Subclassification of cells in S phase in a partially synchronized cell system.

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Abstract
A circadian dependent delay in the incorporation of [3H]TdR [thymidine] into DNA, presumably due to variations in the intracellular pool of [3H]TdR derivatives, was found. This effect may be related to a circadianally varying age distribution of cells in S phase. At any given time the S phase cells showed large variations in DNA synthesis rate; it was possible to identify a mean diurnal variation in the DNA synthesis rate. Differences in the ability of S phase cells to incorporate [3H]TdR are also discussed in relation to flow cytometrical measurements. This contributes to the understanding of the commonly observed phenomenon that flow cytometry estimates of S-fractions are higher than those obtained with autoradiography.