The sequence of stages that a cell passes through between one cell division and the next. The cell cycle can be divided into four main stages: (1) the M phase, which consists of mitosis (nuclear division) and cytokinesis (cytoplasmic division); (2) the G₁ (‘first gap’) phase, in which there is a high rate of biosynthesis and growth; (3) the S phase, in which the DNA content of the cell doubles and the chromosomes replicate; (4) the G₂ (‘second gap’) phase, during which the final preparations for cell division are made.

Interphase consists of the G₁, S, and G₂ phases, which comprise about 90% (16–24 hours) of the total time of the cell cycle in rapidly dividing cells. The M phase lasts about 1–2 hours. A point is reached in the G₁ phase, known as the restriction point, after which the cell becomes committed to passing through the remainder of the cell cycle regardless of the external conditions. (see illustration). The cell cycle is regulated by external and internal signals that determine if the cycle can proceed past several checkpoints. Key to this control mechanism are cyclic fluctuations in cellular concentrations of proteins called cyclins and in the activity of cyclin-dependent kinases. The timing and frequency of cell division are crucial to normal growth and development and vary according to the type of cell. In a mature human, for example, skin cells, which need constant replacement, divide frequently throughout life, whereas nerve and muscle cells exist in a nondividing state called G0. Many other types of tissue cells are in the G0 state, but some can re-enter the cell cycle if new cells are needed, e.g. to repair damaged tissue.

Cell cycle