Vectors Used in Gene Therapy (Retrovirus in Lymphocytes)
Biology · Biotechnology and Its Applications · NEET
In ADA-deficiency gene therapy, a retrovirus (retroviral vector) is used to carry a functional ADA cDNA into the patient's lymphocytes. Lymphocytes are taken from the blood, grown in culture outside the body, the retrovirus inserts the good gene, and the cells are put back into the patient. Memory hook: "Retro carries the gene into the lympho-car" - Retrovirus drives ADA gene into Lymphocytes.
Steps of ADA gene therapy: lymphocytes are taken from the patient, grown in culture, corrected with functional ADA cDNA using a retroviral vector, then returned. Because these cells are not immortal, infusion must be repeated.
Your doubts, answered
Which vector is used to introduce a DNA fragment into human lymphocytes?
A retrovirus (retroviral vector) is used. In ADA gene therapy, a functional ADA cDNA is inserted into lymphocytes using a retroviral vector. Ti plasmid is only for plant cells, and lambda phage and pBR322 are cloning vectors, not gene-delivery vectors for human cells.
Why is a retrovirus chosen and not a Ti plasmid?
A retrovirus can enter animal (human) cells and insert its genes into the host cell DNA, so it can deliver the ADA gene into lymphocytes. Ti plasmid is a vector for plant cells (Agrobacterium), so it cannot be used to transfer genes into human lymphocytes. For NEET, remember: retrovirus = animal cells, Ti plasmid = plant cells.
Why does the ADA patient need periodic (repeated) infusion of lymphocytes?
Because the genetically engineered lymphocytes are not immortal cells. Mature lymphocytes have a limited life span and die off, so the corrected cells are lost over time. The patient must receive fresh genetically engineered lymphocytes again and again. This makes the therapy a treatment, not a permanent cure.
How can gene therapy for ADA become a permanent cure?
If the gene isolated from marrow cells (that produce ADA) is introduced into cells at an early embryonic stage, it could be a permanent cure. Embryonic and stem-type cells can keep dividing and pass on the corrected gene, unlike short-lived mature lymphocytes.
Is the retrovirus here the same as a disease-causing virus?
The retrovirus is used as a disarmed vector - a delivery vehicle. Its own harmful genes are removed and it is used only to carry the functional gene into the cell. NEET 2023 tested this exact idea: disarmed retroviruses deliver genes into animal cells.
⚠️ The NEET trap ✗ Ti plasmid is the vector used to put the ADA gene into human lymphocytes. ✓ A retroviral vector (retrovirus) is used for human lymphocytes; Ti plasmid is only for plant cells. 🧠 Match the vector to the cell type: Retrovirus to animal/human cells, Ti plasmid to plant cells. NTA loves swapping these two.
Real NEET questions
NEET 2018
Which of the following is commonly used as a vector for introducing a DNA fragment in human lymphocytes?
A · lambda phage
B · Ti plasmid
C · Retrovirus ✓
D · pBR322
Solution: In ADA-deficiency gene therapy, lymphocytes from the patient's blood are grown in culture and a functional ADA cDNA is introduced into them using a retroviral vector, then the cells are returned to the patient. Ti plasmid is for plant cells; lambda phage and pBR322 are cloning vectors, not used to transfer genes into human lymphocytes.
NEET 2022
In gene therapy of Adenosine Deaminase (ADA) deficiency, the patient requires periodic infusion of genetically engineered lymphocytes because:
A · Retroviral vector is introduced into these lymphocytes
B · Gene isolated from marrow cells producing ADA is introduced into cells at embryonic stages
C · Lymphocytes from patient's blood are grown in culture, outside the body
D · Genetically engineered lymphocytes are not immortal cells ✓
Solution: The engineered mature lymphocytes are not immortal, so they die off over time. The patient therefore needs periodic (repeated) infusion of fresh genetically engineered lymphocytes. A permanent cure would need the gene put into cells at an early embryonic stage.
Solved Biotechnology and Its Applications NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
A retrovirus, called a retroviral vector. It carries a functional ADA cDNA into the patient's lymphocytes.
Why lymphocytes and not other cells?
ADA deficiency damages the immune system, and lymphocytes are immune cells. They can be easily taken from blood, grown in culture, corrected with the ADA gene, and returned to the patient.
Is ADA gene therapy a permanent cure?
No. Because the corrected lymphocytes are not immortal, they die and the patient needs repeated infusions. A permanent cure may be possible only if the gene is introduced into cells at an early embryonic stage.
When and to whom was the first gene therapy given?
The first clinical gene therapy was given in 1990 to a 4-year-old girl with adenosine deaminase (ADA) deficiency.
What does 'disarmed retrovirus' mean?
A disarmed retrovirus has its own harmful genes removed. It is used only as a delivery vehicle to carry a useful gene into animal cells, as tested in NEET 2023.
