Sequence analysis of the small-subunit RNA allows the relatedness among all organisms to be quantitatively assessed.  These analyses show that one group of prokaryotes, the Archaea, are distinct from the Bacteria and should have molecular properties in common with eukaryotic organisms (the Eucarya).  (Figure adapted from Woese, Kandler, and Wheelis, 1990. Proc. Nat. Acad. Sci. USA 87: 4576-4579).  

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