Abstract

Percent-labeled mitosis curves were obtained at different counting thresholds and emulsion exposure durations in Sarcoma 180 cells grown in vitro and pulse labeled with tritiated thymidine. The shape of the percent-labeled mitosis curve was found to be dependent on counting threshold and on emulsion exposure duration. At low counting thresholds and prolonged emulsion exposure durations more mitoses were labeled than at higher thresholds and at shorter emulsion exposure times. The results could not be attributed to emulsion background noise. The data suggest that the onset and termination of DNA synthesis do not occur abruptly in individual cells as they progress through the cycle; the rate of DNA synthesis and corresponding nuclear tritium uptake rise gradually to a maximum, especially in cells with long cell cycle times. When conditions are such that there is increased detection of low levels of nuclear tritium uptake, the apparent duration of S phase is increased. It is suggested that the identification of distinctive patterns of relative labeling intensity as a function of age within the cycle may prove to be more useful than attempts to measure cell cycle phase durations in the kinetic characterization of growing cell populations.