Mitochondrial Genetics and Functions

INTRODUCTION AND BRIEF HISTORY Mitochondria are complex organelles specialized in respiration and oxidative phosphorylation, which possess their own genetic system and their own protein synthetic machinery. The mitochondrial genome carries the genetic information for only a few, essential mitochondrial components. The formation of mitochondria is a complex process that requires the intricate assembly of components from two different origins: the nucleus and the mitochondria. The majority of the mitochondrial proteins, in mass as well as in number of species, are encoded by the nuclear genome and synthesized on cytoplasmic ribosomes. The minority (~ 5% of all mitochondrial proteins) are encoded and synthesized within the mitochondria. The dual origin is even more apparent if one examines individually the different mitochondrial components, because each of the mitochondrially encoded proteins is eventually assembled into functional complexes comprising nuclear-encoded proteins as well. Despite their numerical importance, thus far very little is known of the mitochondrial components of nuclear origin and of their genes. However, a considerable effort during the past several years has resulted in a very detailed description of the mitochondrial genes and their products to the point that the mitochondrial genome of yeast is, at present, one of the best characterized eukaryotic genomes. This paper is therefore devoted to a detailed description of the mitochondrial genes, their genetic properties, and their functions, with only brief mention of nuclear genes. This apparently limited topic (the mitochondrial genome is, after all, very small) has recently undergone such a complex evolution that, during the past...