Bio24 Practice Problems from Lecture 3-2-10

Bio24 – Spring 2010 Practice Problems from lecture 3/2/10 1) Jahng et al. mutagenized strains of the following genotype: MATa ste2::LEU2 ade2 leu2 then selected for diploids upon mating to unmutagenized strains of the following genotype: MATα ade6 Write out the full diploid genotype. Upon sporulation of resulting diploids, how will the sterility phenotype segregate in spore sacs? Be sure you can explain what is going on. 2) Suppose you start with a strain that has a gpa1 mutation (temperature sensitive so that you can propagate the strain at the permissive temperature), and you mutagenize and look for suppressor mutations? What genes might you find through such a suppressor screen? (ie. what genes might mutate to give a gpa1 suppressor phenotype?) Bio24 – Spring 2010 ANSWERS for Practice Problems from lecture 3/2/10 1) MATa ste2::LEU2 ade2 leu2 ADE6 mutx MTAα STE2 ADE2 LEU2 ade6 MUTx Spore sacs would be obtained in which all four spores would be fertile, or three fertile and one sterile, or two fertile and one sterile. Here are the genotypes. (note that if the MATa locus segregated away from the ste2 allele, this would further decrease the number of sterile spores). As I write this answer I realize that it would have been better to mate to an ade6 leu2 α cell so that LEU2+ prototrophy in a spore could be used to indicate the presence of the ste2 mutation….just a technical detail showing why its advantageous to plan things out on paper before you actually carry them out!) Note that I ignore all auxotrophic markers for simplicity, and I use shorthand “ste2” to designate the ste2::LEU2 allele. PD ste2 ste2 STE2 STE2 mutx mutx MUTx MUTx (fert) (fert) (fert) (fert) NPD ste2 ste2 STE2 STE2 MUTx MUTx mutx mutx (sterile) (sterile) (fert) (fert) Tetratype ste2 mutx (fert) ste2 MUTx (sterile) STE2 mutx (fert) STE2 MUTx (fert) So, it is clear that you do NOT always get 2:2 segregation of sterile:fertile, indicating that there is some interesting genetic interaction going on. 2) A mutation in any component that acts after or at the same step as Gα . Thus, ste4, ste18, ste11, ste12, etc. should all work!