{"id":1117,"date":"2016-12-07T23:01:20","date_gmt":"2016-12-07T23:01:20","guid":{"rendered":"https:\/\/courses.lumenlearning.com\/wm-biology2\/?post_type=chapter&#038;p=1117"},"modified":"2024-04-25T17:55:20","modified_gmt":"2024-04-25T17:55:20","slug":"steps-of-virus-infections","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/wm-biology2\/chapter\/steps-of-virus-infections\/","title":{"raw":"Steps of Virus Infections","rendered":"Steps of Virus Infections"},"content":{"raw":"<div class=\"textbox learning-objectives\">\r\n<h3>Learning Outcomes<\/h3>\r\n<ul>\r\n \t<li>List the steps of replication and explain what occurs at each step<\/li>\r\n<\/ul>\r\n<\/div>\r\n<p id=\"fs-idm96953392\">A virus must use its host-cell processes to replicate. The viral replication cycle can produce dramatic biochemical and structural changes in the host cell, which may cause cell damage. These changes, called\u00a0<span id=\"term804\" data-type=\"term\">cytopathic effects<\/span>, can change cell functions or even destroy the cell. Some infected cells, such as those infected by the common cold virus known as rhinovirus, die through\u00a0<span id=\"term805\" data-type=\"term\">lysis<\/span>\u00a0(bursting) or\u00a0<span id=\"term806\" data-type=\"term\">apoptosis<\/span>\u00a0(programmed cell death or \u201ccell suicide\u201d), releasing all progeny virions at once. The symptoms of viral diseases result both from such cell damage caused by the virus and from the immune response to the virus, which attempts to control and eliminate the virus from the body.<\/p>\r\n<p id=\"fs-idm96283392\">Many animal viruses, such as\u00a0<span id=\"term807\" data-type=\"term\">HIV (human immunodeficiency virus)<\/span>, leave the infected cells of the immune system by a process known as\u00a0<span id=\"term808\" data-type=\"term\"><em data-effect=\"italics\">budding<\/em><\/span>, where virions leave the cell individually. During the budding process, the cell does not undergo lysis and is not immediately killed. However, the damage to the cells that the virus infects may make it impossible for the cells to function normally, even though the cells remain alive for a period of time. Most productive viral infections follow similar steps in the virus replication cycle:\u00a0<em data-effect=\"italics\">attachment, penetration, uncoating, replication, assembly, and release <\/em>(Figure 1).<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"602\"]<img id=\"5\" src=\"https:\/\/openstax.org\/resources\/52f664da282c9abb5213fe97562376a471330d62\" alt=\"The illustration shows the steps of an influenza virus infection. In step 1, influenza virus becomes attached to a target epithelial cell. In step 2, the cell engulfs the virus by endocytosis, and the virus becomes encased in the cell's plasma membrane. In step 3, the membrane dissolves, and the viral contents are released into the cytoplasm. Viral m R N A enters the nucleus, where it is replicated by viral R N A polymerase. In step 4, viral m R N A exits to the cytoplasm, where it is used to make viral proteins. In step 5, new viral particles are released into the extracellular fluid. The cell, which is not killed in the process, continues to make new virus.\" width=\"602\" height=\"425\" data-media-type=\"image\/png\" \/> Figure 1. The influenza reproductive cycle. In influenza virus infection, glycoproteins on the capsid attach to a host epithelial cell. Following this, the virus is engulfed. RNA and proteins are then made and assembled into new virions.[\/caption]\r\n<h2>Attachment<\/h2>\r\n<p id=\"fs-idm143481568\">A virus attaches to a specific receptor site on the host cell membrane through attachment proteins in the capsid or via glycoproteins embedded in the viral envelope. The specificity of this interaction determines the host\u2014and the cells within the host\u2014that can be infected by a particular virus. This can be illustrated by thinking of several keys and several locks, where each key will fit only one specific lock.