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Ch. 12 - The Genetic Code and Transcription
Klug - Essentials of Genetics 10th Edition
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Tutti i libri di testoKlug 10th EditionCh. 12 - The Genetic Code and TranscriptionProblema 11
Capitolo 12, Problema 11

Refer to Table 13.1. Can you hypothesize why a synthetic RNA composed of a mixture of poly U poly A would not stimulate incorporation of ¹⁴C-phenylalanine into protein?

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1
Step 1: Understand the role of synthetic RNA in protein synthesis. Synthetic RNA sequences like poly U or poly A are used to test which amino acids are incorporated into proteins by ribosomes, based on the codon they represent.
Step 2: Recall that poly U RNA consists of repeating uracil nucleotides, which code for the amino acid phenylalanine (Phe) according to the genetic code. This is why poly U stimulates incorporation of ¹⁴C-phenylalanine into protein.
Step 3: Recognize that poly A RNA consists of repeating adenine nucleotides, which code for the amino acid lysine (Lys), not phenylalanine. Therefore, poly A alone would not stimulate incorporation of ¹⁴C-phenylalanine.
Step 4: Consider what happens when you mix poly U and poly A sequences. The mixture contains codons for both phenylalanine and lysine, but the presence of poly A dilutes the poly U codons, reducing the frequency of phenylalanine codons available for translation.
Step 5: Hypothesize that because the synthetic RNA is a mixture of poly U and poly A, the ribosomes encounter fewer phenylalanine codons overall, leading to a reduced or no stimulation of ¹⁴C-phenylalanine incorporation into protein.

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Genetic Code and Codon Specificity

The genetic code consists of codons, three-nucleotide sequences in mRNA that specify particular amino acids. Each codon corresponds to one amino acid, so a synthetic RNA with mixed poly U and poly A sequences would produce ambiguous signals, preventing specific amino acid incorporation like phenylalanine, which is encoded by UUU or UUC codons.
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The Genetic Code

Role of Synthetic RNA in In Vitro Translation

Synthetic RNA sequences are used in experiments to direct protein synthesis by ribosomes. A homopolymer like poly U directs incorporation of phenylalanine, but a mixture of poly U and poly A disrupts this specificity, leading to inefficient or no incorporation of labeled amino acids such as ¹⁴C-phenylalanine.
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RNA

Aminoacyl-tRNA Recognition and Codon-Anticodon Pairing

During translation, aminoacyl-tRNAs recognize specific codons via complementary anticodons, ensuring correct amino acid addition. Mixed RNA sequences interfere with proper codon-anticodon pairing, so tRNA carrying phenylalanine cannot bind effectively, preventing its incorporation into the growing polypeptide chain.
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tRNA
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