Everywhere on Earth, the genetic code at the heart of living things consists of the same four genetic letters. Everywhere, that is, except in a flask of Escherichia coli bacteria on a lab bench in southern California. There, researchers this year engineered the bacteria to incorporate two additional letters into their genetic alphabet. In addition to the natural nucleotides, in which G pairs with C and A pairs with T, the bacterial DNA includes a novel pair: X and Y.
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