How was human insulin produced using recombinant DNA technology?

How to lay out the answer. Follow the frame, then write it in your own words.

1 · Give the need. Insulin from cattle and pigs caused allergy in some patients and supply was limited.
2 · Describe insulin. Two polypeptide chains, A and B, linked by disulphide bridges.
3 · Explain the challenge. In the body insulin is made as proinsulin with a C peptide, which is removed; assembling mature insulin was difficult.
4 · Describe the method.
- DNA sequences for chains A and B prepared.
- Each put into a plasmid of E. coli.
- Chains made separately, extracted.
- Joined by disulphide bonds into human insulin (Eli Lilly, 1983).
5 · Draw proinsulin maturing. [Diagram: proinsulin with A chain, B chain, C peptide and S-S bridges; arrow; mature insulin]
6 · Link to real life. This is the insulin people with diabetes use in India today.
7 · Close in one line. So recombinant insulin is human, safe and made in large amounts.

Scoring tip: Write 'chains A and B', 'disulphide' and 'C peptide' in your answer.