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From what I recall, mitochondrial DNA is very susceptible to damage from oxidative stress because it is a site where free radicals are generated and their DNA isn't packed into chromatin like nucleic DNA is. How then is it possible that mitochondria can stay in-tact when it is repeatedly passed on to offspring?

I would have thought that an accumulation of errors would cause them to malfunction over time.

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If it was the case that we were inheriting damaged mtDNA, we would be inheriting ageing from our parents which in itself is not advantageous in terms of evolution.Considering that the germline mitochondria is purely maternal, nature has evolved a way to switch off the mitochondria of oocytes, and keep it in a simple state, mainly to serve as a template. This template is essentially error free, as the levels of ROS generated, is very low.

Mutations in DNA (mitochondrial or nuclear) in most cells in the body can be harmful to the health of the individual, but will have no influence on the next generation. The genes which we pass onto the next generation are separated off during early development into special ‘germ line’ cells which form sperm and eggs. Great care is taken to minimise the risk of mutation to these genes – genes in germline cells act as a blue print for the next generation. This is essentially why aging is not heritable, and it is a system that works pretty well.

Research has uncovered a rather elegant solution to this problem – those mitochondria that will be passed to the next generation are maintained in an inactive state.Mitochondria are only inherited through the maternal line – every mitochondria in your body came from your mother, and this is true for most animal species. The mitochondria in sperm are generally discarded at some point prior to fertilisation.

Membrane electrical potential, a measure of the activity of the electron transport chain, was reduced in oocytes compared to both sperm and the surrounding tissue. Further, ROS production was 50- and 100-fold lower in the eggs of fruit flies and zebrafish respectively. Finally, they confirmed that oocyte mitochodria in both species exhibit a simpler structure, indicative of reduced activity. So, it seems that in both fish and flies, the mitochondria in egg cells represent little more than a blueprint, ready to be passed on to the next generation error-free. By deactivating ovarian mitochondria, the fidelity of information is ensured across generations, and aging is not heritable.

Taken from: http://blogs.ucl.ac.uk/gee-research/2013/11/11/damage-and-fidelity-the-role-of-the-female-germline-in-mtdna-inheritance/

You can read the article here: de Paula, Wilson BM, et al. "Female and male gamete mitochondria are distinct and complementary in transcription, structure, and genome function." Genome biology and evolution 5.10 (2013): 1969-1977.

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