Non-cellular life

Source: Wikipedia, the free encyclopedia.


This article is about life that has no cellular structure. For the
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 this is for some 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

Viruses

Further information: Virus, Viral evolution, and Virus classification

Viruses were initially described as

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 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

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-stranded

ribozymes, 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-cellular RNA world before the evolution of DNA or 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

In 2024, the possible discovery of viroid-like, but distinct, RNA-based elements called

obelisks was 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

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

References

  1. ^ "What is Non-Cellular Life?". Wise Geek. Conjecture Corporation. 2009. Archived from the original on 21 January 2021. Retrieved 2 August 2009.
  2. ^ "The 7 Characteristics of Life". infohost.nmt.edu. Archived from the original on 19 November 2016. Retrieved 26 January 2017.
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  4. S2CID 8616562. Archived from the original
    (PDF) on 27 January 2012. Retrieved 27 January 2017 – via JBSD.
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  10. ^ "ARS Research Timeline – Tracking the Elusive Viroid". 2 March 2006. Archived from the original on 6 July 2007. Retrieved 18 July 2007.
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  15. 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. PMID 38291328. Archived
    from the original on 30 January 2024. Retrieved 31 January 2024.
  18. doi:10.1126/science.znxt3dk. Archived
    from the original on 30 January 2024. Retrieved 31 January 2024.
  19. S2CID 199534387, archived
    from the original on 8 March 2024, retrieved 2 November 2023
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