Mitochondria and their genomes

Virtually all eukaryotic organisms have the capacity for respiration, the process by which fuel molecules, such as sugars and fats, are converted into carbon dioxide and water. The cell uses the energy derived from this process to drive the synthesis of ATP from phosphate and ADP. The energy derived from the corresponding conversion of ATP back to ADP and phosphate is then used to drive many essential energy consuming biological processes, such the active transport of substances across membranes and the biosynthesis of complex molecules such as DNA, RNA and carbohydrates.

The biochemical processes involving the oxidation of fuel molecules and the coupled synthesis of ATP take place in mitochondria. These organelles are derived from an ancient, ancestral bacterium that participated in a symbiotic event leading to formation of the initial aerobic eukaryotic cell roughly 2 billion years ago. Over the eons since this event, most of the genes of the original symbiotic bacterium have been lost or transferred to the nucleus. In all respiring eukaryotes, however, the mitochondrion has retained genes for a relatively small number of proteins that are expressed via the mitochondrion’s own genetic system, which comprises organelle-specific ribosomes, tRNAs, and DNA and RNA polymerases.

The number of genes retained by the mitochondrion is different in different lines of evolutionary descent. In animals for example, the mitochondrial genome is only 16 kb in size and specifies only 13 proteins. By contrast, in flowering plants the mitochondrial genome is much larger (>200kb), codes for roughly 40 proteins, contains a large amount of non-coding DNA and undergoes frequent recombination. Importantly, mitochondrial genes and genomes are transmitted to offspring differently than their nuclear counterparts; mitochondrial genes are transmitted cytoplasmically; in animals and flowering plants this is manifested as maternal inheritance, i.e. inheritance exclusively from the female parent.