Non-cellular life

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][5]

Hypothesized artificial life may or may not be considered living. (See definition of life.)

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 this article.

History

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.[2][3]

It is not clear if all small viruses have originated from more complex viruses by means of genome size reduction.[2] A viral domain of life may only be relevant to certain large viruses such as nucleocytoplasmic large DNA viruses like the Mimivirus.[6] 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, Bacteria and Archaea.[7] 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.[7]

Viral replication and self-assembly has implications for the study of the origin of life,[8] as it lends further credence to the hypothesis that life could have started as self-assembling organic molecules.[9][10]

Virus

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, Naturae, or Vitae.[11] The taxon Cytota would include three top-level subdivisions of its own, the domains Bacteria, Archaea, and Eukarya.

Viroid

Main article: Viroid

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 commercial 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.[12][13]

Viroid RNA does not code for any protein.[14] 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 a template. Some viroids are ribozymes, having catalytic properties which allow self-cleavage and ligation of unit-size genomes from larger replication intermediates.[15]

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.[16][17] This view was first proposed in the 1980s,[16] and regained popularity in the 2010s to explain crucial intermediate steps in the evolution of life from inanimate matter (Abiogenesis).[18][19]

See also

References

  1. ↑ "What is Non-Cellular Life?". Wise Geek. Conjecture Corporation. 2009. Retrieved 2009-08-02.
  2. 1 2 3 Villarreal, Luis P. (December 2004). "Are Viruses Alive?". Scientific American. Retrieved 2013-04-27.
  3. 1 2 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 2837877Freely accessible. PMID 20198436.
  4. ↑ Luketa, Stefan (2012). "New views on the megaclassification of life" (PDF). Protistology. 7 (4): 218–237.
  5. ↑ Greenspan, Neil (28 January 2013). "Are Viruses Alive?". The Evolution & Medicine Review. Retrieved 2016-04-27.
  6. ↑ Van Etten, James L. (2011). "Giant Viruses". American Scientist. 99 (4): 304. doi:10.1511/2011.91.304.
  7. 1 2 LiveScience.com, "Giant Viruses Are Ancient Living Organisms, Study Suggests", 14 September 2012
  8. ↑ 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 1594570Freely accessible. PMID 16984643.
  9. ↑ 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.
  10. ↑ Nussinov, Mark D.; Vladimir A. Otroshchenkob & 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.
  11. ↑ Witzany, G (2016). "Crucial steps to life: From chemical reactions to code using agents". Biosystems. 140: 49–57. doi:10.1016/j.biosystems.2015.12.007. PMID 26723230.
  12. ↑ Diener TO (August 1971). "Potato spindle tuber "virus". IV. A replicating, low molecular weight RNA". Virology. 45 (2): 411–28. doi:10.1016/0042-6822(71)90342-4. PMID 5095900.
  13. ↑ "ARS Research Timeline – Tracking the Elusive Viroid". 2006-03-02. Retrieved 18 July 2007.
  14. ↑ Tsagris, E. M.; Martínez De Alba, A. E.; Gozmanova, M; Kalantidis, K (2008). "Viroids". Cellular Microbiology. 10 (11): 2168–79. doi:10.1111/j.1462-5822.2008.01231.x. PMID 18764915.
  15. ↑ Daròs, J. A.; Elena, S. F.; Flores, R (2006). "Viroids: An Ariadne's thread into the RNA labyrinth". EMBO Reports. 7 (6): 593–8. doi:10.1038/sj.embor.7400706. PMC 1479586Freely accessible. PMID 16741503.
  16. 1 2 Diener, T. O. (1989). "Circular RNAs: Relics of precellular evolution?". Proceedings of the National Academy of Sciences of the United States of America. 86 (23): 9370–4. PMC 298497Freely accessible. PMID 2480600.
  17. ↑ Villarreal, Luis P. (2005). Viruses and the evolution of life. Washington, D.C.: ASM Press. p. 31. ISBN 1-55581-309-7.
  18. ↑ Flores, R; Gago-Zachert, S; Serra, P; Sanjuán, R; Elena, S. F. (2014). "Viroids: Survivors from the RNA world?". Annual Review of Microbiology. 68: 395–414. doi:10.1146/annurev-micro-091313-103416. PMID 25002087.
  19. ↑ Zimmer, Carl (25 September 2014). "A Tiny Emissary From the Ancient Past.". The New York Times. Retrieved 22 November 2014.
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