Non-cellular life

Non-cellular life is life that exists without a cellular structure for at least part of its life cycle.[1] Historically, most (descriptive) definitions of life postulated that a living organism must be composed of one or more cells,[2] but this is no longer considered necessary, and modern criteria allow for forms of life based on other structural arrangements.[3][4][5][3]

The primary candidates for non-cellular life are viruses. A minority of biologists consider viruses to be living organisms, but most do not. Their primary objection is that no known viruses are capable of autopoiesis, which means they cannot reproduce themselves: they must rely on cells to copy them.[1][6][7][8][9] However, the recent discovery of giant viruses that possess genes for part of the required translation machinery has raised the prospect that they may have had extinct ancestors that could evolve and replicate independently. Most biologists agree that such an ancestor would be a bona fide non-cellular lifeform, but its existence and characteristics are still uncertain.[1][10][11][12][13]

Engineers sometimes use the term "artificial life" to refer to software and robots inspired by biological processes, but these do not satisfy any biological definition of life.

Viruses as non-cellular life

The nature of viruses was unclear for many years following their discovery as pathogens. They were described as poisons or toxins at first, then as "infectious proteins", but with advances in microbiology it became clear that they also possessed genetic material, a defined structure, and the ability to spontaneously assemble from their constituent parts. This spurred extensive debate as to whether they should be regarded as fundamentally organic or inorganic — as very small biological organisms or very large biochemical molecules — and since the 1950s many scientists have thought of viruses as existing at the border between chemistry and life; a gray area between living and nonliving.[6][7][14]

The recent discovery of giant viruses (aka giruses, nucleocytoplasmic large DNA viruses, NCLDVs) has reignited this debate, since they are not only physically larger than previously known viruses, but also possess much more extensive genomes, including genes coding for aminoacyl tRNA synthetases, key proteins involved in translation, which were previously thought to be exclusive to cellular organisms. This raises the prospect that the giant viruses may have had extinct ancestors (or even undiscovered ones) capable of engaging in life processes (such as evolution and replication) independent of cells, and that modern viruses lost those abilities secondarily. The viral lineage including this ancestor would be ancient and may have originated alongside the earliest archaea or before the LUCA. If such a virus is discovered, or its past existence supported by further genetic evidence, most biologists agree that it would constitute a bona fide lifeform, and its descendants (at least the giant viruses, and possibly including all known viruses) could be phylogenetically classified in a fourth domain of life. Discovery of the pandoraviruses, with genomes are even larger than the other giant viruses and exhibiting particularly low homology with the three existing domains, further discredited the traditional view that viruses simply "pick-pocketed" all of their genes from cellular organisms, and further supported the "complex ancestors" hypothesis.[1][10][11][12][13][6][15][16]

Ongoing research is being conducted in this area, using techniques such as phylogenetic bracketing on the giant viruses to infer characteristics of their proposed progenitor.[17]

Furthermore, Viral replication and self-assembly has implications for the study of the origin of life,[18] as it lends further credence to the hypothesis that life could have started as self-assembling organic molecules.[19][20]

Viroids

Viroids are the smallest infectious pathogens known to biologists, consisting solely of short strands of circular, single-stranded RNA without protein coats. They are mostly plant pathogens and some are animal pathogens, from which some 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 realm of sub-viral pathogens.[21][22]

Viroid RNA does not code for any protein.[23] 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.[24]

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.[25][26] This view was first proposed in the 1980s,[25] and regained popularity in the 2010s to explain crucial intermediate steps in the evolution of life from inanimate matter (Abiogenesis).[27][28]

