A possible iterative scenario for viral eukaryogenesis and nuclear viriogenesis. (a) A primitive DNA virus (a bacteriophage ancestor) gets trapped within an RNA cell and becomes a primitive nucleus. (b) Cellular genes are progressively recruited to the enlarging nucleus because of the selective advantages of DNA biochemistry. (c) For a while this situation remains unstable and reversible, allowing new 'pre-eukaryotic viruses' to be created. These viruses reinfect other cells at various stages of this iterative process. (d) This hypothetical scheme provides a mechanism for the emergence of various overlapping but not monophyletic virus lineages as well as for the rapid reassortment of genes from the viral and cellular pools before they reach their 'Darwinian threshold' , that is, (e) the evolution of a stable eukaryotic cell with a fully DNA nuclear genome.