The cell cycle or cell-division cycle (CDC) is the series of events in a eukaryotic cell between one cell division and the next. Thus, it is the process by which a single-cell fertilized egg develops into a mature organism and the process by which hair, skin, blood cells, and some internal organs are renewed. A specialized form of cell division is responsible for cellular differentiation during embryogenesis and morphogenesis, as well as for the maintenance of stem cells during adult life.

The cell cycle consists of four distinct phases: G1 phase, S phase, G2 phase (collectively known as interphase) and M phase. M phase is itself composed of two tightly coupled processes: mitosis, in which the cell’s chromosomes are divided between the two daughter cells, and cytokinesis, in which the cell’s cytoplasm physically divides. Cells that have temporarily or reversibly stopped dividing are said to have entered a state of quiescence called G0 phase, while cells that have permanently stopped dividing due to age or accumulated DNA damage are said to be senescent. Some cell types in mature organisms, such as parenchymal cells of the liver and kidney, enter the G0 phase semi-permanently and can only be induced to begin dividing again under very specific circumstances; other types, such as epithelial cells, continue to divide throughout an organism’s life.

The molecular events that control the cell cycle are ordered and directional; that is, each process occurs in a sequential fashion and it is impossible to “reverse” the cycle. There are two key classes of regulatory molecules that determine a cell’s progress through the cell cycle: cyclins and cyclin-dependent kinases. Leland H. Hartwell, R. Timothy Hunt, and Paul M. Nurse won the 2001 Nobel Prize in Physiology or Medicine for their discovery of these central molecules in the regulation of the cell cycle.