Virus Origins: Conjoined RNA Genomes as Precursors to DNA Genomes

RNA's unprecedented ability to act both as a template for information storage and as an enzymatic molecule has led to the proposal that primitive living systems were based on RNA, with protein synthesis and DNA templates for information storage added later. This chapter reviews the current knowledge about RNA rearrangement and recombination in viruses, and cites evidence for various mechanisms catalyzing these events. RNA recombination can occur in a spontaneous manner, and such a potential, even at low frequency, would expand opportunities for RNA conjunction. The chapter also outlines the significance of work on viroid-like pathogens, circular RNA replication, and their potential relation to early RNA; and relates the early emergence of RNA mosaics to developments leading to today's DNA-based systems of viral gene expression.