Triumph of the Helix
In an epochal discovery 50 years ago, life met its own master molecule
But Watson and Crick were proved right. DNA has achieved superstar status, and its lovely double spiral is an icon of science. DNA has cured deadly diseases and allowed labs to create animals with fantastic new features. It has freed the innocent from death row and caught a president in a tawdry lie. In this 50th-anniversary year, scientists will put the finishing touches on the full 3-billion-letter sequence of our own DNA, and in it they hope to learn what makes us human.
In reality, DNA's star turn has lasted much longer than 50 years. Somehow, this one chemical came to dominate the world at least 3.5 billion years ago, when one-celled microbes were the pinnacle of biology. Its reign has been unbroken since then. Evolution sticks with winners, and DNA is an ideal information storage facility. Its highly stable structure resists degradation but is not so unchanging that it doesn't provide a few chance errors for natural selection to work on. Weak bonds between its two strands allow them to get pulled apart easily for copying--but not too easily. "DNA might sound boring, but it's beautiful in its boringness," says chemist Ryan Mehl of Franklin and Marshall College.
DNA's structure has enough complexity that it couldn't have spontaneously assembled itself in the primordial soup. Scientists think it evolved from another long-chain molecule: RNA, which still plays myriad roles in today's cells. Like DNA, RNA has four "letters" that store information. But it has an extra oxygen unit that makes it highly reactive. That oxygen even attacks RNA itself, making it unreliable for long-term information storage.
RNA's willingness to tangle with other molecules does let it play a bigger role in life's chemistry than DNA can, which might have been an advantage when life had only a handful of molecules. As early as 1968, Orgel and Crick proposed that RNA could have done double duty, both storing genetic information and assembling proteins. Later, as biochemistry grew more elaborate, this primordial RNA would have created a DNA version of itself to keep life's code safe.
The idea of an earlier "RNA world" got a boost in 1982, when scientists found that RNA can act as an enzyme, speeding up chemical reactions like protein synthesis. Less than two years ago, David Bartel's group at the Massachusetts Institute of Technology found another hint that RNA was once the key living molecule: a small RNA enzyme that can build up other stretches of RNA, suggesting that RNA was once able to duplicate itself.
Even RNA, however, could not have emerged straight from the prehistoric muck. "Everyone seems to agree that RNA came before DNA. So what came before RNA?" asks Gerald Joyce of the Scripps Research Institute. "This is the game now." Several labs have tried to concoct simpler molecules that might serve as RNA precursors. Ultimately, they hope to piece together a plausible path from lifeless chemical parts to the DNA-centered life we know.
A different turn. NASA is a major sponsor of such work, which could show how life might begin on other worlds. Researchers are also investigating whether the story could have turned out differently, culminating in something other than the double helix. That could prepare us to recognize alien life and help explain why DNA on Earth works the way it does.
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