Q.What is gene therapy? Explain, using the example of Adenosine Deaminase (ADA) deficiency, how a functional gene is used to correct a genetic disorder.
Concept understanding — Gene Therapy
Gene therapy corrects a hereditary disease by delivering a normal, functional copy of a gene, via a vector, into the cells of a patient carrying a mutant allele, so that the expressed normal gene product restores a normal phenotype; it has mainly targeted single-gene disorders like cystic fibrosis and haemophilia, for which no other effective treatment exists. Two strategies exist — gene augmentation therapy (adding a working gene copy) and gene inhibition therapy (silencing a harmful dominant gene with an antisense gene) — and two levels of application: somatic cell gene therapy (targeting non-reproductive cells like bone marrow, blood or skin; corrects the patient but is not inherited) and germline gene therapy (targeting eggs or sperm; heritable, and far more ethically contentious). The landmark case is ADA-deficiency SCID, first treated clinically by French Anderson in 1990 in a four-year-old girl: lymphocytes are drawn from the patient's blood, infected in culture with a disabled retrovirus carrying a working ADA gene, and reinfused; because these engineered cells are not immortal the patient needs periodic repeat infusions, and only introducing the corrected gene at the early embryonic stage would offer a truly permanent cure.
"Gene therapy definition ADA deficiency SCID example" and "gene therapy class 12 biology important questions" are frequently searched terms in the Biotechnology and its Applications chapter of the NCERT-aligned CBSE Class 12 Biology curriculum, a favourite NEET application-based topic. The 1990 ADA-SCID case discussed here, including why repeat infusions are needed, is a commonly asked case-study question in competitive biology exams.
[!TLDR] Gene therapy delivers a functional gene to correct a genetic disorder; in ADA deficiency, a working ADA gene was inserted into a patient's lymphocytes to restore immune function.
[!ANSWER] Gene therapy corrects a genetic disorder at its source by delivering a normal, functional copy of a gene into a patient's cells, so the cells can produce the protein the defective gene fails to make correctly. In ADA deficiency (caused by a defective/missing ADA gene, leading to severe combined immunodeficiency), lymphocytes were extracted from the patient's blood, given a functional ADA gene in the laboratory, and reinfused, so the corrected lymphocytes could produce functional ADA enzyme and restore some immune function.
Gene therapy is a treatment strategy aimed at fixing the underlying genetic cause of a disorder, rather than only treating its symptoms. Where a patient's own copy of a particular gene is defective, missing, or non-functional, gene therapy attempts to deliver a working, normal copy of that same gene into the patient's cells, so that the patient's own cellular machinery can begin producing the correctly functioning protein that the defective gene had failed to make.
Adenosine deaminase (ADA) deficiency provides the classic illustration. This inherited disorder is caused by a defect or absence of the gene coding for the enzyme adenosine deaminase, which is essential for the normal functioning of certain immune-system cells called lymphocytes; without functional ADA enzyme, an affected individual develops severe combined immunodeficiency (SCID), leaving them extremely vulnerable to infections, since their immune system cannot function properly.
In the gene therapy approach used to treat ADA deficiency, lymphocytes were extracted directly from the patient's own blood. In the laboratory, a normal, functional copy of the ADA gene was introduced into these extracted lymphocytes, using a viral vector engineered to deliver and insert the gene into the cells' genome. These now-corrected lymphocytes, capable of producing functional ADA enzyme, were then reintroduced into the patient's body by infusion, where they could begin producing the missing enzyme and help restore a degree of normal immune function.
[!ANSWER] Gene therapy for ADA deficiency worked by taking a patient's own defective lymphocytes out of the body, giving them a working copy of the ADA gene in the lab, and putting them back -- directly supplying the missing functional gene product at its cellular source, rather than only treating the immune symptoms caused by its absence.
Define gene therapy in general first, then walk through the specific ADA-deficiency example step by step (extract -> correct -> reinfuse).
Do not describe ADA deficiency's gene therapy as a simple drug or enzyme injection -- the treatment specifically involves genetically modifying the patient's own cells to produce the enzyme themselves.
Showing the 12 most recent of 15 on this concept.
- CBSE 2026Set A1 markMCQQ.Gene therapy was first used to remove the deficiency of which of the following enzymes?(a) DNA polymerase(b) Uracil DNA glycosylase(c) Adenosine deaminase(d) Beta galactosidase
›Reveal solutionSolution
The first gene therapy was for adenosine deaminase (ADA) deficiency; the correct option is (c).
The first clinical gene therapy was given in 1990 to a four-year-old girl suffering from adenosine deaminase (ADA) deficiency, which causes a severe immunodeficiency (SCID). ADA deficiency is caused by the deletion or defect of the gene coding for the enzyme adenosine deaminase. In the therapy, lymphocytes from the patient's blood were grown in culture, a functional ADA gene (using a retroviral vector) was introduced into them, and the cells were returned to the patient. Because these cells are not immortal, the treatment must be repeated periodically.
