Chapter 19Chapter 19

The components of a virus.The components of a virus. The differences between lytic and The differences between lytic and

lysogenic cycles.lysogenic cycles.

BacteriaBacteria VirusVirus

Prokaryotic cellProkaryotic cell Most are free-living Most are free-living

(some parasitic)(some parasitic) Relatively large sizeRelatively large size AntibioticsAntibiotics used to kill used to kill

bacteriabacteria

Not a living cell Not a living cell (genes packaged in (genes packaged in protein shell)protein shell)

Intracellular parasiteIntracellular parasite 1/1000 size of 1/1000 size of

bacteriabacteria Vaccines Vaccines used to used to

prevent viral infectionprevent viral infection Antiviral treatmentAntiviral treatment

VirusesViruses Very small (<ribosomes)Very small (<ribosomes) ComponentsComponents = = nucleic acidnucleic acid + + capsidcapsid

Nucleic acidNucleic acid: DNA or RNA : DNA or RNA (double or single-stranded)(double or single-stranded)

CapsidCapsid: protein shell: protein shell SomeSome viruses also have viruses also have viral viral

envelopesenvelopes that surround capsidthat surround capsid Limited Limited host rangehost range (eg. human cold (eg. human cold

virus infects upper respiratory tract)virus infects upper respiratory tract) Reproduce within host cellsReproduce within host cells

Mantra?

Define it!Explain it!Give an example!

Simplified viral replicative cycleSimplified viral replicative cycle

BacteriophageBacteriophage

Virus that infects bacterial cellsVirus that infects bacterial cells

Video: Video: t4 Phage infection

Watch this!Watch this!

Lytic Cycle of T4 Lytic Cycle of T4 PhagePhage

Bacteriophage ReproductionBacteriophage Reproduction Lytic Cycle:Lytic Cycle:

Use host machinery to make copies of virusUse host machinery to make copies of virus Death of host cell by rupturing it (lysis)Death of host cell by rupturing it (lysis) Virulent phages Virulent phages replicate by this methodreplicate by this method

Lysogenic Cycle:Lysogenic Cycle: Phage Phage DNA incorporated into host DNA and DNA incorporated into host DNA and

replicated along with itreplicated along with it Phage DNA = Phage DNA = prophageprophage

Temperate PhageTemperate Phage: uses both methods of : uses both methods of replicationreplication

Lytic Cycle vs. Lysogenic Lytic Cycle vs. Lysogenic CycleCycle

Animal virusesAnimal viruses have a membranous have a membranous envelopeenvelope

Host membrane Host membrane forms around forms around exiting virusexiting virus

Difficult for host Difficult for host immune system to immune system to detect virusdetect virus

Video: Video: How Dengue Virus enters a cell

Watch this!Watch this!

RetrovirusRetrovirus

RNA virus that uses RNA virus that uses reverse transcriptasereverse transcriptase (RNA (RNA DNA) DNA)

Newly made viral DNA inserted into Newly made viral DNA inserted into chromosome of hostchromosome of host

Host transcribes viral DNA (= Host transcribes viral DNA (= provirusprovirus) to ) to make new virus partsmake new virus parts

Example: HIV (Human Immunodeficiency Example: HIV (Human Immunodeficiency Virus)Virus)

HIV = HIV = RetroviruRetroviru

ss

HIVHIV◦ Infects white blood Infects white blood

cellscells◦ HIV+HIV+: provirus (DNA : provirus (DNA

inserted)inserted)◦ AIDSAIDS: active viral : active viral

reproduction reproduction

Video: Video: HIV Life CycleWatch this!Watch this!

Other Human VirusesOther Human Viruses Herpes virusHerpes virus SmallpoxSmallpox

Herpes Simplex Virus 1 (HSV-1)

Herpes Simplex Virus 2 (HSV-2)

Eradicated in 1979 due to worldwide vaccination campaigns

EbolaEbola TransmissionTransmission: contact with contaminated blood or : contact with contaminated blood or

bodily fluidsbodily fluids SymptomsSymptoms: fatigue, fever, severe headache, : fatigue, fever, severe headache,

vomiting, diarrhea, rash, bleedingvomiting, diarrhea, rash, bleeding◦ May appear 2-21 days after exposureMay appear 2-21 days after exposure

EbolaEbola TreatmentTreatment::

◦ Intravenous fluids, balance electrolytesIntravenous fluids, balance electrolytes◦ Experimental: antiviral drugs, plasma Experimental: antiviral drugs, plasma

transfusions from survivors, antibodies (Zmapp)transfusions from survivors, antibodies (Zmapp)◦ No vaccine (yet)No vaccine (yet)

StatisticsStatistics::◦ 2014 Ebola Outbreak 2014 Ebola Outbreak (worldwide): 21,382 (worldwide): 21,382

cases, 8474 deaths (*as of 1/19/15)cases, 8474 deaths (*as of 1/19/15)◦ Seasonal InfluenzaSeasonal Influenza: estimated 36,000 deaths : estimated 36,000 deaths

in U.S. each year (in U.S. each year (2015: mutated H3N2 strain2015: mutated H3N2 strain))

2014 Ebola Outbreak2014 Ebola Outbreak

Cumulative Ebola CasesCumulative Ebola Cases

VaccinesVaccines

Weakened virus or part of pathogen Weakened virus or part of pathogen that triggers immune system that triggers immune system responseresponse

Emerging virusesEmerging viruses = mutation of = mutation of existing virusesexisting viruses

ViroidsViroids Small, circular RNA Small, circular RNA

molecules that infect molecules that infect plantsplants

Cause Cause errors in errors in regulatory systems regulatory systems that control plant that control plant growthgrowth

eg.eg. coconut palms in coconut palms in PhilippinesPhilippines

PrionsPrions Misfolded, infectious Misfolded, infectious proteinsproteins that that

cause misfolding of normal proteinscause misfolding of normal proteins eg. eg. mad cow disease mad cow disease

(BSE),Creutzfeldt-Jakob disease (BSE),Creutzfeldt-Jakob disease (humans), scrapie (sheep)(humans), scrapie (sheep)

Diseases caused by prionsDiseases caused by prions Prions act slowly Prions act slowly – incubation period of at – incubation period of at

least 10 years before symptoms developleast 10 years before symptoms develop Prions are virtually indestructiblePrions are virtually indestructible (cannot (cannot

be denatured by heating)be denatured by heating) No known cure for prion diseasesNo known cure for prion diseases