MCB 421 HOMEWORK #2 ANSWERS FALL 2008
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1. The Salmonella typhimurium strain TR248 is auxotrophic for both histidine and
cysteine due to a mutation in the hisC gene and a mutation in the cysA gene. His+
revertants are found at a frequency of 1 per 107 cells. Cys+ revertants are also found with
a frequency of 1 per 107 cells.
a.) How would you select for His+ revertants only or Cys+ revertants only? (What
kind of medium would you plate the cells on?)
b.) At what frequency would you expect to find revertants that are both His+ and
Cys+? How could you directly select for such double revertants?
c.) His+ Cys+ revertants are actually found at a frequency of 1 per 10 8 cells.
Propose an explanation for this result.
d.) Describe a simple genetic experiment to test your hypothesis.
2. In the following table, briefly diagram or indicate the common properties of each type
of mutation.
Mutation Missense Nonsense Frameshift Deletion Insertion
effect on DNA
effect on
Protein
effect on
Phenotype