✓Final answer(c) Adenosine deaminase.
- CBSE 2026Set ANNUAL1 markQ.Fill in the blank: The first clinical gene therapy was given in 1990 to a four year girl to cure deficiency of ______.
›Reveal solutionSolution
The first approved human gene therapy (1990) treated a girl born with ADA deficiency, a genetic immune disorder.
Adenosine deaminase (ADA) deficiency is caused by deletion of the gene encoding ADA, which is needed for normal immune function; in some cases it can be corrected by bone marrow transplant or enzyme replacement, but the underlying disease is not fully cured this way. In 1990, the first clinical gene therapy was given to a four-year-old girl with ADA deficiency, using lymphocytes carrying a functional ADA cDNA (later also attempted via bone-marrow stem cells for a permanent cure).
✓Final answerAdenosine deaminase (ADA).
- CBSE 2026Set ANNUAL1 markQ.ADA is a genetic disorder due to the absence of an enzyme. Name the enzyme that is absent.
›Reveal solutionSolution
ADA deficiency is named directly for the missing enzyme, adenosine deaminase, whose absence severely compromises the immune system.
Adenosine Deaminase (ADA) deficiency is a genetic disorder caused by the deletion/absence of the gene coding for the enzyme adenosine deaminase. This enzyme is essential for normal functioning of lymphocytes; its absence leads to toxic accumulation of its substrate, deoxyadenosine, which is particularly harmful to developing lymphocytes, causing Severe Combined Immuno-Deficiency (SCID), in which the individual's immune system is unable to function properly.
✓Final answerThe enzyme absent in this genetic disorder is adenosine deaminase (ADA).
- CBSE 2025Set A1 markQ.Write True / False: ADA deficiency can be cured by Lymphocytes.
›Reveal solutionSolution
The statement is True: ADA deficiency is treated by gene therapy performed on the patient's lymphocytes.
Adenosine Deaminase (ADA) deficiency is a genetic (SCID-causing) disorder in which the enzyme ADA, essential for immune function, is missing due to a defective gene. As the first approved gene-therapy procedure (1990), lymphocytes are extracted from the patient's blood, grown in culture outside the body, and a functional copy of the ADA cDNA is introduced into these lymphocytes using a retroviral vector. These genetically corrected lymphocytes are then returned to the patient, allowing them to produce the ADA enzyme. Because the lymphocytes are not permanent (they eventually die), such infusions historically had to be periodically repeated — but the underlying statement that lymphocytes are used to cure/treat ADA deficiency is correct.
✓Final answerTrue.
- CBSE 2025Set ANNUAL1 markMCQQ.The first clinical gene therapy was given for treating(a) Diabetes mellitus(b) Chicken pox(c) Rheumatoid arthritis(d) ADA deficiency
›Reveal solutionSolution
The first clinical application of gene therapy in humans, in 1990, was used to treat adenosine deaminase (ADA) deficiency, a genetic immune disorder.
ADA deficiency arises from a defective/missing gene for the enzyme adenosine deaminase, which is essential for the normal functioning of lymphocytes (immune cells); its loss leads to severe combined immunodeficiency (SCID). In the first approved human gene therapy trial, lymphocytes were taken from the patient's blood, cultured, and a functional copy of the ADA gene was introduced into them using a retroviral vector; these genetically corrected cells were then reintroduced into the patient's body. Because these cells are not permanent, periodic infusions of such genetically engineered lymphocytes were required (a more permanent cure needs the gene to be introduced into bone marrow stem cells at an early embryonic stage).
✓Final answer(d) ADA deficiency.
- CBSE 2025Set ANNUAL1 markMCQQ.The first clinical application of gene therapy over a 4 year old girl was for(a) Adenosine deaminase deficiency(b) Adenosine deficiency(c) Growth deficiency(d) Adenine deficiency
›Reveal solutionSolution
The first-ever human gene therapy trial (1990) treated a 4-year-old girl for adenosine deaminase (ADA) deficiency, an inherited immunodeficiency.
Gene therapy is a biotechnological approach in which a normal, functional gene is introduced into an individual to compensate for a defective/absent gene, aiming to correct a genetic defect. The first clinical application of gene therapy was performed in 1990, on a 4-year-old girl who suffered from Adenosine Deaminase (ADA) deficiency, a rare inherited disorder in which the enzyme adenosine deaminase, essential for the proper functioning of the immune system, is missing (due to the deletion of the ADA gene). In severe cases, this condition can be treated by bone marrow transplantation or enzyme replacement therapy, but as a more permanent solution, gene therapy involves isolating and reintroducing normal lymphocytes (containing a functional ADA gene) grown in culture, back into the patient's body. Since these cells are not immortal, the patient requires periodic reinfusion of such genetically corrected cells, though a permanent cure has since been attempted by introducing the ADA gene into early embryonic/bone-marrow stem cells.
