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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 mycoplasma. 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 capable of protein biosynthesis (such as Mimivirus) have been discovered.
Like viruses, a eukaryotic nucleus contains linear chromosomes with specialized end sequences (bacterial genomes have a circular topology), uses mRNA capping and separates transcription from translation. Eukaryotic nuclei are also capable of cytoplasmic replication. Some large viruses have their own DNA-directed RNA polymerase. A transfer of an 'infectious' nucleus is documented in many parasitic red algae Existing complex eukaryotic DNA viruses could also have originated through nuclear viriogenesis.
Recently, it was suggested that the transition from RNA to DNA genomes first occurred in the viral world. A DNA-based virus may have provided a DNA-based storage for the ancient host that was previously using RNA to store all its genetic information. Viruses could initially adopt 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 one of chloroplast or mitochondria. Following this hypothesis, Archaea, Bacteria, and Eukarya each obtained its DNA informational system from a different virus. The RNA cell at the origin of Eukarya was probably more complex, featuring RNA processing.
A number of precepts in the theory are possible. For instance, a helical virus with a bilipid envelope bears a distinct resemblance to a highly simplified cellular nucleus (i.e.: a DNA chromosome encapsulated within a lipid membrane). To consider the concept logically, a large DNA virus would take control of a bacterial or archaeal cell. Instead of replicating and destroying the host cell, it would remain within the cell. With the virus in control of the host cell's molecular machinery it would effectively become a "nucleus" of sorts. Through the processes of mitosis and cytokinesis, the virus would thus hijack the entire cell—an extremely favourable way to ensure its survival.
- Claverie, Jean-Michel (2006). "Viruses take center stage in cellular evolution". Genome Biology 7 (6): 110. doi:10.1186/gb-2006-7-6-110. PMC 1779534. PMID 16787527.
- Goff, Lynda J.; Coleman, Annette W. (1995). "Fate of Parasite and Host Organelle DNA during Cellular Transformation of Red Algae by Their Parasites". The Plant Cell Online 7 (11): 1899–1911. doi:10.1105/tpc.7.11.1899. JSTOR 3870197. PMC 161048. PMID 12242362.
- Forterre, Patrick (2006). "Three RNA cells for ribosomal lineages and three DNA viruses to replicate their genomes: A hypothesis for the origin of cellular domain". Proceedings of the National Academy of Sciences 103 (10): 3669–74. Bibcode:2006PNAS..103.3669F. doi:10.1073/pnas.0510333103. JSTOR 30048645. PMC 1450140. PMID 16505372.
- Livingstone Bell, Philip John (2001). "Viral Eukaryogenesis: Was the Ancestor of the Nucleus a Complex DNA Virus?". Journal of Molecular Evolution 53 (3): 251–6. doi:10.1007/s002390010215. PMID 11523012.
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