This paper describes a flow-cytometric application of the quenching of fluorescence from 33258 Hoechst stained Chinese hamster ovary-line cells due to the incorporation of 5-bromodeoxyuridine (BrdU) into the cellular deoxyribonucleic acid. Cells were grown for 24 hr in medium containing BrdU in concentrations ranging from 1 x 10-8 to 1 x 10-4 M. For each concentration we measured the average fluorescence as determined by flow cytometry, the extent of BrdU substitution and the effect of the BrdU on cell growth. We determined that a BrdU concentration of 1 x 10-5 M resulted in sufficient substitution to quench the fluorescence from 33258 Hoechst by a factor of 4, allowing discrimination between cycling and noncycling cells. The extent of BrdU substitution after growth for 24 hr in this concentration of BrdU was 64%. These data indicate the feasibility of detecting deoxyribonucleic acid synthesis in whole cells using the 33258 Hoechst-BrdU methodology.
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