Non-cellular life
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- For syncytia organisms, see Syncytium
Non-cellular life is life that exists without a cell structure. This term presumes the phylogenetic classification of viruses as lifeforms.[1]
Hypothesized artificial life, self replicating machines, and most simple molecules capable of self-replication, such as crystals, are not considered living. (See Definition of life).
Some biologists refer to wholly syncytial organisms as 'acellular'[citation needed] because their bodies contain multiple nuclei which are not separated by cell walls, however, these cell-bound organisms are outside the scope of the present article.
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[edit] History
Virus self-assembly has implications for the study of the origin of life,[2] as it lends further credence to the hypothesis that life could have started as self-assembling organic molecules.[3] [4]
The issue of life without cellular structure came again to the fore with the 2003 discovery that the large and complex Mimivirus can make some proteins that are involved in the synthesis of proteins.[5] This discovery suggests the possibility that some viruses may have evolved from earlier forms that could produce proteins independent of a host cell.[6] If so, there may at one time have been a viral domain of life. It is not clear that all small viruses have originated from more complex viruses by means of genome size reduction. A viral domain of life may only be relevant to certain large viruses such as nucleocytoplasmic large DNA viruses like the Mimivirus.[citation needed]
[edit] Virus
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 only two subdivisions of earthly life—also known as terrestrial organisms, Biota, Naturae, or Vitae.
[edit] Viroid
Viroids are plant pathogens that consist of a short stretch of circular, single-stranded RNA without the protein coat that is typical for viruses. Viroid RNA does not code for any protein.[7]
[edit] Speculation
The notion of non-cellular life has also occurred in debates about the possible forms of hypothetical types of biochemistry, especially in the possible habitats of extraterrestrial life.[citation needed]
[edit] See also
[edit] References
- ^ "What is Non-Cellular Life?". Wise Geek. Conjecture Corporation. 2009. http://www.wisegeek.com/what-is-non-cellular-life.htm. Retrieved 2009-08-02.
- ^ Koonin EV, Senkevich TG, Dolja VV (2006). "The ancient Virus World and evolution of cells". Biol. Direct 1: 29. doi:. PMID 16984643. PMC 1594570. http://www.biology-direct.com/content/1//29. Retrieved 2009-08-01.
- ^ 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:. PMID 16044244.
- ^ Nussinov, Mark D.; Vladimir A. Otroshchenkob and 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 . http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6T2K-3W0FTTV-G&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&_docanchor=&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=2ee9045613a39f99b996e96f342023d3. Retrieved 2009-08-02.
- ^ The Mimivirus protein involved in translation
- ^ Luis P. Villarreal (2005). Viruses and the Evolution of Life. New York ASM Press.
- ^ Tsagris EM, de Alba AE, Gozmanova M, Kalantidis K (September 2008). "Viroids". Cell. Microbiol. 10: 2168. doi:. PMID 18764915.
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