text: Organisms in all three domains of life, eukaryotes, bacteria and archaea, are able to carry out de novo biosynthesis of purines.  This ability reflects the essentiality of purines for life.  The biochemical pathway of synthesis is very similar in eukaryotes and bacterial species, but is more variable among archaeal species.  A nearly complete, or complete, set of genes required for purine biosynthesis was determined to be present in 58 of the 65 archaeal species studied.  However, also identified were seven archaeal species with entirely, or nearly entirely, absent purine encoding genes.  Apparently the archaeal species unable to synthesize purines are able to acquire exogenous purines for growth., and are thus similar to purine mutants of eukaryotes, e.g. purine mutants of the Ascomycete fungus Neurospora crassa, that also require exogenous purines for growth.