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, 3H-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.