Chapter 9 (2) Flashcards
Quantification of Viruses
Titer: number of infectious units per volume of fluid.
Plaque assay: analogous to bacterial colony.
-plaques can clear zones that develop on lawns
-each plaque results in single virus
Quantification of Viruses: Number of Plaque
Number of plaque- forming units is almost always lower than direct counts by microscopy.
- inactive virions
- conditions not appropriate for infectivity.
Intact animal methods
Some viruses do not show recognizable changes in cell cultures yet cause death or disease in whole animals.
Virus is diluted.
Animals are infected with viral dilution.
End point is calculated.
Classification of viruses
ssDNA, dsDNA, ssRNA, dsRNA.
- segmented vs. non-segmented genome.
- Virion morphology, enveloped vs. non-enveloped.
- Host range.
Evolution of viruses?
Reductive evolution: virus evolved from independent or intracellular bacteria.
Escaped gene theory: virus evolved from escaped DNA or RNA that gained independence from cellular control.
It’s not east being a virus
- If virus kills host quickly, spreading to new cells will be more difficult.
- Narrow host range (dependence on specific type of host proteins).
- Coordination of viral gene expression and assembly inside host cell.
- Space limitations (genome size by capsid size).
Phases of Virus Replication
- Attachment (adsorption): of virus to a susceptible host cell.
- Entry (penetration): of vision or its nucleic acid.
- Synthesis: of virus nucleic acid and protein by cell metabolism as redirected by virus.
- Assembly: of capsids and packaging of viral genomes into new virions (maturation).
- Release: of mature virions from host cell.
Viral attachment and penetration
Attachment of virion to host cell highly specific.
- Requires complementary receptors on host surface.
- Receptors include proteins, carbohydrates, glycoproteins, lipids, lipoproteins or complexes.
Viral attachment and penetration: Permissive cell
Permissive cell: host cell that allows complete replication cycle of a virus to occur.
Attachment of a virus to its host cell results in changes to both virus and cell surface that facilitate penetration.
Bacteriopahage T4: Attachment and penetration
- Attach via fiber tails.
- Tail fibers retract and tail core makes contact with E. coli cell wall.
- Lysozyme-like enzyme forms small pore in peptidoglycan.
- Tail sheath contracts and viral DNA passes into cytoplasm.
Anti-viral defense systems
Many eukaryotes possess mechanisms to diminish viral infections.
–Immune defense mechanism, RNA interference.
Many bacteria employ restriction-modification systems to evade viral infection.
Anti-anti-viral defense systems
Viral mechanisms to evade bacterial restriction systems.
- Chemical modification of viral DNA.
- Production of proteins that inhibit host cell restriction system.
Baltimore Classification System
David Baltimore, Howard Temin, and Rento Dulbecco discovered retroviruses and reverse transcriptase.
-1975 Nobel Prize
Baltimore developed classification scheme based on relationship of genome to mRNA.
Production of Viral Nucleic Acid and Protein
Virus must replicate genome and make proteins.
Generation of messenger RNA occurs first.
Typically viral genome serves as template or viral mRNA.
Some RNA viruses, viral RNA itself is mRNA.
Sometimes transcriptional enzymes are contained in virion.
Nomenclature of Viral Genomes
Configuration of genome of single-stranded DNA or RNA virus.
- mRNA in plus (+) configuration.
- complement in minus (-) configuration.
- Positive-strand RNA virus: single-stranded RNA genome with same orientation as its mRNA.
