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Viruses Genomes

General virus infomation

In 1898, Friedrich Loeffler and Paul Frosch found evidence that the cause of foot-and-mouth disease in livestock was an infectious particle smaller than any bacteria. This was the first clue to the nature of viruses, genetic entities that lie somewhere in the grey area between living and non-living states.

Viruses are sub-microscopic, obligate intracellular parasites. Virus particles are produced from the assembly of pre-formed components, whereas other agents 'grow' from an increase in the integrated sum of their components and reproduce by division, Virus particles (virions) do not 'grow' or undergo division. Viruses lack the genetic information which encodes apparatus necessary for the generation of metabolic energy or for protein synthesis (ribosomes).

Viruses depend on the host cells that they infect to reproduce. When found outside of host cells, viruses exist in the form of a protein coat or capsid, sometimes enclosed within a membrane. Viruses do not possess any life sustaining characteristics, and do not require any nutrients. In fact, without a proper host viruses lie dormant indefinitely. Infection takes place when a virus comes into contact with its intended host.

All groups of living organisms have viruses that parasitise them. There are plant viruses, animal viruses, fungal viruses, viruses of the Protista, and bacterial viruses (bacteriophages). Viruses (like most parasites) are very host-specific, that is, they generally can infect only one specific host or cell type. Bacteriophages are often specific to a specific strain of bacteria and may not be able to infect all members of a given species.

Viruses generally go through 5 distinct steps as they undergo replication in their host cell:

adsorption - attachment to host cell membrane or wall

penetration - viral genetic material enters the host cell. The rest of the virion may remain outside, for example in bacteriophages, or the complete virion may enter as in the case of most animal viruses.

biosynthesis - the host cell is programmed by viral nucleic acids to produce viral nucleic acids and proteins

maturation - viral proteins and nucleic acids self-assemble into new virions

release - the viruses exits the host cell, often by causing the cell to lyse (rupture)

Viruses may be lytic (they rupture and kill the host cell) or they may be lysogenic and form a stable long term relationship with the host cell without killing it. Lytic viruses are called virulent viruses, and lysogenic viruses are called temperate viruses. When a virus is involved in a lysogenic relationship with a cell its DNA "splices" into the host's DNA where it resides as a so-called provirus (prophage in bacterial viruses). The viral DNA is replicated when the host DNA is replicated, and when the lysogenic cell divides, each daughter cell gets a copy of the provirus. This may go on for many generations of cell division without causing any damage to the host cell. Proviruses may enter a lytic cycle and kill the host cell at any time. This switch from a lysogenic cycle to a lytic cycle is called induction.

Viruses cause a number of diseases in eukaryotes. In humans, smallpox, the common cold, chickenpox, influenza, shingles, herpes, polio, rabies, Ebola, hanta fever, and AIDS are examples of viral diseases. Even some types of cancer, though definitely not all, have been linked to viruses.

Because of their unique properties, specialised techniques are necessary to grow & study viruses.

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