DNA Has Four Bases. Some Viruses Swap in a Fifth.

All life on Earth rests on the same foundation: a four-letter genetic alphabet spelling out a repertoire of three-letter words that specify 20 amino acids. These basic building blocks — the components of DNA and their molecular interpreters — lie at biology’s core. “It’s hard to imagine something more fundamental,” said Floyd Romesberg, a synthetic biologist at the pharmaceutical company Sanofi.

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