Explain how a eukaryotic cell ensures that a polypeptide is only produced from the coding sequence of a gene, rather than from the entire pre-mRNA transcript.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Pre‑mRNA contains non‑coding intervening sequences called introns that must be removed before translation. The coding sequences (exons) are joined together after intron removal to form mature mRNA. This process of intron removal and exon joining is called RNA splicing, a post‑transcriptional processing step. Only the mature mRNA, which contains only exon sequences, is exported from the nucleus and used as a template for translation at ribosomes.
Examiner tips
- Mention introns and exons explicitly; explain splicing as the removal of introns and ligation of exons; state that only mature mRNA leaves the nucleus for translation.
- Use the term ‘post‑transcriptional processing’ and ‘RNA splicing’ as these are key phrases in the mark scheme.
Mark scheme (4 marks)
- Pre-mRNA contains non-coding intervening sequences called introns that must be removed before translation
- The coding sequences (exons) are joined/ligated together after intron removal to form mature mRNA
- This process of intron removal and exon joining is called RNA splicing / post-transcriptional processing
- Only the mature mRNA (containing only exon sequences) is exported from the nucleus and used as a template for translation at ribosomes
Key terms in this question
Related
- All IB DP Biology Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →