CELL CYCLE
The cell cycle is the series of events between the formation of a given cell and its new division or, its death. The cell cycle of a monocellular organism coincides with its life cycle. The growth and division of mammalian cells is controlled by various external factors on the feedback principle.
Four stages are distinguished in the cell cycle. Stage M is composed of the division of the nucleus (mitosis) and the division of the cell's cytoplasm (cytokinesis). After stage M the daughter cells are subjected to the resting stage of a new cycle. The resting stage starts with stage G1, when intensive processes of biosynthesis, which slow down during the mitosis, activate again. During stage S, DNA synthesis occurs. Stage S finishes, when the contents of DNA in the nucleus doubles, and all the chromosomes duplicate. Then the cell enters stage G2, which finishes, when the mitosis begins.
Observable duration of the cell cycle is 8 hours to 100 days, or more, for different cell. The main difference between quickly dividing cells and slowly dividing ones is in duration of stage G1 of their cell cycles. The time from the beginning of stage S till the end of the mitosis is basically constant for all cells.