Duration of DNA Synthesis in Ehrlich Ascites Cells as Estimated by Double-Labeling With C14- and H3-Thymidine and Autoradiography2

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Abstract
Ehrlich ascites tumor cells growing in the peritoneal cavity of male mice were labeled with C14-thymidine, and, after a 1-hour interval, with H3-thymidine. In such a population, the ratio of all cells labeled by C14 only, relative to all other labeled cells, is equal to the ratio: interval between the administration of the radioisotopes/length of the deoxyribonucleic acid (DNA) synthesis phase. Knowing the first three terms of these ratios, one can estimate the length of the DNA synthesis phase. Cells labeled by C14-thymidine only and all other labeled cells were identified by two-emulsion autoradiography, and the length of the DNA synthesis phase was estimated at 10.4 hours. In another experiment, tumor-bearing female mice were given two injections of H3-thymidine; each injection was followed by sampling of the tumor. The difference in the percentage of labeled cells between the two samples gives the fraction of cells entering the DNA synthesis phase in the interval between the administration of the two radioisotopes. From it, and from the fraction of cells labeled by the first injection, the length of the DNA synthesis phase can be estimated as above. It lasted in females 12.5 hours. With tritiated thymidine, the limits of accuracy of the autoradiographs were within the counting error, but with C14-thymidine there was a further error due to uncertainties in the identification of cells.