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39 Cards in this Set

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What kind of organism is a virus?
Obligate intracellular parasite
What are the two components of a virus?
Genome

Protein (capsid or nucleocapsid)
What are the two general categories of viruses?
Nonenveloped
- Lacking envelope

Enveloped
- Lipid membrane
What forms can virus genomes assume?
RNA:
- Single stranded:
- (+) stranded
- (-) stranded
- Double stranded

DNA:
- Single stranded
- Double stranded

*All genetic material can be segmented or nonsegmented
What is a capsid?
The protein coat of non-enveloped viruses
What components make up the viral envelope?
Lipids

Virus-encoded proteins:
What types of proteins are present in the envelope of a virus?
Glycoproteins forming spiked/peplomer external structures

"Membrane" matrix proteins
What are the steps in viral infection of cells?
1. Encounter

2. Attachment (to cell surface)

3. Entry (of viral genome)

4. Replication (of virion components)

5. Assembly (of progeny virions)

6. Release
How does the presence of a virion envelope affect viral infection?
Determines mechanisms of penetration, assembly, and release
How do virions attach to host cells?
VAP (virus attachment protein) binds cell surface receptors
How do virus attachment proteins (VAPs) differ between enveloped and non-enveloped virions?
Enveloped
- Spike

Non-enveloped
- Capsid
What determines the species and organ specificity in viral infection?
Host cell membrane receptors
What step of viral infection does a neutralizing antibody block?
Attachment
How do non-enveloped viruses enter the cell?
Pinocytosis

Endocytosis
How do enveloped viruses enter the cell?
Surface fusion
- Viral envelope becomes part of host cell membrane

Receptor-mediated endocytosis
- Virus taken in via coated endosome
What is uncoating?
Genome made accessible to host cell cytoplasm
What components do infected cells synthesize when producing virus progeny?
Viral mRNA

Viral protein

Viral genome
Describe the process of replication in double-stranded DNA viruses (mechanism)
1. Viral genome DNA transcribed to early mRNA

2. Early mRNA translated to early proteins

3. Early protein DNA polymerase copies viral genome

4. New viral genome transcribed to late mRNA

5. Late mRNA translated to late proteins
Describe the process of replication in single-stranded RNA viruses with positive polarity (mechanism)
1. Viral genome RNA translated into polyprotein

2. Polyprotein cleaved by proteases

3. RNA polymerase (from polyprotein) makes (-) strands from viral genome RNA

4. (-) strands used as template to make more identical (+) strand viral RNA

5. (+) strands can make protein or RNA
Describe the process of replication in single-stranded RNA viruses with negative polarity (mechanism)
1. Original viral RNA transcribed to mRNA via RNA-dependent polymerase (brought with virus)

2. mRNA makes proteins

3. More RNA-dependent polymerase makes (-) strand

4. Original (-) strand used as template to make (+) strand

5. (+) strand used as template to make (-) strand

6. (-) Strand can be used to make mRNA, protein, or stay as genomic RNA
What determines whether RNA viruses are definied as positive or negative stranded?
Whether or not genomic RNA is messenger sense or complementary to mRNA
What are the general types of viral proteins tranlslated from viral mRNAs?
Structural proteins

Nonstructural proteins
What is the difference between structural proteins and nonstructural proteins?
Structural proteins
- Structural part of virion (capsid, glycoprotein, nucleocapsid)

Nonstructural proteins
- Enzymes required for stages of infection
What does RNA-dependent RNA polymerase do and from where does it come?
Synthesizes mRNA from viral RNA

Brought into host cell with the virus
What are virion associated polymerases?
Enzymes incorporated into the virion for use upon infection of a host cell
How are capsids formed?
Either
- Capsid formed and genome inserted
- Capsid built around genome
How do enveloped viruses acquire their envelopes?
Involvement of nucleocapsids or capsids with virus-modified cell membrane of the host
What is budding?
The process by which enveloped viruses exit the host cell out of the plasma membrane
What is the range of viral infection on cells?
None/subtle

Cell death

Cell transformation (ex. immortalized)
What are the general steps of systemic viral infections?
1. Replication at primary site of infection

2. Drainage of virus to local lymph nodes

3. Primary viremia (in blood stream) after further replication

4. Infection of target organ

5. Secondary viremia
How does PCR assist in the identification and measurement of viruses?
Amplify virus sequences
How do immunological techniques assist in the identification and measurement of viruses?
Virus specific antibodies labeled for immunofluorescence or ELISA

Complement fixation assay
- Ab bound to antigen detected by Fc portion fixing complement
How do host range and CPE assist in the identification and measurement of viruses?
Knowledge of cytopathic effects (CPE) and origin can provide presumptive diagnosis
How do plaque assays assist in the identification and measurement of viruses?
Quantitative measure of amount of infectious virus
How do plaque assays work?
Use necrotic areas of cells growing in tissue culture
What are the variations of plaque assays?
Focus assays for transforming viruses

Hemadsorption assays for attaching to red blood cells

Fluorescence assays to detect areas of infected cells
How do hemagglutination assays assist in the identification and measurement of viruses?
VAPs bind RBCs leading to agglutination to give a relative number of particles
How do inhibition assays assist in the identification and measurement of viruses?
Antibodies against VAPs can inhibit hemagglutination and neutralize virus, giving identifying factors of the virus
What are the two types of inhibition assays and how do they differ from each other?
HAI titers (inhibiting hemagglutination)
- Determine end point when Ab in serum is no longer concentrated
- Increases in HAI titer from acute to convalescent sera provide indicdation of infection for that virus

Neutralization
- Protects immunized host
- Ab attachment to VAP
- Can identify isolated virus, test immunity levels, and verify infection by showing an increase from acute to convalescence