Characterization of mitoses with sister chromatid differentiation (SCD) and consequences for the analysis of proliferation kinetics and sister chromatid exchanges in asynchronously growing cells.

Labeling cells with bromodeoxyuridine (BrdU) permits the differentiation of mitoses of the first, second, and third generation after the addition of BrdU. The term "second mitoses" is used for those cells which have incorporated BrdU for two S-phases and which exhibit sister chromatid differentiation (SCD). However, SCD can also be obtained if the cell was in S-phase at the time of BrdU-addition and had already replicated part of its DNA. Such cells with incomplete BrdU-substitution in the first S-phase can only be differentiated from completely substituted ones by the quality of the SCD and are usually also grouped as "second mitoses" in the evaluation of experiments. Due to the heterogeneity of the evaluated "second mitoses", the determination of proliferation delay and the incidence of sister chromatid exchange-induction can depend on the time of chromosome preparation.