ALL four of the key DNA building blocks have now been found in meteorites, suggesting that space rocks may have delivered the compounds to Earth, contributing to the origin of life. DNA has a spiral-staircase structure, in which each step consists of pairs of molecules called nucleobases. Two of these four nucleobases - adenine and guanine, which belong to a group of chemical compounds called purines-were first detected in meteorites in the 1960s.
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