Non-cellular life
Encyclopedia
Non-cellular life is life
that exists without a cellular
structure. This term presumes the phylogenetic scientific classification of virus
es as lifeforms.
Hypothesized artificial life
, self-replicating machine
s, 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 walls, however, these cell-bound organisms are outside the scope of the present article.
, as it lends further credence to the hypothesis
that life could have started as self-assembling organic molecules.
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 protein
s that are involved in the synthesis of proteins. This discovery suggests the possibility that some viruses may have evolved from earlier forms that could produce proteins independent of a host cell. 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.
s 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
.
s are plant
pathogens that consist of a short stretch of circular, single-stranded RNA
without the protein coat
that is typical for virus
es. Viroid RNA does not code for protein
s.
Life
Life is a characteristic that distinguishes objects that have signaling and self-sustaining processes from those that do not, either because such functions have ceased , or else because they lack such functions and are classified as inanimate...
that exists without a cellular
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
structure. This term presumes the phylogenetic scientific classification of virus
Virus
A virus is a small infectious agent that can replicate only inside the living cells of organisms. Viruses infect all types of organisms, from animals and plants to bacteria and archaea...
es as lifeforms.
Hypothesized artificial life
Artificial life
Artificial life is a field of study and an associated art form which examine systems related to life, its processes, and its evolution through simulations using computer models, robotics, and biochemistry. The discipline was named by Christopher Langton, an American computer scientist, in 1986...
, self-replicating machine
Self-replicating machine
A self-replicating machine is an artificial construct that is theoretically capable of autonomously manufacturing a copy of itself using raw materials taken from its environment, thus exhibiting self-replication in a way analogous to that found in nature. The concept of self-replicating machines...
s, and most simple molecules capable of self-replication
Self-replication
Self-replication is any behavior of a dynamical system that yields construction of an identical copy of that dynamical system. Biological cells, given suitable environments, reproduce by cell division. During cell division, DNA is replicated and can be transmitted to offspring during reproduction...
, 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
Cell nucleus
In cell biology, the nucleus is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these...
which are not separated by cell walls, however, these cell-bound organisms are outside the scope of the present article.
History
Viral self-assembly has implications for the study of the origin of lifeAbiogenesis
Abiogenesis or biopoesis is the study of how biological life arises from inorganic matter through natural processes, and the method by which life on Earth arose...
, as it lends further credence to the hypothesis
Hypothesis
A hypothesis is a proposed explanation for a phenomenon. The term derives from the Greek, ὑποτιθέναι – hypotithenai meaning "to put under" or "to suppose". For a hypothesis to be put forward as a scientific hypothesis, the scientific method requires that one can test it...
that life could have started as self-assembling organic molecules.
The issue of life without cellular structure came again to the fore with the 2003 discovery that the large and complex Mimivirus
Mimivirus
Mimivirus is a viral genus containing a single identified species named Acanthamoeba polyphaga mimivirus , or is a group of phylogenetically related large viruses . In colloquial speech, APMV is more commonly referred to as just “mimivirus”...
can make some protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
s that are involved in the synthesis of proteins. This discovery suggests the possibility that some viruses may have evolved from earlier forms that could produce proteins independent of a host cell. If so, there may at one time have been a viral domain
Domain (biology)
In biological taxonomy, a domain is the highest taxonomic rank of organisms, higher than a kingdom. According to the three-domain system of Carl Woese, introduced in 1990, the Tree of Life consists of three domains: Archaea, Bacteria and Eukarya...
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
Nucleocytoplasmic large DNA viruses
The nucleocytoplasmic large DNA viruses or NCLDV refers to a number of families of large eukaryotic DNA viruses:# Asfarviridae# Iridoviridae# Marseilleviridae# Megaviridae# Mimiviridae# Phycodnaviridae# Poxviridae...
like the Mimivirus.
Virus
In discussing the taxonomic domainDomain (biology)
In biological taxonomy, a domain is the highest taxonomic rank of organisms, higher than a kingdom. According to the three-domain system of Carl Woese, introduced in 1990, the Tree of Life consists of three domains: Archaea, Bacteria and Eukarya...
s of life, the terms Acytota or Aphanobionta are occasionally used as the name of a viral
Virus
A virus is a small infectious agent that can replicate only inside the living cells of organisms. Viruses infect all types of organisms, from animals and plants to bacteria and archaea...
kingdom
Kingdom (biology)
In biology, kingdom is a taxonomic rank, which is either the highest rank or in the more recent three-domain system, the rank below domain. Kingdoms are divided into smaller groups called phyla or divisions in botany...
, 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
Nature
Nature, in the broadest sense, is equivalent to the natural world, physical world, or material world. "Nature" refers to the phenomena of the physical world, and also to life in general...
, or Vitae
Vitae
Vitae is a Latin word, meaning or pertaining to life.* The Academia Vitae is a liberal arts college in Deventer, The Netherlands.* Aqua vitae is an archaic name for a concentrated aqueous solution of ethanol....
.
Viroid
ViroidViroid
Viroids are plant pathogens that consist of a short stretch of highly complementary, circular, single-stranded RNA without the protein coat that is typical for viruses. The smallest discovered is a 220 nucleobase scRNA associated with the rice yellow mottle sobemovirus...
s are plant
Plant
Plants are living organisms belonging to the kingdom Plantae. Precise definitions of the kingdom vary, but as the term is used here, plants include familiar organisms such as trees, flowers, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The group is also called green plants or...
pathogens that consist of a short stretch of circular, single-stranded RNA
RNA
Ribonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....
without the protein coat
Capsid
A capsid is the protein shell of a virus. It consists of several oligomeric structural subunits made of protein called protomers. The observable 3-dimensional morphological subunits, which may or may not correspond to individual proteins, are called capsomeres. The capsid encloses the genetic...
that is typical for virus
Virus
A virus is a small infectious agent that can replicate only inside the living cells of organisms. Viruses infect all types of organisms, from animals and plants to bacteria and archaea...
es. Viroid RNA does not code for protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
s.
See also
- Hypothetical types of biochemistry
- PrionPrionA prion is an infectious agent composed of protein in a misfolded form. This is in contrast to all other known infectious agents which must contain nucleic acids . The word prion, coined in 1982 by Stanley B. Prusiner, is a portmanteau derived from the words protein and infection...
- an infectious protein