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Viruses are smaller than bacteria and slightly bigger than many known large proteins. Although there are a great variety of viruses and virus structures, they all tend to have a basic genetic component (either DNA or RNA) usually encapsulated inside a protein cage.
In addition, complex viruses have some mechanisms for mobility similar to the ones seen in cells. A single virus, also called a “virion," does not have the enzymes and proteins needed for reproduction, so it needs to “take over" the reproductive mechanism of another living cell to replicate itself.
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Viruses are “intracellular obligate parasites," think of them as tiny cellular parasites. The first step in virus replication is for the virus to “infect" a host cell. This process varies according to the type of virus but essentially the virus binds to the host cell membrane surface and “injects" its genetic material (DNA or RNA) into the cell.
Once the genetic material of the virus has entered the cell, it will "take over" the cellular machinery (ribosomes and enzymes) to replicate itself. Unlike bacteria and other cells which replicate by processes which yield two cells in each replicating cycle, the replication of viral DNA or RNA is explosive. It yields many hundreds of copies of the virus genetic material and other virus components such as capsule proteins and the proteins needed for mobility structures.
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Once all the virus components (DNA, proteins) have been created, the assembly of the viral genome takes place and it's a spontaneous non-enzymatic process. In this method of “self-assembly" many fully functional viruses are built and ready to infect other cells, and so continuing the replication process. Usually the host cell splits and dies as the viruses are released, although not always. Some viruses bud from the cell without killing it.
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Very successful at what they do viruses can infect any kind of cell, and their ability to change their protein coats makes them a hard target for vaccines trying to stop them in their tracks.