Question

(a) How is insulin different from proinsulin ?

(b) Explain all the stages of how human insulin is produced through rDNA technology.

Hint:

Proinsulin has an extra C peptide that is removed in insulin; rDNA insulin is made by cloning A and B chain genes separately in E. coli and joining the chains.

Answer 1

Step 1: Understanding the Concept:
In the human body, insulin is made first as a larger precursor, proinsulin, and then trimmed to the active hormone.

Step 2: Part (a) - Insulin and proinsulin:
Mature insulin has two short chains, A (21 amino acids) and B (30 amino acids), joined by disulphide bridges.
Proinsulin is a single chain that has an extra C peptide in between. The C peptide is removed during maturation, and this makes insulin active.

Step 3: Part (b) - Stages of rDNA production:
1. Two DNA sequences, one for chain A and one for chain B, are prepared.
2. Each sequence is cut and joined by DNA ligase into a plasmid vector, such as the plasmid of E. coli.
3. The recombinant plasmids are introduced into E. coli (the host) by transformation.
4. Cells that took up the plasmid are selected. They are grown in large bioreactors.
5. The bacteria make chain A and chain B separately.
6. The chains are extracted, purified and mixed. They are joined by disulphide bonds to give human insulin.

Final Answer:
Insulin has chains A and B only, while proinsulin also has the C peptide. Human insulin by rDNA is made by cloning the A and B chain genes in E. coli, growing it, then extracting, purifying and joining the chains.