Viruses  (Page 2/5)

Virions, single virus particles, are very small, about 20–250 nanometers (1 nanometer = 1/1,000,000 mm); although the recent discovery of entities called Pandoraviruses (approx 1 micrometer, or 1/1,000,000 mm in diameter) has shaken that paradigm somewhat. Individual virus particles are the infectious form of a virus outside the host cell. Unlike bacteria (which are about 100 times larger), we cannot see most viruses with a light microscope, with the exception of the Pandoraviruses and some large virions of the poxvirus family ( [link] ).

It was not until the development of the electron microscope in the 1940s that scientists got their first good view of the structure of the tobacco mosaic virus ( [link] ) and others. The surface structure of virions can be observed by both scanning and transmission electron microscopy, whereas the internal structures of the virus can only be observed in images from a transmission electron microscope ( [link] ).

The use of this technology has allowed for the discovery of many viruses of all types of living organisms. They were initially grouped by shared morphology, meaning their size, shape, and distinguishing structures. Later, groups of viruses were classified by the type of nucleic acid they contained, DNA or RNA, and whether their nucleic acid was single- or double-stranded. More recently, molecular analysis of viral replication cycles has further refined their classification. Currently virus classification begins at the level of Order, and proceeds to species level taxonomy using this scheme. The terms in parentheses are the taxon suffixes for that taxonomic level.

Virus classification

• Order (-virales)
• Family (-viridae)
• Subfamily (-virinae)
• Genus (-virus)
• Species (usually XXXX (disease) virus, e.g., Tobacco Mosaic Virus)

A virion consists of a nucleic-acid core, an outer protein coating, and sometimes an outer envelope made of protein and phospholipids derived from the host cell. The most visible difference between members of viral families is their morphology, which is quite diverse. An interesting feature of viral complexity is that the complexity of the host does not correlate to the complexity of the virion. Some of the most complex virion structures are observed in bacteriophages, viruses that infect the simplest living organisms, bacteria.

Viruses come in many shapes and sizes, but these are consistent and distinct for each viral family ( [link] ). All virions have a nucleic-acid genome covered by a protective layer of protein, called a capsid. The capsid is made of protein subunits called capsomeres. Some viral capsids are simple polyhedral “spheres,” whereas others are quite complex in structure. The outer structure surrounding the capsid of some viruses is called the viral envelope. All viruses use some sort of glycoprotein to attach to their host cells at molecules on the cell called viral receptors. The virus exploits these cell-surface molecules, which the cell uses for some other purpose, as a way to recognize and infect specific cell types.