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Past systems of classification

Viruses are classified in several ways: by factors such as their core content ( [link] and [link] ), the structure of their capsids, and whether they have an outer envelope. The type of genetic material (DNA or RNA) and its structure (single- or double-stranded, linear or circular, and segmented or non-segmented) are used to classify the virus core structures.

Virus Classification by Genome Structure and Core
Core Classifications Examples
  • RNA
  • DNA
  • Rabies virus, retroviruses
  • Herpesviruses, smallpox virus
  • Single-stranded
  • Double-stranded
  • Rabies virus, retroviruses
  • Herpesviruses, smallpox virus
  • Linear
  • Circular
  • Rabies virus, retroviruses, herpesviruses, smallpox virus
  • Papillomaviruses, many bacteriophages
  • Non-segmented: genome consists of a single segment of genetic material
  • Segmented: genome is divided into multiple segments
  • Parainfluenza viruses
  • Influenza viruses
Part a (top) is an illustration of the rabies virus, which is bullet-shaped. RNA is coiled inside a capsid, which is encased in a matrix protein-lined viral envelope studded with glycoproteins. Part a (bottom) is a micrograph of a cluster of bullet-shaped rabies viruses. Part b (top) is a micrograph of variola virus, which has DNA encased in a bow-shaped capsid. An oval matrix protein-lined envelope surrounds the capsid. Part b (bottom) shows irregular, bumpy lesions on the arms and legs of a person with smallpox.
Viruses are classified based on their core genetic material and capsid design. (a) Rabies virus has a single-stranded RNA (ssRNA) core and an enveloped helical capsid, whereas (b) variola virus, the causative agent of smallpox, has a double-stranded DNA (dsDNA) core and a complex capsid. Rabies transmission occurs when saliva from an infected mammal enters a wound. The virus travels through neurons in the peripheral nervous system to the central nervous system where it impairs brain function, and then travels to other tissues. The virus can infect any mammal, and most die within weeks of infection. Smallpox is a human virus transmitted by inhalation of the variola virus, localized in the skin, mouth, and throat, which causes a characteristic rash. Before its eradication in 1979, infection resulted in a 30–35 percent mortality rate. (credit “rabies diagram”: modification of work by CDC; “rabies micrograph”: modification of work by Dr. Fred Murphy, CDC; credit “small pox micrograph”: modification of work by Dr. Fred Murphy, Sylvia Whitfield, CDC; credit “smallpox photo”: modification of work by CDC; scale-bar data from Matt Russell)

Viruses can also be classified by the design of their capsids ( [link] and [link] ). Capsids are classified as naked icosahedral, enveloped icosahedral, enveloped helical, naked helical, and complex ( [link] and [link] ). The type of genetic material (DNA or RNA) and its structure (single- or double-stranded, linear or circular, and segmented or non-segmented) are used to classify the virus core structures ( [link] ).

The left illustration shows a 20-sided structure with rods jutting from each apex. The right micrograph shows a cluster of adenoviruses, each about 100 nanometers across.
Adenovirus (left) is depicted with a double-stranded DNA genome enclosed in an icosahedral capsid that is 90–100 nm across. The virus, shown clustered in the micrograph (right), is transmitted orally and causes a variety of illnesses in vertebrates, including human eye and respiratory infections. (credit “adenovirus”: modification of work by Dr. Richard Feldmann, National Cancer Institute; credit “micrograph”: modification of work by Dr. G. William Gary, Jr., CDC; scale-bar data from Matt Russell)
Virus Classification by Capsid Structure
Capsid Classification Examples
Naked icosahedral Hepatitis A virus, polioviruses
Enveloped icosahedral Epstein-Barr virus, herpes simplex virus, rubella virus, yellow fever virus, HIV-1
Enveloped helical Influenza viruses, mumps virus, measles virus, rabies virus
Naked helical Tobacco mosaic virus
Complex with many proteins; some have combinations of icosahedral and helical capsid structures Herpesviruses, smallpox virus, hepatitis B virus, T4 bacteriophage

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Source:  OpenStax, Biology 9-11 grades. OpenStax CNX. Mar 10, 2015 Download for free at https://legacy.cnx.org/content/col11766/1.2
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