virus structure and life cycle Flashcards
by what characteristics are viruses classified?`
- type/structure of nucleic acid
- type/symmetry of virus capsid
- envelope (+/-)
virion
complete viral particle = nucleic acid + capsid
capsid
- protective protein coat formed by self-assembly of identical protein subunits (capsomeres) encoded by viral genome
- only a few proteins
- shape is helical or icosahedral
capsomeres
protein subunits of viral capsid
envelope
- outer covering of virus made of lipid bilayer and glycoproteins derived from the host cell membrane (outer membrane or inner membranes such as nuclear or ER)
- may help viruses avoid immune response
- help viruses bind and enter host cell
compare the characteristics of enveloped vs non-enveloped viruses
enveloped:
- sensitive to dessication in the environment
- transmitted via respiratory, parenteral, and sexual routes
non-enveloped:
- stable to harsh environmental conditions
- transmitted by fecal-oral route (survive well in water) and from surfaces
elements of viral structure
- genome
- nucleoproteins = proteins associated with nucleic acid
- nucleocapsid = association of viral capsid proteins with viral nucleic acid
- matrix/tegument = structural proteins linking envelope with virus core
- envelope
adenovirus shape
icosahedral
orthomyxovirus shape
helical
picornavirus shape
icosahedral
hepadnavirus shape
helical (spheres and filaments)
poxvirus shape
brick
retrovirus shape
icosahedral
characteristics of RNA genomes
- smaller size
- higher error rate
- max size limit
- often exist as quasispecies
- may exist as segmented genomes
benefits of segmented genomes
- reduce the chance that an error will incapacitate the whole genome
- reassortment => evolutionary advantage
characteristics of DNA genomes
- larger size
- high fidelity replication and proofreading ability (except ssDNA)
what are the stages of productive viral infection?
- attachment via host cell type specific glycoproteins on the viral surface; enveloped viruses fuse viral envelope with cell membrane
- penetration
- uncoating and disassembly
- take over host functions for: transcription and translation
- replication
- self assembly
- release (usually via cell lysis)
cell susceptibility vs resistance (with respect to viruses)
susceptible = express receptors needed for viral entry
resistant = lack receptors for viral entry
ss(+)RNA virus life cycle
- ss(+)RNA = mRNA => enters the host cell “translation competent” and translation occurs immediately using host cell cytoplasmic ribosomes
- synthesized polyprotein must be cleaved by viral protease into active protein subunits
- must make a (-)RNA to serve as template for genome replication
ss(-)RNA virus life cycle
- requires viral enzyme (RNA-dependent RNA polymerase) to make mRNA and genomic ss(+)RNA from the ss(-)RNA
- occurs in the cytoplasm
DNA viruses
- replicate in nucleus (except poxviruses, which bring their own replication machinery)
- transcription begins with early mRNA (regulatory proteins and DNA replication machinery needed to take over host cell) then late mRNA (structural proteins)
retrovirus life cycle
- viral RNA dependent DNA polymerase (reverse transcriptase) makes dsDNA from ss(+)RNA
- viral dsDNA integrated into the host cell chromosome (provirus)
- transcription thus regulated by host cell
forms of viral infection
non-permissive = cells lack receptors for viral entry
productive = new infectious virus produced
abortive = no new infectious virus produced
latent = no immediate virus production but potential for later virus production
consequences of viral infection for host cell
- cellular processes compromised
- may induce apoptosis (some viruses block it though)
- usually will end in cell death via cytotoxic effects of viral products or host immune response
clinical consequences of viral infection
- inapparent infection
- illness due to infection (rapid multiplication and lots of cell death)
- illness due to persistent infection (chronic tissue damage)
- cancer (cells transmformed)
genetic bases of latent viral infections
- episomal = plasmid
- proviral = integrated into host cell chromosome
general characteristics of DNA viruses
- dsDNA (not parvovirus)
- linear (not HPV, HBV, polyomavirus)
- isocahedral (not poxvirus)
- nuclear replication (not poxvirus)
general characteristics of RNA viruses
- ssRNA (not reovirus, orthomyxovirus)
- replicate in cytoplasm (not influenza, retroviruses)
- haploid (not retroviruses)
