According to the FDA, mitochondria are membrane enclosed cytoplasmic organelles that are responsible for the synthesis of adenosine triphosphate (ATP) for use in the energy-requiring processes of the cell (FDA, 2014). Interestingly, the prevalent theory on the origin of mitochondria suggests that mitochondrial DNA (mtDNA) is a fossil molecule that proves the occurrence of endosymbiosis. This origin dates back approximately 1.5 billion years ago when protobacteria populated primordial eukaryotic cells and took permanent residence in their new environment (Dimauro & Davidzon, 2005). For this reason, mitochondria have their own DNA and contain their own protein synthesis mechanisms.
The human mitochondrial genome is small. Any given mitochondrion contains about 16.6 kilobases, or approximately 37 genes. However, in any given cell, there could be one-hundred to thousands of mitochondria present that are dedicated to the production of ATP (Dimauro & Davidzon, 2005). This is dependent on the type of tissue the cell is found in. It is important to note that mtDNA synthesis is not associated with nuclear DNA synthesis. In essence, mtDNA replication is not subjected to the numerous checkpoints present in nuclear DNA replication. Thus, the chance of a spontaneous point mutation occurring is much higher in mtDNA (FDA, 2014). Furthermore, while seemingly independent of nuclear DNA, the mitochondria still needs the other organelles present in the cell to survive. Of the 1500 proteins required for the mitochondria to function, most are encoded in the nuclear genome (FDA, 2014).
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