Biology • Year 12 • Module 5 • Lesson 11
Translation — From mRNA to Polypeptide
Build HSC Band 5–6 extended-response technique on the mechanism of translation, using real data from a sickle-cell point mutation and a recombinant insulin production scenario.
1. Extended response — sickle-cell anaemia and a single codon change (Band 5–6)
8 marks Band 5–6
Scenario. In sickle-cell anaemia, a single point mutation in the β-globin gene changes one DNA base, which in turn changes one mRNA codon from GAG (glutamic acid, Glu) to GUG (valine, Val) at amino acid position 6 of the β-globin polypeptide. The resulting haemoglobin (HbS) polymerises under low oxygen, deforming red blood cells into the characteristic sickle shape. Allison's 1954 field study in malaria-endemic regions of East Africa recorded the data below comparing red-cell shape under low oxygen and parasitemia (% red cells carrying Plasmodium falciparum) for individuals with normal haemoglobin (HbA/HbA) and sickle-cell trait carriers (HbA/HbS).
| Genotype | % red cells sickling at low O₂ | Mean parasitemia (% cells infected) |
|---|---|---|
| HbA / HbA (normal) | 0% | 9.8% |
| HbA / HbS (carrier) | 30–40% (variable) | 2.1% |
| HbS / HbS (sickle-cell disease) | > 80% | < 1% |
Data adapted from Allison (1954), British Medical Journal 1: 290-294.
Q1. Analyse and evaluate, using lesson content on translation, how a change of one mRNA codon leads to the observed phenotype, and assess what the data tell us about whether the same translation machinery is operating equally accurately in all three genotypes. In your response you must:
- Describe what a ribosome, mRNA and tRNA each do at the moment the mutated codon is being translated.
- Use codon-anticodon pairing to explain why the new codon results in Val being inserted instead of Glu.
- Link the substituted amino acid to the change in polypeptide and therefore in protein function.
- Use the data to support a judgement about whether translation itself is faulty in the carriers, or whether it is the input (mRNA codon) that is different.
- Reach an evidence-based overall judgement using precise lesson terminology.
2. Extended response — recombinant insulin from E. coli (Band 5–6)
8 marks Band 5–6
Scenario. Since 1982, recombinant human insulin has been manufactured by inserting the human insulin gene into Escherichia coli bacteria. The bacteria transcribe the inserted gene and then translate the resulting mRNA on their own ribosomes, producing the human insulin polypeptide for purification. A pharmaceutical team running such a production line wishes to increase yield. They measure the data below across four production batches.
| Batch | Condition | Insulin polypeptide yield (mg / L culture) | Average completeness of polypeptide (%) |
|---|---|---|---|
| 1 | Standard human mRNA codons | 110 | 96% |
| 2 | mRNA codons optimised for E. coli tRNA pool | 340 | 97% |
| 3 | Standard codons + ribosome inhibitor added (low dose) | 40 | 74% |
| 4 | Standard codons + one engineered premature STOP codon at residue 31 | 0 (no full-length insulin) | 34% |
Hypothetical industrial trial data — typical of codon-optimisation studies (e.g. Welch et al. 2009, PLoS ONE 4: e7002).
Q2. Analyse, using lesson content on translation, why each manipulation in Batches 2–4 produces the observed change in yield and completeness, and justify which batch best demonstrates that "the order of codons on the mRNA determines the order of amino acids in the polypeptide." In your response you must:
- Define the role of the ribosome and of tRNA at the point a codon is being decoded.
- Explain Batch 2's higher yield using the idea that tRNA availability limits codon decoding.
- Explain Batch 3's lower yield + lower completeness in terms of ribosome function.
- Explain Batch 4's zero full-length insulin in terms of codon-anticodon pairing and the STOP codon.
- Choose one batch as the strongest evidence for the lesson's central claim, and justify that choice.
