Virology - Lecture 2 Flashcards
What are four things viruses need to survive?
1) Genetic material
- DNA, RNA
2) The ability to replicate
- Genome replication relies on host or viral genes
3) Gene expression
- Trasncription relies on host or viral genes (RNA polymerase)
- Translation relies on host translational machinery (ribosomes)
4) Structural integrity
- Capsid, envelope
What are the seven stages of the life cycle of a virus?
1) Binding
2) Entry
3) Uncoating
4) Viral gene expression
5) Genome replication
6) Virus assembly
7) Egress
Describe the first phase of viral life cycle (binding)
A viral ligand bind to a cellular receptor.
a host cell or organism is susceptible to infection by a particular virus if the host bears a cognate receptor for the virus
Describe the second phase of viral life cycle (entry)
Fusion
Endocytosis
Cell membrane disrutption
Describe the third phase of viral life cycle (uncoating)
Virus releases genetic material for gene expression and replication
Mediated by viral or cellular enzymes
Describe the fourth phase of viral life cycle (viral gene expression)
mRNA trascription often required
Translation using host ribosomes required
Some viruses encode their own transciptional machinery
Describe the fifth phase of viral life cycle (genome replication)
Virus may use host machinery
Some viruses encode viral replication machinery
ALL RNA viruses encode a viral polymerase for replication
Why do all RNA viruses encode viral polymerase for replication?
Host cells (humans) do not possess polymerase that turn RNA into DNA. Therefore, RNA viruses must possess an RNA dependent RNA-polymerase to carry out transcriptoin o the negative strand into the positive strand and then it can be translated by the host’s ribosomes into proteins.
Describe the sixth phase of viral life cycle (virus assembly)
This phase is spontaneous
Describe the seventh phase of viral life cycle (egress)
Lysis
Exocytosis
Describe susceptibility
The ability of a virus to bind to a cellular receptor
Describe permissivity
the ability of a virus to replicate in a host cell/organism
Describe tropism
The range of cell types, tissues or organisms that are both susceptible and permissive to replication
Ex) Rabies virus has tropism for humans and many animals - it is a neurotropic virus
In the Baltimore Classification System of Viruses:
How many classes are there?
Seven
In the Baltimore Classification System of Viruses: What is class I?
dsDNA viruses
- herpesviruses
- poxviruses
- papillomaviruses
- adenoviruses
In the Baltimore Classification System of Viruses: What is class II?
ssDNA viruses
- parvoviruses
- circoviruses
In the Baltimore Classification System of Viruses: What is common between class I and II (dsDNA & ssDNA)
Most use host DNA-dependent DNA polymerase to replicate genome
Rely on cellular replication machinery
Rely on host transcriptional and translational machinery
In the Baltimore Classification System of Viruses: What is class III?
dsRNA viruses
-reoviruses
In the Baltimore Classification System of Viruses: What is class IV?
(+)ssRNA viruses
- picornaviruses
- flaviviruses
In the Baltimore Classification System of Viruses: What is class V?
(-)ssRNA viruses
- orthomyxoviruses
- rhabdoviruses
In the Baltimore Classification System of Viruses: What is common between class III, IV & V ((dsRNA, (+)ssRNA & (-)ssRNA)?
These classes must encode an RNA-dependent RNA polymerase in their genome
(-)ssRNA viruses must package an RNA-dependent RNA polymerase in each viral particle
In the Baltimore Classification System of Viruses: What is class VI?
(+)ssRNA viruses with a DNA intermediate
-retroviruses
In the Baltimore Classification System of Viruses: What are some characteristics of class VI ((+)ssRNA with DNA intermediate)?
Require a virally encoded reverse transcriptase in the virus particle
Revers transcrition and viral integration are hallmarks of the retroviral family
In the Baltimore Classification System of Viruses: What is class VII?
dsDNA viruses with an RNA intermediate
-Hepatitis B
In the Baltimore Classification System of Viruses: What are some characteristics of class VII (dsDNA with RNA intermediate)?
Require a virally encoded reverse transcriptase for genome replication
Genomic DNA is transcribed into mRNA for viral gene expression
mRNA is revers transcribed by RT to make genome copies during replication
What does a lytic infection result in?
Results in host cell lysis
-rhabdoviruses
What does a lysogenic infection result in?
Results in virus production without cell lysis
-retroviruses, herpesviruses
Describe an acute infection
Infection characterized by a rapid production of virus and/or rapid onset of disease symptoms, followed by clearance by the host
-rhabdoviruses
Describe a persistent infection
Infections occur when the primary infection is not clears
-retroviruses, herpesviruses
Describe a chronic infection
Infections are persistent infections that are eventually cleared
How are viruses classified?
Classified by nucleic acid composition
What are the different types of latent infections?
Clinical and Molecular
Describe clinical latent infections
1) Clinical latency
- A period in which no symptoms of infection are observed
- Viral/pathogen replication may or may not be present
- HIV - after acute phase, patients experience a long period of clinical latency, where no symptoms exist but viral replication occurs and CD4+ T cell counts diminish
Describe molecular latent infections
2) Molecular latency
- A period in which no viral/pathogen replication occurs
- Features of latency vary by pathogen
- Herpesviruses - latent in neuronal or other cell types, express transcripts that prevent viral replication in order to hide from the host immune system
What does PAMP stand for?
pathogen-associated molecular patterns
Describe a PAMP response to infection
Derive from pathogens, common molecular features that indicate infection are: -LPS -flagellin -dsRNA -bacterial cell wall components CpG DNA
What does DAMP stand for?
Danger-associated molecular patterns
Describe a DAMP resonse to infection
Derive from physiological changes that indicate damage
- ROS
- changes in cytoplasmic CA+/K+ concentrations
- ATP influx
- Crystal deposition (MSU, etc.)
- Membrane damage - mitochondrial, lysosomal, cell membrane
