BiologyISC Class 12 · Biotechnology and its Applications

Gene Therapy and Genetically Engineered Insulin: ISC Class 12 Biology

Updated 15 October 2026 · Fact-checked

Human insulin has A and B chains joined by disulphide bridges; it is made as proinsulin, whose C-peptide is removed. In 1983 Eli Lilly made A and B chains separately in E. coli by recombinant DNA technology and joined them. Gene therapy inserts a normal gene to correct a defect; the first case (1990) treated ADA deficiency (SCID).

Lilly made chains A and B separately in E. coli, then joined them with S–S bridges, so no C-peptide had to be removed.

Illustration drawn for QuestionBuddy.

Understand Gene Therapy and Genetically Engineered Insulin

Part A: Genetically engineered insulin.

The problem. Adult-onset diabetes can be managed by regular insulin. Earlier, insulin was extracted from the pancreas of slaughtered cattle and pigs. Animal insulin made some patients develop allergy or other reactions to the foreign protein, and supply depended on slaughter.

Structure of insulin. Two short polypeptide chains, chain A and chain B, linked by disulphide bridges (–S–S–).

Proinsulin and maturation. In mammals insulin is made as a pro-hormone, proinsulin, which has an extra stretch called the C-peptide connecting the A and B chains. During maturation the C-peptide is removed, leaving A and B held by disulphide bonds. Getting insulin assembled into this mature form was the main challenge for rDNA production.

How Eli Lilly solved it (1983).
1. Two DNA sequences corresponding to the A and B chains of human insulin were prepared.
2. Each was introduced into plasmids of E. coli.
3. The bacteria produced chains A and B separately.
4. The chains were extracted and combined by creating disulphide bonds to form human insulin.
No C-peptide had to be removed, because only A and B were made.

Advantages of recombinant (human) insulin. Identical to human insulin, so no unwanted immune or allergic response; can be produced in large amounts by growing bacteria; does not depend on animals. Insulin is a protein, so it cannot be taken orally (it would be digested).

Diagram checklist (proinsulin to insulin). Draw a chain looping as B (top) → C-peptide (curved connector) → A (bottom) with two –S–S– bridges between A and B and one within A; then an arrow 'C-peptide removed' to the mature A and B chains with the bridges intact. Label: A chain, B chain, C-peptide, disulphide bonds, proinsulin, mature insulin.

Part B: Gene therapy.

Definition. A collection of methods that allows correction of a gene defect diagnosed in a child or embryo: a normal gene is delivered into the person's cells or tissues so that it does the job the faulty gene cannot.

The first clinical gene therapy (1990). Given to a 4-year-old girl with adenosine deaminase (ADA) deficiency. ADA is crucial for the immune system; its absence causes severe combined immunodeficiency (SCID). The disorder is due to deletion of the gene for ADA.

Older treatments and their limits. Bone marrow transplantation (cures some children); enzyme replacement therapy (functional ADA injected). Neither is completely curative for all.

Steps of the gene therapy (flow chart).
1. Lymphocytes are taken from the patient's blood and grown in culture outside the body.
2. A functional ADA cDNA is introduced into them using a retroviral vector.
3. The genetically engineered lymphocytes are returned to the patient.
4. Because lymphocytes are not immortal, the patient needs periodic infusions.
Permanent cure: if the ADA gene isolated from marrow cells is introduced into cells at early embryonic stages, the cure could be permanent.

Somatic versus germ-line. The ADA treatment changes body (somatic) cells only, so it is not inherited; changing embryonic or germ cells would pass to offspring, which raises ethical issues.

Link to other applications. About 30 recombinant therapeutics are approved worldwide (12 marketed in India); they avoid unwanted immune responses common with products from non-human sources.

