mcb421Homework 2010 2 Answers-1 - MCB 421 HOMEWORK#2...

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MCB 421 HOMEWORK #2 ANSWERS FALL 2006 Page 1 of 6 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 10 7 cells. Cys + revertants are also found with a frequency of 1 per 10 7 cells. a.) How would you select for His + revertants only or Cys + revertants only? (What kind of medium would you plate the cells on?) ANSWER: Plate greater than 10 7 cells on a minimal plate without His (for His+ revertants) or without Cys (for Cys+). Any cells growing on the plate are revertants. 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? ANSWER: If the reversions are the result of independent events, then the frequency will be the product of the two independent frequencies, i.e. 10 -7 x 10 -7 equals one in 10 14 cells. The revertants could be selected by plating on plates lacking both his and cys. c.) His + Cys + revertants are actually found at a frequency of 1 per 10 8 cells. Propose an explanation for this result. ANSWER: The higher than expected frequency tells us that reversion at both loci must occur through a single event. One explanation for this is that both the His and Cys pathways have a common step that was affected by the mutation. Another explanation is that both the mutations were nonsense mutations and the mutation that reverted the strain mutated a tRNA gene so that the altered tRNA can now read nonsense codons. (The latter explanation will be illustrated in a future lecture). 2. In the following table, briefly diagram or indicate the common properties of each type of mutation.
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MCB 421 HOMEWORK #2 ANSWERS FALL 2006 Page 2 of 6 ANSWER: Mutation Missense Nonsense Frameshift Deletion Insertion effect on DNA base substitution base substitution resulting in a stop codon insertion or deletion of 1 or 2 base pairs loss of multiple base pairs addition of multiple base pairs effect on Protein substituted amino acid truncated polypeptide; inactive protein altered amino acid sequence downstream of mutation; usually truncated polypeptide; inactive protein usually absent; usually inactive protein may insert extra amino acid (if in frame) or cause premature truncation; usually inactive protein effect on Phenotype may result in loss of
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  • Fall '08
  • Gardner,J
  • Mutation, amino acid sequence, CTG Lys Asn Asp Lys Leu Lys Leu Leu Lys Leu, AGT Lys Asn Asp Lys Leu Lys Leu Leu Asn Cys Ile Gln Ser, double mutant FCO

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