Introduction to Viruses
How Are Viruses Classified?
Viruses are extremely diverse. The International Committee on Taxonomy of Viruses (ICTV) has organized them into over 70 different families. Criteria used in classifying viruses include the type of nucleic acid that serves as the genetic material, the shape of the capsid, and whether or not the virus has a membranous envelope.
The genetic material of viruses can be made up of either DNA or RNA. Almost all DNA viruses contain double-stranded DNA as their genome. The DNA can be arranged as linear or circular molecules. Most RNA viruses have single-stranded genomes, which can be found on one or more segments, depending on the virus. When the single RNA strand contains information that is immediately translatable (analogous to mRNA), the virus is categorized as a positive (+) strand RNA virus. RNA viruses that contain the complement of a (+) strand are referred to as negative (-) strand RNA viruses.
Capsids are made up of many copies of protein subunits, often consisting of only a single type of protein. The arrangements of subunits determines the symmetry and shape of the capsid. There are three main types of capsid structure: helical, polyhedral, and complex. For helical viruses, the subunits are assembled in a helix, and the viruses are rod-shaped. The subunits of polyhedral viruses are arranged into an icosahedron (a structure with 20 equilateral triangular faces). These viruses generally have a spherical structure. Complex viruses have capsid structures that do not fit into either of the other categories and are not well understood. Bacteriophages are an example of such a virus.
Some viruses contain an envelope consisting of a lipid bilayer that surrounds the capsid. Viruses acquire an envelope when they bud out through the membrane of a host cell. Virus proteins become inserted into the lipid bilayer and play an important role when viruses infect another host cell. Viruses that lack an envelope are known as naked viruses.
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Polyhedral-shaped SV40 virus. Courtesy: CDC. Retrieved 08-15-2006, from http://phil.cdc.gov/phil/ Image #5629.
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Video and transcript courtesy of Wah Chiu, PhD, National Center for Macromolecular Imaging at Baylor College of Medicine. Funding for the video provided by NCMI, NIH.