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Viruses: intimate parasites
• Are viruses alive?
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Not made of cells, in violation of Cell Theory
Do not grow (but self assemble)
Do not metabolize (but steal energy)
Cannot reproduce w/o a host cell (but other
organisms may require another species in order to
reproduce)
– Can evolve over time
– Some can respond to environmental stimuli
– Have a complex, organized structure
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If not cells, then what?
• Viruses are particles
• Some components are essential
– A genetic material with the blueprint for making more
• Could be: ds DNA, ss DNA, ss RNA, ds RNA
• Space is limited, so genes are few
– A covering to protect the genetic material
• Capsid, made of one or more proteins
• Capsid + nucleic acid: nucleocapsid
• Viruses are obligate intracellular parasites
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Viruses may have other parts
• Envelope: piece of organelle membrane or cell
membrane covering capsid
– Virus is formed by budding, pushes through membrane
taking a piece.
– Viral envelope usually contains viral proteins.
– Envelope makes virus susceptible to some disinfectants
• Spikes (peplomers) extend from envelope
– Used for attachment, escape
• Accessory enzymes
– Reverse transcriptase, RNA
RNA enzymes
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Viral size and shape
• Viruses range from 20 nm to 300 nm
– Ribosomes are about 30 nm
– The smallest known bacteria are about 200 nm
• Viral shapes:
– helical, polyhedral, and complex
http://www.glencoe.com/qe/images/b136/q4323/ch18_0_a.jpg; www.blc.arizona.edu/.../
Figures/Icos_Virus.GIF; http://www.foresight.org/Updates/Update48/Images/T4Schematic.jpg
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Examples of virus shapes
Ebola
Adenovirus
http://www-cgi.cnn.com/HEALTH/9604/16/nfm/ebola.levine/ebola.reston.large.jpg;
http://www.virology.net/Big_Virology/EM/Adeno-FD.jpg
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Specificity
• A recurring theme in biology:
– Enzymes, membrane receptors, antibodies, etc.
• Viruses are limited to certain types of host cells
– Species barriers: rabies not specific, but most are
– Tissue type: rabies specific to nerve, salivary tissue
– Cell type: HIV infection mostly restricted to Helper T
cells, a kind of lymphocyte.
• Different characteristics of host cells involved
– Attachment to cell surface often a major point
• Every type of organism has a virus that infects it?
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Viruses across kingdoms
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• A densovirus newly isolated from the smoky-brown
cockroachPeriplaneta fuliginosa
• Acanthamoeba castellanii Promotion of In Vitro
Survival and Transmission of Coxsackie B3 Viruses
• The causal organism is the Tulip Breaking Virus
(TBV). The pathogen is a potyvirus and is divided
into two strains,
• Genome characterization of Botrytis virus F, a
flexuous rod-shaped mycovirus
Classification
• Microbes problematic, viruses especially so.
– No sexual reproduction, no asexual reproduction, just
“assembly”.
– No clear evolutionary relationships
• Classification scheme (from David Baltimore)
– First, by nucleic acid type, e.g. ds DNA, + sense RNA
– Next, structural characteristics (presence of envelope,
capsid shape), type of organism infected, etc.
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Life cycle of a virus
• Manner of infection and reproduction depends on whether
host is prokaryotic or eukaryotic.
• Life cycle here outlined is general:
– ADSORPTION: following contact,
molecules on surface of virus bind
to particular molecules on host cell.
– PENETRATION: the nucleic acid
must get access to the machinery
of the cell to replicate.
http://www.tthhivclinic.com/Fig_20.gif
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Life cycle continued
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• SYNTHESIS/REPLICATION: once inside the
nucleic acid issues orders leading to
– Replication of the nucleic acid
– Transcription (usually) and translation, producing the
necessary capsid proteins.
• ASSEMBLY: a spontaneous process
– Capsid proteins and nucleic acid combine to make virion.
– Cheap but highly inefficient process.
• RELEASE: successful parasite must spread to others
– Virus causes lysis of cell or pushes through cell
membrane. Virions may acquire an envelope.
Bacteriophage: lytic vs. lysogenic
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• Most bacteriophages multiply then lyse the host cell
– This life cycle is called a lytic cycle
• Others are “temperate”, enter a lysogenic cycle.
– Lysogeny is an effective way to multiply the viral DNA
– Viral DNA inserts into the bacterial chromosome
• Now called a “prophage”
• Bacterial replication also replicates viral DNA
• Prophage may bring new genes for use by bacterium
– Damage to bacterial DNA (e.g. UV) prompts virus to
begin lytic cycle; DNA excises, virus multiplies.
Measuring numbers of virions:
the Plaque Assay
•Virus and host cells are mixed
•Bacteria cover in a Petri dish as a “lawn”,
eukaryotic cells cover bottom of a dish.
•Multiplication of virus leads to release,
spread to and destruction of nearby cells.
•Visible as holes, plaques, on bacterial
lawn; eukaryotic cells in culture are first
stained for easier view to see plaques.
http://dept.kent.edu/biology/Courses/30171/imageQBT.JPG;
news.bbc.co.uk/.../ _230333_cell_culture_300.jpg;
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Growing viruses
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• Obligate intracellular parasites: require a host cell!!
