An Overview of Viruses for Light Microscopists
A 3D modelling article
by Mol Smith 2010
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  Scale  Herpes Virus  T4 Virus  Influenza Virus   Resources and external links
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The Herpes Virus
This is a good 3D model of the Herpes Simplex Virus. one of the herpes virus family, Herpesviridae, that infect humans. There is no known cure for HSV infection. All Animal herpes viruses share these properties: a large double-stranded, linear DNA genome, encased within an icosahedral protein cage called the capsid. The envelope encapsulating the virus particle and DNA, when bonded to specific receptors on the cell's surface, will weld with the host cell membrane and then create a tiny opening through which the virus DNA contents can be introduced (invade) the host cell.

The Herpes virus is symmetrical and looks the same globally, as you can see in the animated rotation above.

In this render of a 3D model above, I have made the external capsoid transparent, so you can clearly see the single DNA strand inside. If you wish to see a SEM image of a Herpes Virus invading a cell, take a look at Dennis Kunkel's image, in this off-site link here:

Where Do Viruses Come From?
No one knows for sure how viruses originated. Three different theories exist:

Regressive hypothesis
Viruses may have once been smaller cells that parasitised larger ones. In time, their parasitism were lost and the virus was able to 'stand-alone' but not reproduce without the appropriate cell DNA/RNA nucleus factory. The bacteria rickettsia and chlamydia cells that, like viruses, can reproduce only inside host cells. They lend support to this hypothesis. This is also called the degeneracy hypothesis.

Cellular origin hypothesis
Some viruses may have evolved from ' vagrant' bits of DNA or RNA that 'escaped' or leaked from the genes of cells or organisms. The escaped DNA could have come from plasmids (pieces of naked DNA that can move between cells) or transposons (molecules of DNA that replicate and move around to different positions within the genes of the cell).This is sometimes called the vagrancy hypothesis.

Co-evolution hypothesis
Viruses may have simply evolved from complex molecules of protein and nucleic acid in parallel and at the same time that cells first appeared on earth.

Although a very controversial theory with very little widespread support, there is also the idea that living organisms (
and therefore possibly also viruses as simpler semi-living molecular structures, {authors addition} ) were introduced to the early Earth by a process called Panspermia: in simple terms - the hypothesis that life on Earth originated from microorganisms from outer space, possibly through the ice-dust clouds of comet tails crossing the Earth's orbital path. There is also the idea that viruses could also have evolved from previous viruses, which doesn't solve their origin, but may account for their large variance and wide diversity. The whole area of Virus origin is highly speculative, not least of course because they cannot leave fossil traces to help us prove their journey down through time.

Viruses (also called Bacteriophages where their host are bacteria) can be classified through properties like host cell range and immunological relationships but the most important characteristics are phage morphology and nucleic acid properties. Genetic material may be DNA or RNA. Most viruses have DNA, usually double-stranded, like the Herpes Virus above. Also, most - but not all - can be placed in one of several morphological groups: tail-less icosahedral phages, those with contractile tails, those with non-contractile tails, and filament phages. . Viruses exist which invade animals, cells, plants, and bacteria. Viruses also exist in large numbers in marine environments. There are also some phages with envelopes, but the most complex ones are those that fall into the group having contractile tails: the T4 Bacteriophage. Let's take a look at a virus like this and take a closer look at its mechanism for releasing genetic material into a host cell.
The T4 Virus...


Comments to the author
Mol Smith are welcomed.

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Published in Mar 2010 Micscape Magazine.
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