Chapter 6: Viruses and Prions Flashcards
Viruses
submicroscopic (itty bitty); always infectious; acellular; obligate intracellular pathogens
Zoonotic Infections
infections/viruses that was originally in an animal and then spread to humans
Bacteriophages
or phages; viruses that infect bacteriaA
Animal Viruses
viruses that infect animals and humans
Virion
single, infectious virus particle; have an exterior protective protein capsid; contain genetic material (DNA or RNA)
Capsid
protein shell that packages and protects the genome; accounts for the bulk of a virions mass; made of capsomere subunits
Helical Capsids
look like a hollow tube
Icosahedral Capsids
look like three dimensional polygons
Complex Capsids
deviations from these two structures
Bacteriophages exhibit a complex capsid structure…
usually have capsids with icosahedral symmetry; often their capsids are associated with additional complex structure tha enable them to inhect their genome into target cells
Enveloped VIruses
have a lipid based envelope that surround the capsid; arise from budding off the host cell (take a portion of the phospholipid bilayer with them)
Naked Viruses
or nonenveloped; lack an envelope; arise from lysing (bursting) the host cell
Animal viruses are either…
enveloped or naked
Bacteriophages lyse cells so…
they are always (usually) naked
Spikes
or peplomers; may protrude from the viral capsid or envelope; glycoprotein extensions that help viruses attach and gain entry to host cells; only bind to specific factors on a given host cell (host specificity and tissue tropism)
Tissue Tropism
the ability for a virus to infect one type of tissue/ cell (say lung) and then migrate and infect other parts (say skin cells)
Influenza A Spikes
hemagglutinin (HA); neuraminidase (NA)
Viral Genes Encode
capsomere proteins; enzymes needed for viral replication; structural factors
Viral Genomes can either be…
RNA or DNA; single or double stranded; single or segmented sections; circular or linear
What is the goal of all viruses?
to get a host cell to make viral proteins, so more virions can be built
Double Stranded DNA Virus
dsDNA; viral DNA is transcribed using host RNA polymerases; mRNA is then translated into protein
Single Stranded DNA Viruses
ssDNA; converted to a double stranded form before transcription
Single Stranded Positive RNA
ssRNA+; ssRNA genome functions as an mRNA; directly translated by host cell ribosomes
Single Stranded Negative RNA
ssRNA-; RNA genome is complementary to mRNA; transcribed into mRNA by RNA dependent RNA polymerases (RdRPs)
Single Stranded Retroviruses
RNA genome is made into DNA by reverse transcriptase; DNA is usually inserted into the host DNA; DNA is then transcribed into mRNA
Double Stranded RNA Genome
dsRNA; transcribed to make mRNA; requires RNA dependent RNA polymerases
Why did viruses exhibit a faster rate of genomic change than living infectious agents?
quick replication time; large quantity of virions are produced; RNA genomes mutate more than DNA (DNA polymerases have proofreading capabilities and RNA polymerases do not)M
Mutations can be…
neutral, beneficial (rarely), or deleterious (normally)
Attenuated Strains
genetic changes that limit infectivity; used in vaccines
Beneficial Mutations may allow the Virus to…
escape host immune sustem detection; broaden host range; expand tropism (the type of cells or tisses the virus infects); increase infectivity
Reassortment
may occur when two different viral strains coinfect a single host cell; leads to new viral strains; ex) influenza virus;
Antigens
things that trigger the immune system response; such as spikes on a virus
Antigenic Drift
frequent and small changes; for influenza it changes HA and NA spikes; eventually leads to a different enough virus that the immune system does not have antibodies to
Antigenic Shift
major genetic reassortment; does not take a long time it is sudden; can lead to increased infectivity or expanded host range; people have no residual immune protection from infections or vaccinations because it is a new strain; sets the stage for a pandemic
Viruses are grouped by the following properties…
1) type of nucleic acid present (DNA or RNA); 2) capsid symmetry (helical, icosahedral, or complex); 3) presence or absence of an envelope; 4) genome architecture (ssDNA, ssRNA, etc)
Host Range
refers to a collection of species that a virus can infect; some viruses infect more than one species, while others infect only one species; ex) measles can only infect humans
Tropism
refers to the tissues or cell specificity (due to viral surface factors); some viruses can infecct a wide range of host cells or tissues (broad tropism); other virsues can infect only one type of host cells or tissue (narrow tropism)
What viruses are the smallest?
rhinoviruses and polioviruses; 30nm
What is one of the largest viruses?
pithovirus; length of 1500nm
What is the highest taxon for viruses?
