Biochemical studies of the division cycle of mammalian cells: Evidence for the premitotic period

In cultures of a murine mast cell tumor DNA synthesis was blocked by the addition of amethopterin, hypoxanthine, and glycine; administration of thymidine 6 hr later resulted in a partial synchronization of cell reproduction. In these partially synchronized cultures, mitotic indices and DNA synthesis, as expressed by the rate of incorporation of labeled thymidine, were determined at regular intervals. The results indicate that there is a distinct time interval (premitotic period) of approximately 1 hr between completion of DNA synthesis and onset of mitosis. This finding is in good agreement with earlier reports based on microphotometric or radioautographic methods.