IN: IN:

►Discuss the following two questions Discuss the following two questions with your group. with your group.

What is a virus?What is a virus? (Come up with a definition.)(Come up with a definition.)

Are viruses alive? Are viruses alive? (Be prepared to defend your answer.)(Be prepared to defend your answer.)

Characteristics of LifeCharacteristics of Life

►Are made of one or more cellsAre made of one or more cells►ReproduceReproduce►Grow and developGrow and develop►Obtain and use energyObtain and use energy►Respond to their environmentRespond to their environment

Viruses = “Viruses = “poisonpoison” ” (Latin); infectious (Latin); infectious

particlesparticles

VirusesViruses►Not Not living (???)living (???) organisms; not in 6 organisms; not in 6

kingdomskingdoms►Lack Lack cell structurescell structures for growth and for growth and

reproductionreproduction►Must replicate in a Must replicate in a living hostliving host►Smaller than bacteria; need electron Smaller than bacteria; need electron

microscope to seemicroscope to see

StructureStructure►Nucleic Acid: Nucleic Acid: DNADNA or or RNARNA, single or , single or

double double strandedstranded►Wrapped in a protein coat called a Wrapped in a protein coat called a

capsidcapsid►Some have an Some have an envelopeenvelope (taken from host (taken from host

cell membrane)cell membrane)

SymmetrySymmetry► Icosahedral – Icosahedral – 20 +20 + sides sides►Helical Helical (Spiral)(Spiral)► Complex (Complex (e.g. head e.g. head & tail or& tail or

enveloped)enveloped)

ClassificationClassification

►Based on… Based on… ►1. 1. RNA or DNARNA or DNA►2. 2. Capsid Capsid ((size, symmetry, presence of size, symmetry, presence of

envelope)envelope)

DNA VirusesDNA Viruses

1.1. DNA RNA proteinsDNA RNA proteins

2.2. Join to host DNA RNA proteinsJoin to host DNA RNA proteins

RNA VirusesRNA Viruses

1.make proteins 1.make proteins directlydirectly from RNA from RNA2.RNA (2.RNA (reverse transcriptase)reverse transcriptase) DNA RNA DNA RNA

proteinsproteins

- - seen in retrovirusesseen in retroviruses

ReproductionReproduction►Obligate intracellular parasites, gain Obligate intracellular parasites, gain

control of control of host cell’s machineryhost cell’s machinery

1.Lytic Cycle 1.Lytic Cycle (all viruses)(all viruses)

--virus attaches to host cell, virus attaches to host cell, injects injects nucleic acidnucleic acid

-DNA/RNA replicates utilizing host cell’s -DNA/RNA replicates utilizing host cell’s enzymesenzymes

-host -host rupturesruptures,, releasing 100’s of releasing 100’s of virusesviruses..

The Lytic cycleThe Lytic cycle

Lysogenic Cycle Lysogenic Cycle (some viruses – HIV, herpes) (some viruses – HIV, herpes) Virus attaches to host cell, injectsVirus attaches to host cell, injects nucleic nucleic

acidacid Viral nucleic acid inserts into Viral nucleic acid inserts into host’s host’s

DNA (provirus)DNA (provirus) Provirus replicates whenever the host cell Provirus replicates whenever the host cell

reproducesreproduces No new No new virusesviruses are produced at this time are produced at this time The provirus does not The provirus does not harmharm the host cell the host cell Upon “activation” (e.g. Upon “activation” (e.g. radiation, radiation,

weakened state, chemicals),weakened state, chemicals), provirus provirus takes over the cell and resumes takes over the cell and resumes lytic cyclelytic cycle..

The Lysogenic CycleThe Lysogenic Cycle

DiseasesDiseases► Spread by air, Spread by air, human contact, insects, animals, and human contact, insects, animals, and

foodfood► Common diseases: flu, cold, chickenpox, measles, Common diseases: flu, cold, chickenpox, measles,

herpes, hepatitis Aherpes, hepatitis A► Serious Diseases: Smallpox, polio, AIDS, hepatitis CSerious Diseases: Smallpox, polio, AIDS, hepatitis C► Some Viruses (e.g. hepatitis B, human papillomavirus)Some Viruses (e.g. hepatitis B, human papillomavirus)

1. Carry 1. Carry oncogenesoncogenes (genes that cause cancer) or (genes that cause cancer) or 2. Stimulate host oncogenes2. Stimulate host oncogenes

► FewFew antiviral drugs antiviral drugs► ImmuneImmune system provides protection; e.g. white blood system provides protection; e.g. white blood

cells, lymohocytes, fever, interferons (interfere with cells, lymohocytes, fever, interferons (interfere with synthesis of viral proteins)synthesis of viral proteins)

► Vaccines Vaccines are made from weakened form of virus; are made from weakened form of virus; stimulate immune responsestimulate immune response

Viral DiseasesViral Diseases

►WartsWarts►HerpesHerpes► Chicken poxChicken pox► SmallpoxSmallpox► The Common ColdThe Common Cold► Influenza (flu)Influenza (flu)► RabiesRabies► AIDSAIDS► CancerCancer

chickenpox

shingles

herpes

smallpox

ViroidsViroids

► Recently isolatedRecently isolated► Viroids = Viroids = naked RNAnaked RNA segments with no protein segments with no protein

coat or envelope, smallest known particles to coat or envelope, smallest known particles to replicate; infect plantsreplicate; infect plants

PrionsPrions► Prions = infectious proteins with Prions = infectious proteins with no no

nucleicnucleic acidacid, clump together inside of cell , clump together inside of cell and eventually and eventually destroy itdestroy it;;

► Cause slow, progressive disease; e.g. kuru, Cause slow, progressive disease; e.g. kuru, “mad cow” disease, Creutzfelt-Jakob “mad cow” disease, Creutzfelt-Jakob diseasedisease

Remember Viruses !!!!Remember Viruses !!!!

►Not considered living things by many scientists…….

►1. No Growth►2. No Homeostasis►3. No Metabolism►4. Reproduce only in a host cell►5. Only Protein and Nucleic Acid

Studying VirusesStudying Viruses

1.1. Virus: “Virus: “poisonpoison” ” Latin); infectious Latin); infectious particlesparticles

2.2. Wendell Stanley Wendell Stanley was the first to was the first to isolate a virus in isolate a virus in 1935 by 1935 by crystallizationcrystallization