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Non-cellular life

From Wikipedia, the free encyclopedia
This article is about life that has no cellular structure. For thesyncytialandplasmodialforms of cellular life sometimes referred to as "acellular", seeMultinucleate.

Non-cellular life,also known asacellular life,islifethat exists without acellularstructure for at least part of itslife cycle.[1]Historically, mostdefinitions of lifepostulated that anorganismmust 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[edit]

Viruses[edit]

Further information:Virus,Viral evolution,andVirus classification

Viruses were initially described aspoisonsortoxins,then as "infectious proteins"; but they possessgenetic 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 biologicalorganismsor very large biochemicalmolecules.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[edit]

Main article:Viroid

If viruses are borderline cases or nonliving, viroids are further from being living organisms. Viroids are the smallest infectious agents, consisting solely of short strands of circular, single-strandedRNAwithout 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 467nucleobases.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 hijacksRNA 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 areribozymes,having catalytic properties which allow self-cleavage and ligation of unit-size genomes from larger replication intermediates.[12]

A possible explanation of their origin is that they represent "living relics" from a hypothetical, ancient, and non-cellularRNA worldbefore the evolution ofDNAor protein.[13][14]This view was first proposed in the 1980s,[13]and regained popularity in the 2010s to explain crucial intermediate steps in the evolution of life from inanimate matter (abiogenesis).[15][16]

Obelisks[edit]

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

First universal common ancestor[edit]

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

See also[edit]

References[edit]

  1. ^"What is Non-Cellular Life?".Wise Geek.Conjecture Corporation. 2009.Archivedfrom the original on 21 January 2021.Retrieved2 August2009.
  2. ^"The 7 Characteristics of Life".infohost.nmt.edu.Archived fromthe originalon 19 November 2016.Retrieved26 January2017.
  3. ^Benner SA (26 January 2017)."Defining Life".Astrobiology.10(10): 1021–1030.Bibcode:2010AsBio..10.1021B.doi:10.1089/ast.2010.0524.ISSN1531-1074.PMC3005285.PMID21162682.
  4. ^Trifonov E (2012)."Definition of Life: Navigation through Uncertainties"(PDF).Journal of Biomolecular Structure & Dynamics.29(4): 647–650.doi:10.1080/073911012010525017.PMID22208269.S2CID8616562.Archived fromthe original(PDF)on 27 January 2012.Retrieved27 January2017– via JBSD.
  5. ^Ma W (26 September 2016)."The essence of life".Biology Direct.11(1): 49.doi:10.1186/s13062-016-0150-5.ISSN1745-6150.PMC5037589.PMID27671203.
  6. ^Villarreal LP (December 2004)."Are Viruses Alive?".Scientific American.291(6): 100–105.Bibcode:2004SciAm.291f.100V.doi:10.1038/scientificamerican1204-100.PMID15597986.Archivedfrom the original on 22 December 2013.Retrieved27 April2013.
  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.PMC2837877.PMID20198436.
  8. ^Lwoff A (1 January 1957)."The Concept of Virus".Microbiology.17(2): 239–253.doi:10.1099/00221287-17-2-239.PMID13481308.
  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.PMID5095900.
  10. ^"ARS Research Timeline – Tracking the Elusive Viroid".2 March 2006.Archivedfrom the original on 6 July 2007.Retrieved18 July2007.
  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.PMID18764915.
  12. ^Daròs JA, Elena SF, Flores R (2006)."Viroids: An Ariadne's thread into the RNA labyrinth".EMBO Reports.7(6): 593–598.doi:10.1038/sj.embor.7400706.PMC1479586.PMID16741503.
  13. ^abDiener 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.PMC298497.PMID2480600.
  14. ^Villarreal LP (2005).Viruses and the evolution of life.Washington, D.C.: ASM Press. p.31.ISBN1-55581-309-7.
  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-091313-103416.hdl:10261/107724.PMID25002087.Archived(PDF)from the original on 22 December 2018.Retrieved15 December2018.
  16. ^Zimmer C(25 September 2014)."A Tiny Emissary From the Ancient Past".The New York Times.Archivedfrom the original on 27 September 2014.Retrieved22 November2014.
  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.PMID38291328.Archivedfrom the original on 30 January 2024.Retrieved31 January2024.
  18. ^Pennisi E (26 January 2024)."'It's insane': New viruslike entities found in human gut microbes ".Science.doi:10.1126/science.znxt3dk.Archivedfrom the original on 30 January 2024.Retrieved31 January2024.
  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,ISBN978-3-030-30363-1,S2CID199534387,archivedfrom the original on 8 March 2024,retrieved2 November2023
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