Rates of incorporation of radioactive molecules during the cell cycle

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Abstract
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.