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Virus Classification

Virus Classification

To understand the features shared among different groups of viruses, a classification scheme is necessary. As most viruses are not thought to have evolved from a common ancestor, however, the methods that scientists use to classify living things are not very useful. Biologists have used several classification systems in the past, based on the morphology and genetics of the different viruses.

However, these earlier classification methods grouped viruses differently, based on which features of the virus they were using to classify them. The most commonly used classification method today is called the Baltimore classification scheme and is based on how messenger RNA (mRNA) is generated in each particular type of virus.

Past Systems of Classification

 In the illustration a viral receptor on the surface of a KSHV virus is attached to an xCT receptor embedded in the plasma membrane.

The KSHV virus binds the xCT receptor on the surface of human cells. xCT receptors protect cells against stress. Stressed cells express more xCT receptors than non-stressed cells. The KSHV virion causes cells to become stressed, thereby increasing expression of the receptor to which it binds. (credit: modification of work by NIAID, NIH)

Viruses are classified in several ways: by factors such as their core content (see the table below and the figure above from Viral Morphology), 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 ClassificationsExamples
  • 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 (see this figure from Viral Morphology and the figure above). Capsids are classified as naked icosahedral, enveloped icosahedral, enveloped helical, naked helical, and complex (see this figure below and this figure below). 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 (see the table below).

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 ClassificationExamples
Naked icosahedralHepatitis A virus, polioviruses
Enveloped icosahedralEpstein-Barr virus, herpes simplex virus, rubella virus, yellow fever virus, HIV-1
Enveloped helicalInfluenza viruses, mumps virus, measles virus, rabies virus
Naked helicalTobacco mosaic virus
Complex with many proteins; some have combinations of icosahedral and helical capsid structuresHerpesviruses, smallpox virus, hepatitis B virus, T4 bacteriophage
Micrograph a shows icosahedral polioviruses arranged in a grid; micrograph b shows two Epstein-Barr viruses with icosahedral capsids encased in an oval membrane; micrograph c shows a mumps virus capsid encased in an irregular membrane; micrograph d shows rectangular tobacco mosaic virus capsids; and micrograph e shows a spherical herpesvirus envelope studded with glycoproteins.

Transmission electron micrographs of various viruses show their structures. The capsid of the (a) polio virus is naked icosahedral; (b) the Epstein-Barr virus capsid is enveloped icosahedral; (c) the mumps virus capsid is an enveloped helix; (d) the tobacco mosaic virus capsid is naked helical; and (e) the herpesvirus capsid is complex. (credit a: modification of work by Dr. Fred Murphy, Sylvia Whitfield; credit b: modification of work by Liza Gross; credit c: modification of work by Dr. F. A. Murphy, CDC; credit d: modification of work by USDA ARS; credit e: modification of work by Linda Stannard, Department of Medical Microbiology, University of Cape Town, South Africa, NASA; scale-bar data from Matt Russell)

Baltimore Classification

The most commonly used system of virus classification was developed by Nobel Prize-winning biologist David Baltimore in the early 1970s. In addition to the differences in morphology and genetics mentioned above, the Baltimore classification scheme groups viruses according to how the mRNA is produced during the replicative cycle of the virus.

Group I viruses contain double-stranded DNA (dsDNA) as their genome. Their mRNA is produced by transcription in much the same way as with cellular DNA. Group II viruses have single-stranded DNA (ssDNA) as their genome. They convert their single-stranded genomes into a dsDNA intermediate before transcription to mRNA can occur. Group III viruses use dsRNA as their genome. The strands separate, and one of them is used as a template for the generation of mRNA using the RNA-dependent RNA polymerase encoded by the virus. Group IV viruses have ssRNA as their genome with a positive polarity.

Positive polarity means that the genomic RNA can serve directly as mRNA. Intermediates of dsRNA, called replicative intermediates, are made in the process of copying the genomic RNA. Multiple, full-length RNA strands of negative polarity (complimentary to the positive-stranded genomic RNA) are formed from these intermediates, which may then serve as templates for the production of RNA with positive polarity, including both full-length genomic RNA and shorter viral mRNAs.

Group V viruses contain ssRNA genomes with a negative polarity, meaning that their sequence is complementary to the mRNA. As with Group IV viruses, dsRNA intermediates are used to make copies of the genome and produce mRNA. In this case, the negative-stranded genome can be converted directly to mRNA. Additionally, full-length positive RNA strands are made to serve as templates for the production of the negative-stranded genome.

Group VI viruses have diploid (two copies) ssRNA genomes that must be converted, using the enzyme reverse transcriptase, to dsDNA; the dsDNA is then transported to the nucleus of the host cell and inserted into the host genome. Then, mRNA can be produced by transcription of the viral DNA that was integrated into the host genome. Group VII viruses have partial dsDNA genomes and make ssRNA intermediates that act as mRNA, but are also converted back into dsDNA genomes by reverse transcriptase, necessary for genome replication. The characteristics of each group in the Baltimore classification are summarized in the table below with examples of each group.

Baltimore Classification
GroupCharacteristicsMode of mRNA ProductionExample
IDouble-stranded DNAmRNA is transcribed directly from the DNA templateHerpes simplex (herpesvirus)
IISingle-stranded DNADNA is converted to double-stranded form before RNA is transcribedCanine parvovirus (parvovirus)
IIIDouble-stranded RNAmRNA is transcribed from the RNA genomeChildhood gastroenteritis (rotavirus)
IVSingle stranded RNA (+)Genome functions as mRNACommon cold (pircornavirus)
VSingle stranded RNA (-)mRNA is transcribed from the RNA genomeRabies (rhabdovirus)
VISingle stranded RNA viruses with reverse transcriptaseReverse transcriptase makes DNA from the RNA genome; DNA is then incorporated in the host genome; mRNA is transcribed from the incorporated DNAHuman immunodeficiency virus (HIV)
VIIDouble stranded DNA viruses with reverse transcriptaseThe viral genome is double-stranded DNA, but viral DNA is replicated through an RNA intermediate; the RNA may serve directly as mRNA or as a template to make mRNAHepatitis B virus (hepadnavirus)

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