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?

Verified step by step guidance

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.
2. 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.
3. 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.
4. 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.
5. 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.

Key Concepts

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.

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.

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.