Viruses are classified in three different ways. One example of virus classification is shape.
There are three main shapes of viruses: rod-like, spherical, and icosahedral. Spherical viruses are spheres, like tennis balls, but with 'arms' of chemical proteins protruding from the protein-covered capsule. An examples of this structure is the human Influenza virus. Rod-like viruses are shaped like paper towel rolls, cylindrical. The Tobacco Mosaic virus is a good example. Icosahedral viruses have twenty sides; Herpes Simplex is an example of icosahedral structure. Rod-like viruses and spherical viruses both have RNA while icosahedral viruses have DNA.
Viruses can also be classified by which nucleic acid they contain. Spherical and rod-like viruses contain RNA, while icosahedral viruses contain DNA. No virus can have both; it is not possible. They use this material to reproduce. DNA is injected into host cells, then the host cell replicates it. In viruses that have RNA, they must first enter the cell and disassemble themselves to release their RNA for replication.
The third way viruses can be classified is by size. Classifying by size also determines how many protein subunits are inside of the virus and how they are arranged. Viruses vary from 20 nanometers up to 1,000 nm. Parvoviridae and Picornaviridae and measure about 20 nm and about 30 nm, classifying them as some of the smallest viruses in the animal world. Some viruses have more than one protein covering, making them larger and more sturdy.
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