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Recombinant insulin is the standard example of genetic engineering in medicine, and NCERT covers it in a few paragraphs. Questions on it test one idea above all: why were the two chains made separately? This post answers that, follows the method from synthetic gene to finished hormone, and works through the numbers, such as chain lengths, gene lengths and bonds, that turn a memorised paragraph into something you understand.
Where is this in the syllabus?
The NEET (UG) 2026 syllabus lists "Human insulin and vaccine production" under applications of biotechnology in health and agriculture. NCERT's Class 12 Chapter 10, Biotechnology and its Applications (2026-27 reprint), section 10.2.1, Genetically Engineered Insulin, is the base text. The tools it relies on, such as plasmids and restriction enzymes, are in our post on biotechnology principles. Check your exam year's syllabus.
Why was animal insulin not good enough?
NCERT explains that insulin for diabetes was earlier extracted from the pancreas of slaughtered cattle and pigs. Insulin from an animal source caused some patients to develop allergy or other reactions to the foreign protein. A bacterium that made human insulin could be grown in large quantity to make as much as needed.
What is insulin made of?
Insulin consists of two short polypeptide chains, A and B, linked by disulphide bridges. In mammals, including humans, it is first made as a pro-hormone, pro-insulin, which contains an extra stretch called the C peptide. The C peptide is removed during maturation, and it is not present in mature insulin.
The UniProt entry for human insulin (P01308) gives the exact layout of the 110-residue precursor:
Signal peptide: residues 1 to 24.
B chain: residues 25 to 54.
C peptide: residues 57 to 87.
A chain: residues 90 to 110.
Three disulphide bonds: two between the B and A chains, and one within the A chain.
Worked example: count the pieces
Use the UniProt positions to answer five quick questions.
Length of each chain. B: 54 − 25 + 1 = 30 amino acids. A: 110 − 90 + 1 = 21. Mature insulin has 30 + 21 = 51 amino acids.
Length of the C peptide. 87 − 57 + 1 = 31 amino acids.
What is cut from pro-insulin? Pro-insulin runs from residue 25 to 110, so 110 − 25 + 1 = 86 amino acids. Removing the chains leaves 86 − 51 = 35, which is the 31-residue C peptide plus the two-residue links on either side of it (55 to 56 and 88 to 89).
How long is a gene for the A chain? Each amino acid needs one three-base codon, so 21 × 3 = 63 bases, plus a three-base stop codon: 66. For the B chain, 30 × 3 + 3 = 93.
How many cysteines form the bridges? Each disulphide bond joins two cysteines, so three bonds use six, and UniProt lists six different cysteine positions.
How were the chains made separately?
NCERT says that the main challenge for producing insulin with rDNA techniques was getting insulin assembled into a mature form. In 1983, it says, Eli Lilly prepared two DNA sequences corresponding to the A and B chains of human insulin and introduced them into plasmids of E. coli. The chains were produced separately, extracted and combined by creating disulphide bonds to form human insulin.
A method of this kind had already been published a few years earlier. Goeddel and colleagues (1979) reported in PNAS that synthetic genes for the human insulin A and B chains were cloned separately in the plasmid pBR322. Each was fused to an E. coli beta-galactosidase gene, which gave efficient expression and a stable precursor protein. The insulin peptides were then cleaved from the beta-galactosidase and purified. The two products were mixed, reduced and reoxidised, and insulin was detected by radioimmunoassay.
The reduce-and-reoxidise step is where the disulphide bonds form. It is the "combined by creating disulphide bonds" in NCERT's summary.
A 1983 review in Science by Johnson described human insulin produced by recombinant DNA technology as the first commercial health care product derived from that technology.
Why can insulin not simply be swallowed?
NCERT asks you to think about whether insulin can be taken orally. Use what you know from digestion: insulin is a protein, and proteins are broken down by digestive enzymes in the stomach and intestine. That is why insulin is usually injected.
Which mistakes cost the most marks?
Saying the C peptide is part of mature insulin. It is removed.
Thinking E. coli was given the pro-insulin gene. In NCERT's account, the A and B chain sequences went in separately.
Mixing up chain lengths. A has 21 amino acids, B has 30.
Calling disulphide bonds peptide bonds. Peptide bonds join amino acids along a chain; disulphide bonds link cysteines, here within and between chains.
Forgetting the reason animal insulin was replaced: allergy and reactions to a foreign protein.
A practice routine
Day 1: Draw pro-insulin as a bar: B, C peptide, A. Mark the three disulphide bonds.
Day 2: Write the five worked-example answers from memory and check the arithmetic.
Day 3: Explain the Goeddel method in four steps: synthetic genes, separate plasmids, fusion protein, joining the chains.
Day 4: Compare insulin with the other applications in the chapter. Our post on biotechnology applications covers Bt crops and gene therapy.
If you find the molecular steps easier to follow when you can see them, Fermi's Simulation Lab has interactive simulations for ideas that are easier to see than to read.
Tonight, without notes, explain to someone why insulin's two chains were made in different bacterial cultures.
