Retardation of cell cycle progression in yeast cells recovering from DNA damage: a study at the single cell level.

Pedigree analyses of individual yeast cells recovering from DNA damage were performed and time intervals between morphological landmark events during the cell cycle (bud emergence and cell separation), were recorded for three generations. The associated nuclear behavior was monitored with the aid of DAPI staining. The following observations were made: All agents tested (X-rays, MMS, EMS, MNNG, nitrous acid) delayed the first bud emergence after treatment, which indicates inhibition of the initiation of DNA replication. Cells that survived X-irradiation progressed further through the cell cycle in a similar way to control cells. Progress of chemically treated cells became extremely asynchronous because surviving cells stayed undivided for periods of varying length. Prolongation of the time between bud emergence and cell separation was most pronounced for cells treated with the alkylating agents MMS and EMS. This is interpreted as retardation of ongoing DNA synthesis by persisting DNA adducts. Cell cycle prolongation in the second and third generation after treatment was observed only with MMS treated cells. In all experiments, individual cells of uniformly treated populations exhibited highly variable responses.