Ch 6 Viruses And Othe Infectious Agents Flashcards
importance of viruses
major cause of disease
vital members of the aquatic ecosystem
can be used to destroy cancer cells
bacteriophages in human guts may regulate bacterial microbiome
important model organism
viruses
infectious agents unique in their simple, acellular organization and pattern of multiplication
extracellular viruses
inactive
cannot reproduce outside of living cells
intracellular viruses
commandeer host cells and use them to synthesize viral components from which mature progeny viruses are assembled and released
bacteriophages
virus that infect bacteria
virion
mature virus particle
contains nucleocapsid - composed of nucleic acid, DNA or RNA, and a protein coat, capsid - which protects the genome
can be enveloped viruses (lipid membrane) or nonenveloped/naked
capsid
- large macromolecule structures which serve as protein coat of virus
- made of protein subunits called protomers
- protect viral genetic material and aid in its transfer btw host cells
- constructed from many copies of one protein
- enveloped viruses require nucleocapsid proteins and extra proteins to anchor to the membrane
- helical, icosahedral, or complex
helical capsid
shaped like hollow tubes with protein walls
- protomers self assemble into rigid tube
- size of capsid is influenced by protomers and genome
icosahedral capsid
most efficient way to enclose a space
- a regular polyhedron with 20 equilateral triangular faces and 12 vertices
assemble in capsomers - ring or knob shaped units made of 5 or 6 protomers
poxiviruses
largest of the animal viruses
complex interior and ovoid- to brick-shaped exterior
binal symmetry
head resembles icosahedral, tail is helical
envelope
outer membranous layer that surrounds a bacterial virus
in animal virus envelopes (lipids and carbs) usually arise from host cell plasma or organelle membranes
peplomers/spiks
envelope proteins are coded for viral genes and often project from the envelope surface
involved in virion attachment to the host cell surface
can have enzymatic activity needed for entry or exit from host cell
segmented genomes
genomes that consist of multiple pieces of RNA
usually all segments are enclosed by the same capsid
five steps of viral multiplication
- attachment (adsorption)
- entry into the host
- synthesis
- assembly
- virion release
attachment (adsorption)
viruses require a host cell in which to multiple so a ligand (virion) attaches to a receptor (host)
- SARS CoV 2 spike protein is the viral ligand that attaches to the human receptor call A C E2
- in plants, no receptors found, instead damage of the host cell is required for entry
receptor determines host preference: tropism - viruses will bind to specific tissue receptors
entry into the host
after attachment, the virus’s genome or entire nucleocapsid enters the cytoplasm
- in some cases, only the genome enters leaving the capsid attached to the outside of the cell
3 methods of entry into the host
- fusion of the viral envelope with host cell’s plasma membrane
- endocytosis
- release of nucleic acid
synthesis
this is the step that differs the most between viruses. genome dictates the events
- ds DNA follows typical synthesis
RNA viruses:
- virus must carry in or synthesize the proteins necessary to complete synthesis
- viral replication complexes - enclose machinery needed for genome replication
- tightly regulate gene expression and protein synthesis
assembly
late proteins are involved
baseplate, tail fibers, and head components are all assembled separately
virion release mechanisms
- host cell lysis (nonenveloped viruses)
- release by budding (enveloped viruses)
budding release
virus-encoded proteins are incorporated into the membrane
then the nucleocapsid is simultaneously released and the enveloped formed by membrane budding
- may use host actin tails to propel through host membrane btw adjacent cells
lysis
common for bacterial viruses and some nonenveloped animal viruses
involves the activity of viral proteins that puncture host plasma membranes and in the case of bacteriophages, degrade the host peptidoglycan
virulent phage
begin to multiply immediately upon entering a bacterial host, and are released from the host by lysis
temperate phage
have two reproductive options:
- reproduce like virulent phages
- remain within the host without destroying it
lysogeny
relationship btw a temperate phage and its host
prophage
form of virus that remain within its host
viral genome either integrated into the bacterial chromosome or free in the cytoplasm
lysogenic bacteria/lysogens
infected bacteria
appear to be perfectly normal but
- cannot be reinfected by the same virus
- prophage is replicated and inherited within the genome of progeny cells
lysogenic conversion
when a temperate phage changes the phenotype of its host
alteration in surface characteristics of the host
two advantages to lysogeny for virus
- allows viral nuclei acid to remain within a host
- high multiplicity of infection can occur but lysogeny allows host cell survival in few uninfected cells
cytopathic effects (CPEs)
viruses that infect eukaryotic cells can cause microscopic or macroscopic degenerative changes or abnormalities in host cells and in tissues that are distinct from lysis
animal viruses are usually cultivated by
inoculating suitable host animals or embryonated eggs
tissue (cell) cultures on monolayers of animal cells
cytopathic effects can be observed
plant viruses are cultivated by
- plant tissue cultures, cultures of separated cells or plant protoplast cultures
- suitable whole plants: rub mixture of virus and an abrasive, may cause localized necrotic lesions or generalized symptoms of infection
plaque assays
dilutions of virus samples made and plated with appropriate host cells
- results expressed as plaque-forming units
viroids
infectious agents that consist only of RNA
- closed, circular ss RNA
- do not encode any proteins
- replication requires host cell
- cause plant diseases
- cause disease by triggering RNA silencing
satellites
similar to viroids but nucleic acid gets enclosed in a capsid and they need a helper virus to replicate
prions
made of only a single protein
cause a variety of neurodegenerative diseases in humans and animals
- scrapie, mad cow disease
