Basic Virology Prestudy Doc - Ryan Flashcards
What is a virus?
viruses are obligate intracellular parasites that replicate by self-assembly of individual components rather than by binary fission.
How can viruses make energy or proteins without a host cell?
they can’t fool
Can a virus contain both DNA and RNA?
no, it only has one or the other
What are the 4 classifying features of virions?
- size
- morphology
- type of genome
- mechanism of replication
What are the different types of viral genomes?
- circular or linear single-strand (ss) +RNA or -RNA (same sense or opposite sense as mRNA)
- linear double-strand (ds) RNA
- linear ss DNA
- circular or linear ds DNA
- segmented RNA (chromosome like)
Which type of viruses typically have larger genomes: DNA or RNA?
DNA
What are capsids?
protein shells that virus genomes are encapsulated in. They are rigid and can withstand environmental stress
How are capsids formed?
they are the result of self-assembly of virally-encoded capsomeres
What are the three types of capsids?
- helical
- icosahedral or spherical
- complex
What effect does shape of the viral genome have on the capsid shape?
none, the capsid shape is determined by the capsomere
what is a nucleocapsid?
genome + capsid
a naked virus
How do viruses obtain lipid-envelopes?
virally-encoded glycoproteins are inserted in the membrane and serve as virus attachment proteins and membrane fusion proteins
What is a nucleocapsid with a lipid membrane called?
enveloped virus
Which are more stable, enveloped viruses or naked viruses?
Naked viruses are more stable.
the lipid envelope makes enveloped viruses more prone to drying, detergents and alcohols, and the gastrointestinal tract
What will likely happen if an enveloped virus makes its way into the gastrointestinal tract?
it will be destroyed. enveloped viruses can not survive there
What are the major steps in viral replication?
- attachment
- penetration
- uncoating
- early transcription+synthesis of nonstructural proteins
- genome replication
- late transcription and structural proteins
- assembly
- release
What are the two ways viruses can penetrate a cell?
endocytosis or membrane fusion
Where do RNA viruses usually get their RNA polymerase? DNA viruses?
RNA viruses usually have virally encoded RNA-dependent RNA polymerase
DNA viruses usually use host RNA polymerase (except poxviruses)
What type of DNA virus does not use host RNA polymerase?
poxviruses
Where does genome replication occur for RNA viruses?
cytoplasm
Where does genome replication occur for most DNA viruses?
nucleus (except poxviruses)
What are the two ways viruses can leave host cells?
cell lysis or budding (for enveloped viruses)
What are the mechanisms by which viruses cause patholgy?
- inhibition of cellular protein synthesis
- inhibition and degradation of cellular DNA
- alteration of cell membrane structure
- disruption of cytoskeleton
- formation of inclusion bodies
- toxicity of virion components
Which type of viral RNA is used immediately as mRNA by cellular ribosomes?
+RNA
-RNA must be used as a template to transcribe a mRNA (+RNA) strand
Describe the genome replication process for +RNA viruses.
- +RNA is immediately translated as a polyprotein, which must then be cleaved into individual proteins
- one of those cleaved protesins is a RNA-dependent RNA polymerase that transcribes -RNA strands from the +RNA
- -RNA strands are used to make more copies of +RNA strands
- +RNA copies are used as mRNA to make structural proteins and are encapsidated to produce nucleocapsids
Describe the genome replication process for -RNA viruses.
- incoming virus particles carry a RNA-dependent RNA polymerase
- +RNA is formed and then translated into proteins, and is used as a template for making more -RNA
- new -RNA copies are encapsidated to produce nucleocapsids
Describe the genome replication process for retroviruses.
retroviruses carry RNA-dependent DNA polymerase (reverse transcriptase)
- +RNA is reverse transcribed into dsDNA and integrated into hose genome
- retrovirus proteins and +RNA genome are produced by host enzymes
Describe the genome replication process for DNA viruses (except poxvirus).
genome is transcribed by host DNA-dependent RNA polymerases. Many of the viruses shut-off and degrade host mRNA, use transcription factors that downregulate polymerase action on host genes.
Large viruses are dependent on virally-encoded DNA-dependent DNA polymerases.
Newly produced DNA is encapsidated to produce nucleocapsids
Where do very large DNA viruses get their DNA polymerase?
it must be carried by them. virally-encoded DNA-dependent DNA polymerase
What is a plaque?
a confluent monolayer of lysed cells in a growth plate from virus activity
What is a lysate?
the suspension of virions in culture mediu that results from unrestricted growth of a virus on a cell monolayer
Are all virus particles produced in a lysate infectious?
No. some are not
What is the particle-to-pfu ratio?
number of physical particles compared to the number of infectious virions
What is a plaque assay?
measures the number of infections virions in a given volume of lysate. Generally measured as plaque-formin units (pfu) per mL of lysate = titer.
Is a biological assay of infectivity
What is MOI?
multiplicity of infection is the ratio of the number of infectious particles to the number of target cells to be infected.
MOI =1 will infect about 60% of monolayer
MOI = 5-10 will ensure total monolayer infection
What are the two parts of a single-cycle growth curve?
- eclipse period
2. latent period
What is the eclipse period of single-cycle growth?
post-penetration phase until virus can be detected intracellularly.
