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