<\/p>\r\n\r\n<h2>Entry<\/h2>\r\n<section id=\"fs-idm43800048\" data-depth=\"2\">\r\n<p id=\"fs-idp37712480\">Viruses may enter a host cell either with or without the viral capsid. The nucleic acid of bacteriophages enters the host cell \u201cnaked,\u201d leaving the capsid outside the cell. Plant and animal viruses can enter through\u00a0<em data-effect=\"italics\">endocytosis<\/em>\u00a0(as you may recall, the cell membrane surrounds and engulfs the entire virus). Some enveloped viruses enter the cell when the viral envelope fuses directly with the cell membrane. Once inside the cell, the viral capsid degrades, and then the viral nucleic acid is released and becomes available for replication and transcription.<\/p>\r\n\r\n<\/section><section id=\"fs-idm200870256\" data-depth=\"2\"><\/section>\r\n<h2>Replication and Assembly<\/h2>\r\n<p id=\"fs-idm16331920\">The replication mechanism depends on the viral genome. DNA viruses usually use host-cell proteins and enzymes to replicate the viral DNA and to transcribe viral mRNA, which is then used to direct viral protein synthesis. RNA viruses usually use the RNA core as a template for synthesis of viral genomic RNA and mRNA. The viral mRNA directs the host cell to synthesize viral enzymes and capsid proteins, and assemble new virions.<\/p>\r\n<p id=\"fs-idm16378990\">Of course, there are exceptions to this pattern. If a host cell does not provide the enzymes necessary for viral replication, viral genes supply the information to direct synthesis of the missing proteins. Retroviruses, such as HIV (group VI of the Baltimore classification scheme), have an RNA genome that must be reverse transcribed into DNA, which then is incorporated into the host cell genome. To convert RNA into DNA, retroviruses must contain genes that encode the virus-specific enzyme reverse transcriptase that transcribes an RNA template to DNA. Reverse transcription never occurs in uninfected host cells\u2014the enzyme reverse transcriptase is only derived from the expression of viral genes within the infected host cells. The fact that HIV produces some of its own enzymes not found in the host has allowed researchers to develop drugs that inhibit these enzymes without affecting the host\u2019s metabolism.<\/p>\r\n<p id=\"fs-idm16331780\">This approach has led to the development of a variety of drugs used to treat HIV and has been effective at reducing the number of infectious virions (copies of viral RNA) in the blood to non-detectable levels in many HIV-infected individuals.<\/p>\r\n\r\n<h2>Egress<\/h2>\r\n<p id=\"fs-idm132883760\">The last stage of viral replication is the release of the new virions produced in the host organism, where they are able to infect adjacent cells and repeat the replication cycle. As you\u2019ve learned, some viruses are released when the host cell dies, and other viruses can leave infected cells by budding through the membrane without directly killing the cell.<\/p>\r\n\r\n<div class=\"textbox\"><section id=\"fs-idm73024256\" data-depth=\"1\"><section id=\"fs-idp131787088\" data-depth=\"2\">\r\n<div id=\"fs-idp1850384\" class=\"interactive ui-has-child-title\" data-type=\"note\" data-has-label=\"true\" data-label=\"\"><section>\r\n<div class=\"os-note-body\">\r\n<p id=\"fs-idp75993600\">Watch this video\u00a0on viruses, identifying structures, modes of transmission, replication, and more:<\/p>\r\nhttps:\/\/youtu.be\/0h5Jd7sgQWY\r\n\r\n<\/div>\r\n<\/section><\/div>\r\n<\/section><\/section><\/div>\r\n<h2>Steps of Virus Infections<\/h2>\r\nThe following activity will show you just how the flu virus can infect your body.\r\n\r\n<iframe src=\"https:\/\/www.oppia.org\/embed\/exploration\/m9eQnqMsifaO\" width=\"700\" height=\"900\"><\/iframe>\r\n\r\n<a href=\".\/steps-of-virus-infections-text-version\/\" target=\"_blank\" rel=\"noopener\">Click here for a text-only version of the activity.