready—the result is uncontrolled cell division, a hallmark of cancer. In this section, we consider how cells control their passage through the cell cycle. Our focus is on control of mitotic cell division. However, many of the same factors also regulate meiotic cell division, a reminder of the close evolutionary connection between these two forms of cell division.