cell cycle

cell cycle 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 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 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. See also cyclin.