Virtually all complex cells – better known as eukaryotes – have at least two separate genomes. The main one sits in the central nucleus. There’s also a smaller one in tiny bean-shaped structures called mitochondria, little batteries that provide the cell with energy. Both sets of genes must work together. Neither functions properly without the other.
Mitochondria came from a free-living bacterium that was engulfed by a larger cell a few billion years ago. The two eventually became one. Their fateful partnership revolutionised life on this planet, giving it a surge of power that allowed it to become complex and big (see here for the full story). But the alliance between mitochondria and their host cells is a delicate one.
Both genomes evolve in very different ways. Mitochondrial genes are only passed down from mother to child, whereas the nuclear genome is a fusion of both mum’s and dad’s genes. This means that mitochondria genes evolve much faster than nuclear ones – around 10 to 30 times faster in animals and up to a hundred thousand times faster in some fungi. These dance partners are naturally drawn to different rhythms.
This is a big and underappreciated problem because the nuclear and mitochondrial genomes cannot afford to clash. In a new paper, Nick Lane, a biochemist at University College London, argues that some of the most fundamental aspects of eukaryotic life are driven by the need to keep these two genomes dancing in time. The pressure to maintain this “mitonuclear match” influences why species stay separate, why we typically have two sexes, how many offspring we produce, and how we age.
Here’s the problem: both sets of genes help to create proteins that sit in the mitochondria and carry out one of the most important of chemical reactions: respiration. The proteins strip electrons from our food and pass them along from one to another. They eventually deposit the electrons onto oxygen; this produces water and releases energy. These ‘electron transfer chains’ are the stuff of life, and they only work if the proteins involved are built correctly.
The proteins in the chain are made of different subunits. Some are built using instructions from nuclear genes, while others are built using mitochondrial genes. They different parts must fit together with nanometre precision. Even a small change in their shape will produce botched proteins that fumble their electrons. If fewer electrons make it to the end of the chain, the mitochondria produce less energy. The leaking electrons can also react with oxygen directly to produce destructive molecules called free radicals.
Monday, February 13, 2012
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