Pellett: Virology - Molecular Aspects Flashcards
Virus
Definition
Genome consists of:
Metabolism on their own:
In host cell:
Infectious, obligate intracellular parasite
Genome consists of DNA or RNA
Metabolically inert on their own (outside the context of a living cell)
Viral genome directs its own replication using cellular systems in a host cell
What are virions?
What is a progeny virion?
Virus particles (virions) are formed by assembly from newly synthesized components within the host cell
A progeny virion is the vehicle of transmission of the genome to the next host cell/organism (disassembly initiates the beginning of the next infectious cycle)
- This disassembly stage is a vulnerable state for the virus
What is a minimal virus?
Minimal Virus: genome with origin of replication and a coat
o DNA or RNA genome
o Proteinaceous coat: also called a capsid
Nucleocapsid:
Envelope:
Virion:
Translation Parasites:
Nucleocapsid: genome + capsid
Envelope: viruses can be enveloped or not
Virion: a complete virus particle with its DNA or RNA core and protein coat as it exists outside the cell; also called a viral particle
Translation Parasites: must use cellular ribosomes in a host in order to replicate
How do we classify viruses?
Nature of genetic material
Capsid Structure
Enveloped or non-enveloped (naked)
Classification
Nature of Genetic Material:
Types of RNA viruses (3):
Types of DNA viruses:
RNA:
- single- and double-stranded
- positive or negative sense
- non-segmented or segmented
DNA:
ss, ds, linear, circular
Positive or negative sense (Baltimore system)
What must all viruses adopt at some point in their replication cycle?
What does it require?
Possible scenarios:
Retrovirus:
At some point in their replication cycle, all viruses must adopt a form that allows transcription of mRNA that can be translated by cellular ribosomes
Requires –ssRNA or dsDNA/dsRNA in order to translate into +mRNA
Possible Scenarios:
o +ssRNA Virus –> –ssRNA –> +mRNA
o +ssRNA Virus –> –ssDNA –> dsDNA –> +mRNA (Retrovirus)
Capsid Structure (3):
o Helical
o Icosahedral
o Complex
Other methods of virus classification:
o Gene organization
o Mode of replication
o Susceptibility to physical and chemical agents (for example, chloroform makes enveloped viruses non infectious)
o Cell tropism (what cells and tissues they infect)
o Pathogenic effects
Examples of RNA and DNA viruses:
RNA Virus Examples: polio virus, influenza virus
DNA Virus Examples: adenovirus, herpesvirus
Virus vs bacteria size
In general: most viruses smaller than most bacteria
How can you distinguish herpesviruses?
Individual virus species may share many properties with their relatives, but yet still have readily distinguished genomes:
Example of differences between herpesviruses shown via Southern blot
Picornaviruses:
Size: Enveloped? DNA: Segmented? What does the genome encode?
Small
Non-enveloped
+RNA genome (needs to be transcribed to negative sense)
Non-segmented: genome encodes single open reading frame that is translated into a large polypeptide and then proteolytically processed to a set of individually functioning proteins
Influenza Viruses:
Enveloped?
DNA:
Segmented?
What does the genome encode?
Enveloped
-RNA genome
Segmented: each gene encoded on individual “chromosomes”;
Genome segmentation is the basis for re-assortment (new strains of virus emerge from infections with 2 different strains)
Adenovirus:
Enveloped?
Size:
DNA:
Transcribed by:
Non-enveloped
Medium sized dsDNA genome
Transcribed by cellular RNA polymerase II (studying this virus led to discovery of mRNA splicing)
Herpesvirus:
Tegument:
Important in helping:
Number of genes:
Enveloped
Tegument: layers of proteins between envelope and capsid
Important in helping the virus control the cell following infection
dsDNA genome (may encode for as many as 200 genes; tightly controlled expression)
The interaction between the virus and the cell (5)
- Get into the cell
- receptors and co-receptors
- what tissues - Deal with host defenses
- innate immunity
- acquired immunity - Control cellular processes
- macromolecular synthesis and stability
- transport
- cell cycle - Replicate
- Get out
Properties of lytic infections
Lytic Infection definition:
Eclipse period:
Burst Size:
Time Scale:
Lytic Infection: making new infectious virions; can be initiated by a single virion
Eclipse Period: interval between disassembly of input virion and production of new infectious progeny
Burst Size: number of infectious virions released per infected cell (100s-1000s)
Time Scale: can be hours or days from time of infection to lytic burst
Steps in lytic infection
Entry Gene Expression Genome replication Assembly/packaging Egress/Release
Lytic Replication
Entry:
Mechanisms with examples:
Entry: attachment to receptor, penetration and uncoating
Mechanisms:
- Endocytosis (poliovirus, influenza virus, adenovirus)
- Genome injection (poliovirus)
- Fusion at cell membrane (HIV, vaccinia virus, herpesvirus)
Lytic Replication
Gene expression:
mRNA synthesis
Protein translation
Lytic Replication
Sites of geneome replication:
Examples:
Cytoplasm: most RNA viruses + poxviruses (DNA)
Nucleus: most DNA viruses + orthomyxoviruses (RNA)
Both: retroviruses, hepadnaviruses
Lytic Replication
Egress/release
Mechanisms (4):
Examples:
Cell lysis (poliovirus, adenovirus)
Budding from cytoplasmic membrane (influenza, HIV)
Cell associated (vaccinia virus)
Vesicle fusion (herpes virus)
Consequences of Infection:
Transformation (4):
Immortalization
Serum independent cell replication
Loss of contact inhibition
Ability to form tumors
Consequences of Infection:
Cytopathic Effect (CPE) (5):
Rounding (lose defined shaped)
Fusion (syncytia formation- giant infected cell)
Intracellular vacuoles and inclusion bodies
Lysis
Cell death
Examples of cytopathic effects in cell culture
Herpes simplex virus - Plaque formation
CMV
RSV - Fusion
Polio replication
Positive-strand RNA genome encoding a single polypeptide that is processed to functioning proteins.
Herpesvirus replication
Type of genome:
Encodes:
# of genes required for lytic replication:
Large double-stranded DNA genome encoding ~80 to ~200 individually transcribed genes that are expressed as one of three major kinetic classes.
Only ~50 of the genes are required for lytic replication in cell culture.
