Key DNA building blocks that previous research mysteriously failed to discover in meteorites have now been discovered in space rocks, suggesting that cosmic impacts could have helped deliver these vital ingredients of life to ancient Earth.
DNA is made up of four main building blocks: nucleobases called adenine (A), thymine (T), cytosine (C), and guanine (G). DNA’s sister molecule, RNA, also uses A, C, and G, but swaps thymine for uracil (U). Scientists wonder if meteorites could have helped deliver these compounds to land have previously looked for nucleobases in space rocks, but until now, scientists had only detected A and G in space rocks, and not T, C, or U.
Nucleobases come in two flavors, known as purines and pyramidines. The nucleobases seen earlier in meteorites are both purines, each of which is made up of a hexagonal molecule fused with a pentagonal molecule. The ones missing from space rocks so far are the pyramidines, which are smaller structures, each made of just one hexagonal molecule.
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For a long time it was a mystery why only purines, not pyramidines, were seen in meteorites. Previous laboratory experiments simulating conditions in outer space suggested that both purines and pyramidines could have been formed during light-triggered chemical reactions within interstellar molecular clouds, and that the compounds could then have been incorporated into asteroids and meteorites during the formation of the solar system. Such chemical reactions may also have occurred directly within the space rocks.
Now scientists have finally detected all the pyramidines and purines found in the DNA and RNA in the meteorites that made it to Earth.
“The presence of all five primary nucleobases in meteorites may contribute to the emergence of genetic functions before the start of life on early Earth,” lead study author Yasuhiro Oba, an astrochemist at the University of Tokyo, told Space.com. Hokkaido in Japan.
The researchers used state-of-the-art analytical techniques originally designed for use in genetic and pharmaceutical research to detect small amounts of nucleobases, down to the parts per billion range. This is at least 10 to 100 times more sensitive than previous methods that tried to detect pyramidines in meteorites, Oba said.
The scientists analyzed samples from three carbon-rich, or carbonaceous, meteorites that previous work suggested might have hosted the types of chemical reactions that created nucleobases: MurchisonMurray and the Tagish Lake Meteorites.
Scientists detected T, C, and U at levels down to a few parts per billion inside the meteorites. These compounds were present in concentrations similar to those predicted by experiments that replicated conditions that existed before the formation of the solar system. In addition to the crucial T, C, and U compounds, the scientists also detected other pyramidines that are not used in DNA or RNA that further show the ability of meteorites to transport these compounds.
“Because of our findings, we can say that nucleobases also show a wide variety in carbonaceous meteorites,” Oba said.
It remains unclear why the pyramidines were so much less abundant in these meteorites than the purines. Oba suggested that a clue might lie in the fact that purines include a pentagonal ring known as an imidazole, while pyramidines do not.
Imidazole and similar molecules were found to be much more abundant than pyramidines in these meteorites, suggesting that they might be easier for natural chemical reactions to synthesize. Furthermore, imidazole can act as a primitive catalyst to trigger chemical reactions, such as the formation of purines instead of pyramidines.
The scientists detailed his findings online April 26 in the journal Nature Communications.
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