Virus classification converted
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This document discusses different ways that viruses can be classified, including by their genome, capsid structure, and method of mRNA production. Viruses are classified based on whether their genetic material is DNA or RNA, single or double stranded, segmented or not, and other properties. They are also classified by capsid structure as naked icosahedral, enveloped icosahedral, enveloped helical, naked helical or complex. The most commonly used system is the Baltimore classification which groups viruses into 7 categories based on how they produce mRNA during replication - whether from DNA, conversion of RNA to DNA, direct transcription of RNA or use of reverse transcriptase.
Virus classification converted
- 1. VIRUS CLASSIFICATION Viruses contain only a few elements by which they can be classified: the viral genome, the type of capsid, and the envelope structure for the enveloped viruses. All of these elements have been used in the past for viral classification. Viral genomes may vary in the type of genetic material (DNA or RNA) and its organization (single- or double-stranded, linear or circular, and segmented or non-segmented). In some viruses, additional proteins needed for replication are associated directly with the genome or contained within the viral capsid. Table 1. Virus Classification by Genome Structure and Core Core Classifications Examples RNA Rabies virus, retroviruses DNA Herpes viruses, smallpox virus Single-stranded Rabies virus, retroviruses Double-stranded Herpes viruses, smallpox virus Linear Rabies virus, retroviruses, herpes viruses, smallpox virus Circular Papilloma viruses, many bacteriophages Non-segmented: genome consists of a single segment of genetic material Para influenza viruses Segmented: genome is divided into multiple segments Influenza viruses Viruses can also be classified by the design of their capsids. Capsids are classified as naked icosahedral, enveloped icosahedral, enveloped helical, naked helical, and complex. Table 2. 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
- 2. 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 Table 3 with examples of each group.
- 3. Table 3. Baltimore Classification Group Characteristics Mode of mRNA Production Example I Double-stranded DNA mRNA is transcribed directly from the DNA template Herpes simplex (herpesvirus) II Single-stranded DNA DNA is converted to double-stranded form before RNA is transcribed Canine parvovirus (parvovirus) III Double-stranded RNA mRNA is transcribed from the RNA genome Childhood gastroenteritis (rotavirus) IV Single stranded RNA (+) Genome functions as mRNA Common cold (pircornavirus) V Single stranded RNA (−) mRNA is transcribed from the RNA genome Rabies (rhabdovirus) VI Single stranded RNA viruses with reverse transcriptase Reverse transcriptase makes DNA from the RNA genome; DNA is then incorporated in the host genome; mRNA is transcribed from the incorporated DNA Human immunodeficiency virus (HIV) VII Double stranded DNA viruses with reverse transcriptase The 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 mRNA Hepatitis B virus (hepadnavirus) References: https://courses.lumenlearning.com/wm-biology2/chapter/virus- classification/