Every cell contains hundreds of mitochondria, the distant descendants of ancient symbiotic bacteria. They have evolved to become cellular components, tightly integrated into many vital functions, but still replicate like bacteria, and still contain a small remnant circular genome, known as mitochondrial DNA. Of the varied tasks undertaken by mitochondria, the most important is the generation of the chemical energy store molecule ATP, used to power cellular operations. This is a necessarily energetic operation and produces oxidative molecules as a byproduct, capable of reacting with and damaging the proteins that make up cellular machinery. This sort of reaction happens constantly and is repaired constantly, as a cell is a fluid bag of countless proteins and other molecules bumping into one another. Too much is harmful, however.
Mitochondrial DNA encodes a few vital proteins, necessary for the correct function of mitochondria, particularly when it comes to the mechanisms of ATP generation. Unfortunately mitochondrial DNA is right next door to the machinery that produces ATP and reactive molecules, it replicates far more frequently than the DNA of the cell nucleus, thus generating errors at a greater rate, and in addition has inferior protective and repair mechanisms in comparison to nuclear DNA. Mutations
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