Q. You find out a mutant yeast strain that cannot grow while provided with pyruvate as its sole carbohydrate, though it grows normally when given glucose as a carbon energy source; you call this the Pyr- trait. Wild-type yeast can grow well using either carbon source; you designate this Pyr+ trait. You suppose that the mutant yeasts have defective mitochondria.

A. Why is mitochondrial function critical for growing on pyruvate?

B. You want to find out whether the mutation responsible for the Pyr- trait is carried in the nuclear or mitochondrial genome. Your experiment is to mate Pyr- haploid cells to Pyr+ haploid cells and examine segregation of the trait in the progeny. After mating, the hybrid diploid cells can grow on pyruvate (Pyr+). You send these diploid cells through meiosis and examine the four haploid cells generated from a single diploid cell. If the mutation is in mitochrondrial DNA, will the four haploid cells probably be all Pyr-, all Pyr+, or a combination? What if the mutation is in nuclear DNA?