phylum; they do not have a domain or kingdom
Bacteriorphage Lytic Replication
1) Attachment (adsorption, phage binds); 2) Penetration (Entry, phage inhect genetic material); 3) replication (synthesis, commandeers host cell); 4) Assembly (maturation, genome packed and structures assemebled); 5) release (host cell lyses and new phages released)
Lysogenic Replication
done by temperate phages; attachment; penetration; phage genome is oncorporated into host cell genome forming a prophage; as the cell divides it copies the prophage; host cell is stressed, the prophage may excise itself from host (bc it will die if the host dies); phage enters the lytic repliation pathway
What are the 6 steps of Animal Virus Replication
attachment, penetration,m uncoating, replication, assembly, and release
Acute Infections
viruses infect a host cell and new virions are made immediately
Persistent INfections
virsues have replication strategies that allow them to avoid immune system clearance; chronic or lantent
Chronic Infections are characterized by…
continous release of virions over time (months or years); slow progressions of disease
Provirus
integration of viral genome into the host cell
Latent Infections
distinguished by flare-ups with intermittent periods of dormancy (latency); during flare up- virions are shed, person experiences symptoms; flare ups can be triggered by stress (fever, sunburn, hormone level changes)
Herpesviridae Family
notorious for causing latent infections; Human herpes virus 1 causes cold sores; HSV 2 causes genital herpes; Variculla zoster virus (HHV3) causes chicken pox and shingles
Oncogenic Viruses
cause 10-15% of cancers; cause cancer by stimulating uncontrolled host cell division and/or decreasing host cell responsiveness to death signals; ex) human papilloma viruses (HPVs) and Human T-Lymphotropic Viruses
Growing Bacteriophages
can be grown in the lab with realitve ease
Plaque Assays
bacteria are gown on a petri plate; pahge infect cells then lyse out of the cell; infect adjacent cells; lysed cells leave a clear zone (plaque)
Plaque Forming Units
are the quanity of bacteriophages in an initial volume of sample
Viral Titer
the quantity of virus present in a given volume of sample
Growing Animal Viruses
animal viruses are more difficult to cultivate than bacteriophages; most animal virsues are grown using tissue culture techniques; live anime hosts (mice, rats, guinea pigs) may be required to support growth of some viruses; embryonted eggs (fertilized eggs) are also useful for growing certain viruses
Specificity
means that the test only detects the virus(es) of interest (no false positives)
Sensitivity
means the test detects very low levels of the target (no false negatives)
Agglutination Tests
purified antibodies linked to tiny latex beads; mixed with the sample; antibodies bind the viral antigen; beads agglutinate
Latex Agglutination Tests
viral antigens linked to tiny latex beads; mixed with the sample; patient antibodies bind the viral antigen; beads agglutinate
Enzyme Linked Immunosorbent Assays (ELISA)
can be adapted to detect either anitgens or antibodies in a sample; target adheres to a surface; change of color indicated binding
Limitations of ELISA and Agglutination Assays
sample being tested must be a liquid; antigens must be fairly well characterized; virsues can undergo an antigenic shift making it no longer detectable; takes time to build up detectablt anitbodies (seroconversion window) ; therefore it is helpful to use a combination of detection methods
Detecting Viral Genetic Material
to perform the test: collect a clinical sample (sputum, blood, cerebrospinal fluid or tissue); DNA and RNA are extracted; very specific segments of viral nucleic acid are detected by fluorescent-labeled probes, sequencing, PCR (polymerase chain reaction)
Difficulties when designing antiviral drugs
viruses are obligate intracellular pathogens; antivirals should be slectively toxic; viruses have fewer chemically distinct targets than living pathogens
Antiviral Drugs treat but do not cure
only limit infections rather than cure them; there are a few effective antiviral agents; prevention of serious viral disease through vaccination is important; vaccines train the immune sys to recognize viruses and are an effective means to limit infection
Postexposure Prophylaxis
laboratory-prepared mixture of injectable antibodies prevents viruses from binding and entering host cells; used shortly after suspected exposure; used to treat rabies and HIV
Docosanol
blocks viral entry into host cells; ised to treat cold cores caused by HHV-1
Palivizumab
injectable antibody preparation; blocks fusion of the respiratory syncytial virus (RSV)
Nuceloside Analogs
block replication; at least a dozen drugs in this class; activated into compounds that mimic normal nucleotides (A,G, C, T, and U); chemical dead end for nucleic acid replication
Acyclovir
example of nucleoside analogs; inhibits DNA replication; effective agianst HHV-1, 2 or caricella zoster vrius
Ribavirin
example of nucleoside analogs; targets RNA polymerases; effective against respiratory syncytial virus and hepatitis C virus
Nucleoside Reverse Transcriptase Inhibitors
NRTIs; target reveerse transcriptase enzymes; azidothymidine
Anitsense Antivirals
shorts sequences of nucleotides complementary to the viral RNA; bind to viral RNA inhibiting translation; targeted RNA is destroyed by cellular enzymes; ex) vitrave
Interferons
naturally occuring substances released by cells in response to viral infections; signla the presence of a virus; neighboring, uninfected cells make defensie changes that limit viral entry and replication; can be produced in the lab and adminitered to help limit the pregression of certain viral infections
Oseltamivir and Zanamivir
tamiflu and relenza; prevent influenza a and B virions from budding off the host cell curface
Prions
infectious proteins; no genetic material; do not replicated; cause transmissible spongiform encephalopathies (TSEs)
Types of Spongiform Encephalopathies
can be inherited or acquired; gerstmann straussler schienker syndrome; fatal familial insomnia (both inherited); creuzfeldt jakob disease (CJD)
Some Neurodegenerative Disease exhibit Prion-like Features
been associated with misfolded proteins in thebrain; alsheimers disease; parkisons disease; amyotrophic laterla sclerosis
