The Gene: An Intimate History - Siddhartha Mukherjee Page 0,100

existed in nature (a bacterial gene combined with a viral gene to form a new genetic element). But the “reading” of genes—the deciphering of the precise sequence of bases in a stretch of DNA—was still an enormous technical hurdle.

Ironically, the very features that enable a cell to read DNA are the features that make it incomprehensible to humans—to chemists, in particular. DNA, as Schrödinger had predicted, was a chemical built to defy chemists, a molecule of exquisite contradictions—monotonous and yet infinitely varied, repetitive to the extreme and yet idiosyncratic to the extreme. Chemists generally piece together the structure of a molecule by breaking the molecule down into smaller and smaller parts, like puzzle pieces, and then assembling the structure from the constituents. But DNA, broken into pieces, degenerates into a garble of four bases—A, C, G, and T. You cannot read a book by dissolving all its words into alphabets. With DNA, as with words, the sequence carries the meaning. Dissolve DNA into its constituent bases, and it turns into a primordial four-letter alphabet soup.

How might a chemist determine the sequence of a gene? In Cambridge, England, in a hutlike laboratory buried half-underground near the fens, Frederick Sanger, the biochemist, had struggled with gene sequencing since the 1960s. Sanger had an obsessive interest in the chemical structures of complex biological molecules. In the early 1950s, Sanger had solved the sequence of a protein—insulin—using a variant of the conventional disintegration method. Insulin, first purified from dozens of pounds of ground-up dog pancreases in 1921 by a Toronto surgeon, Frederick Banting, and his medical student Charles Best, was the grand prize of protein purification—a hormone that, injected into diabetic children, could rapidly reverse their wasting, lethal, sugar-choking disease. By the late 1920s, the pharmaceutical company Eli Lilly was manufacturing grams of insulin out of vast vats of liquefied cow and pig pancreases.

Yet, despite several attempts, insulin remained doggedly resistant to molecular characterization. Sanger brought a chemist’s methodological rigor to the problem: the solution—as any chemist knew—was always in dissolution. Every protein is made of a sequence of amino acids strung into a chain—Methionine-Histidine-Arginine-Lysine or Glycine-Histidine-Arginine-Lysine, and so forth. To identify the sequence of a protein, Sanger realized, he would have to run a sequence of degradation reactions. He would snap off one amino acid from the end of the chain, dissolve it in solvents, and characterize it chemically—Methionine. And he would repeat the process, snapping off the next amino acid: Histidine. The degradation and identification would be repeated again and again—Arginine . . . snap . . . Lysine . . . snap—until he reached the end of the protein. It was like unstringing a necklace, bead by bead—reversing the cycle used by a cell to build a protein. Piece by piece, the disintegration of insulin would reveal the structure of its chain. In 1958, Sanger won the Nobel Prize for this landmark discovery.

Between 1955 and 1962, Sanger used variations of this disintegration method to solve the sequences of several important proteins—but left the problem of DNA sequencing largely untouched. These were his “lean years,” he wrote; he lived in the leeward shadow of his fame. He published rarely—immensely detailed papers on protein sequencing that others characterized as magisterial—but he counted none of these as major successes. In the summer of 1962, Sanger moved to another laboratory in Cambridge—the Medical Research Council (MRC) Building—where he was surrounded by new neighbors, among them Crick, Perutz, and Sydney Brenner, all immersed in the cult of DNA.

The transition of labs marked a seminal transition in Sanger’s focus. Some scientists—Crick, Wilkins—were born into DNA. Others—Watson, Franklin, Brenner—had acquired it. Fred Sanger had DNA thrust upon him.

In the mid-1960s, Sanger switched his focus from proteins to nucleic acids and began to consider DNA sequencing seriously. But the methods that had worked so marvelously for insulin—breaking, dissolving, breaking, dissolving—refused to work for DNA. Proteins are chemically structured such that amino acids can be serially snapped off the chain—but with DNA, no such tools existed. Sanger tried to reconfigure his degradation technique, but the experiments only produced chemical chaos. Cut into pieces and dissolved, DNA turned from genetic information to gobbledygook.

Inspiration came to Sanger unexpectedly in the winter of 1971—in the form of an inversion. He had spent decades learning to break molecules apart to solve their sequence. But what if he turned his own strategy upside down and tried to build DNA, rather than break it down? To solve a gene sequence, Sanger reasoned, one must think like