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Carlson, M., B.C. Osmond, & D. Botstein (1981) Genetic evidence for a silent SUC gene in yeast. Genetics 98: 41-54.

1. Describe how sucrose is fermented (catabolized, utilized) by Saccharomyces.

2. Describe the SUC genes of Saccharomyces.

3. Figure 2 describes a cross between strains DBY473 (MATa SUC2 suc7° his4

URA3) and DBY631 (MATa suc2° SUC7 HIS4 ura3).

(a) Diagram this cross in detail using the format outlined in Chapter 1. Be sure to show the genotype and phenotype of the following: the parental strains; the heterozygous diploid produced by the mating; and the haploid spores of PD, NPD and TT tetrads. Only the SUCIsuc genotype and Suc+/Suc- phenotype of the haploid spores needs to be indicated in the tetrads.

(b) How do the authors use the difference in the HIS4 and URA3 genotypes of the two parental strains to obtain the diploid product of the cross?

4. The S288C suc2° strain used to isolate the Suc+ revertants is 'congenic' to the original S288C SUC2 strain obtained from G. Fink.

(a) How many backcrosses to S288C were made to construct this strain?

(b) In each backcross, what Sue phenotype was selected for from among the haploid segregants?

(c) Because of independent segregation of chromosomes and recombination, each backcross of an individual progeny strain resulting from a cross to the same parental strain increases by 50% the percentage of the subsequent progeny's genome that is identical to that parent's genome. Based on this, calculate the average percent identity between the S288C suc2° strain and the original S288C SUC2 strain.

5. Sucrose fermenting revertants of the S288C suc2° strain were obtained.

(a) Were these Suc+ mutants identified by a selection method or by a screen? Discuss the method.

(b) Was mutagenesis used to increase the frequency of Suc+ revertants or were these spontaneous? What was the frequency of reversion to Suc+?

(c) How did the authors demonstrate that the mutation in the revertant strains represented a change in a single gene? (Give details of any crosses.)

(d) How did the authors demonstrate that the Suc+ reversion mutations were in the suc2° gene?

6. Sucrose fermenting revertants of S288C were obtained only at suc2° and never at any of the other SUC loci.

(a) What possible explanations do the authors give for this result?

(b) Which explanation do they prefer and what experimental evidence do the authors present in support of this explanation?

7. Describe the fundamental difference between the suc2° locus found in strain S288C and the other sue0 loci found in this strain.

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