Abstract
Fluorescence of the dye 33258 Hoechst, when bound to chromosomes, is partially quenched by the incorporation of 5-bromodeoxyuridine into chromosomal deoxyribonucleic acid (DNA). This effect allows microfluorometric analysis of DNA synthesis. Metaphase chromosomes from cultured human leukocytes which have incorporated 5-bromodeoxyuridine for a portion of the DNA synthesis period exhibit reduced 33258 Hoechst fluorescence in 5-bromodeoxyuridine-containing regions. Regions synthesizing DNA during a particular interval can thus be highlighted by the appropriate protocol of 5-bromodeoxyuridine administration. Chromosomes from cells which have replicated twice in medium containing 5-bromodeoxyuridine exhibit one brightly and one dully fluorescing chromatid, reflecting incorporation of 5-bromodeoxyuridine into one or two chains of chromatid DNA, respectively. Sister chromatid exchanges, evident as sharply demarcated reciprocal alterations in fluorescence along chromosomes, can be located relative to quinacrine banding patterns. This fluorometric approach should be useful in many instances as a convenient, high resolution alternative to autoradiography.