Flashcards in Exam #1: Medical Virology Deck (43):
Describe the general characteristics of viruses.
- Obligate parasite i.e. do NOT carry out own metabolism & lack organelles/ ribosomes
- 20-300 nm in size--same size are smallest bacteria
What are the different types of virions? Which is generally more vulnerable to the environment?
- Capsid (protein coat as exterior)
- Enveloped (lipid coat exterior, outside of capsid) & more vulnerable
What is the matrix or tegument of a virus?
Space between the capsid & envelope
What are the different types of genomes that viruses may contain?
- Segmented or continuous
- DNA or RNA
- Double stranded (ds) or Single stranded (ss)
- Positive or negative sense
What is the difference between positive & negative sense?
- Positive= in correction orientation for translation i.e. like an mRNA molecule
- Negative= opposite orientation needed to be a direct template for translation i.e complementary to mRNA
How are viruses classified?
- Genome (which tells how the virus replicates)
- Class of disease caused
What family of viruses contains ssDNA?
What families of viruses contains dsDNA?
What is the family of dsRNA viruses?
What are the families of -ssRNA viruses?
What are the families of +ssRNA viruses?
What are the properties of RNA viruses?
- RNA is more labile (liable to change/easily altered) & transient than DNA
- Replicate quickly
- Cells cannot replicate RNA; thus, RNA viruses must encode an RNA-dependent-RNA polymerase
- Prone to mutation b/c polymerase does NOT have proofreading mechanisms
Clinical impact= RNA viruses are more adaptable to environment & more easily evade the immune system
What are the three major forms of capsids?
- Helical- slinky
- Icosahedral- 20 sided dye
- Complex- less ordered than icosahedral & in large viruses
What is a Naked Capisd?
- Virus does not have an envelope & uses the capsid as its outermost shell
- Spread by lysis--> must kill cell to spread
What are the clinical implications of having a naked capsid?
- Environmentally stable
- Spread easily
- Dry out & retain infectivity
- Survive in adverse conditions
- Resistant to detergents & poor sewage treatment
What is an enveloped virus & what are the characteristics of enveloped viruses?
Enveloped virus= has lipid layer outside of capsid
- Environmentally labile
- Modify cell membrane during replication
- Released by lysis or budding (not killing cell for release)
What are the clinical implications of enveloped viruses?
- Must remain wet
- Cannot survive in GI tract
- Spread in large droplets
- Do NOT need to kill cell to spread
What are the six major steps of the viral lifecycle?
2) Entry-- getting past the cell membrane
3) mRNA production
4) Protein & genome synthesis
5) Virion assembly
Describe the process of attachment & entry of a virus into a host cell.
- Attachment= Molecules on the exterior of the cell bind to the surface of the host cell (via receptors)
- Entry= direct fusion or via high-jacking the receptor-mediated endocytosis pathway
How do dsDNA, ssDNA, dsRNA, +ssRNA, & -ssRNA differ in how they translate mRNA?
- DsDNA= use cellular machinery to produce mRNA
- ssDNA= use cellular DNA repair enzymes to produce dsDNA, then use RNA polymerase
○ Retrovirus use viral reverse transcriptase to make dsDNA
○ Others= use virus as mRNA
- -ssRNA= need enzyme, viral RNA-dependent-RNA polyermase (RdRp) to make +mRNA
- dsRNA also use RdRp
How do DNA viruses combat the fact that cellular DNA replication machinery is not available at all times?
- dsDNA= often contain potent transcriptional activators that act as "turbo boosters"
- encode own DNA replication machinery e.g. Herpes
- encode proteins that push the cell into S-phase
How do + & -ssRNA viruses combat not having RdRp?
- +ssRNA viruses encode a RdRp
- -ssRNA viruses encode & carry RdRp
What is the general pathway to viral assembly & egress?
1) Individual viral proteins form into capsid subunits
2) Subunits combine to form complete capsid
3) Viral genome and other essential virion components are selectively packaged into capsids
How does the egress of lytic viruses differ from non-lytic viruses?
- Lytic= rupture the plasma membrane of infected cells & spill out, killing the host cell in the process
- Non-lytic= egress without lysing
What are the mechanisms that pressure viruses to change & become more effective antigens?
- Point mutation
- Recombination (DNA viruses only)
- Reassortment (segmented) e.g. Influenza has 8 segments & after replication there can be a shuffling of segments-->dramatic changes in antigenicity
What is the difference between acute, chronic, & latent infection types?
Acute= virus killed off by immune system after infection & symptoms resolve
- Common cold
Chronic= infection followed by incomplete immune response (though asymptomatic, patients can infect other & are considered carriers)
Latent= starts similarly to acute & virus killed; however, virus goes dormant & is then reactivated
How do viruses get into the body?
- Direct inoculation (insect bite, trauma, injection)
- Sexual transmission
How do viruses cause cellular injury?
What is the cytopathic effect?
Structural changes in the host cells that are caused by viral invasion
What is a plaque assay?
Plaque assay= diagnostic technique using cytopathic effect
- Infection leads to a "patch" or clearing or plaque
- Note, 1 plaque= 1 virus-- quantification
- ONLY way to determine INFECTIOUS virus
What is the only way to determine if a virus is infectious?
What does a Plaque assay detect? What are the advantages & disadvantages of the technique?
Detects= Infectious virus
Advantages= demonstrates active viral infection
Disadvantages= restricted to viruses that replicate in culture & produce cytopathic effect
What does electron microscopy detect? What are the advantages & disadvantages of the technique?
Detects= Virion particles
Advantages= Helpful in identification of emerging viruses
Disadvantages= expensive & challenging
What does an ELISA assay detect? What are the advantages & disadvantages of the technique?
Detects= Viral proteins & glyocproteins
Advantages= sensitive & quick
Disadvantages= requires a specific antibody
What does PCR detect? What are the advantages & disadvantages of the technique?
Detects= DNA genomes
Advantages= highly sensitive
Disadvantages= DNA sequence information must be available
What does RT-PCR detect? What are the advantages & disadvantages of the technique?
Detects= RNA genomes
Advantages= Highly sensitive
Disadvantages= RNA sequence information must be available
What does a Western Blot detect? What are the advantages & disadvantages of the technique?
Detects= Anti-viral antibodies
Advantages= sensitive & quick
- Time required for immune response
- Difficult to differentiate present from past infections
What is a virion & what are the components of a virion?
Virion= a complete virus particle that consists of:
1) Nuclei acid genome
2) Capsid or protein coat
What is a nucleocapsid?
Capsid that lies interior to the lipid layer of an enveloped virus
In an enveloped virus, where does the lipid membrane come from?
Lipid membrane is derived from host membrane structures that are then modified by the virus
What is the only virus that is able to synthesize its own envelope?
What is the difference between direct fusion & high-jacking the receptor mediated endocytosis pathway for entry?
- Enveloped virus ONLY
- Viral proteins promote fusion with host cell membrane & capsid is released directly into cytoplasm
- Fusion proteins remain embedded in membrane & can cause subsequent fusion reaction with neighboring cells
- Enveloped or capsid virus
- Using the receptor-mediated endocytosis pathway, plasma membrane around entire virion
- If enveloped, viral & plasma membrane fuse prior to release of capsid into the cytoplasm