{"id":"1746","type":{"id":"32","name":"Immunology\/Microbiology","urlId":"immunology-microbiology","subtypes":[],"schema":"http:\/\/schema.org\/Article","numTerms":0},"subtype":{"id":"109","name":"Pathogens - Viruses","urlId":"pathogens---viruses"},"urlId":"viruses-dna-rna","role":"term","course":{"id":"35","urlId":"immunology","name":"Immunology \/ Microbiology"},"children":[],"name":"Viruses - Overview","demo":0,"description":"\u003E ### **Virus Morphology \u0026 Classification**\n\n## Overview\n\nViruses are obligate intracellular parasites: they rely on host cellular energy and proteins for their own replication\n\nThe **nucleocapsid** comprises the viral genome, which consists of either **DNA or RNA,** surrounded by a **capsid.**\n\nThe capsid is a protein coat that protects the genome.\n\nSome virions are enclosed by an envelope, which is made of lipids, proteins, and glycoproteins.\n\nVirions come in a range of sizes, from 20-400 nanometers\n\t\n**Nucleic Acids**\n\nComprise the viral genome.\nCan be linear or circular.\n\nDNA\nSingle- or double-stranded DNA.\n\nRNA\nMost are single-stranded.\nClassified according to the polarity of their RN:\n*Positive-sense* = the RNA is like messenger RNA\n*Negative-sense* = they are like templates for messenger RNA that have to be transcribed into positive RNA. \n\nBe aware that there are exceptions and complications to this classification system, which we\u0027ll learn more about in our tutorial on Virus Replication.\n\t\n**Common capsid shapes**\n\nHelical:\nThe viral genome is enclosed by the helical capsid proteins\n\nIcosahedral capsid:\nComprises 20 equilateral triangles that that form a \u0022shell\u0022 around the genome.\nThe viral genome is inside the capsid.\nViral attachment proteins extend from the corners of the capsid triangles.\n\nBacteriophages:\nComplex capsids to facilitate invasion of bacterial cells. \nComprises a viral genome enclosed within an*icosahedral head, and a tail that facilitates delivery of the viral genome into the bacterium. \n\n**Viral Envelope**\n\nThe viral envelope is linked to the viral core via matrix proteins.\n\nThe envelope is acquired from the host cell during budding (a final step of [viral replication](\/term\/immunology\/viral-replication)).\n\nEnvelope glycoproteins are on the surface.\nThese projections facilitate entry into host cells and stimulate the immune response.\n\n**Naked Capsid Virions vs. Enveloped Capsid Virions**\n\nNaked, aka, non-enveloped capsids:\nRelatively stable.\nResistant to temperature changes, acids and proteases (including those found in the host\u0027s stomach), detergents, and drying.\nThus, they can survive outside the host and are spread easily, for example, via the fecal-oral route.\n\nEnveloped capsids:\nMore fragile.\nLabile to heat, acids, detergents, and drying.\nThus, transmission is often via respiratory or sexual routes. \n\n\u003E ### **KEY DNA and RNA VIRUSES**\n\n## DNA Viruses\n\nMost DNA viruses are double-stranded\n\u2013 Parvoviridae is the single-stranded exception\n\nMost DNA viruses have icosahedral symmetry\n\u2013 Poxviridae, which has a complex shape, is the exception\n\nMost DNA viruses replicate within the host nucleus\n\u2013 Poxiviridae is again the exception, as it replicates in the cytoplasm\n\n**Naked DNA viruses**\n* [Parvoviridae](\/term\/immunology\/parvovirus-b-19-erythrovirus)\n* [Polyomaviridae](\/term\/immunology\/polyomaviruses)\n* [Papillomaviridae](\/term\/immunology\/papillomaviruses-hpv)\n* [Adenoviridae](\/term\/immunology\/adenoviruses)\n\n**Enveloped DNA viruses**\n* [Poxviridae](\/term\/histology\/poxviruses-smallpox)\n* [Herpesviridae](\/term\/immunology\/herpes-simplex-viruses-1-2)\n* Hepadnaviridae\n\n## RNA Viruses\n\nRNA viruses can be positive-stranded, negative-stranded, or ambisense, which means it contains both positive-sense and negative-sense RNA.