Earth's primordial molecule?

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Biologists interested in the rise of organic life have been frustrated by a chicken-and-egg-type riddle. Nucleic acids, the carriers of genetic blueprints, determine what proteins a life form makes. But proteins seem necessary to make the nucleic acids themselves. Which came first?

Discovery of a type of nucleic acid that can act without the help of proteins may point a way out of this dilemma.

The research, carried out at the University of Colorado, has captured wide-spread scientific attention since it was reported in the journal Cell in mid-November. For example, molecular biologist Harry Noller of the University of California at Santa Cruz calls it ''one of the most exciting discoveries of the decade.'' It was made by Thomas R. Cech and colleagues Paula J. Grabowski, Daniel E. Gottsching, Kelly Kruger, Julie Sands, and Arthur J. Zaug.

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Biologists recognize two classes of nucleic acids - RNA (ribonucleic acid) and DNA (deoxyribonucleic acid). DNA carries the basic genetic information that determines the development and form and, to some extent, the behavior of an organism. However, it cannot itself put that information to work. RNA generally is needed to translate those instructions into the many thousands of proteins that underlie biological activity.

Some proteins, called enzymes, speed biochemical reactions, including the formation of DNA and RNA. Up to now, biochemists have considered enzymes essential for RNA to operate. What Cech and his colleagues have found is a type of RNA that, to some extent, can act alone.

Cech says the RNA can itself act as an enzyme. He calls it a ''ribozyme.'' The Colorado biologists discovered it in a single-celled pond animal - a protozoan called Tetrahymenam thermophila. At this writing, they do not know if any other organisms use such RNA. However, they are proceeding on the assumption that they have made a fundamental discovery. Noller agrees. He told the journal Science: ''Cech's discovery is raising our awareness of what RNA can do. You can be sure there is a great deal more to be discovered.''

Cech calls it ''a crack in the door.'' He explains: '' We formerly thought that the molecules that do the work inside the cell were enzymes. Now it looks like ribonucleic acid is perhaps capable of doing some of the work too.''

Specifically, the Tetrahymenam RNA molecules can cut out bits of their own substance and splice themselves together again in a manner that implies that they can recombine themselves into new genetic combinations. This has primordial implications. It suggests that, originally, primitive genetic material could get organic life going without the help of protein enzymes.

Cech warns that such speculation is ''a big jump to go from the Tetrahymenam system.'' Yet he has likened his findings to ''discovering the first enzyme.'' To put it another way, it's like discovering that the ancestor of all chickens didn't come out of an egg after all.

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