<\/a>\r\n<div class=\"textbox tryit\">\r\n<h3>Try It<\/h3>\r\nhttps:\/\/assess.lumenlearning.com\/practice\/30ef639e-b155-4a08-8ef1-bb4d8b49edac\r\n<\/div>","rendered":"<div class=\"textbox learning-objectives\">\n<h3>Learning Outcomes<\/h3>\n<ul>\n<li>List the steps of replication and explain what occurs at each step<\/li>\n<\/ul>\n<\/div>\n<p id=\"fs-idm96953392\">A virus must use its host-cell processes to replicate. The viral replication cycle can produce dramatic biochemical and structural changes in the host cell, which may cause cell damage. These changes, called\u00a0<span id=\"term804\" data-type=\"term\">cytopathic effects<\/span>, can change cell functions or even destroy the cell. Some infected cells, such as those infected by the common cold virus known as rhinovirus, die through\u00a0<span id=\"term805\" data-type=\"term\">lysis<\/span>\u00a0(bursting) or\u00a0<span id=\"term806\" data-type=\"term\">apoptosis<\/span>\u00a0(programmed cell death or \u201ccell suicide\u201d), releasing all progeny virions at once. The symptoms of viral diseases result both from such cell damage caused by the virus and from the immune response to the virus, which attempts to control and eliminate the virus from the body.<\/p>\n<p id=\"fs-idm96283392\">Many animal viruses, such as\u00a0<span id=\"term807\" data-type=\"term\">HIV (human immunodeficiency virus)<\/span>, leave the infected cells of the immune system by a process known as\u00a0<span id=\"term808\" data-type=\"term\"><em data-effect=\"italics\">budding<\/em><\/span>, where virions leave the cell individually. During the budding process, the cell does not undergo lysis and is not immediately killed. However, the damage to the cells that the virus infects may make it impossible for the cells to function normally, even though the cells remain alive for a period of time. Most productive viral infections follow similar steps in the virus replication cycle:\u00a0<em data-effect=\"italics\">attachment, penetration, uncoating, replication, assembly, and release <\/em>(Figure 1).<\/p>\n<div style=\"width: 612px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" id=\"5\" src=\"https:\/\/openstax.org\/resources\/52f664da282c9abb5213fe97562376a471330d62\" alt=\"The illustration shows the steps of an influenza virus infection. In step 1, influenza virus becomes attached to a target epithelial cell. In step 2, the cell engulfs the virus by endocytosis, and the virus becomes encased in the cell's plasma membrane. In step 3, the membrane dissolves, and the viral contents are released into the cytoplasm. Viral m R N A enters the nucleus, where it is replicated by viral R N A polymerase. In step 4, viral m R N A exits to the cytoplasm, where it is used to make viral proteins. In step 5, new viral particles are released into the extracellular fluid. The cell, which is not killed in the process, continues to make new virus.\" width=\"602\" height=\"425\" data-media-type=\"image\/png\" \/><\/p>\n<p class=\"wp-caption-text\">Figure 1. The influenza reproductive cycle. In influenza virus infection, glycoproteins on the capsid attach to a host epithelial cell. Following this, the virus is engulfed. RNA and proteins are then made and assembled into new virions.<\/p>\n<\/div>\n<h2>Attachment<\/h2>\n<p id=\"fs-idm143481568\">A virus attaches to a specific receptor site on the host cell membrane through attachment proteins in the capsid or via glycoproteins embedded in the viral envelope. The specificity of this interaction determines the host\u2014and the cells within the host\u2014that can be infected by a particular virus. This can be illustrated by thinking of several keys and several locks, where each key will fit only one specific lock.<\/p>\n<h2>Entry<\/h2>\n<section id=\"fs-idm43800048\" data-depth=\"2\">\n<p id=\"fs-idp37712480\">Viruses may enter a host cell either with or without the viral capsid. The nucleic acid of bacteriophages enters the host cell \u201cnaked,\u201d leaving the capsid outside the cell. Plant and animal viruses can enter through\u00a0<em data-effect=\"italics\">endocytosis<\/em>\u00a0(as you may recall, the cell membrane surrounds and engulfs the entire virus). Some enveloped viruses enter the cell when the viral envelope fuses directly with the cell membrane. Once inside the cell, the viral capsid degrades, and then the viral nucleic acid is released and becomes available for replication and transcription.