An Introduction to the Viruses Microbiology: A Systems Approach An Introduction to the Viruses Chapter 6 Part 2 Copyright © The McGraw-Hill Companies, Inc) Permission required for reproduction or display. Chapter 6, pages 139 to 167

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 call- quite 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 Enveloped Nonenveloped Double-stranded genome Double-stranded genome Single-stranded genome Poxviruses Adenoviruses (linear dsDNA) Herpesviruses Papovaviruses (circular dsDNA) 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 Nonenveloped Single-stranded genome Single-stranded genome encodes reverse transcriptase Single-stranded genome Double-stranded genome Segmented genome Nonsegmented genome Retroviruses Picornaviruses Reoviruses Orthomyxoviruses Paramyxoviruses Caliciviruses Bunyaviruses Arenaviruses Rhabdoviruses Filoviruses Coronaviruses

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 Polymerases Reverse transcriptase Found in envelope Hemagglutinin Neuraminidase Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Envelope Nucleocapsid

An Introduction to the Viruses Concept Check Microbiology: A Systems Approach What type of genetic material does the influenza virus contain? A. DS DNA B. DS RNA C. SS (+) RNA D. SS (–) RNA Correct answer is D; The influenza virus contains eight pieces of ss (-) RNA KNOWLEDGE (Bloom’s taxonomy): Simple recall of material from the previous slides. Chapter 6, pages 139 to 167

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. Specific interaction between host cell and virus. Host Cell Cytoplasm Cell membrane Spikes 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 Uncoated, thereby freeing the viral RNA into the cell cytoplasm. 3 2 3 N u c l e u s R N A 4 (+) Synthesis: Replication and Protein Production. Under the control of viral genes, the cell synthesizes the basic components of new viruses: RNA molecules, capsomers, spikes. 4 (–) (+) New spikes (+) 5 (+) Assembly. Viral spike proteins are inserted into the cell membrane for the viral envelope; nucleocapsid is formed from RNA and capsomers. (+) New capsomers (+) 5 New RNA 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. 6 (a)

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 Cell membrane Spikes 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 Uncoated, thereby freeing the viral RNA into the cell cytoplasm. 2 3 3 N u c l e u s R N A 4 (+) Synthesis: Replication and Protein Production. Under the control of viral genes, the cell synthesizes the basic components of new viruses: RNA molecules, capsomers, spikes. 4 (–) (+) New spikes (+) (+) 5 Assembly. Viral spike proteins are inserted into the cell membrane for the viral envelope; nucleocapsid is formed from RNA and capsomers. (+) New capsomers 5 New RNA (+) 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. 6 (a)

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