Viral Life Cycles Flashcards
What are the three fundamental properties of viral propagation?
- All viral genomes are packaged inside particles (virions) that mediate their transmission from host-to-host
- The viral genome contains the information for initiating and completing an infectious cycle within the susceptible permissive cell
- All viruses are able to establish themselves in a host population so that virus survival is ensured
WHat are the phases of the viral life cycle?
First attachment and penetration
-infectious particles are still detectable at this point
- Eclipse Period
- uncoating
- infectious particles not detectable - Maturation
infectious particles being becoming detectable - Release
- Progeny virions released from host cell to begin new cycle
What are the 7 key steps of the viral replication cycle?
- Attachment
- Penetration
- Uncoating
- Replication
- Assembly
- Maturation/Processing
- Release
What is attachment?
Also known as adsorption
1) Cell surface receptors on host cell used by virus to locate host and attach
2) Sometimes, co-receptors are needed for attachment
3) Host range (broad/narrow) determined by the type(s) of cell surface receptors used by virus
Some viruses need to bind to more than one receptor (co-receptor) for entry. How many does HIV and HSV-1 need?
HIV
- Primary receptor = CD4
- Secondary receptor = Chemokine Cell receptor
HSV-1
- initially bind to glycosaminoglycan sugars on cell surface
- also bind to protein receptor to initiate entry
What is penetration?
Entry accomplished via two methods:
1) Receptor mediated endocytic entry
- MOST common
- Cell Surface receptors target virus and engulf
- Low pH of endosome causes capsid to be released within
2) Direct ligand-mediated fusion with plasma membrane
- Fuses envelope w/ host plasma membrane
Both methods are pH dependent and some viruses can utilize either method of entry
In which method of entry does the capsid deposit directly into the host cell and in which method does the entire virion enter the host cell?
Direct Fusion
- deposits capsid into host cell
- capsid uncoats and releases DNA
Endocytosis
- entire virion enters host cell
- acidification degrades endosome and releases capsid
- capsid then uncoats and releases DNA
What method does Equine Herpes Virus 1 (EHV-1) use to gain entry to a host cell?
*EHV-1 is studies by Dr. Frampton
Both Direct fusion and Endocytosis
What is uncoating?
Disassembly and Localization
-release the viral genome into the cytoplasm or nucleus
The encaspidated (coated) genome is inactive -need to uncoat nucleocapsid (infectious particles no longer detected)
How do viruses get their DNA into the host nucleus to replicate?
Many DNA viruses ride along cellular microtubules to make the journey from entry to host nucleus
-kynesin+dynein transport the capsid to the nucleus
How do we know vinblastine inhibits EHV-1 infection?
% infection went down and cell viability remained over 100%
What are the four type of viral genomes and what is needed in addition to replication?
dsDNA and ssDNA
dsRNA and ssRNA
In addition to replicating their genomes, all viruses must also generate viral mRNAs
-these are translated into viral proteins using host cell machinery
What is assembly?
All of the components of the virus assembled into a particle
Occurs when an appropriate concentration of virus proteins and genomic nucleic acids are reached and localized at specific sites in infected cell
SOme partles self-assemble
What is maturation/processing?
Putting on the capsid and envelope
What is release?
newly formed viruses are released to the outside environment by:
1) Upon cell lysis for cytopathic/lytic viruses
2) Budding
