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Fluorescence of the dye 33258 Hoechst bound to chromosomes is partially quenched by the introduction of BrdU into chromosomal DNA. This finding has allowed microfluorometric detection of DNA synthesis in human metaphase chromosomes. Incorporation, shortly before cell harvest, of a pulse of thymidine into chromosomes otherwise substituted with BrdU results in sharply defined foci of bright 33258 Hoechst fluorescence, corresponding to regions of late DNA replication. The latereplicating X chromosome can thereby be clearly identified, and the time course of appearance of its fluorescent bands can be compared with that of its earlier replicating homologue. Growth of cells in medium containing BrdU for two generations allows fluorometric documentation of the semiconservative distribution of newly replicated DNA between sister chromatids, and regions of sister chromated exchange are demarcated. This approach should constitute, in many instances, a convenient, high-resolution fluorometric alternative to autoradiography.
Samuel A. Latt (1973) studied this question.