Overview
Maternal spindle transfer (MST) is a form of mitochondrial replacement therapy in which the spindle-chromosome complex carrying the mother's nuclear DNA is removed from her egg and inserted into a donor egg that has had its own nucleus removed but retains healthy mitochondria; the reconstructed egg is then fertilised. It prevents the inheritance of mitochondrial diseases, and because the child carries mitochondrial DNA from a donor it is popularly called the three-parent baby technique.
Mitochondria and the problem MST solves
Mitochondria, mtDNA and maternal inheritance
Every cell contains tiny structures called mitochondria, the power plants that supply the cell with energy. Unusually, mitochondria carry their own small set of genes, the mitochondrial DNA or mtDNA, separate from the main DNA in the cell nucleus.
A child inherits mtDNA almost entirely from the mother, through the egg. This means that any fault in a mother's mitochondrial genes can be passed down to all of her children, which is the central problem that maternal spindle transfer is designed to solve.
Mitochondrial diseases and why they are hard to treat
When the mtDNA carries harmful mutations, the affected cells cannot make enough energy. The result is a group of serious, often incurable conditions called mitochondrial diseases, which can damage the brain, heart, muscles and other energy-hungry organs.
Because these faults sit in the mitochondria rather than the nucleus, ordinary genetic counselling cannot remove them, and an affected mother risks passing the disease to her children. This is why scientists developed mitochondrial replacement as a way to break the chain of inheritance.
Mitochondrial replacement therapy
What mitochondrial replacement therapy is
Mitochondrial replacement therapy (MRT), also called mitochondrial donation, is the replacement of faulty mitochondria in a cell in order to prevent or reduce disease. The healthy mitochondria are supplied by a third person, an egg donor.
The aim is precise: keep the nuclear DNA of the intended parents, which carries almost all inherited traits, while swapping out only the small set of disease-causing mitochondria. The result is an embryo with the parents' genetic identity but the donor's healthy mitochondria.
The 'three-parent baby' and the DNA the child inherits
Because some of the child's mtDNA then comes from a third party, the technique is popularly called the three-parent baby method. The label is striking but can mislead about how much the donor actually contributes.
The donor supplies only the mitochondrial genes, a tiny fraction of the total. The child's appearance, personality and almost all inherited characteristics come from the nuclear DNA of the two intended parents, with the donor adding healthy cellular power plants alone.
The maternal spindle transfer technique
How maternal spindle transfer works
In maternal spindle transfer, an egg is taken from the mother, and at a stage of cell division called metaphase II the spindle-chromosome complex, which holds her nuclear DNA, is carefully removed.
This spindle is then inserted into a donor egg whose own nucleus has already been removed but which keeps its healthy mitochondria. The reconstructed egg, carrying the mother's nuclear DNA and the donor's mitochondria, is then fertilised with the father's sperm to form a healthy embryo.
Maternal spindle transfer versus pronuclear transfer
Maternal spindle transfer is one of two main techniques of mitochondrial donation. The other is pronuclear transfer, in which the eggs are first fertilised, and then the pronuclei of the parents are moved from the fertilised egg into a donor egg with healthy mitochondria.
The key difference is timing. Maternal spindle transfer acts before fertilisation, while pronuclear transfer acts just after it. Some find spindle transfer ethically simpler because it does not involve creating and then dismantling a fertilised egg.
Benefits, ethics and regulation
Benefits and the diseases it prevents
The clear benefit of MRT is that it lets a woman who carries faulty mitochondrial genes have a genetically related child without passing on a mitochondrial disease. It offers hope to families with a history of these otherwise untreatable conditions.
It also advances the wider science of reproductive medicine and cell biology. By showing that nuclear and mitochondrial material can be recombined safely, the technique opens new ground in understanding and preventing inherited disease.
Ethical debate and regulatory status
The technique also raises real concerns. Critics worry about long-term safety, about a child having genetic material from three people, and about a possible slide towards selecting other traits, so most countries allow it only under tight controls.
The United Kingdom became the first country to legalise the procedure, in 2015, permitting it case by case for serious mitochondrial disease, and the first such babies have since been born there. In India, where assisted reproduction is closely regulated, the technique is not in routine clinical use.
