VirusesViruses

I. What are they (and what aren’t they)?I. What are they (and what aren’t they)?

II. Virus structure and classificationII. Virus structure and classification

III. Viral infectionIII. Viral infection

HerpesvirusHerpesvirus

Foot and Mouth Disease virusFoot and Mouth Disease virus

Ebola virusEbola virus

Characteristics of VirusesCharacteristics of Viruses

Somewhere between life and non-lifeSomewhere between life and non-life

Can’t reproduce or __________________ independentlyCan’t reproduce or __________________ independently

But But cancan direct living cells to do so for them direct living cells to do so for them

““___________________ intracellular parasites”___________________ intracellular parasites”

““Infectious particles” rather than “organisms”, “active” or Infectious particles” rather than “organisms”, “active” or

“inactive” rather than “alive” or “dead”.“inactive” rather than “alive” or “dead”.

Bacteriophage Phi X 174Bacteriophage Phi X 174

General StructureGeneral Structure

• Small -- _______________ nm Small -- _______________ nm

in diameterin diameter

• All have All have nucleocapsidnucleocapsid

composed of ______________ composed of ______________

and nucleic acidand nucleic acid

• Some have a phospholipid Some have a phospholipid

bilayer membrane (bilayer membrane (envelopeenvelope))

NucleocapsidNucleocapsid

DNA or RNADNA or RNA

CapsidCapsid

Enveloped virusEnveloped virusSpikeSpike

EnvelopeEnvelope

Viral envelopeViral envelope

• Common in animal and plant virusesCommon in animal and plant viruses

• Membrane is required for ______________Membrane is required for ______________

• Viral envelopes are actually portions of the Viral envelopes are actually portions of the

infected host cell _____________________ infected host cell _____________________

(viruses do not form their own)(viruses do not form their own)

• Often replace ______________ proteins Often replace ______________ proteins

with with viralviral glycoproteins glycoproteins

Nucleocapsid architectureNucleocapsid architecture

Two most common shapes: Two most common shapes: helicalhelical & &

icosahedralicosahedral. .

Both are regular, geometric shapes determined by Both are regular, geometric shapes determined by

the structure and orientation of proteins in the the structure and orientation of proteins in the

________________________________________________

Helical structureHelical structure

Capsomeres

(___________

building blocks)

Virus _________

Helical structure: Tobacco Mosaic Virus Helical structure: Tobacco Mosaic Virus

TMV is TMV is

composed of composed of

only_________ only_________

types of types of

molecules: a molecules: a

single ________ single ________

molecule and molecule and

one type of one type of

proteinprotein

Icosahedral structureIcosahedral structure

Note that in both of these architectures, only one or two types of Note that in both of these architectures, only one or two types of

proteins used. Means that the virus needs only one or two proteins used. Means that the virus needs only one or two

protein-coding ________________.protein-coding ________________.

Composed of Composed of only one or two only one or two different proteins different proteins + nucleic acid+ nucleic acid

AdenovirusesAdenoviruses -- non-enveloped icosahedral viruses which are generally -- non-enveloped icosahedral viruses which are generally

mild pathogens in humans, birds, etc. causing ___________________ mild pathogens in humans, birds, etc. causing ___________________

(‘pink eye’) and _________________ illnesses.(‘pink eye’) and _________________ illnesses.

Complex virusesComplex viruses

A number of types with somewhat more ___________ A number of types with somewhat more ___________ structuresstructures

One is typical bacteriophage, with icosahedral head, helical One is typical bacteriophage, with icosahedral head, helical tail, fibers for attachmenttail, fibers for attachment

Bacteriophage Bacteriophage

lambdalambda

Nucleic acid contentNucleic acid content

Viral genomes are very smallViral genomes are very small ___________ base pairs (enough to encode 3 or ___________ base pairs (enough to encode 3 or

4 proteins) to 200,000 bp4 proteins) to 200,000 bp

Small compared to bacteria (~10Small compared to bacteria (~1066 bp) and bp) and

human (~2 X 10human (~2 X 1099 bp). bp).

Characterized by __________________ genesCharacterized by __________________ genes

Viral genomic DNAViral genomic DNA

mRNA transcripts mRNA transcripts (overlapping)(overlapping)

Viral genomes. The genomes of viruses can be either _________ or

________ (or some use both at different stages in their life cycle)

Viral genomes. The genomes of viruses can be either _________ or

________ (or some use both at different stages in their life cycle)

E. coli bacteriophagesE. coli bacteriophages

Viral classificationViral classification

Viral classification (cont.)Viral classification (cont.)

Viral Infection -- OverviewViral Infection -- Overview

1) Adsorption1) Adsorption

2) ____________ (either of nucleocapsid or 2) ____________ (either of nucleocapsid or

of nucleic acid only)of nucleic acid only)

3a) Viral _______________ (lytic phase) 3a) Viral _______________ (lytic phase) oror

3b) 3b) IntegrationIntegration of viral genome into host cell of viral genome into host cell

genome (lysogenic phase)genome (lysogenic phase)

4) Exit from cell4) Exit from cell

AdsorptionAdsorption

Specific interaction of viral surface with cell Specific interaction of viral surface with cell

surface.surface.

This is the reason viruses only infect certain cell This is the reason viruses only infect certain cell

types, for example: types, for example: HIV only infects cells (such as T cells) with the CD4 HIV only infects cells (such as T cells) with the CD4

glycoprotein on their surface.glycoprotein on their surface.

Influenza virus (an orthomyxovirus) only infects Influenza virus (an orthomyxovirus) only infects

mucosal cells of upper ____________________ tract.mucosal cells of upper ____________________ tract.

Bacteriophages are highly ___________________ for Bacteriophages are highly ___________________ for

certain hosts (e.g. coliphages only infect certain hosts (e.g. coliphages only infect E. coliE. coli))

EntryEntry

1) ______________ of nucleic acid (e.g. T4 phage)1) ______________ of nucleic acid (e.g. T4 phage)

2) Entry of ___________ nucleocapsid (most 2) Entry of ___________ nucleocapsid (most

common in animal and plant viruses)common in animal and plant viruses)

EndocytosisEndocytosis::

Virus in vescicleVirus in vescicle

Vesicle and capsid break down, releasing nucleic acidVesicle and capsid break down, releasing nucleic acid

Cell membraneCell membrane

Membrane ____________ (only in enveloped viruses)Membrane ____________ (only in enveloped viruses)

OutOut

InIn

OutOut

InIn

OutOut

InIn

InIn

OutOut

Cell membraneCell membrane

Viral replicationViral replication

• Once inside, one of several things occurs, Once inside, one of several things occurs, depending on the virus.depending on the virus.

• However, the end result is always ________ However, the end result is always ________ ________________________________

• Involves:Involves:1) Replication of the nucleic acid1) Replication of the nucleic acid

2) Synthesis of capsid proteins2) Synthesis of capsid proteins

3) ________________ of nucleocapsids3) ________________ of nucleocapsids

General stages of (lytic) virus replicationGeneral stages of (lytic) virus replication

One-step growth curve of virus replicationOne-step growth curve of virus replication

Lysogeny or ‘integration’ vs. lytic pathwaysLysogeny or ‘integration’ vs. lytic pathways

ExitExit• Cell lysis orCell lysis or• BuddingBudding

HIV movieHIV movie

QuickTime™ and aCinepak decompressor

are needed to see this picture.