Question

 

a) (i) What is it about a mutant phenotype that classifies the mutation as “temperature-sensitive”?

(ii) How is this type of phenotype explained by protein behaviour?

b) Would the following types of mutations most likely have: severe effects, mild effects, or no effect on protein function? Briefly explain your answers. Note that there may be more than one correct answer, but you need only to give one sensible answer and explain your reasoning. 

(i) A nonsense mutation occurring in sequences encoding amino acids near the carboxy terminus of the protein

(ii) A missense mutation affecting the active site of the protein

(iii) A frameshift mutation near the 5’ end of the open reading frame

Answer 1

a)

(i) A mutation is temperature-sensitive only when it is lethal under certain conditions. The individuals develop and multiply normally at permissive temperature but when the temperature rises for longer duration, the organisms die. The temperature at which lethal allele is expressed is known as restrictive temperature.

(ii) At permissive temperature, the protein folds normally or remain properly folded and cannot be differentiated from wild-type protein. However, loss of protein function occurs due to denaturation at the non-permissive temperature.

 

b)

(i) A nonsense mutation is one in which sense codon is changed to a stop codon. Nonsense mutation occurring in sequences encoding amino acids near the carboxy terminus of the protein will have mild effects if none of the last few amino acids are important for enzyme function.

(ii) A missense mutation is one in which the amino acid is changed to produce a different amino acid. A missense mutation affecting the active site of the protein will have severe effects as it is likely to destroy the enzyme activity.

(iii) A frameshift mutation near the 5’ end of the open reading frame will have severe effects as there will be no functional protein.