Gene Therapy and Genetically Engineered Insulin: key facts

Insulin maturation: 1.Proinsulin: B chain, C-peptide, A chain
2.C-peptide removed
3.Mature insulin: A and B chains joined by disulphide bonds; C-peptide is absent in mature insulin.
Eli Lilly route: 1.DNA sequences for the A and B chains prepared
2.put into plasmids of E. coli
3.bacteria make the A and B chains separately
4.chains extracted and joined by disulphide bonds; 1983.
ADA gene therapy: 1.Lymphocytes taken from the patient's blood
2.grown in culture
3.functional ADA cDNA added with a retroviral vector
4.cells returned to the patient
5.repeated periodically; Periodic infusions needed.
Defect: 1.ADA gene deleted
2.no adenosine deaminase
3.severe combined immunodeficiency (SCID); First gene therapy, 1990.

How to answer Gene Therapy and Genetically Engineered Insulin questions

ISC asks the proinsulin-to-insulin change, the technique and bonds, Eli Lilly's method, why recombinant insulin is preferred, and the gene therapy for ADA deficiency (SCID) with its limits.

1For insulin maturation, say: C-peptide removed; A and B chains remain joined by disulphide bonds.
2Name the technique as recombinant DNA technology (genetic engineering), not PCR.
3For Eli Lilly, give four steps: DNA for A and B, plasmids of E. coli, separate chains, joined by disulphide bonds.
4For gene therapy, define it, name ADA deficiency (SCID), then list the steps with the retroviral vector.
5Explain the limitation (lymphocytes not immortal) and the possible permanent cure (early embryonic cells).
6For advantages, compare with animal insulin: no allergy, unlimited supply.

Gene Therapy and Genetically Engineered Insulin in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Why patients switched to recombinant insulin

Suppose a diabetic patient who has used insulin extracted from pig pancreas for years develops itching and swelling at injection sites, and her doctor switches her to recombinant human insulin.

Explain the reaction and why the switch helps.

Try it first, then show the solutionSolution

Pig insulin differs slightly from human insulin, so her immune system treats it as a foreign protein and reacts (allergy).
Recombinant insulin is made from the human A and B chain sequences in E. coli, so it is identical to her own insulin.
It does not trigger an immune response, and supply does not depend on slaughtered animals.
It must still be injected because a protein taken by mouth would be digested.

Answer: The reaction was to a foreign protein; recombinant human insulin is identical to her own and avoids it.

Memory hook: Human gene, human insulin, no alarm.

Tricks to remember Gene Therapy and Genetically Engineered Insulin

Short memory aids. Say each one aloud once, then use it on a question the same day.

A-B joined, C cut
A and B stay, joined by S-S; C (the connector) is cut out.
Use it when: Insulin maturation.
1983 insulin, 1990 gene therapy
Insulin first (1983), then the first gene therapy seven years later (1990).
Use it when: Dates in answers.
ADA = Always Defending Antibodies
Without ADA, the immune system collapses: SCID.
Use it when: Linking ADA to immunodeficiency.

Gene Therapy and Genetically Engineered Insulin: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Insulin structure · 2 marks · Short answerBoard level

Describe the structure of mature human insulin. What is the C-peptide?

Try it first, then show the solutionSolution

Two short polypeptide chains, A and B, linked by disulphide bridges (1 mark).
The C-peptide is an extra stretch in proinsulin connecting A and B; it is removed during maturation and is absent from mature insulin (1 mark).

Answer: A and B chains joined by S-S bonds; C-peptide is the removed connector of proinsulin.

Example 2: Define gene therapy · 2 marks · Short answerBoard level

What is gene therapy? Name the disease treated by the first clinical gene therapy and its cause.

Try it first, then show the solutionSolution

Methods that correct a diagnosed gene defect by delivering a normal gene into the person's cells or tissues to compensate for the non-functional gene (1 mark).
ADA deficiency (a form of SCID), caused by deletion of the gene for adenosine deaminase (1 mark).

Answer: Delivery of a normal gene to correct a defect; ADA deficiency due to ADA gene deletion.

Example 3: Eli Lilly's method · 3 marks · Short answerHarder board level

Explain how human insulin was produced by Eli Lilly using recombinant DNA technology. Why was it not necessary to remove the C-peptide?

Try it first, then show the solutionSolution

Two DNA sequences coding for the A and B chains were prepared and introduced into plasmids of E. coli (1 mark).
The bacteria produced A and B chains separately; these were extracted and combined by forming disulphide bonds to give human insulin (1 mark).
Only the A and B chains were made, never proinsulin, so there was no C-peptide to remove (1 mark).

