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Viral structures

In general, virions (viral particles) are small and cannot be observed using a regular light microscope. They are much smaller than prokaryotic and eukaryotic cells; this is an adaptation allowing viruses to infect these larger cells (see [link] ). The size of a virion can range from 20 nm for small viruses up to 900 nm for typical, large viruses (see [link] ). Recent discoveries, however, have identified new giant viral species , such as Pandoravirus salinus and Pithovirus sibericum , with sizes approaching that of a bacterial cell. N. Philippe et al. “Pandoraviruses: Amoeba Viruses with Genomes up to 2.5 Mb Reaching that of Parasitic Eukaryotes.” Science 341, no. 6143 (2013): 281–286.

Figure a is an electron micrograph showing a virus on the surface of a bacterial cell. The virus has a large head region, a thick neck and thin spider-like legs attached to the bacterium. Figure b is a drawing that labels the outside of the head as the capsid with the viral genome inside. The neck as the sheath and the legs as tail fibers.
(a) In this transmission electron micrograph, a bacteriophage (a virus that infects bacteria) is dwarfed by the bacterial cell it infects. (b) An illustration of the bacteriophage in the micrograph. (credit a: modification of work by U.S. Department of Energy, Office of Science, LBL, PBD)
A scale showing sizes of various sentities. The largest is a frog egg ad 1 mm. Human egg cells and pllen are approximately 400 µm. Typical plant ant animal cells reange from 10 to 100 µm. Red blood cells are uner 10 µm. Mitochondria and bacteria are approximately 1 µm. Smallpox is approximately 500 nm. Flu virus is approximately 100 nm. Polio virus is approximately 50 nm. Proteins range from 5 – 10 nm. Lipids range from 1 – 5 nm. Atoms are approximately 0.1 nm.
The size of a virus is small relative to the size of most bacterial and eukaryotic cells and their organelles.

In 1935, after the development of the electron microscope, Wendell Stanley was the first scientist to crystallize the structure of the tobacco mosaic virus and discovered that it is composed of RNA and protein. In 1943, he isolated Influenza B virus , which contributed to the development of an influenza (flu) vaccine. Stanley’s discoveries unlocked the mystery of the nature of viruses that had been puzzling scientists for over 40 years and his contributions to the field of virology led to him being awarded the Nobel Prize in 1946.

As a result of continuing research into the nature of viruses, we now know they consist of a nucleic acid (either RNA or DNA, but never both) surrounded by a protein coat called a capsid (see [link] ). The interior of the capsid is not filled with cytosol, as in a cell, but instead it contains the bare necessities in terms of genome and enzymes needed to direct the synthesis of new virions. Each capsid is composed of protein subunits called capsomere s made of one or more different types of capsomere proteins that interlock to form the closely packed capsid.

There are two categories of viruses based on general composition. Viruses formed from only a nucleic acid and capsid are called naked virus es or nonenveloped virus es . Viruses formed with a nucleic-acid packed capsid surrounded by a lipid layer are called enveloped virus es (see [link] ). The viral envelope is a small portion of phospholipid membrane obtained as the virion buds from a host cell. The viral envelope may either be intracellular or cytoplasmic in origin.

Extending outward and away from the capsid on some naked viruses and enveloped viruses are protein structures called spike s . At the tips of these spikes are structures that allow the virus to attach and enter a cell, like the influenza virus hemagglutinin spikes (H) or enzymes like the neuraminidase (N) influenza virus spikes that allow the virus to detach from the cell surface during release of new virions. Influenza viruses are often identified by their H and N spikes. For example, H1N1 influenza viruses were responsible for the pandemics in 1918 and 2009, J. Cohen. “What’s Old Is New: 1918 Virus Matches 2009 H1N1 Strain. Science 327, no. 5973 (2010): 1563–1564. H2N2 for the pandemic in 1957, and H3N2 for the pandemic in 1968.

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Source:  OpenStax, Microbiology. OpenStax CNX. Nov 01, 2016 Download for free at http://cnx.org/content/col12087/1.4
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