Non-cellular life

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This article is about non-cellular organisms. For cell-like, non-cell structures, see Syncytium.

Non-cellular life is life that exists without a cellular structure. This term presumes the phylogenetic scientific classification of viruses as lifeforms,[1] which is a controversial issue.[2][3][4]

Hypothesized artificial life, self-replicating machines, and most simple molecules capable of self-replication are not usually considered living. (See definition of life.)

Some biologists refer to wholly syncytial organisms as "acellular"[citation needed] 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.

History[edit]

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.[2][3]

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.[5] This discovery suggests the possibility that some viruses may have evolved from earlier forms that could produce proteins independent of a host cell.[6] 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.[7] 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.[8] 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.[8]

Viral self-assembly has implications for the study of the origin of life,[9] as it lends further credence to the hypothesis that life could have started as self-assembling organic molecules.[10][11]

Virus[edit]

Main article: Virus

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, Biota, Naturae, or Vitae. The Taxon Cytota would include three top-level subdivisions of its own, the Domains Bacteria, Archaea, and Eukarya.

Viroid[edit]

Main article: Viroid

Viroids are plant pathogens that consist of a short stretch of circular, single-stranded RNA without the protein coat that is typical for viruses. Viroid RNA does not code for proteins.[12]

See also[edit]

References[edit]

  1. ^ "What is Non-Cellular Life?". Wise Geek. Conjecture Corporation. 2009. Retrieved 2009-08-02. 
  2. ^ a b Villarreal, Luis P. (December 2004). "Are Viruses Alive?". Scientific American. Retrieved 2013-04-27. 
  3. ^ a b Forterre, Patrick (3 March 2010). "Defining Life: The Virus Viewpoint". Orig Life Evol Biosph. 40 (2): 151–160. doi:10.1007/s11084-010-9194-1. PMC 2837877. PMID 20198436. 
  4. ^ Greenspan, Neil (28 January 2013). "Are Viruses Alive?". The Evolution & Medicine Review. Retrieved 2013-04-27. 
  5. ^ The Mimivirus protein involved in translation
  6. ^ Luis P. Villarreal (2005). Viruses and the Evolution of Life. New York ASM Press.
  7. ^ American Scientist, "Giant Viruses", James L. Van Etten, July-August 2011, Volume 99, Number 4
  8. ^ a b LiveScience.com, "Giant Viruses Are Ancient Living Organisms, Study Suggests", 14 September 2012
  9. ^ Koonin EV, Senkevich TG, Dolja VV (2006). "The ancient Virus World and evolution of cells". Biol. Direct 1: 29. doi:10.1186/1745-6150-1-29. PMC 1594570. PMID 16984643. Retrieved 2009-08-01. 
  10. ^ Vlassov AV, Kazakov SA, Johnston BH, Landweber LF (August 2005). "The RNA world on ice: a new scenario for the emergence of RNA information". J. Mol. Evol. 61 (2): 264–73. doi:10.1007/s00239-004-0362-7. PMID 16044244. 
  11. ^ Nussinov, Mark D.; Vladimir A. Otroshchenkob and Salvatore Santoli (1997). "Emerging Concepts of Self-organization and the Living State". Biosystems 42 (2–3): 111–118. doi:10.1016/S0303-2647(96)01699-1. PMID 9184757. Retrieved 2009-08-02. 
  12. ^ Tsagris EM, de Alba AE, Gozmanova M, Kalantidis K (September 2008). "Viroids". Cell. Microbiol. 10 (11): 2168–79. doi:10.1111/j.1462-5822.2008.01231.x. PMID 18764915.