<\/p>\n<\/section>\n<section id=\"fs-idm200870256\" data-depth=\"2\"><\/section>\n<h2>Replication and Assembly<\/h2>\n<p id=\"fs-idm16331920\">The replication mechanism depends on the viral genome. DNA viruses usually use host-cell proteins and enzymes to replicate the viral DNA and to transcribe viral mRNA, which is then used to direct viral protein synthesis. RNA viruses usually use the RNA core as a template for synthesis of viral genomic RNA and mRNA. The viral mRNA directs the host cell to synthesize viral enzymes and capsid proteins, and assemble new virions.<\/p>\n<p id=\"fs-idm16378990\">Of course, there are exceptions to this pattern. If a host cell does not provide the enzymes necessary for viral replication, viral genes supply the information to direct synthesis of the missing proteins. Retroviruses, such as HIV (group VI of the Baltimore classification scheme), have an RNA genome that must be reverse transcribed into DNA, which then is incorporated into the host cell genome. To convert RNA into DNA, retroviruses must contain genes that encode the virus-specific enzyme reverse transcriptase that transcribes an RNA template to DNA. Reverse transcription never occurs in uninfected host cells\u2014the enzyme reverse transcriptase is only derived from the expression of viral genes within the infected host cells. The fact that HIV produces some of its own enzymes not found in the host has allowed researchers to develop drugs that inhibit these enzymes without affecting the host\u2019s metabolism.<\/p>\n<p id=\"fs-idm16331780\">This approach has led to the development of a variety of drugs used to treat HIV and has been effective at reducing the number of infectious virions (copies of viral RNA) in the blood to non-detectable levels in many HIV-infected individuals.<\/p>\n<h2>Egress<\/h2>\n<p id=\"fs-idm132883760\">The last stage of viral replication is the release of the new virions produced in the host organism, where they are able to infect adjacent cells and repeat the replication cycle. As you\u2019ve learned, some viruses are released when the host cell dies, and other viruses can leave infected cells by budding through the membrane without directly killing the cell.<\/p>\n<div class=\"textbox\">\n<section id=\"fs-idm73024256\" data-depth=\"1\">\n<section id=\"fs-idp131787088\" data-depth=\"2\">\n<div id=\"fs-idp1850384\" class=\"interactive ui-has-child-title\" data-type=\"note\" data-has-label=\"true\" data-label=\"\">\n<section>\n<div class=\"os-note-body\">\n<p id=\"fs-idp75993600\">Watch this video\u00a0on viruses, identifying structures, modes of transmission, replication, and more:<\/p>\n<p><iframe loading=\"lazy\" id=\"oembed-1\" title=\"Viruses\" width=\"500\" height=\"281\" src=\"https:\/\/www.youtube.com\/embed\/0h5Jd7sgQWY?feature=oembed&#38;rel=0\" frameborder=\"0\" allowfullscreen=\"allowfullscreen\"><\/iframe><\/p>\n<\/div>\n<\/section>\n<\/div>\n<\/section>\n<\/section>\n<\/div>\n<h2>Steps of Virus Infections<\/h2>\n<p>The following activity will show you just how the flu virus can infect your body.<\/p>\n<p><iframe loading=\"lazy\" src=\"https:\/\/www.oppia.org\/embed\/exploration\/m9eQnqMsifaO\" width=\"700\" height=\"900\"><\/iframe><\/p>\n<p><a href=\".\/steps-of-virus-infections-text-version\/\" target=\"_blank\" rel=\"noopener\">Click here for a text-only version of the activity.<\/a><\/p>\n<div class=\"textbox tryit\">\n<h3>Try It<\/h3>\n<p>\t<iframe id=\"assessment_practice_30ef639e-b155-4a08-8ef1-bb4d8b49edac\" class=\"resizable\" src=\"https:\/\/assess.lumenlearning.com\/practice\/30ef639e-b155-4a08-8ef1-bb4d8b49edac?iframe_resize_id=assessment_practice_id_30ef639e-b155-4a08-8ef1-bb4d8b49edac\" frameborder=\"0\" style=\"border:none;width:100%;height:100%;min-height:300px;\"><br \/>\n\t<\/iframe>\n<\/div>\n\n\t\t\t <section class=\"citations-section\" role=\"contentinfo\">\n\t\t\t <h3>Candela Citations<\/h3>\n\t\t\t\t\t <div>\n\t\t\t\t\t\t <div id=\"citation-list-1117\">\n\t\t\t\t\t\t\t <div class=\"licensing\"><div class=\"license-attribution-dropdown-subheading\">CC licensed content, Shared previously<\/div><ul class=\"citation-list\"><li>Biology 