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how many infectious diseases are caused by viruses?
more than 80%
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viruses are defined as
obligate intracellular parasites - they live outside a cellular host
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viruses have what goal?
productive infection
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viruses can infect:
- bacteria (bacteriophages)
- plant cells
- animal cells (humans included in this group
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intact viral particle is called:
virion
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capsid
protein coat surrounding viral nucleic acid
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capsid made up of:
capsomeres
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virion structure must overcome 2 basic problems:
- must be strong enough to protect viral nucleic acid
- must be able to release viral nucleic acid for infection
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nucleocapsid
nucleic acid plus capsid
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envelope
phospholipid bilayer with embedded glycoproteins surrounding capsid in enveloped virus
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virion
complete infectious viral structure: nucleic acid plus capsid for non-enveloped virus; nucleic acid plus capsid plus envelope for enveloped virus
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how do capsomeres give capsid structural symmetry?
bond together
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viruses possess what types of symmetry?
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2 shapes of helical viruses:
- rod - straight and relatively rigid
- filamentous - flexible, curved, or coiled
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Classification of viruses
- type of genetic material
- capsid shape
- number of capsomeres
- size of capsid
- presence/absence of envelope
- type of host it infects
- type of disease it produces
- target cell
- immunologic and antigenic properties
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icosahedral viruses' shape derived from?
20 triangular faces that make up capsid - capsid insert has 12 points of symmetry
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2 types icosahedral viruses
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viral envelopes
form when viral glycoproteins and oligosaccharides associate with plasma membrane of host cell
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all envelopes have:
phospholipid bilayer
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envelopes vary in:
- size
- morphology
- complexity
- composition
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envelope glycoproteins
firmly embedded in the envelope bilayer, facilitated by domains of host membrane proteins called "spanners"
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envelope glycoproteins form:
spikes or other structures on the outside of the virion that can be used to attach to host cell
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viral genomes packaged in one of three ways:
- directly in the capsid
- enclosed in special proteins
- enclosed in proteins from host cell
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infection cycle first worked out in
bacteriophages
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animal virus infections can be either:
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bacteriophages go through 5 steps in replication process, ending in lysis of the cell:
- attachment
- penetration
- uncoating
- biosynthesis
- maturation
- release
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what happens in a lytic infection?
host cells fill with virions and burst
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lysogenic infections also known as:
latent infections
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what happens in lysogenic infections?
viral genome becomes incorporated into the host cell's DNA, remaining this way for an extended period - host cell lives
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attachment and penetration of a lytic infection involve:
endocytosis
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virion attachment occurs when?
when virion binds to specific receptors on a host cell
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some viruses require what to attach?
co-receptor - without it, no infection
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viral-host cell interactions occur through:
random collisions
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lytic infections produce:
maximum number of virions
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host cell must be what for infection to succeed?
permissive - must contain all components required to make new virions
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viral infections at apical cell surface usually cause what kind of infection?
acute infection
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viral infection at basolateral cell surface becomes?
systemic
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lipid rafts
- specific areas on the host cell membrane to which many viruses attach
- rich in cholesterol, fatty acids, and other lipids
- more reliable for stable attachment
- site of release for many viruses
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virus-receptor binding is:
high affinity - conformational interactions
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receptors can determine
host range of virus
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receptors used:
- some viruses use more than one type
- some receptors shared by many viruses
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non-enveloped virus binding
takes place between viral capsid and receptor
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enveloped virus binding
takes place between viral envelope proteins and receptor
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penetration
once attached, virus must gain entry to the host cell
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uncoating occurs in 3 places
- at plasma membrane
- in cytoplasm
- at nuclear membrane
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non-enveloped virus enclosed in:
vesicle - early endosome (fuse with or become late endosomes)
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late endosomes fuse with:
lysosome where uncoating begins
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enveloped viruses - envelope fuses with
host cell membrane - fusion mediated by specialized fusion proteins of host cell, resulting in formation of fusion pore (large opening allowing viral entry)
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viral infection requires:
compartmentalization
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newly synthesized viral components moved to other locations for:
assembly of viral particles
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viral components moved in:
vesicles using host cell microtubules (specialized host cell proteins sometimes used)
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DNA viruses use what to cross nuclear membrane
host cell import pathways - pathways form pores in nuclear membrane
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RNA viruses use what to convert RNA to DNA?
reverse transcriptase
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newly converted viral DNA put into:
pre-integration complex - this moves into host cell nucleus during mitosis when nuclear membrane broken down
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biosynthesis - double-stranded DNA viruses
- one strand of viral DNA transcribed into mRNA
- uses either host cell or viral RNA polymerase
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biosynthesis - single-stranded DNA
- viral strand used as a template to make complementary strand of DNA
- uses host cell's DNA polymerase
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complementary copy (in single-stranded DNA biosynthesis)
- transcribed into mRNA
- used to make new copies of viral genome
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replication of DNA virus genomes requires:
- synthesis of at least one viral protein
- expression of several viral genes
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replication of DNA virus genomes is performed by:
host cell machinery
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latent DNA viruses require:
much less DNA replication
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virus does what to host DNA synthesis?
- inhibits
- all polymerases and proteins concentrate on viral DNA synthesis
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replication compartments
- specialized sites formed in host cell
- essentially viral factories
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replication compartments contain:
both DNA templates and host cell replication machinery
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compartmentalization allows:
exponential viral replication
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in latent DNA viruses, viral genome inserted where?
into host chromosome
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how many viral genes expressed in latent viral replication?
- small number
- limited number of viral genomes replicated
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