QUESTION 138 (5 marks)
A primary RNA transcript is written 5′ to 3′ as exon 1 AUGGA, intron UCUCU, exon 2 ACUUUAA. These two exons form the complete coding region, with one start and one terminal stop codon. Ignore the cap and poly-A tail. Use the code excerpt.
| RNA codon | Meaning |
|---|---|
| AUG | Methionine/start |
| GAA | Glutamate |
| CUU | Leucine |
| UAA | Stop |
a) Write the mature coding RNA with direction shown and codons grouped. [2 marks]
b) Determine its polypeptide sequence and indicate where translation stops. [2 marks]
c) Identify the codon that uses bases from both exons. [1 mark]
Practice marking scheme
Answer
Spliced RNA: 5′-AUG GAA CUU UAA-3′. Polypeptide: methionine–glutamate–leucine, then stop. The GAA codon crosses the exon junction.
Working
Removing UCUCU joins AUGGA directly to ACUUUAA, producing AUGGAACUUUAA. Reading triplets from the start gives AUG/GAA/CUU/UAA. The last two bases of exon 1 and first base of exon 2 form GAA, showing why codon grouping must be applied after splicing. UAA terminates translation and supplies no amino acid.
Marking criteria
- Removes the intron and joins the exons in the stated order. [1 mark]
- Writes 5′-AUG GAA CUU UAA-3′. [1 mark]
- Identifies methionine–glutamate–leucine. [1 mark]
- States stop at UAA, with no amino acid added. [1 mark]
- Identifies GAA as spanning the exon junction. [1 mark]
Practice question aligned to the current QCAA syllabus; review the worked solution and marking criteria.
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