2e. <strong>Provided by<\/strong>: OpenStax. <strong>Located at<\/strong>: <a target=\"_blank\" href=\"http:\/\/cnx.org\/contents\/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8\">http:\/\/cnx.org\/contents\/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8<\/a>. <strong>License<\/strong>: <em><a target=\"_blank\" rel=\"license\" href=\"https:\/\/creativecommons.org\/licenses\/by\/4.0\/\">CC BY: Attribution<\/a><\/em>. <strong>License Terms<\/strong>: Access for free at https:\/\/openstax.org\/books\/biology-2e\/pages\/1-introduction<\/li><li>Modification of the virus replication cycle. <strong>Provided by<\/strong>: OpenStax College. <strong>Located at<\/strong>: <a target=\"_blank\" href=\"http:\/\/cnx.org\/contents\/e42bd376-624b-4c0f-972f-e0c57998e765@4.4\">http:\/\/cnx.org\/contents\/e42bd376-624b-4c0f-972f-e0c57998e765@4.4<\/a>. <strong>Project<\/strong>: Microbiology. <strong>License<\/strong>: <em><a target=\"_blank\" rel=\"license\" href=\"https:\/\/creativecommons.org\/licenses\/by\/4.0\/\">CC BY: Attribution<\/a><\/em>. <strong>License Terms<\/strong>: Download for free at http:\/\/cnx.org\/contents\/e42bd376-624b-4c0f-972f-e0c57998e765@4.4<\/li><\/ul><div class=\"license-attribution-dropdown-subheading\">All rights reserved content<\/div><ul class=\"citation-list\"><li>Flu Attack! How A Virus Invades Your Body. <strong>Authored by<\/strong>: NPR. <strong>Located at<\/strong>: <a target=\"_blank\" href=\"https:\/\/youtu.be\/Rpj0emEGShQ\">https:\/\/youtu.be\/Rpj0emEGShQ<\/a>. <strong>License<\/strong>: <em>All Rights Reserved<\/em>. <strong>License Terms<\/strong>: Standard YouTube License<\/li><\/ul><\/div>\n\t\t\t\t\t\t <\/div>\n\t\t\t\t\t <\/div>\n\t\t\t <\/section>","protected":false},"author":17,"menu_order":7,"template":"","meta":{"_candela_citation":"[{\"type\":\"cc\",\"description\":\"Biology 2e\",\"author\":\"\",\"organization\":\"OpenStax\",\"url\":\"http:\/\/cnx.org\/contents\/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8\",\"project\":\"\",\"license\":\"cc-by\",\"license_terms\":\"Access for free at https:\/\/openstax.org\/books\/biology-2e\/pages\/1-introduction\"},{\"type\":\"copyrighted_video\",\"description\":\"Flu Attack! How A Virus Invades Your Body\",\"author\":\"NPR\",\"organization\":\"\",\"url\":\"https:\/\/youtu.be\/Rpj0emEGShQ\",\"project\":\"\",\"license\":\"arr\",\"license_terms\":\"Standard YouTube License\"},{\"type\":\"cc\",\"description\":\"Modification of the virus replication cycle\",\"author\":\"\",\"organization\":\"OpenStax College\",\"url\":\"http:\/\/cnx.org\/contents\/e42bd376-624b-4c0f-972f-e0c57998e765@4.4\",\"project\":\"Microbiology\",\"license\":\"cc-by\",\"license_terms\":\"Download for free at http:\/\/cnx.org\/contents\/e42bd376-624b-4c0f-972f-e0c57998e765@4.4\"}]","CANDELA_OUTCOMES_GUID":"ace477de-e75a-46ee-865a-8022f344ceee, 014a9b13-cf74-4e58-9602-b5feec13c8b1","pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":""},"chapter-type":[],"contributor":[],"license":[],"class_list":["post-1117","chapter","type-chapter","status-publish","hentry"],"part":1098,"_links":{"self":[{"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/pressbooks\/v2\/chapters\/1117","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/wp\/v2\/users\/17"}],"version-history":[{"count":19,"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/pressbooks\/v2\/chapters\/1117\/revisions"}],"predecessor-version":[{"id":8259,"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/pressbooks\/v2\/chapters\/1117\/revisions\/8259"}],"part":[{"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/pressbooks\/v2\/parts\/1098"}],"metadata":[{"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/pressbooks\/v2\/chapters\/1117\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/wp\/v2\/media?parent=1117"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/pressbooks\/v2\/chapter-type?post=1117"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/wp\/v2\/contributor?post=1117"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/wp\/v2\/license?post=1117"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}