QUESTION 72 (5 marks)
A translated mRNA segment begins . Mutant P loses the fourth nucleotide; mutant Q loses nucleotides 4–6. Read from the original start codon.
a) Write the first four complete codons for P after the deletion. [2 marks]
b) Identify which mutant has a frameshift and explain why. [2 marks]
c) Explain why Q can still change the polypeptide even though it does not cause a frameshift. [1 marks]
Practice marking scheme
Answer
a) AUG AAG GCU CUU. b) P: deletion of one nucleotide changes downstream triplet grouping; Q removes three and preserves the downstream frame. c) Q removes a codon/amino acid.
Working
Deleting the fourth A leaves AUGAAGGCUCUUAA, regrouped as AUG AAG GCU CUU with two bases remaining. The one-base deletion shifts all subsequent triplet boundaries. Removing the whole AAA codon leaves AUG GGC UCU UAA, so Q retains the downstream frame but loses the amino acid encoded by AAA.
Marking criteria
- Writes AUG AAG as the first two complete codons. [1 mark]
- Writes GCU CUU as the next two. [1 mark]
- Identifies P as the frameshift mutant. [1 mark]
- Explains one-base versus three-base deletion and downstream reading frame. [1 mark]
- States that Q deletes one codon/amino acid. [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.