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Type of Viruses
- - Naked nucleocapsid virus
- - nucleicacid
- - capsid
- - Envelope Virus
- - nucleic acid
- -capsid
- -envelope
- -spikes
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What are Viruses?
- unique group of biological entities known to infect every type of cell
- - unusual structure and behavior
- - all possess a protein capsid surrounding some type of nucleic acid
- infectious particles
- obligate intracelluar parasites
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What is a capsid?
- - protective outer shell that surrounds viral nucleic acid
- - capsid spikes are used for attachement to host cells
- - composed of capsomer subunits
- - two types of capsids
- - helical
- - icosahedral
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Helical Capsid
- - rod-shaped capsomers bond forming a hollow disc similar to a bracelet
- - form a continuous helix
- - nucleic acid coils inside
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Types of Helical Viruses
- - naked helical virus
- - tobacco virus
- - nucleocapsid is rigid and tightly wound into a cylinder-shaped package
- - Enveloped helical virus
- - influenza, measles, rabies
- - nucleocapsid is more flexible
- - envelope comes from host cell membrane
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Icosahedron capsid
- three dimensional, 20 sided with 12 evenly spaced corners
- - variation in capsomer numbers
- - polio virus 32 capsomers
- - adenovirus 240 capsomers
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Types of Icosahedral Viruses
- - naked icosahedral viruses
- - papillomavirus
- - Enveloped icosahedral viruses
- - herpesvirus
- - envelope comes from host cell membrane
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Envelope
- - most common among animal viruses
- - lipid and proteins
- - during release of animal viruses, a part of the host membrane is taken
- - has envelope spikes for attachment of viruses to the next host cell
- - enable pleomorphic shape of the viruses
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Function of Capsid/Envelope
- protect nucleic acid from the host's acid and protein digesting enzymes
- assist in binding and penetrating host cell
- stimulate the host's immune system
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Complex Viruses
- - intricate structure
- - pox virus
- - several layers of lipoproteins
- - course surface fibrils
- - bacteriophage
- - polydedral head
- - helical tail
- - fibers for attachment
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Nucleic Acid
- viruses contain either DNA or RNA but not both
- possess only the genes to invade and regulate the metabolic activity of host cells
- no viral metabolic genes; the virus uses the host's metabolic resources
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Clinically important DNA viruses
- DNA viruses:
- - Enveloped
- - double stranded genome
- - Non-enveloped
- - single stranded genome
- - double stranded genome
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Clinically Important RNA Viruses
- RNA Viruses:
- - enveloped - non-enveloped
- - single stranded genome - single stranded genome
- - segmented genome - double stranded genome
- - non segmented genome
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Multiplication Cycle
- - adsorption (attachment)
- - penetration/uncoating
- - synthesis
- - assembly
- - release
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Adsorption
- - viral attachment to host cell is mediated by spike proteins
- - spikes recognize only certain host receptors
- - basis for specificity
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Viral Penetration/ Uncoating
- - naked viruses are endocytosed
- - requires degradation of cell membrane and capsid prior to uncoating
- -enveloped viruses
- - viral envelope fuses to host cell membrane
- - allows only capsid to directly enter cytosol
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Release of Enveloped Viruses
- - liberated by budding or exocytosis
- - allows them to retain a portion of the host cell membrane
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Cytopathic Effects
- - Damage to the host cell due to a viral infection
- - inclusion bodies: compact masses of viruses of damaged cellular material
- - syncytia: fusion of multiple host cells into a single, large, multinucleated cell
- - chronic latent state: viruses remain dormant in a certain cell type periodically becoming reactivated
- - transformation: viral geneic material permanently alter host genome
- - oncogenes
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Bacteriophage
- - virus that specifically infects bacteria
- - multiplication is similar to animal viruses except:
- - pentration occurs via injecting DNA
- - can form a dormant prophage (lysongeny)
- - release (lyses) destroys the cell
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Viral Release
- viral enzymes weaken host cell membrane
- leads to rupture the cell (lyses)
- release numerous virions
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Lysogeny
- insertion of viral DNA into the bacterial host genome
- allows viral DNA to be copied with every cell division of bacteria
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Cultivating Animal Viruses
- eggs provide an intact, self-supporting unit for viral maintenance
- viruses can be injected through shell into various areas within embryonic egg
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Normal Versus Infected Cells
- - tissues are grown on culture media
- - normal cells form an even monolayer of undisturbed cells
- viruses cause an aggregation of cells forming visible plaques
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Non cellular Infectious Agents
- prions
- satellite viruses
- viroids
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What are Prions?
- protein particle with no nucleic acid, no envelope, no capsid
- - Diseases
- - creutzfeldt- Jakob
- - "mad cow disease"
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Satellite Viruses
- - dependent on other viruses for replication
- - delta agent: naked RNA only expressed in the presence of hepatitis B virus potentially worsening the damage to liver cells
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Virods
- - plant pathogens
- - tomatoes, potatoes, cucumbers
- 1/10th the size of normal viruses
- naked strands of RNA, no capsid
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