Question

Several auxotrophic mutant strains (A?, B?, C?) in yeast are obtained in a genetic screen for mutant alleles that disrupt the ability of the cells to grow in media without supplied tryptophan (an essential amino acid needed to make proteins). All 3 mutant strains grow on rich media, and also on minimal media supplemented with added tryptophan. All three cannot grow on minimal media alone. Part of the tryptophan biosynthetic pathway (three steps) may be represented in simplified form in the figure, below (numbers refer to the enzyme that catalyzes each step): Strain A grows if the minimal media is supplemented with compound ZZZ, but not with XXX or YYY. Strain B grows if the minimal media is supplemented with compounds YYY or ZZZ, but not with XXX. Strain C grows only if the minimal media is supplemented with tryptophan, but not ZZZ, XXX or YYY. In terms of the pathway shown, the normal alleles A?, B?, and C? of the three genes mutated in the auxotrophic strains encode enzymes that catalyze which order of steps, respectively? XXX --1-->YYY--2-->ZZZ--3--> tryptophan 2, 3, 1 2, 1, 3 1, 2, 3 3, 2, 1

          Several auxotrophic mutant strains (A?, B?, C?) in yeast are obtained in a genetic screen for mutant alleles that disrupt the ability of the cells to grow in media without supplied tryptophan (an essential amino acid needed to make proteins). All 3 mutant strains grow on rich media, and also on minimal media supplemented with added tryptophan. All three cannot grow on minimal media alone. Part of the tryptophan biosynthetic pathway (three steps) may be represented in simplified form in the figure, below (numbers refer to the enzyme that catalyzes each step):

Strain A grows if the minimal media is supplemented with compound ZZZ, but not with XXX or YYY. Strain B grows if the minimal media is supplemented with compounds YYY or ZZZ, but not with XXX. Strain C grows only if the minimal media is supplemented with tryptophan, but not ZZZ, XXX or YYY.

In terms of the pathway shown, the normal alleles A?, B?, and C? of the three genes mutated in the auxotrophic strains encode enzymes that catalyze which order of steps, respectively?

XXX --1-->YYY--2-->ZZZ--3--> tryptophan

2, 3, 1
2, 1, 3
1, 2, 3
3, 2, 1

Several auxotrophic mutant strains (A?, B?, C?) in yeast are obtained in a genetic screen for mutant alleles that disrupt the ability of the cells to grow in media without supplied tryptophan (an essential amino acid needed to make proteins). All 3 mutant strains grow on rich media, and also on minimal media supplemented with added tryptophan. All three cannot grow on minimal media alone. Part of the tryptophan biosynthetic pathway (three steps) may be represented in simplified form in the figure, below (numbers refer to the enzyme that catalyzes each step):

Strain A grows if the minimal media is supplemented with compound ZZZ, but not with XXX or YYY. Strain B grows if the minimal media is supplemented with compounds YYY or ZZZ, but not with XXX. Strain C grows only if the minimal media is supplemented with tryptophan, but not ZZZ, XXX or YYY.

In terms of the pathway shown, the normal alleles A?, B?, and C? of the three genes mutated in the auxotrophic strains encode enzymes that catalyze which order of steps, respectively?

XXX –1–>YYY–2–>ZZZ–3–> tryptophan

2, 3, 1
2, 1, 3
1, 2, 3
3, 2, 1

Added by Sabrina M.

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