Geneticists are studying a malfunctioning protein that causes impaired endothelial cell migration and angiogenesis. They have noticed that the amino acid sequence of the protein is truncated compared to normal controls. The mutated mRNA strand is isolated, and analysis shows a nonsense mutation located near the 3' end of the coding region. The 3' terminal coding sequence of the abnormal mRNA strand is shown below. Which of the following tRNA anticodons is responsible for adding the last amino acid to the truncated polypeptide during protein translation?

Question

Geneticists are studying a malfunctioning protein that causes impaired endothelial cell migration and angiogenesis.  They have noticed that the amino acid sequence of the protein is truncated compared to normal controls.  The mutated mRNA strand is isolated, and analysis shows a nonsense mutation located near the 3' end of the coding region.  The 3' terminal coding sequence of the abnormal mRNA strand is shown below.  Which of the following tRNA anticodons is responsible for adding the last amino acid to the truncated polypeptide during protein translation?

Quizplus · Accepted Answer

11ec160b_3767_bfb0_9249_29ddd1062b15_MD0001_00 One codon (AUG) signals initiation of protein synthesis and 3 codons (UAA, UAG, and UGA) stop protein synthesis. Stop codons function only to terminate translation; they do not add amino acids to the polypeptide chain. Transfer RNA molecules (tRNA) transport amino acids to the ribosome and ensure placement of the proper amino acid. One end of the molecule serves as the amino acid binding site; the opposite end contains a specific nucleic acid sequence known as the anticodon that is complementary to one or more mRNA codons (due to base wobbling). The type of amino acid that is bound to each tRNA molecule is determined by its anticodon sequence; this ensures that the proper amino acid is added at each mRNA codon during protein synthesis. Translation of the mRNA template proceeds in the 5' to 3' direction. The mRNA of the dysfunctional protein in the question stem contains an extra stop codon (UAA) before the normal stop codon (UAG) at the end of the template. During protein translation, the first stop codon encountered will bind a release factor, halting protein synthesis. Therefore, the codon just prior to the first stop codon will be the last codon to add an amino acid. In this case, (5'-UUG-3') is the last codon to add an amino acid to the truncated protein, and this amino acid will be carried by the 5'-CAA-3' anticodon (codon-anticodon binding occurs in opposite directions [ie, 5' to 3' binds 3' to 5']). (Choice A) Because complementary sequences align in antiparallel fashion, during translation the tRNA anticodons will bind the 5' to 3' mRNA in the (opposite) 3' to 5' direction. Therefore, the 5'-UUG-3' mRNA codon will bind the 5'-CAA-3' tRNA anticodon, not the 5'-AAC-3' anticodon. (Choices B and E) The 5'-AUC-3' and 5'-UCG-3' anticodons will bind to the 5'-GAU-3' and 5'-CGA-3' codons, respectively (5' to 3' binds 3' to 5'). These are not present in the above mRNA sequence. (Choice D) The last codon shown in the above mRNA sequence is 5'-UAG-3' (a stop codon), with the second-to-last codon being 5'-AGC-3'. Therefore, 5'-GCU-3' would be the tRNA anticodon responsible for adding the last amino acid to the normal (non-truncated) protein. Educational objective: Translation of the mRNA template proceeds in the 5' to 3' direction. Because complementary sequences align in antiparallel fashion, during translation tRNA anticodons will be oriented in the opposite 3' to 5' direction. Stop codons (UAA, UAG, and UGA) halt protein synthesis by binding a release factor; they do not add amino acids to the polypeptide chain.

Geneticists Are Studying a Malfunctioning Protein That Causes Impaired Endothelial