How does an enzyme know when it is making a mistake? The problem of accuracy in biosynthesis has been studied for a number of years. In DNA synthesis, for example, if too many errors occur a system cannot reproduce viable copies of itself. Too few and the system can never evolve. Solid‐state theorist John Hopfield at Princeton has proposed that many biological systems do “kinetic proofreading,” a method of replication in which the system looks twice so that it can correct an initial mistake. Recently several groups have shown experimentally that such proofreading does appear to be taking place.

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