25. Shown here are the amino acid sequences of the wild-type and three mutant fo

Question

25. Shown here are the amino acid sequences of the wild-type and three mutant forms of a short protein. Use this information to answer the following questions:

Wild type: mer-trp-tyr-arg-gly-ser-pro-thr

Mutant 1: met-trp

Mutant 2: met-trp-his-arg-gly-ser-pro-thr

Mutant 3: met-cys-ile-val-val-val-gln-his


Using Figure 13-7, predict the type of mutation that occurred leading to each altered protein.


For each mutant protein, determine the specific ribonucleotide change that led to its synthesis.


The wild-type RNA consists of nine triplets. What is the role of the ninth triplet?


For the first eight wild-type triplets, which, if any, can you determine specifically from an analysis of the mutant proteins? In each case, explain why or why not.


Another mutation (Mutant 4) is isolated. Its amino acid sequence is unchanged, but mutant cells produce abnormally low amounts of the wild-type proteins. As specifically as you can, predict where in the gene this mutation exists.

Explanation / Answer

Mutant 1: The protein stops with tryptophan. This indicates a point mutation in the codon of Tyrosine which converts it into a stop codon. The codons for tyrosine are UAU and UAC. The stop codons are UAA or UAG.

Mutant 2: Only one amino acid change is seen. The change is from tyrosine to histidine. This is also a point mutation as the codons for tyrosine are UAU and UAC. The codons for histidine are CAU and CAC.

Mutant 3: A frameshift mutation has occured with a deletion and an insertion. This causes the change in the whole sequence following the deletion. a "G" has been deleted in the tryptophan codon of UGG. Due to this deletion, the reading frame has shifted and so different amino acids make up the protein. As the total number of amino acids do not change from the wildtype protein, this indicates that an insertion has also occurred.

The ninth triplet in the wildtype RNA is the stop codon.

I guess your question is whether we can determine the sequence of the wildtype triplets from the mutant strains. This is a bit difficult as each amino acid has more than one codon. without knowing which of the codons is used it is difficult to determine the actual sequence. Some amino acids like tryptophan have only one codon. Only this sequence can be easily determined. Hence, the second triplet sequence can be determined.

The mutant 4 probably has a mutation in the promoter of this group of genes. If the promoter is mutated or repressed the genes will be transcribed lesser.A strong promoter induces continuous transcription of genes.

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