Are we all extraterrestrials? Scientists discover traces of DNA in space.

Fully-formed building blocks of DNA have been found in meteorites, suggesting an extraterrestrial origin for some of the chemicals deemed necessary for organic life.

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    Scientists have found a large variety of nucleobases, essential building blocks of DNA, in meteorites. The team found adenine and guanine, two of the four components of DNA, as well as hypoxanthine and xanthine. DNA resembles a spiral ladder; adenine and guanine connect with two other nucleobases to form the rungs of the ladder. Hypoxanthine and xanthine are not found in DNA, but are used in other biological processes.
    Chris Smith / Goddard Space Flight Center / NASA
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If you're out watching the Perseid meteor shower tonight, you'll be witnessing a process that scientists have increasingly come to regard as the vehicle for delivering basic building blocks of organic life to Earth.

A team of scientists led by NASA's Michael Callahan appears to have added some critical components to the list of space-borne building blocks raining onto Earth's surface: a class of chemicals called nucleobases. Nucleobases combine to carry the genetic information found in DNA molecules, the molecules that orchestrate an organism's form and functions.

In results reported this week in the Proceedings of the National Academy of Sciences, the team analyzed the chemical makeup of a dozen carbon-rich meteorites plucked from different locations in Antarctica and Australia.

They not only found nucleobases widely found in organisms on Earth, compounds such as adenine and guanine, two of the four bases found in DNA. They also found related compounds, which the team dubbed "nucleobase analogues," that aren't found on Earth and in effect are new to science.

Over the years, scientists have found amino acids, which are needed to form proteins, with unambiguous space-based origins. And meteorites have carried the chemicals necessary to make cell walls.

Even nucleobases have been detected in meteorites before, notes Dr. Callahan. But scientists have had a hard time convincing themselves and others that the nucleobases came from space. They could represent contamination a space rock picked up after it landed.

"We do see biological nucleobases in ice and soil samples" collected from the places where the meteorites were found, he says. "That's only natural."

But, he adds, the mix and distribution of the nucleobases typically found in ice and soil are far different that those found in the meteorites the team analyzed.

And those new-to-science compounds are definitely not earth contamination. "We don't see any nucleobase analogues whatsoever" in the ice and soil, he says.

Many biologically important molecules form from water, hydrogen cyanide, and ammonia – all substances that are abundant in the clouds of dust and gas that collapse to form stars and planets.

As an additional test, the team conducted simple experiments in the lab in which they mixed ammonia, hydrogen cyanide, and water and ended up with the same variety of nucleobases and nucleobase analogues they found in meteorites.

They concluded that some of the key components of DNA may have been delivered to Earth ready-made by chemical reactions that took place on asteroids or comets as the solar system evolved.

And if it can happen in our solar system, Callahan suggests, under the right conditions these building blocks can find their way to hospitable planets elsewhere.

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