Mitochondrial DNA

Despite the majority of DNA being stored as chromosomes within the nucleus of every cell, there is a fraction of DNA found in mitochondria, known as mitochondrial DNA, or mtDNA. It contains 37 genes which are all involved in allowing the normal functioning of the organelle, with 13 being involved in producing enzymes to carry out oxidative phosphorylation (energy / ATP production) and the remaining 24 carrying instructions for making transfer and ribosomal RNA.

MtDNA is inherited exclusively from the mother, as it is transmitted through the female egg, and will be passed on to both sons and daughters, but only her daughters will be able to continue passing it on. Given that mtDNA is considered nonrecombiant (does not combine with any other DNA), it remains virtually unchanged through the direct maternal line over hundreds and even thousands of generations. Consequently, many people will have almost identical mtDNA, which suggests they share a common maternal ancestor. As a result, mtDNA is almost completely useless when it comes to determining biological parents, as there is no connection between a father’s mtDNA and his offspring’s, and many women have very similar mtDNA, making it almost impossible to distinguish a difference. Only in certain circumstances, such as the offspring having completely different mtDNA to their supposed mother, is it useful in determining biological relations.

Disorders and diseases caused by mutations in mtDNA often affect multiple organ systems, and more specifically have the greatest effects on the organs which require the most energy, e.g. the brain or the heart. One example of  disease caused by mutations is Leigh Syndrome, which is a severe neurological disorder.

Symptoms usually present themselves within the first year of life, although there have been reported cases of patients being in early adulthood before any sign of Leigh syndrome becoming apparent. The first symptoms commonly include vomiting and difficulty swallowing, resulting in stinted growth and issues gaining weight. In addition, movement becomes more and more difficult due to muscle weakness, involuntary muscle contractions, and trouble with maintaining balance. In more advanced cases of Leigh Syndrome, muscle weakness may also affect movement of the eyes, and severe breathing problems which may result in acute respiratory failure. Sufferers of the disease tend to only live for a few years, often dying as a result of respiratory problems.

Whilst the majority of Leigh Syndrome patients inherit it through an autosomal recessive pattern, in which both parents are carriers, around 20% of cases are caused by a mitochondrial pattern. Such cases are inherited from the mother only, and whilst it is not certain that children will inherit the disease, the chances are fairly high. This is the reason that last year, for the first time, a baby was born in Mexico and was declared to be the biological child of three people – two mothers and one father. The technique, pronuclear transfer, with which the embryo was created, aimed to prevent the mother of the child passing on any mitochondrial DNA, to remove the risk of passing on Leigh Syndrome.

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