PS10KEY - Professor Hinck Bio 110 Fall problem set 11 1 The...

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Professor Hinck Bio 110, Fall, problem set 11 1) The budding yeast Saccharomyces cerevisiae and the fission yeast Schizosaccharomyces pombe provide facile genetic systems for studying a wide range of eukaryotic cell biological processes. If cell-cycle progression is essential for cell viability, as it is in these yeasts, how is it possible to isolate cells that are defective in cell-cycle genes? ANS: A yeast mutant that is defective in a cell-cycle gene can be isolated if its phenotype is conditionally defective—that is, it can be isolated if the product of the mutant gene fails to function under one set of conditions, but functions adequately under another. Most conditional cell-cycle mutants are temperature sensitive: the mutant protein fails to function at high temperature, but performs well enough at low temperature to allow the cell to divide. 2) You have isolated a temperature sensitive mutant of the Cdc28 gene, which encodes Cdk1 in budding yeast. At the restrictive temperature, the mutant Cdk1 binds the G1 cyclins, Cln1 and Cln2, so weakly that colonies do not form. When a cDNA library on a high copy number plasmid was expressed in the mutant strain, three different cDNAs were found that allowed colony formation at the restrictive temperature. One was the cDNA from the wildtype Cdc28 gene; the other two were cDNAs that encoded cyclins Cln1 and Cln2. If the cDNAs for the cyclin genes were transferred to a plasmid that was maintained at one copy per cell, neither one allowed the Cdc28 mutant to grow at high temperature. Suggest a mechanism whereby high-level expression of Cln1 or Cln2 would allow survival of mutant Cdc28 cells at high temperature, whereas low level expression would not. ANS: At the restrictive temperature, the mutant Cdk1 binds Cln1 and Cln2
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PS10KEY - Professor Hinck Bio 110 Fall problem set 11 1 The...

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