Abstract
A number of applications of the detection of deoxyribonucleic acid synthesis by fluorescence microscopy are illustrated. These include (a) the analysis of sister chromatid exchanges and sister chromatid segregation at mitosis, (b) the location of chromosome regions containing deoxyribonucleic acid with an asymmetric distribution of thymine residues between polynucleotide chains and (c) the detection of late replicating regions in metaphase chromosomes. The suppression of 33258 Hoechst fluorescence by 5-bromodeoxyuridine incorporated biosynthetically into interphase nuclei is demonstrated both in fixed cytologic preparations and in unfixed cultured cells. Many of the cytologic observations described might form a basis for future biochemical studies.