Origin of mutations in haploid and diploid organisms

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To obtain insights into the mechanisms of spontaneous mutations in haploid and diploid organisms, we have characterized the genetic alterations that inactivate either the tonB gene in haploid Escherichia coli and CAN1 gene in haploid or heterozygously situated in diploid Saccharomyces cerevisiae cells.In E.coli strains containing deletions in genes encoding three translesion polymerases, and defective in MutS and DNA polymerase I (Poll) mismatch repair, we estimated the rate and specificity of spontaneous mutations. The results suggested that unlike the case previously known pertaining to postreplicative MutS mismatch repair for frameshifts and transitions and Poll mismatch repair for frameshifts and deletions, Poll can recognize and correct transition mismatches. The spectrum of spontaneous mutations in deficiencies in three translesion polymerases, MutS mismatch repair and Poll mismatch repair strain is considered to represent the spectrum of in vivo DNA pol III replication errors. T … More he mutation rate of this strain was 219×10^<-8>, about a 100-fold increase relative to the wild- type strain.In S.cerevisiae, the mutation rate in haploid cells was 9.08×10^<-7>,100-fold lower than that in diploid cells (1.03×10^<-4>). In haploid cells, 75% were base substitutions and 22% frameshifts. The base substitutions were both transitions (33%) and transversions (42%), with G : C→A:T and G : C→T : A dominating. Minus frameshifts (12%) and plus frameshifts (10%) were also observed at run and nonrun bases and at A : T and G : C pairs with almost equal efficiency. An analysis of chromosome structure in diploid yeast cells indicated that crossover was the predominant event followed by gene conversion and chromosome loss. Taken together, we argued that genetic alterations leading to spontaneous phenotypic changes in wild-type diploid yeast cells occurred through two steps; replication-dependent alterations of bases in either allele then recombination-dependent transfer of the mutated allele to the intact one. Less