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[[Image:Virus.jpg|right|350px|thumb|Microscopic view of a virus.]]

[[Image:Virus.jpg|right|350px|thumb|Microscopic view of a virus.]]

A '''virus''' is a microscopic, non-[[cell]]ular, infectious agent that has two characteristics: It has genetic material (either [[DNA]] or [[RNA]]) inside a protective [[protein]] coat.<ref>Wile, Dr. Jay L. ''Exploring Creation With Biology''. Apologia Educational Ministries, Inc. 1998</ref>

A '''virus''' is a microscopic, non-[[cell]]ular, infectious agent that has two characteristics: It has genetic material (either [[DNA]] or [[RNA]]) inside a protective [[protein]] coat (called a "capsid").<ref>Wile, Dr. Jay L. ''Exploring Creation With Biology''. Apologia Educational Ministries, Inc. 1998</ref> Uniquely, viruses may contain single or double stranded nucleic acid. All other organisms are based on double stranded DNA.

To reproduce itself, a virus needs to infect a living cell. The genetic material is inserted into the cell, and the reproductive machinery of that cell is then effectively "hijacked": the cell starts to produce components of new virus particles. These components spontaneously self-assemble and the new virus particles can then infect other cells, where the same process happens.

To reproduce itself, a virus needs to infect a living cell. The genetic material is inserted into the cell, and the reproductive machinery of that cell is then effectively "hijacked": the cell starts to produce components of new virus particles. These components spontaneously self-assemble and the new virus particles can then infect other cells, where the same process happens.

==Forms of replication==

==Forms of replication==

This infection cycle of replication has two forms, lysogenic and lytic. In the lysogenic a virus will implant its DNA in the host cell's and wait until the cell is under stress and then begin to copy more of itself. This method allows for the virus gene (called the pro-phage) to continue through multiple generations of cells until the correct replicating time has arrived. Viruses that use this cycle are said to be 'latent' and are often retroviruses (their genetic material is RNA, as opposed to DNA). An example of this type of virus would be the HIV virus. The other method (the lytic cycle) involves a virus directly injecting its genetic material into the cell and immediately forcing it to make viral copies. After about 200 viruses have been produced the cell lyses (explodes) and the copies are now free to infect more cells. Many of these viruses are bacteriophages.  This is because bacterium have a cell wall that does not allow the absorption of the virus like the [[plasma membrane]] of animal cells do, forcing the virus to inject only its genetic material.

Broadly speaking, there are two types of replicative cycles viruses undertake; lytic and lysogenic. The cycles are described below.<br />

 

'''Lytic'''<br />

The first step to viral replication is entry into the cell. The virus uses proteins in either its capsid or envelope to bind to a protein on the surface of the cell. Each virus can only use certain proteins which influences which cells they are able to infect (a property called 'tropism'). One inside, the virus must shed its protein coat and prepare to be replicated. The process of replication will differ based upon whether or not the virus is DNA based or RNA based. Following replication virus particles self assemble and leave the cell, either by bursting through the membrane or budding off leaving the membrane intact.

 

Lysogenic replication, on the other hand, involves the virus integrating into the host genome. The virus is then replicated every time the genome replicates. In times of stress the virus genome can excise itself from the host genome and enter the lytic cycle.

==Are viruses alive?==

==Are viruses alive?==