Evidence that SCEs induced by mutagens do not occur at the same locus in successive cell cycles: lack of cancellation in three-way stained CHO chromosomes.

An approach based on the synchronization of CHO cells after a first cell cycle incorporating a relatively low amount of bromodeoxyuridine (BrdUrd) into DNA, followed by mutagenic treatment and subsequent culture for second and third generations of BrdUrd incorporation for the scoring of sister chromatid exchanges (SCEs) per cell cycle in three-way differentially (TWD) stained chromosomes, has been used to investigate the possible cancellation of SCEs. Cancellation is expected to occur if two mutagen-induced SCEs occur at exactly the same site in subsequent rounds of replication. Lesions in DNA seem to persist and are able to induce SCE throughout two cell cycles after treatment with the three mutagens tested--mitomycin C (MMC), ethyl methanesulfonate (EMS) and ultraviolet (UV) light--though this latter agent was shown as only moderately persistent. Our results seem to indicate that SCEs induced by these mutagens do not take place at the same locus in successive cell generations, as assessed by a lack of SCE cancellation.