Acellular

This article is about non-cellular organisms. For cell-like, non-cell structures, see Syncytium.

Non-cellular life is life that exists without a cellular structure. This term presumes the phylogenetic scientific classification of viruses as lifeforms,[1] which is a controversial issue.[2][3][4]

Hypothesized artificial life, self-replicating machines, and most simple molecules capable of self-replication, such as crystals, are not usually considered living. (See definition of life.)

Some biologists refer to wholly syncytial organisms as "acellular" because their bodies contain multiple nuclei which are not separated by cell membranes, however these cell-bound organisms are outside the scope of the present article.

History

For about 100 years, the scientific community has repeatedly changed its collective mind over what viruses are. First seen as poisons, then as life-forms, then biological chemicals, and today many scientists think of viruses as existing at the border between chemistry and life: a gray area between living and nonliving.[2]

The issue of life without cellular structure came again to the fore with the 2003 discovery that the large and complex Mimivirus can synthesize 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.[7] A 2012 study suggests that the giant viruses, such as Mimivirus, are a separate domain of life, alongside the traditional three of eukarya, prokarya and archaea, by studying the protein folding structure made by the viruses.[8] The study concluded that giant viruses have evolved from more complex organisms into their highly parasitic form, and are an ancient lineage, alongside that of the other domains.[8]

Viral self-assembly has implications for the study of the origin of life,[9] as it lends further credence to the hypothesis that life could have started as self-assembling organic molecules.[10][11]

Virus

Main article: 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 two top-level subdivisions of life, whereby life as a whole would be known as organisms, Biota, Naturae, or Vitae. The Taxon Cytota would include three top-level subdivisions of its own, the Domains Bacteria, Archaea, and Eukarya.

Viroid

Main article: 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 proteins.[12]

Controversy

The initial interest in viruses stemmed from their association with diseases—the word "virus" has its roots in the Latin term for "poison."[2][3] Their demotion to inert chemicals came after 1935, when Wendell M. Stanley and his colleagues, at what is now the Rockefeller University in New York City, crystallized a virus—the tobacco mosaic virus—for the first time. They saw that it consisted of a package of complex biochemicals but it lacked essential systems necessary for metabolic functions, the biochemical activity of life. Stanley shared the 1946 Nobel Prize in chemistry—not in physiology or medicine—for this work.[2]

Further research by Stanley and others established that a virus consists of nucleic acids (DNA or RNA) enclosed in a protein coat that may also shelter viral proteins involved in infection. By that description, a virus seems more like a chemistry set than an organism.[2]

The seemingly simple question of whether or not viruses are alive, raises a fundamental issue: What exactly defines "life?" A precise scientific definition of life is an elusive thing.[3][4] (See definition of life.) Although viruses challenge our concept of what "living" means, they are vital members of the web of life. Viruses have their own, ancient evolutionary history, dating to the very origin of cellular life. It has been recognized that viruses have played (and still play) a major innovative role in the evolution of cellular organisms.[3] Most evolutionary biologists look on viruses as coming from host genes that somehow escaped the host and acquired a protein coat. In this view, viruses are fugitive host genes that have degenerated into parasites.[2] But by viewing viruses as inanimate, these investigators place them in the same category of environmental influences.[2]

See also

Viruses portal

References