The big idea: Many polypeptides cannot work straight after translation. They must be modified first: insulin is made in two stages.
The points to remember
- Translation makes pre-proinsulin, a single polypeptide.
- Stage 1, in the rough ER: the signal peptide is removed, giving proinsulin.
- Proinsulin folds and disulfide bonds form.
- Stage 2, in the Golgi apparatus and vesicles: the C-peptide is cut out.
- Insulin is two chains, A (21 amino acids) and B (30), joined by disulfide bonds.
Remember it as: Pre-pro-insulin: cut the pre, cut the pro, keep the insulin.
Real example: collagen. Collagen chains must have some of their proline amino acids modified after translation, by an enzyme that needs vitamin C. Without vitamin C, collagen is weak: this is the disease scurvy.
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Why modification matters: Translation gives the amino acid sequence; modification gives the working shape.
| Modification | Example |
|---|---|
| Parts cut off | Signal peptide and C-peptide removed from pre-proinsulin |
| Bonds added | Disulfide bonds hold the A and B chains of insulin together |
| Amino acids changed | Proline in collagen modified (needs vitamin C) |
Order and place: Signal peptide first, in the rough ER; C-peptide second, in the Golgi apparatus and its vesicles. Do not swap them.
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How it is asked: The two stages from pre-proinsulin to insulin, or why a protein takes time to work after its mRNA is made.
IB-style questionOutline[3 marks]
Outline how pre-proinsulin is modified to produce insulin.
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See the answer broken into clear points, with guidance on how each point earns marks.