Answer: Separate A and B chains made in E. coli and joined by disulphide bonds; no proinsulin, so no C-peptide.

Example 4: ADA gene therapy steps · 3 marks · Short answerHarder board level

Describe the steps of gene therapy for ADA deficiency. Why is it not a permanent cure, and how could it become one?

Try it first, then show the solutionSolution

Lymphocytes from the patient's blood are grown in culture; functional ADA cDNA is introduced into them with a retroviral vector; the cells are returned to the patient (1½ marks).
Lymphocytes are not immortal, so the corrected cells die and periodic infusions are needed (½ mark).
If the ADA gene isolated from marrow cells is introduced into cells at early embryonic stages, the cure could be permanent (1 mark).

Answer: Engineered lymphocytes returned; not permanent as cells die; embryonic-stage gene transfer could cure permanently.

Example 5: Recombinant versus animal insulin · 3 marks · Short answerHarder board level

Give three reasons why recombinant human insulin is preferred over insulin from cattle and pigs.

Try it first, then show the solutionSolution

Animal insulin is a foreign protein that caused allergy or other reactions in some patients; recombinant insulin is identical to human insulin (1 mark).
Bacteria can be grown in large quantities, giving as much insulin as needed (1 mark).
It does not depend on slaughtered animals and is free of animal-borne contaminants (1 mark).

Answer: No immune reaction, large-scale supply, no dependence on animals.

Example 6: Comparing treatments for SCID · 4 marks · Short answerHOTS / competency

Suppose two children have ADA deficiency. Child X receives weekly injections of functional ADA enzyme; child Y receives her own lymphocytes carrying a functional ADA cDNA. (a) Name the two treatments. (b) Why must both be repeated? (c) Which treatment changes the child's cells genetically, and would that change pass to her children? Explain.

Try it first, then show the solutionSolution

(a) X: enzyme replacement therapy; Y: gene therapy (somatic) (1 mark).
(b) X: injected enzyme is used up and broken down; Y: the engineered lymphocytes are not immortal and die (1 mark).
(c) Y's treatment changes her lymphocytes genetically (1 mark).
It would not pass to her children, because lymphocytes are somatic cells, not germ cells (1 mark).

Answer: (a) Enzyme replacement; gene therapy. (b) Enzyme degraded; cells die. (c) Y's lymphocytes; not inherited (somatic).

Where students lose marks in Gene Therapy and Genetically Engineered Insulin

The slips examiners see most often, and what to write instead.

Mistake: Not mentioning removal of the C-peptide
In 2024 examiners reported that candidates did not mention removal of the C-peptide when asked how proinsulin becomes functional.
Do this: Write: C-peptide removed; A and B remain joined by disulphide bonds.
Usually costs: 1 mark.
Mistake: Wrong technique or bond
Examiners (2024) saw PCR instead of recombinant DNA technology, and sulphur, sulphide or hydrogen bonds instead of disulphide bonds.
Do this: Technique: recombinant DNA technology; bonds: disulphide (S-S).
Usually costs: 1 mark each.
Mistake: Wrong disease for the first gene therapy
In 2024 examiners reported haemophilia, sickle-cell anaemia or AIDS named instead of SCID.
Do this: SCID due to ADA deficiency.
Usually costs: 1 mark.
Mistake: Calling the lymphocyte treatment permanent
Answers often miss that engineered lymphocytes are not immortal.
Do this: State the need for periodic infusions.
Usually costs: 1 mark.

Exam tips

Draw proinsulin with the C-peptide clearly labelled, and show the S-S bridges.
Write 'adenosine deaminase' in full once.
Mention 'retroviral vector' and 'cDNA' in the ADA steps.
Give the years 1983 (insulin) and 1990 (gene therapy) only if sure; they are not essential for marks.
Separate somatic gene therapy from germ-line change in ethics answers.

