2005_pset1_ans - 7.06 Problem Set#1 Spring 2005 1 As a new...

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1 7.06 Problem Set #1, Spring 2005 1. As a new graduate student in a yeast lab, you decide to do a genetic screen looking for genes involved in mitosis, an essential biological process. After screening 1000 mutagenized haploid yeast strains at 23 o C and 36 o C, you find 20 strains that have defects in mitosis. These cells do not grow at the higher temperature but grow normally at the lower temperature. a. Why do you use haploid, and not diploid, yeasts for the screen? This allows you to isolate single mutations that cause recessive phenotypes. If you used diploid yeast, both copies of a gene would have to be mutated to yield a recessive phenotype, and it is very unlikely that both copies of the same gene will be mutated in one cell that has been randomly mutagenized. b. What is the name for the category of mutants that you have found? Conditional mutants, and more specifically, temperature sensitive mutants. c. Why can’t you isolate mutants that have a defect in mitosis at all temperatures? Because mitosis is an essential process, mutants with a permanent defect in mitosis will be non-viable. Screening for temperature sensitive mutants allows researchers to isolate strains that are viable at the permissive temperature but that have interesting lethal phenotypes at the restrictive temperature. d. How would you determine the total number of genes that are mutated in your collection of 20 mutant yeast strains? Perform a complementation test. Mate each haploid mutant strain with all others at the permissive temperature until you have mated all possible pairs. Analyze each resulting diploid by shifting the diploid to the restrictive temperature and determining the phenotype. If there is still a mitotic defect in the diploid, then the two haploid strains did not complement each other and the two haploid strains had mutations in the same gene. (You mated A– yeast to A– yeast to yield A-/A- diploids.) If the mitotic defect is corrected in the diploid, then the two haploid strains complement each other. In this case, the two haploid strains had mutations in different genes. (You mated A– B+ yeast to A+ B– yeast to yield A+ B-/A- B+ diploids.)
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2 You decide to pursue your favorite gene from this screen, which you name YFG1 . You use genetic mapping techniques to figure out which gene in the yeast genome YFG1 is, and then you find the DNA sequence of YFG1 using the yeast genome database. You are now interested to see if YFG1 plays a role in mitosis in other organisms. e. How can you find out if YFG1 is present in the sequenced genomes of other organisms? Using the BLAST search on the NCBI website at http://www.ncbi.nlm.nih.gov/BLAST/ , you can see if a gene with homology to YFG1 is present in another organism with a sequenced genome. f.
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2005_pset1_ans - 7.06 Problem Set#1 Spring 2005 1 As a new...

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