The cell cycle is an ordered set of events, culminating in cell growth and division. The cell cycle of eukaryotes can be divided in two brief periods: interphase, during which the cell grows, accumulating nutrients needed for mitosis and duplicating its DNA, and the mitosis (M) phase, during which the cell splits itself into two distinct cells, often called daughter cells. By studying molecular events in cells, interphase is divided into three stages, G1, S, and G2. Thus the cell cycle consists of four phases: G1, S, G2, M.G1 phase is from the end of the previous M phase until the beginning of DNA synthesis, and G stands for gap. During this phase the biosynthetic activities of the cell, which had been considerably slowed down during M phase, resume at a high rate. This phase is marked by synthesis of various enzymes that are required in S phase, mainly those needed for DNA replication.
Early work in frog and invertebrate embryos suggested that cell cycle events are triggered by the activity of a biochemical oscillator centered on cyclin-CDK complexes. The cyclin/CDK complexes induce two processes, duplication of centrosomes and DNA during interphase, and mitosis. The roles of individual cyclins were tested by adding recombinant proteins to cyclin- biologidepleted extracts. Cyclin E supports DNA replication and centrosome duplication, cyclin A supports both of these processes and mitosis, and cyclin B supports mitosis alone.