Rates of incorporation of radioactive molecules during the cell cycle

We report measurements of the incorporation of radioactive molecules during short labeling periods, as a function of cell‐cycle stage, using a cell‐sorter‐based technique that does not require cell synchronization. We have determined: (1) tritiated thymidine (³H‐TdR) incorporation throughout S‐phase in Lewis lung tumor cells in vitro both before and after treatment with cytosine arabinoside; (2) ³H‐TdR incorporation throughout S‐phase in KHT tumor cells in vitro and in vivo; (3) ³H‐TdR incorporation throughout S‐phase in Chinese hamster ovary cells and compared it with DNA synthesis throughout S‐phase; (4) a mathematical expression describing ³H‐TdR incorporation throughout S‐phase in Chinese hamster M3‐1 cells; and (5) the simultaneous incorporation of ³H‐TdR and ³⁵S‐methionine as they are related to cell size and DNA content in S49 mouse lymphoma cells. In asynchronously growing cells in vitro and in vivo, ³HH‐TdR incorporation was generally low in early and late S‐phase and highest in mid‐S‐phase. However, in Lewis lung tumor cells treated with cytosine arabinoside ³H‐TdR incorporation was highest in early and late S‐phase and lowest in mid‐S‐phase. Incorporation of ³⁵S‐methionine increased continuously with cell size and DNA content. Incorporation of ³H‐TdR in CHO cells was proportional to DNA synthesis.