Problem Set 2: Yeast
Due: Monday, October 24, 5pm (NO LATE PROBLEM SETS ACCEPTED!!!)
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Turn in problem sets to the Agard lab, room S414, in the box on Elena’s desk
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. 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. The normal turn of events is that one will see that mutants with alleles of the
same gene will not complement. That is, one will see ALLELIC
NONCOMPLEMENTATION or NONALLELIC COMPLEMENTATION.
However, occasionally you’ll see
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allelic complementation or
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nonallelic noncomplementation.
Allelic complementation: This is defined as a situation in which two different
alleles of the same gene will complement each other for some phenotype.
Nonallelic noncomplementation: This refers to a situation in which two
recessive mutations in different genes, when they are each present in a diploid as
heterozygotes (each over a wildtype allele), nonetheless generate a phenotype of
the same type seen for the single alleles in a haploid.
Examine the complementation table below.
A) How do you know that this is not a simple case of “normal”
complementation?
B) Which scenario might we be seeing here, allelic complementation or
nonallelic noncomplementation? Come up with an explanation (in terms
of which interactions are displaying which types of complementation) that
could explain these data. Note that there are many possible answers, but
one is quite simple.
C) Without having identified any of the genes affected in these mutants, how
could you confirm the answer you gave in part (B)?