Viral eukaryogenesis  Viral eukaryogenesis is the hypothesis that the cell nucleus of eukaryotic life forms evolved from a large DNA virus in a form of endosymbiosis within a methanogenic archaeon. The virus later evolved into the eukaryotic nucleus by acquiring genes from the host genome and eventually usurping its role. The hypothesis was proposed by Philip Bell in 2001 and gained support as large, complex DNA viruses (such as "Mimivirus") capable of protein biosynthesis were discovered. Recent supporting evidence includes the discovery that, upon the infection of a bacterial cell, the giant bacteriophage 201Phi2-1 (of genus "Phikzvirus") assembles a nucleus-like structure that segregates proteins according to function. This nucleus-like structure and its key properties have been found conserved in the related phages. The viral eukaryogenesis hypothesis has inflamed the longstanding debate over whether viruses are living organisms. Many biologists do not consider viruses to be alive, but the hypothesis posits that viruses are the originators of the DNA genetic mechanism shared by all eukaryotes alive today (and possibly that of prokaryotes as well). Critics of the theory point out that the similarities between DNA viruses and nuclei can be taken as evidence either of viral eukaryogensis or of its converse, nuclear viriogenesis: that complex eukaryotic DNA viruses could have originated from infectious nuclei. The viral eukaryogenesis hypothesis posits that eukaryotes are composed of three ancestral elements: a viral component that became the modern nucleus; a prokaryotic cell (an archaeon according to the eocyte hypothesis) which donated the cytoplasm and cell membrane of modern cells; and another prokaryotic cell that, by endocytosis, became the modern mitochondrion or chloroplast. In 2006, researchers suggested that the transition from RNA to DNA genomes first occurred in the viral world. A DNA-based virus may have provided storage for an ancient host that had previously used RNA to store its genetic information (such host is called ribocell or ribocyte). Viruses may initially have adopted DNA as a way to resist RNA-degrading enzymes in the host cells. Hence, the contribution from such a new component may have been as significant as the contribution from chloroplasts or mitochondria. Following this hypothesis, archaea, bacteria, and eukaryotes each obtained their DNA informational system from a different virus.