You discover a mutant yeast strain that cannot grow when provided with pyruvate as its sole...

You discover a mutant yeast strain that cannot grow when provided with pyruvate as its sole carbohydrate, although 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 the Pyr+ trait. You suspect that the mutant yeast have defective mitochondria.

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

B. You want to determine 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 Pyr-/Pyr+ 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?

Expert Solution

A.

Glycolysis:

Glc --------> Pyruvate

It can occur in the presence as well as the absence of oxygen. It occurs in the cytosol and does not require mitochondria.

Pyruvate enters TCA cycle as acetyl Co-A to produce NADH and FADH2 which generate ATP through oxidative phosphorylation. Pyruvate oxidative decarboxylation, TCA cycle, and ETC occur in mitochondria. So, if an organism has to survive on pyruvate, it must depend upon mitochondrial function.

B.

If Pyr gene is located in the nuclear genome, F2 would show 1:1 segregation. This is because one Pyr+ allele would go into one haploid cell and Pyr- allele would enter the other haploid cell.

If Pyr gene is located in the mitochondrial genome, all the F2 progeny would be Pyr+. The diploid parent is Pyr+/Pyr-. So, its cytoplasm contains a mix of mitochondria (Sometimes non-functional mitochondria are eliminated from the cell...!). So, all the daughter cells would acquire the parent cytoplasm (a mix of +/-). Hence all the progeny would show Pyr+ phenotype.

gladiator answered 1 year ago

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