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Eukaryotes, prokaryotes: who's first?

Eukaryotes, prokaryotes: Who's first?

In plants and animals, the vast majority of DNA is never translated into protein. These stretches of silent DNA, called introns, may be a clue in evolution's mystery of first appearances: Are intron-laden eukaryotes (higher organisms) the most ancient living things? Or does that honor go to the less elaborate prokaryotes (bacteria), in which what you see in DNA more closely resembles what you get in protein?

Periannan Senapathy of the National Institutes of Health in Bethesda, Md., reports that a statistical analysis shows eukaryotes to be the older of the two. Senapathy says eukaryotic DNA is similar to a randomly generated series of DNA subunits--and, he hypothesizes, to the randomly organized bits of genetic material in the "primordial soup.'

According to the report, which appears in the April PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES (No. 8), there are generally no more than 200 consecutive coding bits (codons) of DNA in random sequences before a bit that would read as a "stop' message during copying. "In the first primitive cells,' Senapathy says, "the main selective pressure must have been to generate long coding sequences from short' ones, to make longer proteins. To avoid the interfering "stop' codons, Senapathy says, primordial eukaryotes evolved a splicing mechanism. The stretches whose information is deleted by the splicing machinery are the introns. Prokaryotes, with sequences of coding bits that often far exceed the limits in random DNA, may have evolved from eukaryotes, losing both introns and the splicing mechanism used to circumvent them.

It is the resemblance between eukaryotic gene structure and the hypothetically random structure of such sequences in the "primordial soup' that makes Senapathy argue for the precedence of primordial eukaryotes. But, says biologist Lynn Margulis of Boston University, "There are strong arguments against random pieces of DNA in the primordial soup. [Senapathy's study shows] probably bona fide phenomena that I see much more as a consequence of how eukaryotic cells are put together, and how they function. To go from there to primitiveness, or earliness in a geological sense--it just isn't warranted by the data.'
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Title Annotation:statistical analysis on most ancient living things
Publication:Science News
Date:May 3, 1986
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