Pedigree Analyses of Yeast Cells Recovering from DNA Damage Allow Assignment of Lethal Events to Individual Post-Treatment Generations

AUTOR(ES)

Klein, F.

RESUMO

Haploid cells of Saccharomyces cerevisiae were treated with different DNA damaging agents at various doses. A study of the progeny of individual such cells (by pedigree analyses up to the third generation) allowed the assignment of lethal events to distinct post treatment generations. By microscopically inspecting those cells which were not able to form visible colonies we could discriminate between cells dying from immediately effective lethal hits and those generating microcolonies (three to several hundred cells) probably as a consequence of lethal mutation(s). The experimentally obtained numbers of lethal events (which we call apparent lethal fixations) were mathematically transformed into mean probabilities of lethal fixations as taking place in cells of certain post treatment generations. Such analyses give detailed insight into the kinetics of lethality as a consequence of different kinds of DNA damage. For example, X-irradiated cells lost viability mainly by lethal hits (which we call 00-fixations); only at a higher dose also lethal mutations fixed in the cells that were in direct contact with the mutagen (which we call 0-fixations), but not in later generations, occured. Ethyl methanesulfonate (EMS)-treated cells were hit by 00-fixations in a dose dependent manner; 0-fixations were not detected for any dose of EMS applied; the probability for fixation of lethal mutations was found equally high for cells of the first and second post treatment generation and, unexpectedly, was well above control in the third post-treatment generation. The distribution of all sorts of lethal fixations taken together, which occurred in the EMS-damaged cell families, was not random. The small but significant surplus of pedigrees without any lethal fixation on the one hand and of pedigrees with more than one lethal fixation on the other hand may indicate the existence of unequally sensitive subpopulations. For comparison analyses of cells treated with methyl methanesulfonate, N-methyl-N'-nitro-N-nitrosoguan-idine N-methyl-N'-nitro-N-nitrosoguanidine and nitrous acid are also reported.

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