A study was made of the timing of DNA synthesis in the mitotic cycle under conditions where the average mitotic cycle of populations of human amnion and kitten lung cells in culture was variable. Three types of experiments were performed: (a) Autoradiographs were made of incorporated tritiated thymidine in cells whose mitotic histories were recorded microcinematographically allowing the measurement of telophase + G1 along with the total length of the mitotic cycle. (b) Measurement of the G2 + prophase part of the mitotic cycle was performed under various conditions by exposing cells to tritiated thymidine and observing the increase in labeled metaphases plus anaphases as a function of time. (c) The effect of a change in pH on parts of the mitotic cycle was tested by continuously photographing a single colony of cells first at pH 7.8 and then at pH 7.1. All of our data point to the same conclusion; namely, that within a population of cells with a given generation time, the length of each of the measurable parts of the mitotic cycle has a particular distribution of values and that, when there is a change in the generation time, under our conditions only the T + G1 distribution changes.

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