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All cellular life on Earth is based on DNA, which transfers information—about everything from hair color to personality traits—from one generation to the next. The four chemical bases that convey this information are adenine (A), cytosine (C), guanine (G), and thymine (T).

The other essential “information molecule” on Earth is RNA, in which thymine (T) is replaced by uracil (U). RNA has a one-string structure rather than a double-string structure like DNA. The first cellular life on our planet is thought to have relied exclusively on this means of transferring genetic information—in the so-called “RNA world”—and even today there are viruses (like the one that causes COVID) that only use RNA.

In a paper recently published in Science, a research group led by Dona Sleiman from the Institute Pasteur in Paris has discovered that some viruses show more variation in their genetic coding than was previously known. In the RNA of these viruses, adenine (A) is replaced with Z, where Z stands for diaminopurine.

This follows an earlier study by Zunyi Yang and colleagues at the Foundation for Applied Molecular Evolution in Gainesville, Florida, showing that an artificial genetic system could be created by adding two additional non-standard bases to ordinary DNA. Amazingly, the artificial six-base system continued to evolve rather than reverting back to the natural four-base system. This implies that the DNA we take as standard—made of A, C, G, and T—is just one of many viable solutions to the challenge of biological information transfer.

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