Mitochondria that serve as the "power plant" in cells of animals and plants started out more as energy "parasites" before they turned beneficial, scientists say.

Parasitic bacteria considered first cousins to mitochondria first operated in the form of energy parasites inside cells before they became beneficial, they say.

That's the finding of a study at the University of Virginia that utilized state-of-the-art DNA technology to sequence the genomes in 18 types of bacteria closely related to mitochondria.

The suggestion that mitochondria first started out as parasitic bacteria within cells may explain their origin 2 billion years ago and how they got taken up by host cells to become organelle "powerhouses" within the nucleus of all plant and animal cells, the researchers suggest in the journal PLOS One.

Mitochondria provide cells with a compound known as adenosine triphosphate, which biologists consider the very "fuel" behind all life.

"We believe this study has the potential to change the way we think about the event that led to mitochondria," says biologists and lead study author Martin Wu. "We are saying that the current theories -- all claiming that the relationship between the bacteria and the host cell at the very beginning of the symbiosis was mutually beneficial -- are likely wrong."

Rather, he says, the initial relationship was an antagonistic one, with the bacteria being parasitic.

"We reconstructed the gene content of mitochondrial ancestors, by sequencing DNAs of its close relatives, and we predict it to be a parasite that actually stole energy in the form of ATP from its host -- completely opposite to the current role of mitochondria," Wu said.

It was only when later evolution caused the mitochondria to switch the direction of ATP transport within cells that it became the beneficial cellular component it is today, he says.

Without mitochondria providing energy to the rest of a cell in which it exists, early life on earth could not have evolved to create the amazing biodiversity around us today, he says.

The scientists also identified human genes which originated in mitochondria, a finding they say may help give insights into the genetics of dysfunction in human mitochondria assumed to contribute to a number of diseases such as "Alzheimer's disease, Parkinson's disease and diabetes" and a number of other aging-related diseases, the researchers reported.

Researchers have also studied mitochondrial DNA for use in forensics, the history of human evolution and genealogy.

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