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

Edward Jenner – 1700’s, Smallpox vaccine

Vaca = Cow,

noticed milkmaids immune to smallpox

Louis Pasteur – rabies vaccine, 1800’s

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

Gene Therapy for CF

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)

Prion Disease – ‘holes’ in brain tissue