A new method for the identification of SCE's per cell cycle in BrdUrd-substituted chromosomes.

If cells are allowed to replicate in the presence of different concentrations of BrdUrd throughout three consecutive cell cycles, FPG stained third division chromosomes show three different levels of staining. This fact allows identification of four different types of SCE's, corresponding to the first-, second- and third-cycle exchanges. Finally, the relationship between the degree of BrdUrd-substitution and the frequency of SCE's per cell cycle supports that most of the SCE's detected in BrdUrd-substituted chromosomes are induced by this thymidine analogue incorporated into DNA.