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)
