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Virus, or “Poison”Virus, or “Poison”
Is it alive? Not really…Is it alive? Not really… Depend on cells to reproduce Depend on cells to reproduce obligate obligate
intracellular parasitesintracellular parasites Inert organic particles when outside cellsInert organic particles when outside cells Viruses for all - infect bacteria, protists, Viruses for all - infect bacteria, protists,
and all higher plants and animalsand all higher plants and animals Infect Infect specific cell typesspecific cell types in host in host
Virus size comparison at Cellsalive.com
Wendell Stanley – crystallized TMV (Tobacco Mosaic Virus), 1900’s,
showed that viruses are NOT small bacteria
Viruses - StructureViruses - Structure nucleic acid nucleic acid (RNA or DNA) – instructions (RNA or DNA) – instructions
for making proteinsfor making proteins protein protein capsidcapsid may have plasma membrane, or may have plasma membrane, or envelopeenvelope Surface proteins for attachment to host Surface proteins for attachment to host
cellcell Little or no Little or no metabolism:metabolism:
Who will read the DNA/RNA and make viral proteins that Who will read the DNA/RNA and make viral proteins that assemble into a viral capsid?assemble into a viral capsid?
If a virus has an envelope (membrane), where does it come If a virus has an envelope (membrane), where does it come from?from?
The host cell
The host cell’s plasma membrane
Influenza Virus
Surface proteins(protein)
(nucleic acid,instructions formaking proteins)
(membrane from host cell)
Criteria used to Classify Viruses
1.Type of nucleic acid 2.Presence or absence of an envelope3.Shape/morphology of the capsid4.Specific surface proteins different strains
Lytic Cycle:Cells lyses and releasesnew viral particles
Viral Replication by Host Cell
2. Entry
3. Replication
4. Assembly5. Lysis / Cytolysis
1. Adsorption
Lysogenic Cycle:Period of dormancyuntil stimulus causeslytic cycle
Viral Replication by Host Cell
Integration of viral DNA into host’s DNA Provirus (in this case a prophage)
Viral Replication by Host Cell
(compare & contrast based on lecture diagrams)Lytic Cycle Lysogenic Cycle
1. Viral DNA/RNA transcribed and translated by host cell
2. Cells lyse and releasesnew viral particles
1. Period of dormancyuntil stimulus causes cell to transcribe and translate viral DNA (viral DNA is incorporated into cell’s DNA)
2. Cells lyse & release new viral particles
RetrovirusesRetroviruses
RNARNA core core Replication – cells make viral Replication – cells make viral
DNA from RNA (reverse DNA from RNA (reverse order)order)
Enzyme: Enzyme: reverse reverse transcriptase transcriptase – virus contains – virus contains itit
Ex.: Ex.: HIVHIV and tumor producing and tumor producing virusesviruses
HIV inside humanT4 lymphocyte
Provirus (viral DNA in cell’s chromosome)
Retrovirus injects RNA and reverse transcriptase enzyme into host cell;
viral DNA is integrated into cell’s DNA (provirus forms) and may remain dormant for long various periods of time (lysogenic).
Reverse transcriptaseNew retroviruses
(Cystic Fibrosis)
Viruses can be used to insert genetic material into cells.Ex.: Gene therapy to cure genetic diseases
Possible Evidence of Virus EvolutionPossible Evidence of Virus Evolution
Today = Plasmid in BacteriaToday = Plasmid in Bacteria
Today = jumping genes in Corn
“Chunks” of DNA that can move around and be independent: Plasmids and Transposons
Viral Disease ExamplesViral Disease Examples Common coldCommon cold InfluenzaInfluenza RhinovirusRhinovirus Smallpox (eradicated)Smallpox (eradicated) Chicken poxChicken pox RabiesRabies Polio (still out there…)Polio (still out there…) MumpsMumps
RubellaRubella Ebola (90% Ebola (90%
lethality)lethality) Lassa feverLassa fever Dengue feverDengue fever Many other Many other
hemorrhagic hemorrhagic feversfevers
The list goes on The list goes on and on and on….and on and on….
Are Viruses Considered to be Living or Non-Living?
Characteristic No, Viruses do not display this trait Yes, Viruses do display this trait
Nucleic Acid
Growth
Mutate
Production of ATP
Cell Organelles
Homeostasis
Metabolism
IndependentReproduction
Other Infectious ParticlesOther Infectious Particles ViroidsViroids – single strand of RNA, no capsids – single strand of RNA, no capsids
Ex.: plant diseasesEx.: plant diseases
Prion causes proper proteins to fold incorrectly cannot do their ‘job’ in the cell,
collect inside cell cell death
PrionsPrions – infectious protein molecules – infectious protein moleculesEx.: CJD, scrapie, mad cow disease (BSE)Ex.: CJD, scrapie, mad cow disease (BSE)