Suppose you found a naturally occurring mRNA composed of alternating C and U res

Question

Suppose you found a naturally occurring mRNA composed of alternating C and U residues. Which of the following peptides could be formed? The codons for leucine are CUX and the codons for serine are UCX where X is any ribonucleotide. A. A polypeptide containing alternating leucine and serine residues. B. A polypeptide containing either leucine or serine residues. C. A polypeptide containing only leucine residues D. A polypeptide containing only serine residues E. Cannot be determined from the information provided

Explanation / Answer

Since the naturally occurring mRNA has alternating C and U residues, the probable sequence is as follows:

CUCUCUCUCUCUCUCUCU

Given, the codon for leucine is CUX

Codon for serine is UCX

X is any RNA, in this case either C or U.

Now since 3 amino acids make up one codon, we start reading the above probable sequence using 3 amino acids at a time. We get the following:

CUC, UCU, CUC, UCU, CUC, UCU

Hence, we get a polypeptide containing alternating leucine and serine residues. Hence the correct answer is A.

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