7. Viruses Flashcards
(30 cards)
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
