A virus is a piece of genetic instructions, usually in a protective coat. Virus particles are tiny; a cell can manufacture and contain as many as a thousand of them before breaking open. They were first discovered when biologists observed that some disease-causing agents were able to pass through a filter too fine for bacteria. They can be small because they contain almost none of the machinery of a cell, only a smallish quantity of DNA or RNA.
Viruses are not living things. When they are outside of their host cell, they are just very complex molecular particles that have no metabolism and no way to reproduce. In our computer metaphor, they're like software with no hardware, floppy disks or diskettes without a computer. Having no independent metabolism they can remain viable indefinitely, under the right circumstances. "Some of them can even be crystallized, like minerals. In this state they can survive for years unchanged — until they are wetted and placed into contact with their particular hosts"
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The viruses that infect bacteria are more specifically called bacteriophages, or simply phages. The kind and amount of genetic instructions in phages vary from 3,600 RNA nucleotides to 166,000 DNA nucleotide pairs
(9). To restate these dimensions in terms of our computer analogy, the computer viruses that infect handheld calculators range in size from 900 bytes to over 40 kilobytes. For comparison, the simplest handheld calculator (a bacterium) has about 200 kilobytes of stored programs.
Herpesvirus by Linda Stannard: All the Virology on the WWW
The viruses that infect eukaryotic cells vary in size also. The poliovirus has 7,600 RNA nucleotides; the vaccinia (cowpox) virus has 240,000 DNA nucleotide pairs
(10). To use computer terms again, the computer viruses that infect personal computers range in size from 1.9 kilobytes to 60 kilobytes. For comparison, a very simple personal computer (a yeast cell) has genetic instructions equivalent to about 8 megabytes. An advanced personal computer (a human cell) contains about 1.5 gigabytes of stored information, counting the backup copy and the "silent" DNA.
When a virus attaches to its host cell, the host may take the whole virus into its cytoplasm where the virus's protective coat is removed. However, some bacteriophages use a different invasion method. They remain outside the cell and a chemical trigger causes them to inject their genome into the host's cytoplasm. Either way, the virus's genome enters the cytoplasm of the host cell.
Once inside, the virus causes the machinery of the host cell to enter one of two cycles, the lytic cycle or the lysogenic cycle. In the lytic cycle, which leads to cell degradation, the host begins to carry out the reproductive instructions in the invading virus's genome. Those instructions are, in summary, "make more of me." The host becomes a slave to the invader; it drops everything and begins to manufacture copies of the virus. After many copies have been made, the cell breaks open and dies, and many viruses are released. This is the normal way in which a virus causes symptoms of disease in its host.
In the lysogenic cycle the host cell does not make more viruses, but simply harbors the entire viral genome in the cell, usually by incorporating it into the cell's genome. If the virus is an RNA virus, as many are, the RNA must first undergo "reverse transcription" into DNA. While harboring the viral genes, the cell may grow and multiply normally, carrying the new instructions harmlessly along with it. A virus carried in this manner is said to be latent. Recently scientists have learned that even during latency, some of the virus's genes can be expressed