• Whole organism
– Animal models, human volunteers
• Ethically, humans require consent, safety, pay
• Eggs: aseptic incubator
– Various cells and membranes support growth of viruses
– Shell provides protection from bacterial contamination
– Used for large batches of viruses for vaccines
• Egg allergies a problem sometimes
http://www.fao.org/ag/againfo/foto/egg-facts.gif
Growing viruses continued
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• Organ/tissue/cell culture
– Minced tissue, separated into cells by enzyme treatment
– Grown in sterile plastic dish with nutrient solution
– Cells prepared this way grow until dish is covered, stop.
• Scrape up, use some to inoculate new culture
• Limited number of rounds of replication
– Transformed cells, with cancer properties, grow forever.
• Must be subcultured when dish bottom is covered
• Cell culture major reason for advances in virology
Viruses and disease
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• Most of discomfort, some of damage, is due to host
defense response to viral infection.
• Cytopathic effects: damage caused by viruses
– Cells change shape, change size (round up, swell)
– Cells become sticky (clump, fuse to form syncytia)
• Syncytia are giant, multinucleated cells
– Cells change internally (vacuoles, inclusions)
• Location, appearance of inclusions characteristic of
certain viruses; comprised of viral parts or cell debris
– Cells die.
Other effects of viruses
• Transformation: cells become cancer-like
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Lose contact inhibition
Continue to multiply
In vivo, produce tumors
Certain Herpes-type viruses, genital wart virus, etc.
• Some viruses cause birth defects = teratogenesis
– Damage to critical cells in fetus, failure to form parts
– Typically, virus is mild and able to cross placenta
– Rubella (German measles virus), CMV
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NOT viruses
• Viroids: naked RNA
– Cause of infectious disease in plants
• Prions: infectious protein particles
– Got Stanley Prusiner the Nobel Prize, but still
controversial.
– Cause of Mad Cow Disease, Scrapie, Elk Wasting
disease, Creutzfeldt-Jakob disease, Kuru, etc.
– Transmissible spongioform encephalopathies
– Misshapen protein causes normal protein to also fold
incorrectly, increasing their numbers
– Replication without the presence of nucleic acid
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Attack and defense
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• Virus has ways of attacking
– Specific binding to host cell receptor
• Receptors involved in normal cell functions
– Virus may regulate cell division for its own
replication
– Insertion of viral DNA into chromosome allows virus
to hide from immune system
– Causes adjacent cells to fuse, allows cell to cell
spread without leaving cell.
Defense by host
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• Host has ways of defending
– Cell mediated immunity (T cells): infected cells
killed.
– Antibodies intercept virions between cells, in fluids
– Interferon produces anti-viral state, prevents
replication
– Natural killer cells activated
A molecular biology lesson
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• DNA is copied faithfully
– DNA polymerase has 3’ to 5’ exonuclease activity, a
“backspace key” which deletes mistakes.
– Other mechanisms exist to maintain fidelity.
• RNA fidelity is not maintained
– RNA polymerase does not backspace
– Methods for monitoring RNA don’t exist
• Many RNA viruses show high mutation rate
– Many variants, immunity difficult.
Picornaviruses
• Small RNA viruses (“pico” = very small)
– About 25 nm, near the size of a ribosome
– Two kinds
• Enteric viruses
– includes Hepatitis A and polio
• Only some cases of polio result in paralysis
– Cause of many cases of “stomach flu”
• Rhinoviruses: major cause of common cold
– Rhino means nose
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The Common cold
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• Rhinoviruses have many serotypes
– Variants, caused by easy mutation of RNA
– Immune system can’t recognize all differences, but
some protection with age.
– Multiplies in narrow temperature range, nose/sinus
cooler than body temperature
• Other cold viruses
– Coronavirus (best known cousin causes SARS)
– Adenovirus (DNA virus), some serotypes cause GI
infections
Foodborne viruses
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• Norovirus (Norwalk)
– Restaurants, nursing homes, cruise ships
• Rotavirus- similar transmission
• Enteric Picornaviruses
– Hepatitis A virus
• Attacks GI especially liver
– Poliovirus – famous for paralysis, but only in a small
percentage; target for extinction.
• Spread by fecal-oral route
Arkansas Arboviruses
• Not an official taxonomic group, but short for
“arthropod-borne”
– Includes Flaviviruses, Togaviruses, and others.
– Zoonotic, spread from animals to people by
arthropod vectors, especially mosquitoes.
• Reservoirs may be birds, various mammals
– Result in two main types of illnesses
• Encephalitis, inflammation of the brain
• Hemorrhagic fever: high fever with bleeding
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Arkansas Arboviruses
• Encephalitis: spread by skeeters
– Eastern Equine encephalitis;
• Togavirus; summer 2005, outbreak in NE US
• Also infects, kills horses. Most dangerous.
– St. Louis encephalitis,
• Flaviviral diseases; Human disease.
• Usually not serious.
– West Nile virus
• Flavivirus; imported to US, spread from NYC
• Disease mostly in young and elderly
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Rabies
• Rhabdovirus- bullet shaped RNA virus
• Reservoir: animals of family Carnivora
– Foxes, bats, skunks, racoons, dogs and cats
• Virus elicits change in behavior of host
– Virus multiplies in salivary glands and brain
– Animals become agitated and bite, spread virus
– Humans show “hydrophobia”
• Long incubation, slow development
– Depends on dose, location.
– Vaccine AFTER inoculation
– # of survivors w/o treatment: 5
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Structure of rabies virus
http://www.rabies.net/_img/pict/pict_40.rabies_virus_structure__bullet_shape_model.jpg
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