section22_ak - MIT Department of Biology 7.014 Introductory...

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MIT Department of Biology 7.014 Introductory Biology, Spring 2005 Recitation Section 22 Answer Key May 2-3, 2005 From Yeast to Humans—Essential Genes on the Evolutionary Continium. A. CBS protein in yeast and humans Recall that earlier in the term we considered the human gene CBS and its yeast analog cys4. Recall that these genes each encode the protein cystathionine β -synthase that is responsible for converting homocysteine into cystathionine in the cellular pathway of creating cysteine. 1. What is cysteine? Is it important for organism’s survival? Cysteine is an amino acid. It is used in production of proteins, and is thus, essential for organism’s survival. However, external supplementation can sometimes alleviate the phenotype associated with inability to make own cysteine. 2. What would you expect to be the result of complete absence of the protein product of the yeast CYS4 gene to be? What about the same question for the human CBS protein? Yeast will not be able to make cysteine, resulting in cysteine auxotrophy. Humans lacking CBS protein should also be unable to make cysteine. Human phenotype may be more complex than just cysteine auxotrophy. 3. Would you expect cells that contain no functional copy of CBS enzyme to accumulate some kind of a compound? If no, why not? If yes, what kind of a compound would you expect that compound to be? These cells should accumulate the compound that is the product of the previous step in the cysteine biosynthesis pathway. The compound is homocysteine, an intermediate in the cycteine biosynthesis pathway and one of the reactants in the step catalyzed by the CBS protein (the other reactant is serine). 4. In the experiments we discussed earlier in the term, what was the phenotype of the cys4 mutants on complete media? On complete media cys4 mutants had the same phenotype as the wild type yeast. 5.
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section22_ak - MIT Department of Biology 7.014 Introductory...

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