Some biologists refer to wholly syncytial organisms as "acellular" because their bodies contain multiple nuclei which are not separated by cell membranes, however these cell-bound organisms are outside the scope of the present article.
For about 100 years, the scientific community has repeatedly changed its collective mind over what viruses are. First seen as poisons, then as life-forms, then biological chemicals, and today many scientists think of viruses as existing at the border between chemistry and life: a gray area between living and nonliving.
The issue of life without cellular structure came again to the fore with the 2003 discovery that the large and complex Mimivirus can synthesize proteins. This discovery suggests the possibility that some viruses may have evolved from earlier forms that could produce proteins independent of a host cell. If so, there may at one time have been a viral domain of life. It is not clear that all small viruses have originated from more complex viruses by means of genome size reduction. A viral domain of life may only be relevant to certain large viruses such as nucleocytoplasmic large DNA viruses like the Mimivirus. A 2012 study suggests that the giant viruses, such as Mimivirus, are a separate domain of life, alongside the traditional three of eukarya, prokarya and archaea, by studying the protein folding structure made by the viruses. The study concluded that giant viruses have evolved from more complex organisms into their highly parasitic form, and are an ancient lineage, alongside that of the other domains.
In discussing the taxonomic domains of life, the terms 'Acytota' or 'Aphanobionta' are occasionally used as the name of a viral kingdom, domain, or empire. The corresponding cellular life name would be Cytota. Non-cellular organisms and cellular life would be the two top-level subdivisions of life, whereby life as a whole would be known as organisms, Naturae, or Vitae. The Taxon Cytota would include three top-level subdivisions of its own, the Domains Bacteria, Archaea, and Eukarya.
Viroids are the smallest infectious pathogens known, consisting solely of short strands of circular, single-stranded RNA without protein coats. They are mostly plant pathogens, some of which are of economical importance. Viroid genomes are extremely small in size, ranging from 246 to 467 nucleobases. In comparison, the genome of the smallest known viruses capable of causing an infection by themselves are around 2,000 nucleobases in size. Viroids are the first known representatives of a new biological domain of sub-viral pathogens.
Viroid RNA does not code for any protein. Its replication mechanism uses RNA polymerase II, a host cell enzyme normally associated with synthesis of messenger RNA from DNA, which instead catalyzes "rolling circle" synthesis of new RNA using the viroid's RNA as template. Some viroids are ribozymes, having catalytic properties which allow self-cleavage and ligation of unit-size genomes from larger replication intermediates.
With Diener’s 1989 hypothesis—that viroids may represent “living relics” from a hypothetical, ancient, and non-cellular RNA world before the evolution of DNA or protein—--viroids have attained significance far beyond plant virology to evolutionary biology, by representing plausible molecules capable of explaining crucial intermediate steps in the evolution of life from inanimate matter (Abiogenesis).
While Diener’s hypothesis was dormant for 25 years, it has recently been resurrected in a review articleand as a popularized version by a New York Times science writer, in which, however, the author mistakenly credits the "living relics" hypothesis of viroids to Flores et al.
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