Proteins are built from combinations of twenty smaller subunits, amino acids, similar to the way English words arise from various combinations of twenty-six letters. And much as sensible words require letters to be arranged in specific orders, usable proteins require amino acids to be linked in specific sequences. If such assembly were left to blind chance, the likelihood that the requisite amino acids would happen to bump into one another in just the right way to build a particular protein would be next to nothing. The sheer number of ways that twenty distinct amino acids can be linked in a
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