By Andy Extance
DNA and RNA, the two major modern forms of genetic code underpinning all of earthly biology, could have coexisted in strict pairings on our planet before life arose here, scientists in England, Scotland and Poland say. Using a hydrogen cyanide–based chemical system intended to mimic conditions in Earth’s early history, the researchers made four bases, the molecular “letters” of the genetic alphabet. Strung together, these bases form gene sequences that cells translate into proteins. But surprisingly, the team found that of the four bases their experiments consistently made, two were in a form found in DNA, whereas the other two were of a kind seen in RNA.
The study, published in Nature and conducted by John Sutherland of the Medical Research Council Laboratory of Molecular Biology in Cambridge, England, and his colleagues, further undermines the so-called RNA world hypothesis. This idea, long one of the most prominent in origins-of-life research, posits that RNA formed the basis of Earth’s biosphere long before DNA and other molecules important to life emerged. Yet to date, scant evidence has been found of chemical pathways to make the RNA-exclusive system that rigid versions of the idea adopt or that could lead to DNA. “People have tended to think of RNA as the parent of DNA,” Sutherland says. “This [paper] suggests that they are molecular siblings.”
Other scientists who were not involved with the study question the plausibility of the conditions used in this hydrogen cyanide-based route, however. Frances Westall, director of the exobiology group at the French National Center for Scientific Research’s Center for Molecular Biophysics in Orléans, notes that forming the bases requires very specific conditions. Mixtures would need to dry out and be exposed to ultraviolet light—two hurdles most easily surmounted on dry land, which was in short supply during our planet’s ocean-covered early days more than four billion years ago. “These conditions certainly existed on the early Earth,” Westall says. “They would not have been that common because there was not that much exposed landmass.” Although she adds that the study is “clever” and “not completely impossible,” she concludes that “there are other, better hypotheses as to locations for the emergence of life and prebiotic molecules.”
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