Corresponds to uncoating, early transcription, and genome replication steps; ends at virus assembly
What is the latent period of single-cycle growth?
post-penetration phase until virus can be detected extracellularly.
includes the eclipse period and corresponds to uncoating, early transcription, genome replication, virus assembly, and release
Why do viral mutations occur at a relatively high frequency?
there are a large number of genome copies produced in every infected cell.
There are many polymerase errors, especially in RNA viruses
What is complementation?
an exchange of proteins.
picks up something in the cell, but can’t reproduce it later in other cells
What is recombination?
an exchange of genetic material on the same segment of genome.
What type of virus frequently undergoes recombination?
DNA viruses.
Can recombination occur in RNA viruses?
Not truly, but they can exchange genetic material in a similar fashion
What is reassortment?
an exchange of genetic material on different segments of genome.
Has the potential to create “novel” viruses with segments of both parents. This is how influenza creates new strains
What is the most common route of viral infection?
inhalation
What is syncytia formation?
What when an enveloped virus fuses an infected cell with an uninfected cell
What is viremia?
presence of virions in blood
How can viruses enter the CNS?
they can circumvent the blood-brain barrier, go through cerebral spinal fluid, or by direct uptake in peripheral nerves
What is the incubation period?
the period postinfection prior to the onset of symptoms
How long is the typical incubation period for viruses that do not require secondary spread?
1-2 days
What is the typical timespan for viruses that require secondary spread?
12-14 days
How long is the incubation period for HIV?
months to years
Can a patient be infectious during the incubation period?
yes
What are examples of secondary spread?
bloodstream, lymph, CNS spread
What is the acute phase of an infection?
the symptomatic phase of infection. Most viral infections completely resolve after acute phse
What is chronic infection?
virus is produced at low levels, but may not continue to cause disease
What is a latent infection?
virus genome remains in cells indefinitely but virus particles are not produced, except during reactivation
What is a transforming infection?
intact or partial viral genome integrates into cellular DNA or is otherwise maintained in the cell and immortalizes the cell, alters its growth properties (eg, oncogenic viruses).
What type of RNA viruses produce transforming infections?
retroviruses, hepatitis C
What type of DNA viruses produce transforming infections?
hep B, papilloma virus, polyoma virus, adenovirus type 2, epstein-barr, herpes, pox
What type of receptors can recognize viral PAMPs?
Toll-like receptors and PRRs recognize viral PAMPs to induce IFN alpha and beta
What is the main viral PAMP?
dsRNA
List the most common viral PAMPs.
dsRNA, unmethylated DNA and 5; modified ssRNA
What do IFNs do in a viral infection?
they bind to surrounding, uninfected cells and induce pathways to prevent virus replication
What pathways do IFNs induce in neighboring uninfected cells?
- protein kinase pathway (PKR) inactivates translation initiation factor eIF-2, inhibiting viral protein translation
- 2-5A system activates RNase L which cleaves RNA, destroying RNA genomes or inhibiting viral translation
- Mx pathway proteins are GTPases that inhibit RNA polymerase activity
What does the PKR pathway do?
inactivates translation initiation factor eIF-2, inhibiting viral protein translation
What does the 2-5A system do?
2-5A system activates RNase L which cleaves RNA, destroying RNA genomes or inhibiting viral translation
What does the Mx pathway do?
Mx pathway proteins are GTPases that inhibit RNA polymerase activity
Which cells respond first to viral infections?
NK cells, nonspecifically
Which cells are the major cellular response to primary viral infections?
T cells. They target infected cells for lysis though antigen presentation
How do antibodies help in viral infections?
they can neutralize virus binding or facilitate the lysis of enveloped viruses with complement.
Are children defective in antibody production more susceptible to viral infections and if so why?
No they are not. Cellular immunity is the main and dominant method of viral immunity
How do viruses evade immune defense mechanisms?
antigen variation, inhibition of antigen presentation, cytokine homologs that down regulate the cellular response, latent infection in neurons, where there is no MHC class I
Which viral response immunomolecules cause flu-like symptoms?
IFN and lymphokines
What part of the viral immune response causes inflammation?
T cells, macrophages and PMNs
What immune response causes immune complex disease?
antibody and complement systems
What viral immune response causes hemorrhagic disease?
T cells, antibody, and complement
What are the three types of antivirals?
- vaccines
- immune globulin
- drugs
When are vaccines not practical?
When there are a large number of virus strains or if the virus undergoes a lot of antigenic variation due to high mutation rates
What are the three basic types of vaccines?
- live, attenuated virus
- killed virus
- subunit (recombinant DNA)
Which type of vaccine causes subclinical infection?
attenuated vaccine
What are the advantages and disadvantages of attenuated virus vaccines?
advants: cheap and strong, long-lasting response (IgG, IgA, Tcell)
Disadvants: easily damaged, dangerous for immunocompromised patients, can revert to virulance in rare cases
Can killed virus vaccines cause illness?
no
What are the advantages and disadvantages of killed virus vaccines?
advants: very stable, rare side effects, cannot revert to virulance
disadvants: more expensive, shorter term immunity that is limited to mostly IgG
What type of vaccine are Hep B and HPV?
subunit vaccines.
How are subunit vaccines made?
composed of single viral proteins that are expressed in yeast using recombinant DNA tech
What are the advantages and disadvantages of subunit vaccines?
advants: cannot cause disease, are not derived from blood
disadvants: requires multiple injections.
What is immune globulin?
used in both pre and postexposure prophylaxis.