Significance and the exam
Significance for medicine and reproductive science
Maternal spindle transfer marks a turning point in the prevention of inherited disease. For the first time, a class of conditions passed down through the mother can be stopped before a child is even conceived.
It also sharpens an important public debate about how far medicine should go in shaping heredity. Balancing the promise of disease prevention against ethical limits is exactly the kind of science-and-society question that policy must keep addressing.
Maternal spindle transfer in the UPSC exam
MST is a favourite science and technology topic, usually tested on the technique and its implications. The high-yield points are few and precise.
- MST is a form of mitochondrial replacement therapy (mitochondrial donation).
- It moves the mother’s nuclear DNA into a donor egg with healthy mitochondria.
- It prevents the inheritance of mitochondrial diseases.
- It is popularly, if loosely, called the ‘three-parent baby’ technique.
| Feature | Detail |
|---|---|
| Purpose | prevent inherited mitochondrial disease |
| DNA from parents | nuclear DNA (almost all traits) |
| DNA from donor | mitochondrial DNA only |
| Other main technique | pronuclear transfer |
| First legalised in | the United Kingdom, 2015 |
A precise answer distinguishes nuclear from mitochondrial DNA, contrasts spindle transfer with pronuclear transfer, and weighs the medical benefit against the ethical concerns, the balance this article builds throughout.
Prelims MCQ practice
Each question below tests one specific concept on the topic. Click to reveal the answer and a full option-wise explanation.
Q1. Maternal spindle transfer is a technique of:
- Gene editing of the nucleus
- Mitochondrial replacement therapy
- Cloning
- Stem-cell transplantation
Show answer and explanation
Answer: Mitochondrial replacement therapy
Explanation.
Maternal spindle transfer is one of the two main techniques of mitochondrial replacement therapy (mitochondrial donation). Hence (b).
Q2. In maternal spindle transfer, the material moved from the mother's egg into the donor egg is the:
- Mitochondria
- Spindle-chromosome complex (nuclear DNA)
- Cytoplasm only
- Ribosomes
Show answer and explanation
Answer: Spindle-chromosome complex (nuclear DNA)
Explanation.
The spindle-chromosome complex, which carries the mother's nuclear DNA, is transferred into a donor egg whose nucleus has been removed. Hence (b).
Q3. With reference to mitochondrial DNA (mtDNA), consider the following statements:
- Mitochondria carry a small set of their own genes.
- A child inherits mtDNA almost entirely from the mother.
- Mitochondrial DNA is located inside the cell nucleus.
Which of the statements given above is/are correct?
- 1 and 2 only
- 2 and 3 only
- 1 and 3 only
- 1, 2 and 3
Show answer and explanation
Answer: 1 and 2 only
Explanation.
Statements 1 and 2 are correct. Statement 3 is wrong: mtDNA lies in the mitochondria, not in the nucleus. Hence 1 and 2 only.
Q4. The 'three-parent baby' description of mitochondrial donation arises because the child inherits:
- Nuclear DNA from three people
- Mitochondrial DNA from a third-party donor
- Equal DNA from three parents
- Cytoplasm from the father
Show answer and explanation
Answer: Mitochondrial DNA from a third-party donor
Explanation.
The child gets nuclear DNA from the two parents and only the small mitochondrial DNA from the donor, hence the loose label 'three-parent'. Hence (b).
Q5. How does maternal spindle transfer differ from pronuclear transfer?
- MST is done after fertilisation
- MST is done before fertilisation, on the unfertilised egg
- MST uses the father's mitochondria
- MST does not use a donor egg
Show answer and explanation
Answer: MST is done before fertilisation, on the unfertilised egg
Explanation.
Maternal spindle transfer acts before fertilisation, whereas pronuclear transfer moves the pronuclei after the egg is fertilised. Hence (b).
Q6. Which country became the first to legally permit mitochondrial replacement therapy?
- United States
- United Kingdom
- Japan
- Australia
Show answer and explanation
Answer: United Kingdom
Explanation.
The United Kingdom became the first country to legalise mitochondrial replacement therapy, in 2015. Hence (b).
Sources and Further Reading
Editorial Disclaimer
This article explains maternal spindle transfer for UPSC preparation, drawing on standard scientific references. The technique and its regulation reflect the cited authorities.
