Here’s the history as they now see it: the free-living, oxygen-breathing ancestors of mitochondria were infected with some nasty T3/T7 viruses. Most of the time the viruses were fatal. But some mutant tried to replicate itself inside a proto-mitochondrion and failed. Its genes were trapped in the genome of its host. Its host was able to reproduce, and one of its descendants took up residence inside the cell of a eukaryote. At some point after this merger, a mutation caused the virus’s DNA and RNA copying genes to come back online. They took over the job of making these molecules, and the mitochondria’s own genes for this job were later stripped out of its genome.
It’s a plausible hypothesis for a number of reasons. Filee and Forterre didn’t just pull the notion that viral genes can become active again out of a hat; this sort of viral resurrection has been documented in other species. Not only is the hypothesis plausible, but it’s a tantalizing as well. It suggests that we are chimeras built from the DNA of eukaryotes, bacteria, and viruses, all mixed together through a natural version of genetic engineering. Forterre even argues that these sorts of results are going to turn out to be the tip of the iceberg. Like many scientists, he believes that before life was based on DNA, the Earth was inhabited by RNA-based life. He argues that DNA was an invention of viruses of these RNA-based organisms, which the RNA-based organisms then seized for their own use. All this may not make you any fonder of the chickenpox you may have had as a kid, but it may at least give you a feeling of kinship.
September 16, 2005
Viruses, mitochondria, cells, etc.
Carl Zimmer posts a fascinating description on new research about the ever-more-fascinating stuff that is found in our cells. It was first thought that cells are nice containers, designed to keep our DNA safely shielded in their nuclei, but it turns out that our cells contain DNA fragments of quite diverse origins, which have managed to strike up an alliance for their joint proliferation: