EVIDENCE THAT SCES INDUCED BY MUTAGENS DO NOT OCCUR AT THE SAME LOCUSIN SUCCESSIVE CELL-CYCLES - LACK OF CANCELLATION IN 3-WAY STAINED CHOCHROMOSOMES

An approach based on the synchronization of CHO cells after a first ce ll cycle incorporating a relatively low amount of bromodeoxyuridine (B rdUrd) into DNA, followed by mutagenic treatment and subsequent cultur e for second and third generations of BrdUrd incorporation for the sco ring 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 subseque nt rounds of replication. Lesions in DNA seem to persist and are able to induce SCE throughout two cell cycles after treatment with the thre e mutagens tested-mitomycin C (MMC), ethyl methanesulfonate (EMS) and ultraviolet (UV) light-though this latter agent was shown as only mode rately persistent. Our results seem to indicate that SCEs induced by t hese mutagens do not take place at the same locus in successive cell g enerations, as assessed by a lack of SCE cancellation. (C) 1994 Wiley- Liss, Inc.