by Darian Peters

Created on: December 09, 2008   Last Updated: May 29, 2009

Viruses are a type of microorganism with interesting internal structure and replication cycle that are responsible for causing many well known diseases such as influenza and HIV, for example. It is debatable whether they count as living creatures or not but they are certainly subject to natural selection and will evolve into new forms over time in response to the selective pressures upon them, sometimes including cunning new structural changes and strategies for survival into their repertoire.

In general a virus is simply an infectious particle that contains genetic material (DNA or RNA). They are much smaller than the cells that they infect. At 10(-7)m they are a couple of orders of magnitude smaller than the average prokaryotic cell, and about three orders of magnitude smaller than the typical eukaryotic cells, like human cells.

The nucleic material at the heart of the virus is protected by a protein capsid. This capsid can be helical or maybe icosahedral (which makes the virus look almost spherical) or it may exist in a more complex form with tails or an envelope (this being constructed from lipid made from host cell membranes).

Viruses can infect animal, plant, and bacterial cells in their efforts to survive and reproduce. They are able to hijack the machinery of the cell by splicing their own genetic material into that of the cell and using the cell's own natural processes of development and reproduction to replicate themselves. Eventually they cause the host cell to burst and the new viral particles spill out before then putting the finishing touches to themselves as fully constructed viruses that can then go in search of new host cells to infect.

Helical viruses have a single strand of RNA (or sometimes DNA) with capsid subunits stacked around the central axis of the nuclei material which they are bound to. The nucleic acid is negatively charged and the capsid subunits are positive so they attract each other. Icosahedral viruses have a spherical look because the capsid protomers fit together with such perfect symmetry.

Enveloped viruses, such as influenza and HIV, use an infected host cell membrane to create a lipid bilayer called a viral envelope. They basically steal material from the host cell. This can help the virus to protect itself in the environment of the cell against enzymes and chemicals that would otherwise destroy it or impair its function. The envelope also contains natural receptor molecules, such as glycoproteins, that other host cells will recognise and bind to. This gives the virus the chance to infect them as well.

Complex viruses may have tails or outer walls rather than having just a simple structure such as a helix or an icosahedron. Poxiviruses, for example, are large complex viruses with a nucleoid, surrounded by a membrane, and two mysterious lateral bodies. But whatever their complexity, viruses are a highly effective type of entities at surviving and replicating themselves. They display adaptation and plasticity in the anatomical forms that they take and in the strategies that they use.

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