7. Viruses Flashcards
Virus
genetic element that requires a living (host) cell for its replication
Virology
the study of viruses
viral particle (virion)
extracellular form of a virus
- > exists outside of host + facilitates transmission from one host cell to another
- > contains nucleic acid genome surrounded by a protein coat + sometimes other layers of materials
viral shapes
Polyhedral
Spherical
Helical
Complex
Viral sizes
much smaller than eukaryotic cells (~ 20 to 300 nm)
Viral genomes
- > most are smaller than those of cells
- > Either DNA or RNA genomes
- > single-stranded (ss) or double stranded (ds)
- > some circular but most are linear
- DNA Viruses (A - A)
(ss DNA + ds DNA) - RNA Viruses (A - A)
(ss RNA + ds RNA) - RNA DNA Viruses
(ss RNA -> Retrovirus)
(ds DNA Hepadnavirus)
Viral Hosts and Taxonomy
Viruses can be classified on the basis of the hosts they infect
- > Bacterial Viruses (bacteriophages)
- > Archael viruses
- > Animal Viruses
- > Plant viruses
- > other viruses
Viral structure
composed of protein molecules which are arranged in a precise and repetitive pattern around the nucleic acid
- Capsid
The protein shell that surrounds the genome of a virus particle
- Capsomere
Subunit of the capsid smalled morphological unit visible with an electron microscope
- Nucleocapsid
Complete complex of nucleic acid and protein packaged in a virion
- Enveloped Virus
Virus that contains additional layers around the nucleocapsid (often a lipid membrane)
-> Naked virus
(nucleic acid and capsid - composed of capsomeres)
Have a nucleocapsid, but no envelope
-> Nucleocapsid - constructed in highly symmetric ways
-> Enveloped Virus
(Envelope with capsid and anucleic acid)
membranes surrounds the nucleocapsid
(Lipid bilayer with embedded proteins)
(Envelope makes initial contact with host cell)
- Helical symmetry
rod-shaped viruses
e.g. tobacco mosaic virus
(virus RNA + structural subunits - capsomere)
- > Length of virus is determined by the length of nucleic acid
- > Width of virus determined by size and packaging of protein subunits
- Icosahedral symmetry
Spherical viruses with 20 faces
e. g. human papillomavirus
- > most efficient arrangement of subunits in a closed shell
- Complex Virus
Virions composed of several parts, each with separate shapes and symmetries
- > Bacterial viruses contain complicated structures
- > Icosahedral heads and helical tails
The Virus host
- > Viruses replicate only in certain types of cells (host-specific)
- > Bacterial Viruses are easier to grow (model systems)
- > Animal Viruses (and some plant viruses) can be cultivated in tissue or cell structures
- > Plant viruses typically are most difficult because study often required growth of whole part
Quantification of Viruses
Titer:
number of infectious units per volume of fluid
Plaque assay:
A way to measure virus infectivity
Plaques:
are clear zones that develop on lawns of host cells (can be bacterial cells or animal cells tissue culture)
-> Each plaque results from infection by a single virus particle (analogous to bacterial colony)
- Plaque assay
-> a way to measure virus infectivity
Plaque assay using bacterial cells
- > Mixture containing melten top agar, bacterial cells and diluted phage suspension is being poured onto solidified nutrient agar plate
- > let solidify
- > sandwich of top agar and nutrient agar
- > incubate
- > phage plaques and lawn of host cells
- Efficiency of plating is used in quantitative virology
-> expressed as plaque-forming units (PFU)
-> the number of PFUs is almost always lower than direct counts by electron microscopy
(inactive virions)
(conditions not appropriate for infectivity)
efficiency of plaque formation:
Plaques formed expressed as the pecentage of virions present in ample viewed under microscope
- Intact animal method
Some animal viruses do not show recognizable changes in cell cultures yet causes death or disease in whole animals
- > dilution series of whole sample
- > animals are then infected with the viral dilutions and monitored for death or disease
- > end point is calculated
Lethal Dose: LD 50
Infection Dose: ID 50
General features of Virus replication
- Attachment (absorption):
of the virus to a susceptible host cell - Penetration (Injection):
Entry of the virion or its nucleic acid - Synthesis of nucleic acid and protein:
Synthesis of virus nucleic acid and protein by host cell metabolism as redirected by virus - Assembly and packaging:
of capsids and packaging of viral genomes into new virions (maturation) - Release (lysis):
of mature virions from host cell
-> Virus replication typically characterized by a one-step growth curve
Latent period:
eclipse plus maturation
Burst size:
number of virions released
Viral attachment and penetration
Attachment of virion to host cell is highly specific
- > Requires complementary receptors on the surface of a susceptible host and its infecting virus
- > Receptors on host cell carry out normal functions for cell (e.g. proteins involved in cell to cell interaction)
Receptors include:
host cell proteins, carbohydrates, glycoproteins, lipids, lipoproteins or complexes)
-> the attachment of a virus to its host cell results in changes to both virus and host cell surface that facilitate penetration
Permissive cell:
host cell that allows the complete replication cycle of a virus to occur
Production of viral nucleic acid and protein
The Baltimore Classification Scheme:
Viruses classified by type of viral genome and its relationship to mRNA
Class I -> ds DNA Viruses
Class II -> ss DNA viruses
Class III -> ds RNA
Class IV + V -> ss RNA (+ or -)
Class VI -> retrovirus (reverse transcriptase)
Class VII -> ds DNA viruses that replicate through an RNA intermediate
Positive (+) sense viral RNA is similar to mRNA + thus can be immediately translated to proteins by the host cell
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Temperate Viruses only in bacteria
Can undergo a stable genetic relationship with the bacterial host (lysogenic pathway)
- > can also kill cells through lytic pathway
- > under certain conditions lysogenic viruses may revert to the lytic pathway and begin to produce virions
- Lysogeny
State where most virus genes are not expressed and virus genome (prophage) is replicated in synchrony with host chromosome
- Lysogen
a bacterium containing prophage
Animal Viruses
- Persistent infection
- Latent or slow infection
- A chronic infection that is eventually cleared
e.g. Varicella-zoster virus.
Measles virus
Human cytomegalovirus
- lasts the life of the host
e.g. Herpes virus family
Herpesviridae, all of which establish latent infection. Establish episomal latency in neurons and leave linear genetic material floating in the cytoplasm
Episomal latency
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