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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, for some, this is no longer considered necessary, and modern criteria allow for forms of life based on other structural arrangements.
Life-cycle of a typical virus (left to right); following infection of a cell by a single virus, hundreds of offspring are released. When a virus infects a cell, the virus forces it to make thousands more viruses. It does this by making the cell copy the virus's DNA or RNA, making viral proteins, which all assemble to form new virus particles. [37]
This mechanism has evolved to favour those viruses that infect only cells in which they are capable of replication. Attachment to the receptor can induce the viral envelope protein to undergo changes that result in the fusion of viral and cellular membranes, or changes of non-enveloped virus surface proteins that allow the virus to enter. [76]
The first universal common ancestor (FUCA) is a hypothetical non-cellular ancestor to LUCA and other now-extinct sister lineages. Whether the genesis of viruses falls before or after the LUCA–as well as the diversity of extant viruses and their hosts–remains a subject of investigation.
Non-cellular life, most notably the viruses, is not included in this system. Alternatives to the three-domain system include the earlier two-empire system (with the empires Prokaryota and Eukaryota), and the eocyte hypothesis (with two domains of Bacteria and Archaea, with Eukarya included as a branch of Archaea).
Viruses are non-living and do not have the means to reproduce on their own, instead depending on their host cell's machinery to do this. Thus, viruses do not code for most of the proteins required for their replication and the translation of their mRNA into viral proteins, but use proteins encoded by the host cell for this purpose. [2]
Cell theory has traditionally been accepted as the governing theory of all life, [1] but some biologists consider non-cellular entities such as viruses living organisms [2] and thus disagree with the universal application of cell theory to all forms of life.
Virosphere (virus diversity, virus world, global virosphere) was coined to refer to all those places in which viruses are found or which are affected by viruses. [1] [2] However, more recently virosphere has also been used to refer to the pool of viruses that occurs in all hosts and all environments, [3] as well as viruses associated with specific types of hosts (prokaryotic virosphere, [4 ...