Problem Set 2: Yeast
Due: Monday 10 / 22, in class
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Please be explicit and detailed in your answers in order to receive full credit.
Please type out answers (you can draw in any diagrams).
Have your name, question #, and page number (example: John Smith, Question
#1, Page 1 of 2) on each page.
Start each question on a new page and staple each question together separately.
1. In class, I explained why homologous recombination is required for meiosis: it
allows cohesive tension between sister chromatids to be translated into cohesion
between homologues. If a mutant that is unable to do homologous recombination
is sporulated it produces almost entirely dead progeny. However, if you plate out
all the tetrads (you don’t even need to dissect) a small number are viable (less
than 1%). This could actually be used to map a gene to a chromosome. If one
had a temperature sensitive mutant at an unknown locus, how could you use a
recombination-defective mutant background to map this temperature sensitive
trait to a chromosome (i.e. find out which chromosome the gene encoding the
mutation is on)? Explain what other strains you would need to do this.
2. As mentioned in class, yeast have two mating types determined by the MAT
locus, which encodes transcription factor genes (see diagram next page). MAT a
cells make all the gene products required to mate as a cells. Mat cells make all
the gene products to form  cells. In diploids, the combination of the
transcription factors encoded by MAT a and MAT turn off the haploid-specific
genes and turns on the diploid-specific genes. That is how it knows it is a diploid
for the purposes of things like sporulation.
You would like to isolate mutants that are defective in mating:
A) How could you set up this screen? Explain in some detail.
B) You have isolated 5 mating-defective mutants in this screen. You would
like to determine how many genes these represent (i.e. do you have
multiple alleles of any genes). You decide to do this by determining
whether any of the mutations are linked to each other, figuring that if two
mutants’ defects are very tightly linked, they are likely to be affecting in
the same gene. (At this point you are not identifying the gene affected in
each mutant. That is, you don’t know where the mutation is.) The
problem is that, by definition, these are mating defective mutants! How
could you go back and change the screen in part (A) to allow you to mate
your mutants?
C) How could you determine dominance? Again, this is without “cloning”
your gene (that is, you have a mutant, but you don’t know the affected
gene).
3.
Gene conversion was discovered in yeast. Yet, there was years of work preceding
this studying genetics in peas, fruitflies and mice. Why wasn’t gene conversion found in
these organisms? (Hint, it isn’t just because yeast geneticists are better)