Taxonomy

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

See also

References

  1. 1 2 3 4 "What is Non-Cellular Life?". Wise Geek. Conjecture Corporation. 2009. Retrieved 2009-08-02.
  2. ↑ "The 7 Characteristics of Life". infohost.nmt.edu. Retrieved 2017-01-26.
  3. 1 2 Benner, Steven A. (2017-01-26). "Defining Life". Astrobiology. 10 (10): 1021–1030. Bibcode:2010AsBio..10.1021B. doi:10.1089/ast.2010.0524. ISSN 1531-1074. PMC 3005285. PMID 21162682.
  4. ↑ Trifonov, Edward (2012). "Definition of Life: Navigation through Uncertainties" (PDF). Journal of Biomolecular Structure & Dynamics. 29 (4): 647–650. doi:10.1080/073911012010525017 – via JBSD.
  5. ↑ Ma, Wentao (2016-09-26). "The essence of life". Biology Direct. 11. doi:10.1186/s13062-016-0150-5. ISSN 1745-6150. PMC 5037589. PMID 27671203.
  6. 1 2 3 Villarreal, Luis P. (December 2004). "Are Viruses Alive?". Scientific American. Retrieved 2013-04-27.
  7. 1 2 Forterre, Patrick (3 March 2010). "Defining Life: The Virus Viewpoint". Orig Life Evol Biosph. 40 (2): 151–160. Bibcode:2010OLEB...40..151F. doi:10.1007/s11084-010-9194-1. PMC 2837877. PMID 20198436.
  8. ↑ Luketa, Stefan (2012). "New views on the megaclassification of life" (PDF). Protistology. 7 (4): 218–237.
  9. ↑ Greenspan, Neil (28 January 2013). "Are Viruses Alive?". The Evolution & Medicine Review. Retrieved 2016-04-27.
  10. 1 2 Legendre, Matthieu; Arslan, Defne; Abergel, Chantal; Claverie, Jean-Michel (2012-01-01). "Genomics of Megavirus and the elusive fourth domain of Life". Communicative & Integrative Biology. 5 (1): 102–106. doi:10.4161/cib.18624. ISSN 1942-0889. PMC 3291303. PMID 22482024.
  11. 1 2 Boyer, Mickaël; Madoui, Mohammed-Amine; Gimenez, Gregory; La Scola, Bernard; Raoult, Didier (2010-12-02). "Phylogenetic and Phyletic Studies of Informational Genes in Genomes Highlight Existence of a 4th Domain of Life Including Giant Viruses". PLoS ONE. 5 (12): e15530. Bibcode:2010PLoSO...515530B. doi:10.1371/journal.pone.0015530. ISSN 1932-6203. PMC 2996410. PMID 21151962.
  12. 1 2 Claverie, Jean-Michel; Abergel, Chantal (2010-10-01). "Mimivirus: the emerging paradox of quasi-autonomous viruses". Trends in Genetics. 26 (10): 431–437. doi:10.1016/j.tig.2010.07.003. ISSN 0168-9525. PMID 20696492.
  13. 1 2 Forterre, Patrick; Prangishvili, David (2009-09-01). "The origin of viruses". Research in Microbiology. 160 (7): 466–472. doi:10.1016/j.resmic.2009.07.008. ISSN 1769-7123. PMID 19647075.
  14. ↑ Lwoff, A. (1957-01-01). "The Concept of Virus". Microbiology. 17 (2): 239–253. doi:10.1099/00221287-17-2-239.
  15. ↑ Zimmer, Carl (2013-07-18). "Changing View on Viruses: Not So Small After All". The New York Times. ISSN 0362-4331. Retrieved 2017-01-26.
  16. ↑ Nasir, Arshan; Kim, Kyung Mo; Caetano-Anolles, Gustavo (2012-08-24). "Giant viruses coexisted with the cellular ancestors and represent a distinct supergroup along with superkingdoms Archaea, Bacteria and Eukarya". BMC Evolutionary Biology. 12: 156. doi:10.1186/1471-2148-12-156. ISSN 1471-2148. PMC 3570343. PMID 22920653.
  17. ↑ Colson, Philippe; Gimenez, Gregory; Boyer, Mickaël; Fournous, Ghislain; Raoult, Didier (2011-04-29). "The giant Cafeteria roenbergensis virus that infects a widespread marine phagocytic protist is a new member of the fourth domain of Life". PLoS One. 6 (4): e18935. Bibcode:2011PLoSO...618935C. doi:10.1371/journal.pone.0018935. ISSN 1932-6203. PMC 3084725. PMID 21559486.
  18. ↑ 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.
  19. ↑ 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. Bibcode:2005JMolE..61..264V. doi:10.1007/s00239-004-0362-7. PMID 16044244.
  20. ↑ 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.
  21. ↑ 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.
  22. ↑ "ARS Research Timeline – Tracking the Elusive Viroid". 2006-03-02. Retrieved 18 July 2007.
  23. ↑ 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.
  24. ↑ 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 1479586. PMID 16741503.
  25. 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. Bibcode:1989PNAS...86.9370D. doi:10.1073/pnas.86.23.9370. PMC 298497. PMID 2480600.
  26. ↑ Villarreal, Luis P. (2005). Viruses and the evolution of life. Washington, D.C.: ASM Press. p. 31. ISBN 1-55581-309-7.
  27. ↑ 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.
  28. ↑ Zimmer, Carl (25 September 2014). "A Tiny Emissary From the Ancient Past". The New York Times. Retrieved 22 November 2014.
  29. ↑ 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.
This article is issued from Wikipedia. The text is licensed under Creative Commons - Attribution - Sharealike. Additional terms may apply for the media files.