✓Final answer(a) Adenosine deaminase deficiency
- CBSE 2025Set ANNUAL1 markMCQQ.A boy is born with a hereditary disease, what corrective method will you suggest ?(i) Gene cloning(ii) Nuclear transfer(iii) Organism cloning(iv) Gene therapy
›Reveal solutionSolution
Gene therapy is the corrective method for hereditary genetic disorders — it works by delivering a normal, functional copy of the defective gene into the patient's cells.
Gene therapy is a collection of methods that allows correction of a gene defect diagnosed in a child or embryo. In this approach, genes are inserted into a person's cells and tissues to treat a disease that has resulted from a faulty/mutated gene. A normal, healthy copy of the gene is introduced into the patient's cells (often using a vector, e.g. a modified retrovirus) to compensate for the non-functional gene, and this corrected cell population is expected to produce the normal protein needed. The classic example is the treatment of Adenosine Deaminase (ADA) deficiency (a form of SCID) by gene therapy.
Gene cloning, nuclear transfer, and organism cloning are technologies for producing copies of genes/organisms, not corrective treatments for a hereditary disease in an already-born individual.
✓Final answerGene therapy (Option iv) is the corrective method for a hereditary disease.
- CBSE 2024Set D1 markQ.Fill in the blank: ______ is caused by mutation in the gene related to the adenosine deaminase enzyme.
›Reveal solutionSolution
Severe Combined Immunodeficiency (SCID), specifically ADA deficiency, results from a mutated/defective gene for the enzyme adenosine deaminase.
Adenosine deaminase (ADA) is an enzyme essential for the normal functioning of lymphocytes (immune cells). In some children, this enzyme is deficient due to the deletion of the gene that codes for it, and in others the defect is in the ADA gene itself (a genetic — often autosomal recessive — mutation).
This deficiency leads to Severe Combined Immunodeficiency (SCID), in which the affected individual has little to no functioning immune system and is highly vulnerable to infections.
ADA deficiency is historically significant as the very first human disease treated by gene therapy (in 1990), in which functional lymphocytes bearing a normal ADA gene (introduced via a retroviral vector) were injected back into the patient — though, since lymphocytes are not immortal, this required periodic infusion; a permanent cure requires introducing the correct gene into bone marrow stem cells at an early embryonic stage.
✓Final answerSCID (Severe Combined Immunodeficiency) — caused by mutation/deficiency of the adenosine deaminase (ADA) gene.
- CBSE 2023Set 57/1/11 markMCQQ.Assertion (A) : A patient of ADA deficiency undergoing treatment for gene therapy requires periodic infusion of genetically engineered lymphocytes. Reason (R) : Lymphocytes are immortal.(a) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A).(b) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A).(c) Assertion (A) is true, but Reason (R) is false.(d) Assertion (A) is false, but Reason (R) is true.
›Reveal solutionSolution
The assertion is true — ADA deficiency patients do need periodic infusions of engineered lymphocytes — but the reason given is false; lymphocytes are not immortal, which is exactly why the infusions must be repeated.
Let’s begin with the context. Severe Combined Immunodeficiency (SCID) caused by adenosine deaminase (ADA) deficiency was one of the earliest targets for gene therapy. The NCERT textbook describes the first successful clinical trial: lymphocytes were taken from the patient, the normal ADA gene was inserted into them using a retroviral vector, and these corrected cells were then infused back into the patient. This is the treatment the assertion refers to.
Now, why must these infusions be periodic? The reason lies in the nature of the cells used. Lymphocytes are a type of white blood cell, but they are not immortal. They have a finite lifespan — they divide, function for a while, and then die. Over time, the population of corrected lymphocytes declines, and the patient’s ADA levels drop again. So the treatment has to be repeated at intervals to maintain a sufficient number of functioning cells.
NoteThe textbook also mentions that later approaches shifted to using bone marrow stem cells for gene therapy, because stem cells can divide throughout life and produce a continuous supply of corrected blood cells — avoiding the need for repeated infusions.
The reason given in the question — “Lymphocytes are immortal” — is simply incorrect. No normal human cell is immortal. Immortality in cells is a property of cancer cell lines (like HeLa cells) or certain stem cells under specific conditions, but not of ordinary lymphocytes. The NCERT does not state that lymphocytes are immortal; in fact, the need for repeated infusions implicitly confirms their limited lifespan.
ImportantThe key point: the assertion is true because the treatment uses lymphocytes, which are not self-renewing. The reason is false because lymphocytes are not immortal. Therefore, the reason cannot explain the assertion.