Practice questions on Gene Therapy and Genetically Engineered Insulin

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
The disorder treated by the first clinical gene therapy, in 1990, was:
(a) haemophilia (b) sickle-cell anaemia (c) ADA deficiency (d) AIDS
Hint
An immunodeficiency.
Answer
(c) ADA deficiency.
Q2 · 2 marks · Short answerBoard level
Name the bonds that hold the A and B chains of insulin together and the technique used to make human insulin commercially.
Hint
S-S.
Answer
Disulphide bonds (bridges). Recombinant DNA technology (genetic engineering).
Q3 · 1 mark · Assertion-reasonHarder board level
Assertion (A): Insulin cannot be given to diabetic patients by mouth.
Reason (R): Insulin is a protein and would be digested in the alimentary canal.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Proteases.
Answer
(a).
Q4 · 2 marks · Short answerHarder board level
Why was a retrovirus chosen as the vector in ADA gene therapy?
Hint
Integration.
Answer
Retroviruses naturally deliver genes into animal cells and integrate their DNA into the host genome; a disarmed retrovirus can therefore carry the ADA cDNA into lymphocytes so that the cells express functional ADA.
Q5 · 2 marks · Short answerHarder board level
Name two treatments for ADA deficiency other than gene therapy, and state why they are not satisfactory.
Hint
Marrow; injection.
Answer
Bone marrow transplantation and enzyme replacement therapy (functional ADA injected). Neither is completely curative for all patients.
Q6 · 2 marks · Short answerHarder board level
What was the main challenge in producing insulin by rDNA techniques, and how did Eli Lilly overcome it?
Hint
Assembly.
Answer
Getting insulin assembled into its mature form (proinsulin would need its C-peptide removed). Eli Lilly made the A and B chains separately in E. coli and joined them chemically by forming disulphide bonds.
Q7 · 3 marks · Short answerHOTS / competency
Suppose doctors could correct the ADA gene in an early embryo instead of in lymphocytes. Give one advantage and two ethical concerns of this approach.
Hint
All cells, all descendants.
Answer
Advantage: all cells, including marrow stem cells, would carry the functional gene, giving a permanent cure without repeated infusions. Concerns: the change would pass to future generations (germ-line), whose consent cannot be taken; unknown long-term side effects and the risk of misuse for non-medical traits.
Q8 · 3 marks · Case-basedHOTS / competency
Read and answer.
Suppose a diagram shows a polypeptide in which chain B is linked through a segment C to chain A, with disulphide bridges between A and B. After processing, segment C is absent.
(a) Name the molecule before and after processing. (1)
(b) What is segment C? (1)
(c) In Eli Lilly's method, was this processing step needed? Why? (1)
Hint
Pro-hormone.
Answer
(a) Proinsulin; mature insulin. (b) The C-peptide, removed during maturation. (c) No: the A and B chains were made separately in E. coli and joined by disulphide bonds, so no proinsulin was formed.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Which company produced human insulin by rDNA technology?Show answer
Eli Lilly (1983).
2.In which host were the insulin chains produced?Show answer
E. coli (in plasmids).
3.What is removed from proinsulin?Show answer
The C-peptide.
4.Which enzyme is missing in the first gene-therapy patient?Show answer
Adenosine deaminase (ADA).
5.Which cells were engineered in ADA gene therapy?Show answer
The patient's lymphocytes.
6.Why are periodic infusions needed?Show answer
The engineered lymphocytes are not immortal.

Gene Therapy and Genetically Engineered Insulin: frequently asked questions

How is human insulin made by genetic engineering?

DNA sequences for the A and B chains are put into E. coli plasmids; the bacteria make the two chains separately, which are extracted and joined by disulphide bonds.

What happens to proinsulin in the body?

Its C-peptide is removed, leaving the A and B chains linked by disulphide bridges as mature insulin.

What is gene therapy?

A set of methods that corrects a gene defect by delivering a normal gene into a person's cells to compensate for the faulty gene.

What is SCID?

Severe combined immunodeficiency; one form is caused by deletion of the gene for adenosine deaminase (ADA), which is needed for the immune system.

Can gene therapy for ADA deficiency be permanent?

With lymphocytes it is not, as the cells die; introducing the gene into cells at early embryonic stages could give a permanent cure.