\n\nMost RNA viruses are single-stranded\n\u2013 Reoviridae is the exception.\n\nMany RNA viruses display helical symmetry\n\u2013  Several exceptions: \n    Reoviridae\n    Picornaviridae\n    Togaviridae\n    Flaviviridae\n    Caliciviridae\n    Rhabdoviridae (has a distinctive bullet-shaped capsid)\n\n* Most RNA viruses replicate in the host cytoplasm\n\u2013 [Retroviridae](\/term\/pathology\/hiv-life-cycle) and Orhtomyxoviridae are the exceptions, because they replicate in the host nucleus.\n\n***Naked positive-stranded RNA viruses***\n* [Picornaviridae](\/term\/immunology\/picornaviruses)\n* Caliciviridae\n\n***Enveloped positive-stranded RNA viruses***\n* [Togaviridae](\/term\/immunology\/togaviridae)\n* [Flaviviridae](\/term\/immunology\/flaviviridae)\n* [Coronaviridae](\/term\/immunology\/coronaviridae)\n* [Retroviridae](\/term\/general-biology\/hiv-essentials) (which are positive-stranded RNA via DNA).\n\n***Negative-stranded RNA viruses are enveloped***\n* [Rhabdoviridae](\/term\/immunology\/rabies-virus)\n* [Filoviridae](\/term\/immunology\/filoviruses-marburg-ebola-viruses)\n* [Orthomyxoviridae](\/term\/immunology\/influenza-orthomyxovirus)\n* [Paramyxoviridae](\/term\/immunology\/paramyxoviridae-measles-mumps-parainfluenza)\n* [Bunyaviridae](\/term\/immunology\/bunyaviruses)\n* [Arenaviridae](\/term\/immunology\/arenaviruses)\n\n***Double stranded, segmented RNA virus*** \n* [Reoviridae](\/term\/immunology\/reoviridae) \n\u2013  Has a double capsid\n\n\n\u003E ### **Summary of Virus Morphology**\n\n\n## Naked, aka, non-enveloped capsids:\n\nHelical arrangement: the nucleic acids are wrapped in capsid proteins to form a helical shape\n\u2013 The Tobacco mosaic virus is an example of this morphology\n\nIcosahedral: the nucleic acids in the center and viral attachment proteins projecting from the corners. \n\u2013 Several of the DNA viruses have this morphology.\n\n## Enveloped capsids:\n\nSimilar to enveloped capsids\nAdditional outer layer of lipids and glycoproteins\n\u2013 Envelope glycoproteins on their surfaces.","keywords":[],"images":[{"id":"5637","url":"term\/images\/viruses-5637.png","title":"Viruses","demo":false},{"id":"6351","url":"term\/images\/viral-envelope-6351.png","title":"viral envelope","demo":false},{"id":"6350","url":"term\/images\/viral-genome-viral-capsid-6350.png","title":"viral genome, viral capsid","demo":false},{"id":"3177","url":"term\/images\/dna-virus-rna-virus-3177.png","title":"dna virus, rna virus","demo":false}],"radiographs":[],"curricula":[{"id":"35","level":"graduate","category":"individual","urlId":"immunology","name":"Immunology \/ Microbiology","subjectGroups":[{"id":"624","name":"Viruses - Introduction","urlId":"viral-infections","subjects":[{"id":"2295","urlId":"viruses","name":"Viruses - Biology","tutorials":[{"id":"1534","urlId":"viral-replication","title":"Viral Replication","hasQuizQuestions":0,"hasNotes":0,"hasStarterImage":0,"hasSlides":0},{"id":"1533","urlId":"intro-viruses","title":"Virus 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