A synthetically prepared mRNA containing repetitive AU sequences, when incubated with mammalian cell extract containing ribosomes, tRNAs, and all necessary factors for protein synthesis, will most likely produce:

(2) Two peptides with alternating sequences of two amino acids.

Explanation:

• The repetitive AU sequence in mRNA corresponds to repeating codons of AUA and UAU or similar combinations depending on the reading frame.

• Since the mRNA consists of alternating adenine (A) and uracil (U) nucleotides, the ribosome will read codons that alternate between two different triplets.

• Each codon codes for a specific amino acid, so the resulting peptide will have an alternating sequence of two amino acids corresponding to these codons.

• Without an initiation codon, translation can begin at multiple sites, but the repetitive nature of the sequence means the peptides formed will have repeating alternating amino acids.

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Conclusion

Given a synthetic mRNA with repetitive AU sequences and a mammalian translation system, the ribosomes will produce two peptides with alternating sequences of two amino acids. This outcome is due to the alternating nature of the codons formed by the AU repeats, resulting in peptides with alternating amino acid residues.

Correct answer: (2) Two peptides with alternating sequence of two amino acids.