Explain how the wobble hypothesis accounts for the fact that the number of tRNA molecules required for translation is less than the number of sense codons in the genetic code.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
The genetic code has 61 sense codons but fewer tRNA anticodons are needed because of the wobble hypothesis.
1. The wobble position is the third base of the codon (first base of the anticodon).
2. Base‑pairing rules are relaxed here, so a single tRNA anticodon can pair with more than one codon.
3. The wobble base can hydrogen‑bond with two or more complementary bases.
4. Thus synonymous codons that differ only at the third position are read by the same tRNA, reducing the number of distinct tRNA molecules required.
1. The wobble position is the third base of the codon (first base of the anticodon).
2. Base‑pairing rules are relaxed here, so a single tRNA anticodon can pair with more than one codon.
3. The wobble base can hydrogen‑bond with two or more complementary bases.
4. Thus synonymous codons that differ only at the third position are read by the same tRNA, reducing the number of distinct tRNA molecules required.
Examiner tips
- Use the term ‘wobble position’ and specify it is the third codon base.
- Explain that relaxed pairing allows one tRNA to recognise multiple codons.
- Show the link to synonymous codons to justify fewer tRNAs.
- Keep the answer concise – 4 points, one per sentence.
Common mistakes
- Confusing the wobble position with the first codon base.
- Failing to mention synonymous codons.
- Using vague language like ‘some flexibility’ instead of ‘relaxed base‑pairing’.
Mark scheme (4 marks)
- The genetic code contains 61 sense (non-stop) codons but far fewer tRNA anticodon types are needed to decode them.
- The wobble position is the third (3′) base of the codon / first (5′) base of the anticodon, where base-pairing rules are relaxed.
- One tRNA anticodon can recognise more than one codon because the base at the wobble position can form hydrogen bonds with more than one complementary base.
- This means that synonymous codons (codons coding for the same amino acid that differ only at the third position) can be read by the same tRNA, reducing the total number of distinct tRNA molecules needed.
Key terms in this question
wobble hypothesis · sense codon
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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