Magazine article Science News

Early Earth May Have Had Two Key RNA Bases

Magazine article Science News

Early Earth May Have Had Two Key RNA Bases

Article excerpt

At some time more than 3 billion years ago, nucleic acids formed in the primordial soup that simmered on primitive Earth. These large molecules, notably RNA and DNA, evolved into the genetic basis of life.

But that prompts the question, What sequence of reactions triggered the evolution of nucleic acids?

The emergence of RNA, believed to be life's chief precursor, presupposes that its basic building blocks--adenine, guanine, cytosine, and uracil--formed first. Although scientists have shown that the purine bases, adenine and guanine, form readily under simulated early Earth conditions, they have also noted that the pyrimidine bases, cytosine and uracil, do not.

Now, Michael P. Robertson and Stanley L. Miller, both biochemists at the University of California, San Diego, have found a way around this obstacle. They describe in the June 29 Nature an "efficient prebiotic route" for the synthesis of cytosine and uracil.

"What we're envisioning," says Robertson, "is a tidal pool filled with water and urea. As the water evaporates, the urea becomes highly concentrated. The concentrated urea would then have reacted with cyanoacetaldehyde [also present in the primordial soup] to form cytosine. The cytosine would then have gone on to form uracil."

The key to solving the problem, says Miller, lies in concentrating the urea. At low concentrations, little cytosine forms. But seawater pools slowly evaporating under a hot noonday sun can become saturated enough to spawn the requisite chemical reactions. …

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