Infectious Agents and the Immune System Flashcards
definition of a virus
packaged set of genes that is inert outside a living cell
capable of invading cells
replicates in cells by disassembly, synthesis of component parts, and reassembly
needs cell for protein and nucleic acid synthesis and energy
virion
the extracellular form of a virus
is disassembled to release the viral genome
properties of virion
size
shape and symmetry
numbers and sizes of capsomers
enveloped or not
properties of the viral genome
DNA or RNA
ds or ss (+ or - sense)
linear or circular
segmented or not
size
icosahedral symmetry
roughly spherical with the genome tightly packed within
subunits of the capsid are located around the vertices or face of an icosahedron
20 triangular faces, 30 edges, and 12 verticies
2-, 3-, and 5-fold axes of symmetry
helical symmetry
genome is coated with proteins that assume a helical lattice
the sizes of the proteins and subunit interactions will determine the pitch and diameter of the helix as well as the rigidity of the helix
the size of the RNA will in part determine the length of the helix
What do virus-cell interactions determine?
pathology
host response
viral evasion
targets for therapy
resistant vs. susceptible
resistant cells lack the receptors for viral entry
susceptible cells allow virus to enter and either express genes or establish their genomes inside the cell
productive infection
a full viral replicative cycle occurs and viral progeny are produced
necessary to generate enough virus to cause disease
usually associated with cell death- important aspect of pathogenesis
abortive infection
viral genes are expressed but infectious progeny do not result, the cell probably dies
latent infection
the viral genome is established inside the cell, some genes may be expressed but viral replication does not occur and the cell remains viable despite altered function
stages in productive infection
attachment to cell
penetration of cell
disassembly, release of genome
expression of viral genes
replication of genome
assembly of progeny virions
release from cell
attachment (adsorption)
birus binds to a cell surgace through interaction of virion proteins with specific cell surface “receptors”
some viruses recognize more than one receptor, permitting use of different receptors on different cell types or alternate routes of entry into a single cell type
penetration
energy dependent and requires:
translocation of virus or viral genome across the plasma membrane
fusion of the virion envelope with the plasma membrane
endocytosis of the virus particle followed by translocation across the endosome membrane or fusion with the endosome membrane
disassembly, release of genome
the virus particle is further disassembled so that the geonome becomes accessible for translation or transcription and later for genome replication
the viral genome will be delivered to the cell cytoplasm for most RNA viruses and to the cell nucleus for most DNA viruses
eclipse or replication
the periods during which viral nucleic acids and proteins are being syntehsized but infectious virus cannot be detected in the infected cell
transcription of viral mRNA
syntehsis of proteins
replication of viral genome
assembly
when progeny viral genomes and viral structural proteins have accumulated to sufficient levels, assembly of progeny virions can take place
this occurs in the cell compartment where genome replication occurred
release from cell
it is thought that most non-enveloped viruses are released from infected cells only when the cell lyses though the use of cell excretory pathways for release has not been ruled out
release of many enveloped viruses occurs coincident with acquiring the envelope by budding through the plasma membrane
other enveloped viruses acquire an envelope by budding through an internal cell membrane into a secretory compartment and are then secreted from the cell
eclipse period
the period between active viral gene expression and genome replication
infectious units do not reappear until the end of the eclipse period when there is sufficient viral genomes and structural proteins for assembly of new virions
types of nucleic acids and replication strategies
double stranded DNA
single stranded DNA
double stranded RNA
single stranded (+) RNA
single stranded (-) RNA
single stranded RNA + DNA
double stranded DNA + RNA
double stranded DNA virus replication
ex. adenoviruses and herpesviruses
same sense mRNA replication
ex. polioviruses
single strand negative sense RNA virus replication
need to make + strand template to replicate and make mRNA
ex. orthomyoxoviruses (influenza)
RNA retrovirus replication
RNA genome in virons but goes through a double strand DNA intermediate in cells
ex. retroviruses, HIV
consequences of productive infection to the cell
production is usually incompatible with cell survival because of cytotoxic effects of viral effects including the ones that give the virus a competitive advantage for cellular biosynthetic machinery
induction of innate responses can result in death of cells
in some inefections such as hepatitis B, the cell doesn’t die but continually releases virus
poliovirus protease
cleaves cell cap-binding protein and inhibiting translation of capped mRNAs (poliovirus enters through internal ribosome entry sites)
herpesvirus protein
shuts down splicing of mRNAs (most herpesvirus late mRNAs are unspliced)
latent infections
comm for many DNA viruses
no viral genome replication except in concert with cell division
few if any viral proteins expressed, those expressed can be non-immunogenic
latent state is compatible with cell survival and normal cell functions though some functions may be altered
latent virus is a reservoir for reactivation of infectious virus
