Download Problem 1. (10 Points) List all possible single

Problem 1. (10 Points) List all possible single-nucleotide substitutions causing missense mutations for an AGG
arginine codon and indicate the amino that is substituted for arginine in each mutant.
AGG can change to GGG (Gly) or TGG (Trp) or ATG (Met) or ACG (Thr) or AAG (Lys) or
AGT (Ser) or AGC (Ser) 10 - 5 - 0
Problem 2. (10 Points) List all possible single-nucleotide substitutions causing silent mutations for an AGG
arginine codon.
AGG can change to AGA or CGG 10 - 5 - 0
Problem 3. (10 Points) A protein has the sequence Met-Thr-X-Lys-Val where “X” indicates that the amino acid
is unknown. Four new strains containing single-nucleotide substitutions in the gene encoding this protein were
isolated and the corresponding mutant proteins were sequenced. The protein sequence of mutant 1 was MetThr-Arg-Lys-Val. The protein sequence for mutant 2 was Met-Thr-Ser-Lys-Val. The protein sequence of
mutant 3 was Met-Thr-Leu-Lys-Val. The protein sequence for mutant 4 was Met-Thr-Gly-Lys-Val. Based on
this information, what amino acid is corresponds to “X” in the wild type protein sequence.
The wild type amino acid in the third position was Trp (TGG) which can mutate to Arg (CGG)
or Ser (TCG) or Leu (TTG) or Gly (GGG). 10 - 5 - 0
Problem 4. You are studying four his- bacterial strains (A, B, and C), each of which carries a different type of
mutation in the same gene required for histidine biosynthesis. You treat bacteria from each strain with buffer
only or buffer plus a base analog. You plate 108 bacteria from each treatment on medium that lacks histidine,
allow colonies to grow and get the following results:
Strain A: The buffer only plate has 10 colonies; the buffer plus mutagen plate has 10 colonies.
Strain B: The buffer only plate has 10 colonies; the buffer plus mutagen plate has 100 colonies.
Strain C: The buffer only plate has 100 colonies; buffer plus mutagen has 100 colonies.
Use this information to answer the following questions.
a) (10 Points) Give one example of a type of mutation that is in the histidine biosynthesis gene in Strain A.
Expalin.
Base analogs cause transition mutations, therefore, strain A has something other than a
transition mutaiton (deletion or insertion = frameshift or a transversion) 10 - 5 - 0
b) (10 Points) What type of mutation is in the histidine biosynthesis gene in Strains B? Explain.
Strain B has a transition mutation. 10 - 5 - 0
c) (10 points) Provide an explanation for the results obtained with strain E
The strain C culture contained or produced (depending on how you want to think about it)
more his+ mutant bacteria per 10 to the 8th cells plated that A or B. You could argue that
this culture was grown differently that A or B and because of fluctuation, yielded a higher
frequency of his+ bacteria as in Luria and Delbruk's fluctuation tes t (not what I was going for
but is a reasonable way to look at it as far as I'm concerned) or you could argue that strain C
exhibits a higher frequency of spontaneous mutations because it caries an additional
mutation in a gene leading to increased mutation rate. The most obvious examples from
your reading are mutations in gene in DNA repair pathways. 10 - 5 - 0
Problem 5. The figure shows two petri plates containing
medium that lacks histidine. The plates have Salmonella
bacterial colonies and a filter disk in the center. 10 million his –
bacteria were plated on both plates. The his – mutation in these
bacteria is a frameshift mutation. The filter disk in the plate on
the right was soaked in buffer only. The filter disk in the plate
on the left was soaked in buffer plus an unknown mutagen. Use
this information to answer the following questions.
a) (10 Points) Name a specific mutagen (use an example from your textbook) that could correspond to the
mutagen added to the filter disk in the plate on the left.
An acridine dye or other "intercalating" agent would be a good answer here. Since I
mentioned that alkylating agents are good "broad-spectrum" mutagens that give all kinds of
mutations I will also give full credit for that. 10-5-0
b) (10 Points) Which plate or plates contain mutants created by spontaneous mutations?
Explain.
Both plates contain mutant created by spontaneous mutations. Mutagens act by increasing
the rate or frequency of mutations but do not affect the spontaneous mutation rate. 10-5-0
c) (10 Points) Why are there colonies close to the filter on the plate on the right but not the plate on the left?
The filter on the right was soaked in buffer only, which can be assumed to have not effect on
bacterial growth or mutation. Therefore, spontaneous his+ mutants produced colonies
regardless of where they were located on the plate (close to or far away from the filter). The
filter on the left was soaked in mutagen, which clearly induces frameshift mutations. The
concentration of the mutagen will behighest closest to the filter. The lack of his+ colonies
close to the filter could be explained by lethality due to high mutation rates or simple toxicity
of the mutagen.
10 - 5 - 0