|Why we grow old and die - the mitochondrion and the "vicious circle" theory of ageing.
The picture above is of a single mitochondrion. Mitochondria are the power-houses of both plant and animal cells, and contain electron transport chains and genes ("mtDNA"). Electron transport - redox chemistry - is bad for genes, causing mutations that increase the proportion of electron transfers that do genetic damage; these mutations are their own cause and effect. This "vicious circle" is now widely discussed as an explanation of ageing. Whatever the reason for the persistence, in evolution, of genes in mitochondria and chloroplasts, it had better be a good one - these are the wrong places to keep a delicate genetic apparatus. In a paper in the Journal of Theoretical Biology John Allen has proposed that mitochondrial and chloroplast genes have been retained because there is an over-riding need for their regulation by the electron transport chains that they help to build. A more recent paper in the same journal proposes that the fresh mitochondria and chloroplasts of each new generation come only from the mother because her egg cells contain mitochondrial and chloroplast genes that are switched off, so that no damaging electron transport can occur. These ideas are being tested.