The legacy of mitochondrial DNA

Introduction Present in all nucleated cells, mitochondria are essential subcellular organelles that play a crucial role in several different biochemical processes, including energy production. Mitochondria are believed to be evolutionary relics of ancient bacterial symbionts [1], and an important legacy of this history is the persistence within these organelles of a small genome, termed mitochondrial DNA (mtDNA). MtDNA is the only extranuclear source of DNA in the cell and it follows a different mode of inheritance from nuclear DNA. We highlight the important role of mitochondria in reproduction and why this small molecule of DNA presents so many interesting and important challenges particularly in reproductive biology. Mitochondria are double-membraned structures which are central to a multitude of biological functions in all nucleated mammalian cells, including the regulation of apoptotic cell death, the control of cytosolic calcium concentration, and the biogenesis of iron–sulfur clusters. Mitochondria are also the primary source of endogenous reactive oxygen species and they house several critical biochemical pathways, including the tricarboxylic acid cycle and part of the urea cycle. However, arguably the most important function of mitochondria is the production of ATP, the energy carrier of the cell, via oxidative phosphorylation (OXPHOS). OXPHOS requires the coordinated activity of five multi-subunit enzyme complexes located in the inner mitochondrial membrane. Electrons, resulting from the oxidation of fat and carbohydrates, are transported along complexes I–IV, thus creating an electrochemical gradient for protons across the inner mitochondrial membrane that drives the synthesis of ATP by complex V (ATP synthase).