Non-cellular life, or acellular life is

life Life is a quality that distinguishes matter that has biological processes, such as signaling and self-sustaining processes, from that which does not, and is defined by the capacity for growth, reaction to stimuli, metabolism, energy ...

that exists without a cellular structure for at least part of its life cycle. Historically, most (descriptive) definitions of life postulated that an

organism In biology, an organism () is any living system that functions as an individual entity. All organisms are composed of cells ( cell theory). Organisms are classified by taxonomy into groups such as multicellular animals, plants, and fu ...

must be composed of one or more cells, but this is no longer considered necessary, and modern criteria allow for forms of life based on other structural arrangements. The primary candidates for non-cellular life are

virus A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea. Since Dmitri Ivanovsk ...

es. Some biologists consider viruses to be organisms, but others do not. Their primary objection is that no known viruses are capable of autonomous

reproduction Reproduction (or procreation or breeding) is the biological process by which new individual organisms – "offspring" – are produced from their "parent" or parents. Reproduction is a fundamental feature of all known life; each individual or ...

: they must rely on cells to copy them. Engineers sometimes use the term " artificial life" to refer to

software Software is a set of computer programs and associated documentation and data. This is in contrast to hardware, from which the system is built and which actually performs the work. At the lowest programming level, executable code consist ...

and

robot A robot is a machine—especially one programmable by a computer—capable of carrying out a complex series of actions automatically. A robot can be guided by an external control device, or the control may be embedded within. Robots may be ...

s inspired by biological processes, but these do not satisfy any biological definition of life.

Viruses as non-cellular life

The nature of viruses was unclear for many years following their discovery as

pathogen In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a g ...

s. They were described as

poison Poison is a chemical substance that has a detrimental effect to life. The term is used in a wide range of scientific fields and industries, where it is often specifically defined. It may also be applied colloquially or figuratively, with a broa ...

s or

toxin A toxin is a naturally occurring organic poison produced by metabolic activities of living cells or organisms. Toxins occur especially as a protein or conjugated protein. The term toxin was first used by organic chemist Ludwig Brieger (1849 ...

s 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 Organic may refer to: * Organic, of or relating to an organism, a living entity * Organic, of or relating to an anatomical organ Chemistry * Organic matter, matter that has come from a once-living organism, is capable of decay or is the product ...

or inorganic — as very small biological

s or very large biochemical

molecule A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and b ...

s — 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.

Viral replication Viral replication is the formation of biological viruses during the infection process in the target host cells. Viruses must first get into the cell before viral replication can occur. Through the generation of abundant copies of its genome an ...

and self-assembly has implications for the study of the origin of life, as it lends further credence to the

hypothesis A hypothesis (plural hypotheses) is a proposed explanation for a phenomenon. For a hypothesis to be a scientific hypothesis, the scientific method requires that one can test it. Scientists generally base scientific hypotheses on previous obse ...

that life could have started as self-assembling organic molecules.

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 Nucleobases, also known as ''nitrogenous bases'' or often simply ''bases'', are nitrogen-containing biological compounds that form nucleosides, which, in turn, are components of nucleotides, with all of these monomers constituting the bas ...

. 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. Viroid RNA does not code for any protein. Its replication mechanism hijacks

RNA polymerase II RNA polymerase II (RNAP II and Pol II) is a multiprotein complex that transcribes DNA into precursors of messenger RNA (mRNA) and most small nuclear RNA (snRNA) and microRNA. It is one of the three RNAP enzymes found in the nucleus of euka ...

, 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. 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. This view was first proposed in the 1980s, and regained popularity in the 2010s to explain crucial intermediate steps in the evolution of life from inanimate matter ( Abiogenesis).

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 An empire is a "political unit" made up of several territories and peoples, "usually created by conquest, and divided between a dominant center and subordinate peripheries". The center of the empire (sometimes referred to as the metropole) ex ...

. The corresponding cellular life name would be

Cytota The cell is the basic structural and functional unit of life forms. Every cell consists of a cytoplasm enclosed within a membrane, and contains many biomolecules such as proteins, DNA and RNA, as well as many small molecules of nutrients and ...

. 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 ''Vitae'' is a Latin word, meaning or pertaining to life. * Aqua vitae, archaic name for a concentrated aqueous solution of ethanol, distilled spirits * Arbor vitae (disambiguation), "tree of life" * ''De Brevitate Vitae'', work of Roman philo ...

. The taxon Cytota would include three top-level subdivisions of its own, the domains

Bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were am ...

Archaea Archaea ( ; singular archaeon ) is a domain of single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotes. Archaea were initially classified as bacteria, receiving the name archaebacteria (in the Archaeba ...

, and Eukarya.

References

{{portal bar, Chemistry, Evolutionary biology, Science, Viruses Biological classification Life Virology Viruses

Viruses as non-cellular life

Viroids

Taxonomy

See also

References