When we think about our genes, we most commonly think of the nuclear genome, the 23 pairs of chromosomes that determine our characteristics but the nuclear genome is not the only genome we possess.
Mitochondria are present in most of cells and their main function is to assist in respiration. They also contain a genome known as mitochondrial DNA (mtDNA). Like nuclear DNA it is double stranded but the similarities end there. MtDNA is circular with around 16,000 base pairs in 37 genes. These genes encode for 13 proteins, 22 tRNA’s and 2 rRNA’s. They are not enveloped or packaged into chromatin and some parts of mtDNA do not follow the universal coding rules. It also tends to have a higher mutation rate than nuclear DNA. MtDNA has a maternal inheritance pattern, rather than the usual inheritance pattern in the nuclear genome, meaning that all offspring inherit mtDNA from their mother.
Just as mutations in nuclear DNA cause various diseases and syndrome, mutations in mtDNA can affect vital organs in the body. The most common defects in mtDNA cause migraines, strokes, epilepsy, dementia, heart failure, diabetes, thyroid problems and cataracts. It can also cause anaemia and hepatic failure in children. Pearson (deletion at 4977bp) and Leigh’s syndrome (point mutation at 8993) are some of the syndromes that are caused by mtDNA mutations.
In the 1990’s at Saint Barnabas Medical Center in New Jersey, a technique known as cytoplasmic transfer was carried out in order to assist reproduction in infertile women. Cytoplasmic transfer takes the cytoplasm of a donor egg and inserts it into the egg of the mother. It is estimated that around 30 children were born using this technique in the last couple of decades. Although the nuclear DNA in the egg is still that of the mother’s as it is unaltered by the transfer. However, mitochondria reside in the cytoplasm of the cell and so the donor’s mitochondria are transferred into the mother’s egg. Synchronous transfer is when the donor cytoplasm is transferred when both egg’s are in the same developmental stage and asynchronous transfer is when the donor cytoplasm is transferred when the egg’s are in different developmental stages.
The effects of cytoplasmic transfer haven’t been studied in detail and while little side effects were encountered during the procedure, no further study has been carried out on the children that were born using this method. During the procedure, 2 foetuses were not carried to term. One was aborted and one was a miscarriage but both had a sex chromosomes consist with those of Turner syndrome (45 XO).
Later on this year, an investigation is due to be launched in order to look at the effects on the children today. Since the technique was stopped by the US government in 2002, no more children were born using this technique after that. However, if the technique is proved to be safe, it could provide benefits to infertile women. Following the results of the study, the technique could also be used in Britain.
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