Biology:Non-cellular life

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Short description: Life that has no cellular structure


Adenovirus illustration

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 this is for some no longer considered necessary, and modern criteria allow for forms of life based on other structural arrangements.[3][4][5]

The primary candidates for non-cellular life are viruses. Some biologists consider viruses to be organisms, but others do not. Their primary objection is that no known viruses are capable of autonomous reproduction; they must rely on cells to copy them.[1][6][7][8][9]

Viruses as non-cellular life

Further information: Biology:Virus, Biology:Viral evolution, and Biology:Biology:Virus classification

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][10]

Viral replication and self-assembly has implications for the study of the origin of life,[11] as it lends further credence to the hypotheses that cells and viruses could have started as a pool of replicators where selfish genetic information was parasitizing on producers in RNA world,[4] as two strategies to survive, gained in response to environmental conditions,[5] or as self-assembling organic molecules.[12][13]

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.[14][15]

Viroid RNA does not code for any protein.[16] 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.[17]

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.[18][19] This view was first proposed in the 1980s,[18] and regained popularity in the 2010s to explain crucial intermediate steps in the evolution of life from inanimate matter (abiogenesis).[20][21]

Obelisks

In January 2024, the discovery of viroid-like, but distinct, RNA-based elements called obelisks was announced. Obelisks were found in 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 a typical rod-like secondary structure of their RNA.[22][23]

Taxonomy

In discussing the taxonomic domains of life, the terms "Acytota" and "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, Biota or Vitae.[24] The taxon Cytota would include three top-level subdivisions of its own, the domains Bacteria, Archaea, and Eukarya.[citation needed]

The First universal common ancestor (FUCA) is an example of proposed non-cellular lifeform in taxonomy, as it is the earliest ancestor of LUCA, its sister lineages, and every currently living cell.[25]

See also


References

  1. ↑ 1.0 1.1 "What is Non-Cellular Life?". Wise Geek. Conjecture Corporation. 2009. http://www.wisegeek.com/what-is-non-cellular-life.htm. 
  2. ↑ "The 7 Characteristics of Life". https://infohost.nmt.edu/~klathrop/7characterisitcs_of_life.htm. 
  3. ↑ Benner, Steven A. (2017-01-26). "Defining Life". Astrobiology 10 (10): 1021–1030. doi:10.1089/ast.2010.0524. ISSN 1531-1074. PMID 21162682. Bibcode: 2010AsBio..10.1021B. 
  4. ↑ 4.0 4.1 Trifonov, Edward (2012). "Definition of Life: Navigation through Uncertainties". Journal of Biomolecular Structure & Dynamics 29 (4): 647–650. doi:10.1080/073911012010525017. PMID 22208269. http://www.jbsdonline.com/mc_images/category/4317/21-trifonov-jbsd_29_4_2012.pdf. 
  5. ↑ 5.0 5.1 Ma, Wentao (2016-09-26). "The essence of life". Biology Direct 11 (1): 49. doi:10.1186/s13062-016-0150-5. ISSN 1745-6150. PMID 27671203. 
  6. ↑ 6.0 6.1 Villarreal, Luis P. (December 2004). "Are Viruses Alive?". Scientific American 291 (6): 100–105. doi:10.1038/scientificamerican1204-100. PMID 15597986. Bibcode: 2004SciAm.291f.100V. http://www.scientificamerican.com/article.cfm?id=are-viruses-alive-2004. Retrieved 2013-04-27. 
  7. ↑ 7.0 7.1 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. PMID 20198436. Bibcode: 2010OLEB...40..151F. 
  8. ↑ Luketa, Stefan (2012). "New views on the megaclassification of life". Protistology 7 (4): 218–237. http://protistology.ifmo.ru/num7_4/luketa_protistology_7-4.pdf. 
  9. ↑ Greenspan, Neil (28 January 2013). "Are Viruses Alive?". The Evolution & Medicine Review. https://evmedreview.com/are-viruses-alive/. Retrieved 2016-04-27. 
  10. ↑ Lwoff, A. (1957-01-01). "The Concept of Virus". Microbiology 17 (2): 239–253. doi:10.1099/00221287-17-2-239. PMID 13481308. 
  11. ↑ 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. PMID 16984643. 
  12. ↑ 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. Bibcode: 2005JMolE..61..264V. 
  13. ↑ 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. 
  14. ↑ "Potato spindle tuber "virus". IV. A replicating, low molecular weight RNA". Virology 45 (2): 411–28. August 1971. doi:10.1016/0042-6822(71)90342-4. PMID 5095900. 
  15. ↑ "ARS Research Timeline – Tracking the Elusive Viroid". 2006-03-02. http://www.ars.usda.gov/is/timeline/viroid.htm. 
  16. ↑ 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. 
  17. ↑ 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. PMID 16741503. 
  18. ↑ 18.0 18.1 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. doi:10.1073/pnas.86.23.9370. PMID 2480600. Bibcode: 1989PNAS...86.9370D. 
  19. ↑ Villarreal, Luis P. (2005). Viruses and the evolution of life. Washington, D.C.: ASM Press. p. 31. ISBN 1-55581-309-7. https://archive.org/details/virusesevolution0000vill. 
  20. ↑ 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. https://digital.csic.es/bitstream/10261/107724/1/Annu.%20Rev.%20Microbiol.%20Flores%20et%20al%202014.pdf. 
  21. ↑ Zimmer, Carl (25 September 2014). "A Tiny Emissary From the Ancient Past.". The New York Times. https://www.nytimes.com/2014/09/25/science/a-tiny-emissary-from-the-ancient-past.html. 
  22. ↑ Sidik, Saima (29 January 2024). "‘Wildly weird’ RNA bits discovered infesting the microbes in our guts". doi:10.1038/d41586-024-00266-7. https://www.nature.com/articles/d41586-024-00266-7. 
  23. ↑ Pennisi, Elizabeth (26 January 2024). "‘It’s insane’: New viruslike entities found in human gut microbes". doi:10.1126/science.znxt3dk. https://www.science.org/content/article/it-s-insane-new-viruslike-entities-found-human-gut-microbes. 
  24. ↑ 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. Bibcode: 2016BiSys.140...49W. https://philpapers.org/rec/GUECST-2. 
  25. ↑ Prosdocimi, Francisco; José, Marco V.; de Farias, Sávio Torres (2019), Pontarotti, Pierre, ed., "The First Universal Common Ancestor (FUCA) as the Earliest Ancestor of LUCA's (Last UCA) Lineage" (in en), 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, https://doi.org/10.1007/978-3-030-30363-1_3, retrieved 2023-11-02 



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