Q1 — Sample Band 6 response (8 marks), annotated
Translation is the cytoplasmic process in which a ribosome reads an mRNA molecule three bases at a time, while transfer RNAs (tRNAs) deliver specific amino acids whose order is determined by codon-anticodon pairing. At codon 6 of the β-globin mRNA, the ribosome sits on the codon being read; a charged tRNA arrives with an anticodon complementary to that codon; once the next tRNA arrives at the adjacent site, the ribosome catalyses formation of a peptide bond between the two amino acids. [1 — describes ribosome, mRNA and tRNA at the mutated codon]
In a normal individual the codon at position 6 is GAG, so the tRNA with anticodon CUC pairs with it and delivers glutamic acid (Glu). In sickle-cell anaemia the mutated codon is GUG, so a tRNA with the complementary anticodon CAC pairs with it and delivers valine (Val) instead. [1 — codon-anticodon pairing for the new codon explains Val insertion]
The result is a polypeptide in which one amino acid at position 6 has been substituted (Glu → Val). Because Glu is negatively charged and hydrophilic while Val is hydrophobic, this single change creates a sticky hydrophobic patch on the outside of the β-globin protein. Under low oxygen, these patches stick to each other and HbS molecules polymerise into long fibres, deforming red blood cells into the sickle shape. [1 — links the substituted amino acid to polypeptide and protein function] [1 — uses precise lesson terms: codon, anticodon, polypeptide]
The data make an important distinction visible. The HbA/HbS carriers show roughly 30–40% sickling under low O₂ (because half their β-globin is HbS), while HbS/HbS shows > 80%. The proportion of sickling tracks the proportion of mutated mRNA being translated — not any malfunction of the ribosome or tRNAs themselves. The translation machinery is operating just as accurately in all three genotypes; it is simply being given a different message. [1 — uses the data to judge that translation itself is not faulty]
This is the lesson's central point in action: codon-anticodon matching faithfully translates whatever sequence is presented to it. Accuracy of translation in HbS individuals is not the problem — fidelity of the message is. [1 — explicit overall judgement in precise lesson terms]
The data also show why the mutation persists at high frequency in malaria-endemic regions: carriers have markedly lower parasitemia (2.1% vs 9.8%) because P. falciparum reproduces poorly in HbS-containing red cells. So although the mutation produces a non-functional polypeptide variant, the resulting phenotype confers a survival advantage where malaria is common — a striking demonstration that "correct translation" of a sequence does not, on its own, guarantee that the protein produced is fit for every environment. [1 — extends evaluation to fitness context]
Award the final mark for a sophisticated response that integrates accurate translation mechanism, codon-anticodon detail, polypeptide-to-protein link, and data-grounded judgement throughout — not just for a concluding sentence. [1 — synthesis quality]
Marking notes (8 marks): Award 1 mark per criterion as annotated above. Cap at 6 if any of (ribosome role, codon-anticodon pairing on GUG, Glu→Val substitution, data-grounded judgement) is missing.
Q2 — Sample Band 6 response (8 marks), annotated
During translation, the ribosome reads an mRNA codon three bases at a time and a charged tRNA whose anticodon is complementary to that codon delivers the specified amino acid. Once two adjacent amino acids are held in position, the ribosome catalyses the peptide bond that lengthens the polypeptide. [1 — ribosome and tRNA roles defined precisely]
Batch 2 — codon optimisation (340 mg/L, 97% complete). E. coli has its own tRNA pool, biased toward the codons it normally uses. The standard human mRNA contains some codons whose matching tRNAs are rare in E. coli; ribosomes stall whenever they reach those codons, slowing translation. Recoding the same protein sequence using codons that match the abundant E. coli tRNAs means a complementary anticodon is almost always immediately available, so the ribosome moves smoothly along the mRNA and yield rises ≈ 3-fold. Completeness barely changes because tRNA shortages slow translation but do not introduce wrong amino acids. [1 — Batch 2 explained via tRNA availability and codon-anticodon pairing]
Batch 3 — ribosome inhibitor (40 mg/L, 74% complete). A drug that interferes with ribosome function reduces both how fast the ribosome reads codons and how reliably it elongates the chain. Some ribosomes stall and dissociate part-way through, releasing truncated polypeptides; this drops the average completeness to 74% and slashes the yield of full-length insulin. This shows that translation depends on a functional ribosome to coordinate the process — without it the mRNA carries the code but cannot be read. [1 — Batch 3 explained in terms of ribosome function] [1 — uses lesson Card 1 framing]
Batch 4 — engineered premature STOP at residue 31 (0 mg/L full-length, 34% complete). A STOP codon does not match any tRNA anticodon. When the ribosome reaches it, no amino acid can be delivered; instead the polypeptide is released early. Every ribosome that begins reading this mRNA terminates at residue 31, so the average completeness drops to ≈ 31/86 ≈ 36% — close to the 34% observed — and no full-length insulin is produced at all. [1 — Batch 4 explained via codon-anticodon pairing failing at STOP]
Best evidence for "codon order determines amino acid order". Batch 4 is the strongest evidence. In Batch 2, the protein sequence is unchanged — the experiment only changes which codons code for those amino acids, not the resulting polypeptide. In Batch 3, the rate of translation changes but again the sequence is unchanged. Only in Batch 4 is the actual order of codons changed (by inserting a STOP); only there is the order of amino acids in the polypeptide changed (truncated at residue 31). This directly demonstrates the lesson's claim that the order of codons on the mRNA determines the order of amino acids in the polypeptide. [1 — explicit justified choice of Batch 4]
Together these four batches show that translation requires both a functional ribosome and an adequate tRNA pool, and that the precise sequence of codons specifies the precise sequence of amino acids — exactly the framing in Cards 1–4 of the lesson. [1 — overall synthesis linking back to lesson framework] [1 — uses precise terminology throughout: codon, anticodon, ribosome, tRNA, polypeptide, peptide bond, STOP]
Marking notes (8 marks): 1 mark per criterion as annotated. Cap at 6 if the chosen "best evidence" batch is not justified, or if either Batch 2's tRNA pool reasoning or Batch 4's STOP codon reasoning is missing.