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By contrast, the first determination of the exact amino-acid sequence of a protein by Sanger”—the sequencing of bovine insulin between 1949 and 1955—“was absolutely essential. One could not even have begun to think seriously about the genetic code until it had been revealed, to begin with, that a protein is beyond the shadow of doubt a polypeptide in which the amino-acid residues really are arranged in a definite, constant, genetically determined sequence—and yet a sequence with no rule by which it determined itself. So that therefore it had to have a code—that is, complete instructions expressed in some manner—to tell it how to exist, you see. Suppose that instead Sanger had found—and that’s what many biochemists would have guessed, in those days, that he would find—that there were general rules of assembly, that a polypeptide was made of a repetitive sequence of amino acids, for example lysine, aspartic acid, glutamine, threonine, repeated however many times. Then that would have been a chemical rule”—of the kind that governs the assembly of sugars into monotonous polysaccharides, for example—“and so you didn’t need a genetic code. Or you needed only a partial code. But Sanger’s discovery, since it revealed a sequence that had no rule, where—” The sequence was full of information, I said, nowhere redundant, no part of it predictable from other parts. “Exactly,” Monod said. “And so to explain the presence of all that information in the protein, you absolutely needed the code.
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Horace Freeland Judson (The Eighth Day of Creation: Makers of the Revolution in Biology)