Virus: Encyclopedia II - Virus - Lifeform debate

Virus - Lifeform debate

A virus makes use of existing host enzymes and other molecules of a host cell to create more virus particles (virions). Some viruses encode part or all of their own genome replication machinery and are not entirely reliant on host polymerases for replication of their genetic material. Such viruses can be targeted by antiviral drugs that specifically inhibit the virally encoded replicase molecule(s). Viruses rely on host cell ribosomes for the production of viral proteins and utilize several distinct strategies to make the host cell synthesize the viral proteins. For example, at least some +RNA viruses use Internal Ribosome Entry Site (IRES) segments to drive the translation from their genomic +RNA molecule. Viruses are neither unicellular nor multicellular organisms; they are somewhere between being living and non-living. Viruses have genes and show inheritance, but are reliant on host cells to produce new generations of viruses. Many viruses have similarities to complex molecules. Because viruses are dependent on host cells for their replication they are generally not classified as "living". Whether or not they are "alive", they are obligate parasites, and have no form which can reproduce independently of their host. Like most parasites, they have a specific host range, sometimes specific to one species (or even limited cell types of one species) and sometimes more general.

Some viruses form by self-assembly of protein and nucleic acid molecules. These macromolecules are assembled within host cells from smaller organic compounds. Virus self-assembly has implications for the study of the origin of life. Some viruses also incorporate lipids from the host cell membrane when their core protein-nucleic acid complex buds from the surface of a host cell. Concerning whether viruses are alive or not, if the requirement for autonomous self-reproduction is abandoned, it can be argued strongly that viruses are indeed alive. Some small viruses are more efficient than most cellular life forms as their ratio of functions to working parts is so high. If viruses are alive then the prospect of creating artificial life is enhanced or at least the standards required to call something artificially alive are reduced.

Adapted from the Wikipedia article "Lifeform debate", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki