1. Passing on mitochondrial diseases from parent to child can be prevented by mitochondrial replacement therapy either before or after in vitro fertilization of egg. 

2. A child inherits mitochondrial diseases entirely from mother and not from father. 

Which of the statements given above is/are correct? 

(a) 1 only 

(b) 2 only 

(c) Both 1 and 2 

(d) Neither 1 nor 2 

Ans: c

Explanation: 

Children inherit mitochondrial DNA (mDNA) exclusively from their mothers. If a mother has a mDNA mutation, there is a possibility that she may pass this mutation on to her offspring. Mitochondrial replacement therapy (MRT) is an advanced technique used in in vitro fertilisation (IVF), which involves fertilising an egg with sperm in a controlled laboratory environment. This therapy enables women with mDNA mutations to conceive and give birth to children while reducing the risk of transmitting the mutation to their descendants.   

MRT is performed using one of two methods: maternal spindle transfer or pronuclear transfer. 

Maternal Spindle Transfer (MST) 

Maternal Spindle Transfer (MST) is an innovative reproductive technique designed to prevent the transmission of mitochondrial DNA (mDNA) mutations from a mother to her child. The process begins with a careful selection of donor eggs. Each donor egg undergoes thorough screening to ensure that it is completely free from any mDNA mutations, guaranteeing a healthy source of mitochondria.

Once a suitable donor egg is identified, the next step involves the removal of the egg’s nuclear DNA (nDNA), effectively vacating the donor egg of its genetic material while preserving its healthy mitochondria. This crucial step leaves the donor egg with robust mDNA, which is essential for cellular energy production.

Following this, the mother’s nDNA, which carries her genetic traits, is precisely transplanted into the now-empty donor egg. This combination creates an egg that contains the biological mother’s nDNA and the donor’s healthy mDNA. 

The final phase of MST involves fertilizing the transformed donor egg using in vitro fertilization (IVF) techniques. Once fertilization occurs, the resulting embryo is carefully implanted into the mother’s uterus, allowing for the possibility of a healthy pregnancy while minimizing the risk of passing on mitochondrial diseases.

Pronuclear Transfer (PNT)

Pronuclear Transfer (PNT) is a sophisticated reproductive technique that is akin to Maternal Spindle Transfer (MST), but it utilises an egg fertilised in a laboratory setting by the mother and father’s genetic material. The process unfolds in several detailed steps:

Creation of the Lab-Fertilized Egg: Initially, a lab-fertilized egg is formed by combining the biological mother’s egg with sperm from the biological father. This step ensures that the resulting zygote contains the intended nuclear DNA (nDNA) from both parents.

Selection of a Donor Egg: Next, a suitable donor egg is meticulously selected. This egg must be screened thoroughly to guarantee it is devoid of any mitochondrial DNA (mDNA) mutations, ensuring the highest chances of a healthy pregnancy.

Removal of Donor nDNA: The selected donor egg, which possesses healthy mDNA, undergoes a process where its nDNA is carefully extracted. This step is crucial, as it prepares the donor egg to receive the genetic material from the lab-fertilized egg without introducing any foreign nuclear DNA.

Transplantation of nDNA: Following the extraction, the nDNA from the lab-fertilized egg is transplanted into the donor egg. This action effectively reconstitutes the egg with the biological parents’ genetic information while retaining the donor’s healthy mDNA.

Implantation into the Mother’s Uterus: Finally, the reconstructed egg is implanted into the biological mother’s uterus through in vitro fertilisation (IVF). This step marks the culmination of the PNT process, with the hope of achieving a successful pregnancy.

Through these carefully orchestrated steps, Pronuclear Transfer aims to provide couples with the opportunity to conceive healthy children while minimising the risk of inherited mitochondrial diseases.

Mitochondrial Inheritance

Mitochondria are organelle present in the cytoplasm of the cells which are primarily responsible for energy production for all cellular activities. 

Mitochondria contain their own distinct DNA, called mitochondrial DNA (mtDNA), which directs mitochondrial activities. Mitochondrial DNA has the same molecular structure and function as the DNA found in the nucleus, but it’s organized somewhat differently

All cells contain several hundred of these oval-shaped structures that convert energy (derived from the breakdown of nutrients) to a form that can be used by cells.

Each mitochondrion contains several copies of a ring-shaped DNA molecule, or chromosome. While mitochondrial DNA (mtDNA) is distinct from chromosomal DNA, its molecular structure and functions are the same. The entire molecule has been sequenced and is known to contain around 40 genes that direct the conversion of energy within cells.

Like the DNA in a cell’s nucleus, mtDNA is subject to mutations, and some of these mutations cause certain genetic disorders that result from impaired energy conversion.

 Importantly, animals of both sexes inherit all their mtDNA, and thus all mitochondrial traits, from their mothers. Because mtDNA is inherited from only one parent, meiosis and recombination don’t occur. 

Hence, option c is correct.

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