Triumph of the Helix
In an epochal discovery 50 years ago, life met its own master molecule
Physicist Erwin Schrodinger had a knack for taking complex ideas and making them easy to understand. To illustrate the wacky world of quantum physics, for example, he told a story about an unfortunate feline that was both dead and alive--his famous "cat paradox." And in 1944, he helped unravel the central mystery of biology in a little book called What Is Life?
This daunting question didn't put off Schrodinger. Perhaps, he speculated, the genetic material in cells' chromosomes contains some kind of chemical Morse code. With just a few simple letters, this code could store enough information to explain the dizzying diversity of life.
Without knowing it, Schrodinger had hit on an eerily prescient description of deoxyribonucleic acid, or DNA. Scientists already knew of DNA, but most viewed it as just one more ingredient in the gunk inside cells. Less than 10 years later, however, James Watson and Francis Crick worked out the structure--the fabled double helix--that allows DNA to do just what Schrodinger envisioned. That discovery established DNA as life's master molecule and ultimately made it the master molecule of biology and medicine as well.
Now, half a century later in a world transformed by DNA, scientists are still trying to work out how this molecule became nature's record keeper. They are also wondering whether life could arise without DNA--whether some other form of the molecule, or even something completely different, could do the same job. Stanford University chemist Eric Kool says he and others are asking: "Could you just monkey with the structure a little bit and still get life? Or could you entirely redesign it from scratch and still get life?"
Fifty years ago, DNA seemed an unlikely master molecule. For one thing, it was chemically simple, even boring, compared with the complex proteins also found in the cell nucleus. But earlier experiments showing that DNA from one strain of bacteria could permanently alter another convinced Watson and Crick that it might hold the secret of heredity. They'd also seen some X-ray pictures that suggested DNA had a spiral shape. On Feb. 28, 1953, Watson cleared off his desk and began fiddling with cardboard models of four key parts of DNA--adenine, guanine, cytosine, and thymine.
Suddenly the puzzle pieces fit into place: A linked with T and C with G. The two pairs looked identical, and Watson realized they could form the steps of a spiral staircase. This double helix could duplicate itself by unzipping into two strands, each a template for building another helix with the same sequence of letters. It was just what was needed from the molecule of heredity. In wry understatement, Watson and Crick wrote that their structure had "novel features which are of considerable biological interest."
Truth and beauty. The beauty of the structure convinced some scientists right away. Leslie Orgel, now at Salk Institute for Biological Studies, remembers feeling awestruck when he saw the model in 1953. But he adds, "The world of biochemistry was extremely slow to pick up on it." Few newspapers spotlighted the discovery, and scientists had doubts about the structure. How could DNA unzip itself without getting all tangled up? And how could those few letters serve as code for all the complex proteins of life?
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