"Unusual SCD" explained by differences in BrdU incorporation during subsequent cell cycles.

In the course of three consecutive cell cycles, V79 cells were labeled with BrdU by different labeling protocols. Cells treated for three cycles with bromodeoxyuridine (BrdU) showed third division metaphases, with the typical appearance after fluorescent plus Giemsa (FPG) staining (i.e., 75% of the chromatids showed light staining, 25% showed dark staining). The same staining pattern is achieved by the second labeling protocol, during which the cells have replicated for two cycles in the presence of BrdU and, during the last cycle, in the absence of BrdU. Cells that have replicated only for one cell cycle in BrdU-containing medium, and the following two cycles in normal medium, depict just the opposite staining pattern (i.e., 75% dark, 25% light). These experiments explain how the variation of BrdU substitution in the DNA leads to altered FPG staining. Unexpected staining patterns ("unusual SCD") are also observed after seemingly permanent BrdU substitution. This phenomenon, which has been found in cancer cells, is due to the decrease in BrdU concentration and not to a peculiarity of the cells investigated.