QUESTION 70 (6 marks)
A eukaryotic primary RNA transcript contains three exons of 90, 120 and 150 nucleotides, separated by two introns of 80 and 100 nucleotides. Ignore the 5′ cap and poly-A tail in length calculations.
a) Calculate primary-transcript length and spliced-RNA length. [2 marks]
b) Describe three changes during processing that produce a typical mature mRNA. [3 marks]
c) Explain why spliced-RNA length alone does not always determine the exact number of amino acids in the protein. [1 marks]
Practice marking scheme
Answer
a) Primary: 540 nucleotides; spliced: 360 nucleotides. b) Introns removed/exons joined; 5′ cap added; poly-A tail added. c) Mature RNA may contain untranslated regions and a stop codon.
Working
Total primary length is 90 + 120 + 150 + 80 + 100 = 540; removing the introns leaves 360. RNA processing joins exons, adds a cap at the 5′ end and adds a poly-A tail at the 3′ end. Mature mRNA can include regions outside the translated coding sequence, and the stop codon does not code for an amino acid; therefore total length divided by three need not equal protein length.
Marking criteria
- Calculates primary length as 540. [1 mark]
- Calculates spliced length as 360. [1 mark]
- Describes removal of introns and joining of exons. [1 mark]
- Describes addition of a 5′ cap. [1 mark]
- Describes addition of a poly-A tail. [1 mark]
- Recognises untranslated regions and/or the non-amino-acid stop 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.