Let’s look at the options:
- (a) would require both statements to be true and the reason to correctly explain the assertion — but the reason is false.
- (b) would require both to be true but unrelated — again, the reason is false.
- (c) says assertion true, reason false — this matches our analysis.
- (d) says assertion false, reason true — both are wrong.
The correct choice is therefore (c).
✓Final answerIn short, the assertion is correct — ADA gene therapy using lymphocytes does require periodic infusions because the cells are not permanent — but the reason is false, as lymphocytes are not immortal. Hence, option (c) is the right answer.
- CBSE 2023Set ZOOLOGY1 markMCQQ.When an abnormal gene is replaced by normal gene, what do you call it?(a) Gene mutation(b) Gene cloning(c) Gene therapy(d) Gene ligation
›Reveal solutionSolution
Gene therapy is the technique of correcting a genetic defect by delivering a normal, functional gene to replace or supplement the patient's abnormal (mutant) gene.
Gene therapy is a collection of methods that allow correction of a gene defect diagnosed in a child/embryo. Genetic material (a normal, healthy copy of the gene) is inserted into an individual's cells and tissues to treat a disease caused by a defective (mutant) gene. The classic example taught in NCERT is the treatment of Adenosine Deaminase (ADA) deficiency, caused by the deletion of the gene for ADA, an enzyme crucial for immune function:
- Lymphocytes from the patient's blood are grown in culture outside the body.
- A functional ADA cDNA is introduced into these lymphocytes using a retroviral vector.
- These genetically corrected cells are returned to the patient.
Because such lymphocytes are not immortal, the patient needs periodic infusion of such genetically engineered cells; a permanent cure would require introducing the normal gene into bone-marrow stem cells at an early embryonic stage.
This is distinct from gene mutation (a change in DNA sequence), gene cloning (making identical copies of a gene) and gene ligation (joining DNA fragments with DNA ligase, a step used during cloning, not therapy).
✓Final answer(c) Gene therapy.
- CBSE 2023Set ANNUAL1 markQ.Write function of Adenosine deaminase.
›Reveal solutionSolution
ADA breaks down adenosine in purine metabolism; without it, toxic metabolites accumulate and destroy lymphocytes, crippling the immune system - the basis of the first successful gene therapy case.
Adenosine deaminase (ADA) converts adenosine to inosine as part of normal purine metabolism. This function is critical for the proper working of lymphocytes (a type of white blood cell important for the immune system). A genetic deficiency of ADA causes accumulation of toxic deoxyadenosine, which is lethal to lymphocytes, resulting in Severe Combined Immuno-Deficiency (SCID), commonly known as 'bubble boy disease', where the affected individual has little to no functional immune protection.
ADA deficiency can be treated by bone marrow transplantation, periodic enzyme replacement, or by gene therapy - in which lymphocytes from the patient's blood are cultured, a functional ADA cDNA is introduced into them using a retroviral vector, and the cells are returned to the patient's body. However, since these lymphocytes are not immortal, the patient needs periodic infusions of such genetically engineered cells; a permanent cure could be achieved if the ADA gene is introduced into bone marrow cells at an early embryonic stage.
✓Final answerADA is required for normal immune-cell function; its deficiency causes SCID.
- CBSE 2022Set HE2201 markQ.Match the following (Column I with Column II): Gene therapy — choose the matching term from:(a) Obelia(b) Hydra(c) Transgenic(d) Gender linked disease(e) Amoebiasis(f) Antimicrobial substance(g) Male sterilization(h) Genetic disorder.
›Reveal solutionSolution
Gene therapy corrects a genetic defect by introducing a normal, functional copy of the gene, so it matches (h) Genetic disorder; (a) Obelia is the intended distractor.
Gene therapy is a collection of biotechnological methods that correct a genetic defect by delivering a normal, functional gene into a patient's cells to compensate for the non-functional one. The classic example is the treatment of adenosine deaminase (ADA) deficiency, in which a functional ADA gene is introduced into the patient's lymphocytes or bone-marrow stem cells. Because its whole purpose is to treat inherited genetic defects, gene therapy pairs with (h) Genetic disorder.
Working through the rest of the set for consistency: Budding → (b) Hydra, Vasectomy → (g) Male sterilization, Colour blindness → (d) Gender-linked disease, Entamoeba histolytica → (e) Amoebiasis, Interferon → (f) Antimicrobial substance, and Transgenic animal → (c) Transgenic. This leaves (a) Obelia — a colonial marine cnidarian — as the single unmatched distractor (a plausible decoy alongside Hydra for 'Budding', since Obelia also reproduces by budding).
[!ANSWER]
Gene therapy → (h) Genetic disorder; (a) Obelia is the leftover distractor.
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