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 struggled to understand 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.
It is not clear if 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 on viruses' protein folding and structure, suggests that the giant viruses, such as Mimivirus, are a separate domain of life, alongside the traditional three of Eukarya, Prokarya and Archaea. The study suggests that giant viruses have evolved from more complex organisms into their highly parasitic form, and are an ancient lineage, alongside that of the other three domains.
Viral replication and self-assembly has implications for the study of the origin of life, as it lends further credence to the hypothesis that life could have started as self-assembling organic molecules.
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 economic 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 hijacks 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.
Viroids attained significance beyond plant virology since one possible explanation of their origin is that they represent “living relics” from a hypothetical, ancient, and non-cellular RNA world before the evolution of DNA or protein. This view was first proposed in the 1980s, and regained popularity in the 2010s to explain crucial intermediate steps in the evolution of life from inanimate matter (Abiogenesis).
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