3. A mutation in DNA generates a UGA stop codon in the middle of the RNA coding

Question

3. A mutation in DNA generates a UGA stop codon in the middle of the RNA coding for a particular protein. A second mutation in the cell leads to a single nucleotide change in a tRNA that allows the correct translation of the protein; that is, the second mutation suppresses the defect caused by the first. The altered tRNA translates the UGA as tryptophan. What nucleotide change has probably occurred in the mutant tRNA molecule? What consequences would the presence of such a mutant tRNA have for the translation of the normal genes in this cell?

Explanation / Answer

What nucleotide change has probably occurred in the tRNA molecule as a consequence of the second mutation? That's a toughie. Not sure what your teacher wants to hear. Maybe: A change in the anticodon of a tRNA that carries a tyrosine, specifically the anticodon that matched the codon UAU. This would perhaps be a change from 3'-AUA to 3'-AUU on the anticodon, but more likely it would involve replacing the nucleotide at the 5' position with a nonstandard, more wobbly, base, so the aminoacyl-tRNA-synthetase for this tRNA would still work. > What consequences would the presence of such a mutant tRNA have for the translation of all the normal genes in this cell? Genes that had normally been terminated on the mRNA by the UAA codon would now produce a polypeptide with a tyrosine stuck on the end -- and may continue even longer if the next codon isn't another form of STOP.

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