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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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Prions
Normal cell protein
Misfolded version: prion
www.stanford.edu/.../2004anderson/index.html
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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
• 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 act
• Using antibodies as tags
• Quickly before B &T cells ready
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Pathogenic Viruses
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Name of virus
what family it belongs to
what disease it causes, organ system affected
DNA or RNA?
Route of transmission; reservoirs, vectors
Viral virulence factors (selected viruses)
Immunizations
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A molecular biology lesson
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• RNA mutates more readily than DNA
– Copying mistakes by the enzymes are not corrected
– Not double stranded, so no mismatches noticed that
can be fixed.
– Many variants result
– Less chance of lasting immunity
– Harder to create vaccines
HIV: Human Immunodeficiency Virus
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• Host range
– Main types of cells infected: T helper cells and
dendritic cells (including macrophages, microglia)
• Have CD4 and CCR5 glycoproteins on surface
• Infection process
– RNA is copied into cDNA by reverse transcriptase
– cDNA inserts into host chromosome
– New RNA made
– Protein precursor made, then processed; assembly
occurs
– Virions bud through cell membrane
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HIV life cycle
1. Binding to host cell
2. Copying RNA into
DNA by Rev Trans
3. Integration into host
DNA
4. Transcription
5. Translation
6. Assembly and exit
by budding
www.aidsmeds.com/images/HIVLifeCycle1.gif
Disease process
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• Chronic infection
– T cells continually made, continually destroyed
– Eventually, host loses
• AIDS diagnosis:
– Acquired Immuno Deficiency Syndrome
• CD4 cell count below 200/µl;
• opportunistic infections
• Examples of opportunistic “infections”
– Pneumocystis carinii pneumonia (PCP pneumonia)
– Kaposi’s sarcoma; Tuberculosis; several others
Prevention and Treatment
• Prevention is easy
– Practice monogamous sex, avoid shared needles
– HIV cannot be spread by casual contact, skeeters
• Drug treatment
– Nucleoside analogs such as AZT
– Protease inhibitors prevent processing of viral
proteins
Nifty animation at:
http://www.hopkins-aids.edu/hiv_lifecycle/hivcycle_txt.html
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