Protein synthesis in suspension cultures of human cells in logarithmic growth was inhibited with puromycin or chloramphenicol, and the growth rate and mitotic index were measured as a function of time. The mitotic index remained constant for about 1 hour after addition of inhibitor; this indicates that any protein synthesis necessary for mitosis is completed before the beginning of prophase. For rates of protein synthesis equal to or greater than 0.3 that of untreated cells, the index decreased over a 5- to-7-hour period and then remained constant. The final value of the index relative to that of the uninhibited control was approximately equal to the relative rate of protein synthesis. The period from the end of DNA synthesis to mitosis (G2) was increased by partial inhibition of protein synthesis. A mathematical model of the inhibition process has been formulated which predicts the shape of the mitotic index curves and the increase in the G2 period. An interpretation of the model is that the rate-limiting step is the synthesis of an enzyme which catalyzes the formation of a compound necessary to initiate mitosis.

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