QUESTION 69 (6 marks)
A short coding region on a DNA template is . The genetic code excerpt is provided.
a) Write the mRNA sequence with direction shown. [2 marks]
b) Determine the polypeptide sequence encoded by this region, indicating where translation stops. [2 marks]
c) Describe how a tRNA molecule contributes to translation. [2 marks]
Practice marking scheme
Answer
a) 5′-AUG UUU CCG UGA-3′. b) Methionine–phenylalanine–proline, then stop. c) A tRNA carries a specific amino acid and matches its anticodon to an mRNA codon.
Working
Transcription forms complementary RNA using U in place of T. The mRNA is read in codons from 5′ to 3′. AUG specifies methionine, UUU phenylalanine and CCG proline; UGA is a stop signal and does not add an amino acid. tRNA anticodons pair with appropriate codons at the ribosome, bringing amino acids for incorporation into the growing chain.
Marking criteria
- Writes AUG UUU CCG UGA. [1 mark]
- Shows 5′ to 3′ direction. [1 mark]
- Gives methionine–phenylalanine–proline in order. [1 mark]
- Recognises UGA as a stop signal rather than an amino acid. [1 mark]
- States that tRNA carries a specific amino acid. [1 mark]
- Describes anticodon pairing with the corresponding mRNA codon. [1 mark]
Practice question aligned to the current QCAA syllabus; review the worked solution and marking criteria.
View the QCAA syllabusCompare your working with the guide above.