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Mitochondrial DNA Mixes It Up
14 May 2004 (All day)
"Mitochondrial Eve," the hypothetical mother of all modern humans who lived about 150,000 years ago, might be lying about her age. A key assumption in determining how long ago she lived--that molecules of mitochondrial DNA do not swap segments with one another--is false, researchers now say. Their findings call into question a multitude of findings in evolution, early human migration, and even the relations between languages.
Researchers have long counted on the stability of DNA in mitochondria--cells' energy-producing factories--to measure the time between events in the distant past. Unlike the DNA in chromosomes, which are a mix of maternal and paternal genes, mitochondrial DNA is inherited directly from mom. Moreover, mitochondrial DNA molecules were not thought to swap sequences of DNA with one another. (Chromosomes, in contrast, routinely do this, creating novel assortments of genes in each new generation.)
The only changes to mitochondrial DNA, therefore, seemed to be spontaneous mutations. And because mutations pile up at a predictable rate, the number of mitochondrial DNA differences between, say, a modern human and an ancestor can be used to calculate how long ago the two groups diverged. Or so scientists thought.
Several years ago, however, researchers made an unusual discovery: a man who had inherited some mitochondrial DNA from his father, as well as his mother. In the current study, Konstantin Khrapko of Harvard Medical School in Boston and colleagues took advantage of the rare glitch to test the assumption that mitochondrial DNA doesn't recombine. The research team members sequenced the man's mitochondrial DNA and compared it to some from his parents. They found stretches of paternal DNA mixed in with stretches of maternal DNA.
Additional experiments showed that this recombination happened when enzymes that copy mitochondrial DNA stopped copying mom's DNA, jumped to dad's DNA, and began copying from the same site--and vice versa--the team reports in the 14 May issue of Science.
"The data are quite convincing," says molecular biologist R. Sanders Williams of Duke University in Durham, North Carolina. "The implications are that this is going on all the time in our cells." Mitochondrial DNA's history "is clearly not as clean as people had thought. Or people had wished," says molecular biologist Eric Shron of Columbia University in New York City. As yet, it's not known how recombination changes mitochondrial DNA, says Shron, so it's too early to say whether the molecular clock has been running fast or slow.