Early forms of life are thought to have used RNA as their sole source of information before DNA existed. The endosymbiotic theory says that millions of years ago a multicellular(eukaryote) organism engulfed a single-celled(prokaryote) organism. Instead of killing the prokaryote, the eukaryote allowed it to remain and incorporated it into its own environment. It is hypothesized that this is the molecular basis for the existence of RNA in eukaryotes.
DNA and RNA differ by one atom. Both eukaryotes and prokaryotes use the same genetic code, and it is repetative in both, to signal the construction of proteins.