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An Introduction to the Viruses Chapter 6 Part 2 Copyright © The McGraw-Hill Companies, Inc) Permission required for reproduction or display. Nucleic Acids: At the Core of a Virus • Genome- the sum total of the genetic information carried by an organism • Contain DNA or RNA, not both • Number of viral genes compared with a callquite small • They only have the genes necessary to invade host cells and redirect their activity • Some viruses are exceptions to typical DNA and RNA formations • Parvoviruses contain single-stranded DNA • Reoviruses contain double-stranded RNA DNA Viruses of Eukaryotes Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Table 6.2 Medically Relevant DNA Virus Groups DNA Viruses Nonenveloped Enveloped Double-stranded genome Double-stranded genome Poxviruses Adenoviruses (linear dsDNA) Herpesviruses Papovaviruses (circular dsDNA) Single-stranded genome Parvoviruses Source: Adapted from: Poxviridae from Buller et al., National Institute of Allergy & Infectious Disease, Department of Health & Human Services. RNA Viruses • Mostly single-stranded • Positive-sense RNA: genomes that are ready for immediate translation into proteins • Negative-sense RNA: genomes have to be converted into the proper form to be made into proteins • Segmented- individual genes exist on separate pieces of RNA RNA Viruses of Eukaryotes Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Table 6.3 Medically Relevant RNA Viruses RNA Viruses Enveloped Single-stranded genome Segmented genome Orthomyxoviruses Single-stranded genome encodes reverse transcriptase Nonsegmented genome Retroviruses Paramyxoviruses Bunyaviruses Arenaviruses Rhabdoviruses Filoviruses Coronaviruses Nonenveloped Single-stranded genome Picornaviruses Caliciviruses Double-stranded genome Reoviruses Viral Genomes • One kind of nucleic acid (either RNA or DNA) • DS DNA Adenovirus, Herpes virus, Poxvirus • SS DNA Parvovirus • DS RNA Reovirus • SS RNA (+) Poliovirus, Rhinovirus • SS RNA (-) Influenza, Rabies virus • SS RNA (+) HIV (becomes DNA in cell) • SS RNA (+) Hepatitis B (becomes DNA in virus) Viral Enzymes • Found in nucleocapsid Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. • Polymerases • Reverse transcriptase • Found in envelope • Hemagglutinin Envelope • Neuraminidase Nucleocapsid Concept Check What type of genetic material does the influenza virus contain? A. DS DNA B. DS RNA C. SS (+) RNA D. SS (–) RNA 6.4 How Viruses are Classified and Named • Main criteria • Structure • Chemical composition • Similarities in genetic makeup • International Committee on the Taxonomy of Viruses, 2005 • 3 orders • 73 families “-viridae” • 287 genera “-virus” • Some virologists use a species naming system, but it is not an official designation Viral Classification 6.5 Modes of Viral Multiplication Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. • Host cell is required for viral multiplication. Host Cell Cytoplasm Spikes Cell membrane 1 1 Adsorption. The virus attaches to its host cell by specific binding of its spikes to cell receptors. Receptors 2 Penetration. The virus is engulfed into a vesicle and its envelope is 3 Uncoated, thereby freeing the viral RNA into the cell cytoplasm. 2 3 • Specific interaction between host cell and virus. (+) 4 RNA 4 (–) (+) New spikes (+) (+) (+) New RNA 5 Assembly. Viral spike proteins are inserted into the cell membrane for the viral envelope; nucleocapsid is formed from RNA and capsomers. New capsomers (+) 6 Release. Enveloped viruses bud off of the membrane, carrying away an envelope with the spikes. This complete virus or virion is ready to infect another cell. (a) Synthesis: Replication and Protein Production. Under the control of viral genes, the cell synthesizes the basic components of new viruses: RNA molecules, capsomers, spikes. 5 6 Multiplication Cycles in Animal Viruses • Adsorption – specific attachment • Penetration – entry of viral genome • Uncoating – release of viral genome • Synthesis – new viral products made • Assembly – new viruses are made in the cell • Release – often causes the host cell to lyse Adsorption • Virus encounters susceptible host cells • Adsorbs specifically to receptor sites on the cell membrane • Because of the exact fit required, viruses have a limited host range Penetration • Flexible cell membrane of the host is penetrated by the whole virus or its nucleic acid • Endocytosis: entire virus engulfed by the cell and enclosed in a vacuole or vesicle • The viral envelope can also directly fuse with the host cell membrane Uncoating • Enzymes in the vacuole dissolve the envelope and capsid • The virus is now uncoated Synthesis • Free viral nucleic acid exerts control over the host’s synthetic and metabolic machinery • DNA viruses- enter host cell’s nucleus where they are replicated and assembled • RNA viruses- replicated and assembled in the cytoplasm Synthesis RNA virus DNA virus Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Host Cell Cytoplasm Spikes Cell membrane 1 1 Adsorption. The virus attaches to its host cell by specific binding of its spikes to cell receptors. Receptors 2 Penetration. The virus is engulfed into a vesicle and its envelope is 3 Uncoated, thereby freeing the viral RNA into the cell cytoplasm. 2 3 (+) 4 RNA 4 (–) (+) New spikes (+) (+) (+) New RNA 5 Assembly. Viral spike proteins are inserted into the cell membrane for the viral envelope; nucleocapsid is formed from RNA and capsomers. New capsomers (+) 6 Release. Enveloped viruses bud off of the membrane, carrying away an envelope with the spikes. This complete virus or virion is ready to infect another cell. (a) Synthesis: Replication and Protein Production. Under the control of viral genes, the cell synthesizes the basic components of new viruses: RNA molecules, capsomers, spikes. 5 6 Assembly and Release • Mature virus particles are constructed from the growing pool of parts • Nonenveloped and complex viruses are released when the cell lyses or ruptures • Enveloped viruses are liberated by budding or exocytosis • Anywhere from 3,000 to 100,000 virions may be released, depending on the virus • Entire length of cycle- anywhere from 8 to 36 hours Release of Enveloped Virus