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any member of a unique class of infectious agents, which were originally distinguished by their smallness (hence, they were described as “filtrable” because of their ability to pass through fine ceramic filters that blocked all cells, including bacteria) and their inability to replicate outside of and without assistance of a living host cell.
Unlike cellular organisms, viruses do not contain all the biochemical mechanisms for their own replication; they replicate by using the biochemical mechanisms of a host cell to synthesize and assemble their separate components.(Some do contain or produce essential enzymes when there is no cellular enzyme that will serve.) When a complete virus particle () comes in contact with a host cell, only the viral nucleic acid and, in some viruses, a few enzymes are injected into the host cell.Within the host cell the genetic material of a DNA virus is replicated and transcribed into messenger RNA by host cell enzymes, and proteins coded for by viral genes are synthesized by host cell ribosomes.These are the proteins that form the (protein coat); there may also be a few enzymes or regulatory proteins involved in assembling the capsid around newly synthesized viral nucleic acid, in controlling the biochemical mechanisms of the host cell, and in lysing the host cell when new virions have been assembled.Some of these may already have been present within the initial virus, and others may be coded for by the viral genome for production within the host cell.Because host cells do not have the ability to replicate “viral RNA” but are able to transcribe messenger RNA, RNA viruses must contain enzymes to produce genetic material for new virions.
For certain viruses the RNA is replicated by a viral enzyme ( that degrades the source RNA.In viruses that have membranes, membrane-bound viral proteins are synthesized by the host cell and move, like host cell membrane proteins, to the cell surface.When these proteins assemble to form the capsid, part of the host cell membrane is pinched off to form the envelope of the virion.Some viruses have only a few genes coding for capsid proteins.Other more complex ones may have a few hundred genes.But no virus has the thousands of genes required by even the simplest cells.