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Non-cellular life

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This article is about life that has no cellular structure. For the syncytial and plasmodial forms of cellular life sometimes referred to as "acellular", see Multinucleate.

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

Nucleic acid-containing infectious agents

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Viruses

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Further information: Virus, Viral evolution, and Virus classification
an image showing types of virus
Types of virus

Researchers initially described viruses as "poisons" or "toxins", then as "infectious proteins"; but they possess genetic material, a defined structure, and the ability to spontaneously assemble from their constituent parts. This has spurred extensive debate as to whether they should be regarded as fundamentally organic or inorganic — as very small biological organisms or as very large biochemical molecules. Without their hosts, they are not able to perform any of the functions of life - such as respiration, growth, or reproduction. 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][8]

Viroids

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Main article: Viroid

If viruses are borderline cases or nonliving, viroids are further from being living organisms. Viroids are some of the smallest infectious agents, consisting solely of short strands of circular, single-stranded RNA without protein coats. They are only known to infect flowering plants, of 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 viruses capable of causing an infection are around 2,000 nucleobases in size.[9][10] Viroid RNA does not code for any protein.[11] 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.[12]

A possible explanation of the origin of viroids sees them as "living relics" from a hypothetical, ancient, and non-cellular RNA world before the evolution of DNA or of protein.[13][14] This view, first proposed in the 1980s,[13] regained popularity in the 2010s to explain crucial intermediate steps in the evolution of life from inanimate matter (abiogenesis).[15][16]

Obelisks

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In 2024, researchers announced the possible discovery of viroid-like, but distinct, RNA-based elements dubbed obelisks. Obelisks, found in sequence databases of the human microbiome, are possibly hosted in gut bacteria. They differ from viroids in that they code for two distinct proteins, dubbed "oblins", and for the predicted rod-like secondary structure of their RNA.[17][18]

First universal common ancestor

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The first universal common ancestor is an example of a proposed non-cellular lifeform, as it is the earliest ancestor of the last universal common ancestor, its sister lineages, and every currently living cell.[19]

See also

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References

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  1. ^ "What is Non-Cellular Life?". Wise Geek. Conjecture Corporation. 2009. Archived from the original on 21 January 2021. Retrieved 2 August 2009.[unreliable source?]
  2. ^ "The 7 Characteristics of Life". infohost.nmt.edu. Archived from the original on 19 November 2016. Retrieved 26 January 2017.[better source needed]
  3. ^ Benner SA (26 January 2017). "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 E (2012). "Definition of Life: Navigation through Uncertainties" (PDF). Journal of Biomolecular Structure & Dynamics. 29 (4): 647–650. doi:10.1080/073911012010525017. PMID 22208269. S2CID 8616562. Archived from the original on 27 January 2012. Retrieved 27 January 2017 – via JBSD.{{cite journal}}: CS1 maint: unfit URL (link)
  5. ^ Ma W (26 September 2016). "The essence of life". Biology Direct. 11 (1): 49. doi:10.1186/s13062-016-0150-5. ISSN 1745-6150. PMC 5037589. PMID 27671203.
  6. ^ Villarreal LP (December 2004). "Are Viruses Alive?". Scientific American. 291 (6): 100–105. Bibcode:2004SciAm.291f.100V. doi:10.1038/scientificamerican1204-100. PMID 15597986. Archived from the original on 22 December 2013. Retrieved 27 April 2013.
  7. ^ Forterre P (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. ^ Lwoff A (1 January 1957). "The Concept of Virus". Microbiology. 17 (2): 239–253. doi:10.1099/00221287-17-2-239. PMID 13481308.
  9. ^ 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.
  10. ^ "ARS Research Timeline – Tracking the Elusive Viroid". 2 March 2006. Archived from the original on 6 July 2007. Retrieved 18 July 2007.
  11. ^ Tsagris EM, Martínez De Alba AE, Gozmanova M, Kalantidis K (2008). "Viroids". Cellular Microbiology. 10 (11): 2168–79. doi:10.1111/j.1462-5822.2008.01231.x. PMID 18764915.
  12. ^ Daròs JA, Elena SF, Flores (2006). "Viroids: An Ariadne's thread into the RNA labyrinth". EMBO Reports. 7 (6): 593–598. doi:10.1038/sj.embor.7400706. PMC 1479586. PMID 16741503.
  13. ^ a b Diener TO (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.
  14. ^ Villarreal LP (2005). Viruses and the evolution of life. Washington, D.C.: ASM Press. p. 31. ISBN 1-55581-309-7.
  15. ^ Flores R, Gago-Zachert S, Serra P, Sanjuán R, Elena SF (2014). "Viroids: Survivors from the RNA world?" (PDF). Annual Review of Microbiology. 68: 395–414. doi:10.1146/annurev-micro-09113-103416. hdl:10261/107724. PMID 25002087. Archived (PDF) from the original on 22 December 2018. Retrieved 15 December 2018.
  16. ^ Zimmer C (25 September 2014). "A Tiny Emissary From the Ancient Past". The New York Times. Archived from the original on 27 September 2014. Retrieved 22 November 2014.
  17. ^ Sidik S (29 January 2024). "'Wildly weird' RNA bits discovered infesting the microbes in our guts". Nature. doi:10.1038/d41586-024-00266-7. PMID 38291328. Archived from the original on 30 January 2024. Retrieved 31 January 2024.
  18. ^ Pennisi E (26 January 2024). "'It's insane': New viruslike entities found in human gut microbes". Science. doi:10.1126/science.znxt3dk. Archived from the original on 30 January 2024. Retrieved 31 January 2024.
  19. ^ Prosdocimi F, José MV, de Farias ST (2019), Pontarotti P (ed.), "The First Universal Common Ancestor (FUCA) as the Earliest Ancestor of LUCA's (Last UCA) Lineage", Evolution, Origin of Life, Concepts and Methods, Cham: Springer International Publishing, pp. 43–54, doi:10.1007/978-3-030-30363-1_3, ISBN 978-3-030-30363-1, S2CID 199534387, archived from the original on 8 March 2024, retrieved 2 November 2023
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