Comet Clues Lead to RNA World Revelations

저자：   업로드：2016-05-23  조회수：

    “The cosmos is within us,” said Carl Sagan. “We are made of star stuff.” Memorable words, but they don’t explain how “star stuff”—pretty much all the chemical elements—could have come together in various ways to form life’s chemical building blocks—proteins and nucleic acids. No doubt Sagan was aware that the story of chemical evolution still needed to be worked out in detail, so if he were still with us today, he might have amended his words in light of new information. He might have said that we are made of “comet stuff.”

    New information has indeed been found. And appropriately enough, it has come from outer space. Recent discoveries made by the European Space Agency's Rosetta mission to the comet 67/P/Churyumov-Gerasimenko (67P/C-G) have led scientists to propose an efficient mechanism for the prebiotic synthesis of a vital class compounds—the large purine bases A and G. These bases presumably arose from simple organic compounds, and, in living organisms, they help encode the genetic information stored in RNA and DNA. They also form part of the molecules adenosoine triphosphate (ATP) and guanosine triphosphate (GTP), both of which serve as energy sources for biochemical reactions and as molecular switches in the control of protein function.

    Yet how could A and G have accumulated in the first place? Such questions are entertained by scientists working on the "RNA World" hypothesis, which posits that RNA was one of the first self-replicating molecules leading to the origin of life. Among these scientists are chemists at Ludwig-Maximilians-Universitaet (LMU) in Munich. They analyzed simple organic molecules that were detected on the surface of the 67P/C-G comet, and they developed a plausible mechanism for how these molecules could have served as purine base precursors.

    The LMU scientists, led by Thomas Carell, presented their work May 13 in the journal Science, in an article entitled “A High-Yielding, Strictly Regioselective Prebiotic Purine Nucleoside Formation Pathway.” In this article, the LMU scientists assert that their mechanism is more efficient than one that had been proposed earlier. The earlier mechanism called for a single prebiotic pathway for purine nucleosides, but it lacked regioselectivity and would have produced low yields.

    “We report that the condensation of formamidopyrimidines (FaPys) with sugars provides the natural N-9 nucleosides with extreme regioselectivity and in good yields (60%),” the authors of the Science article wrote. “The FaPys are available from formic acid and aminopyrimidines, which are in turn available from prebiotic molecules that were also detected during the Rosetta comet mission.”

    The LSU authors added that this nucleoside formation pathway can be fused to sugar-forming reactions to produce pentosides, providing a plausible scenario of how purine nucleosides may have formed under prebiotic conditions.