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VIRUSES

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Are Viruses Living orAre Viruses Living or

NonNon--living?living? Viruses are both and neither

They have some properties of lifebut not others For example, viruses can be killed,

even crystallized like table salt

However, they cant maintain aconstant internal state(homeostasis).

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What a Virus Isnt

Not a bacterium...

Not independent... Cannot survive in absence of a living cell

within which to replicate...

Antibiotics generally dont work on them...

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Whatare Viruses?Whatare Viruses?

A virus is a non-

cellular particle madeup of genetic materialand protein that can

invade living cells.

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What Viruses Are...

Infectious agents composed mainly of nucleic acid

with a protein coat (capsid)

Visible with electron microscope (10-200 nM)

Carry on normal cell-like function (unless free, then

infectious)

In infectious form: no growth; no respiration??? Can enter living plant, animal or bacterial cell

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Discovery of VirusesDiscovery of VirusesBeijerinck (1897)coined the Latin namevirus meaning poison

He studied filteredplant juices & foundthey caused healthyplants to become sick

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Tobacco Mosaic VirusTobacco Mosaic Virus

Wendell Stanley(1935) crystallized

sap from sicktobacco plants

He discovered

viruses were made ofnucleic acid andprotein

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SmallpoxSmallpox

Edward Jenner (1796)developed a smallpoxvaccine using milder

cowpox virusesDeadly viruses aresaid to be virulent

Smallpox has beeneradicated in theworld today

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Viewing VirusesViewing Viruses

Viruses areViruses are smallersmallerthan the smallest cellthan the smallest cell

Measured inMeasured in

nanometersnanometersViruses couldnt beViruses couldnt beseen until theseen until the electronelectronmicroscopemicroscope waswasinvented in theinvented in the 2020thth

centurycentury

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Electron Microscopy

Mitra, K. & Frank, J., 2006. Ribosome dynamics: insights from atomic structure modeling into cryo-electron

microscopy maps.Annual review of biophysics and biomolecular structure, 35, 299-317.

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X-ray Crysta

llography of Viruses

Symmetry of protein shells makes them uniquely

well-suited to crystallographic methods

Viruses are the largest assemblies of biological

macromolecules whose structures have been

determined at high resolution

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ViralViralStructureStructure

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Virus Appearence? 1. Capsid

2. Core and genetic material (DNA/RNA)

Capsid: outer shell of the virus which encloses genetic material

(link: chemical structure of capsid helps determine immune

response to virus)

capsid is made of many identical individual proteins

protein core under capsid protecting genetic material

sometimes an additional covering (lipid bilayer w/embedded

proteins) on outside known as an envelope ( like a baseball)

various forms: rods, filaments, spheres, cubes, crystals

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CharacteristicsCharacteristics

Non living structures

Noncellular

Contain a protein coat called thecapsid

Have a nucleic acid core containing

DNA or RNA Capable of reproducing only when

inside a HOST cell

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CharacteristicsCharacteristics

Some viruses areenclosed in anprotective envelope

Some viruses may havespikes to help attach tothe host cell

Most viruses infectonly SPECIFIC hostcells

CAPSID

ENVELOPE

DNA

SPIKES

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CharacteristicsCharacteristics

Viral capsids(coats) are madeof individualprotein subunits

Individual

subunits arecalled capsomeres CAPSOMERES

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Capsid

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capsomere: unit/molecule associated with capsid structure

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CharacteristicsCharacteristics

Outside of host cells,viruses are inactive

Lack ribosomes and

enzymes needed formetabolism

Use the raw materials

and enzymes of the hostcell to be able toreproduce

EBOLAVIRUS

HIVVIRUS

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Size of VirusesSize of Viruses

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Viral ShapesViral Shapes

Viruses come in a variety ofshapes

Some may be helical shape likethe Ebola virus

Some may be polyhedral

shapes like the influenza virusOthers have more complex

shapes like bacteriophages

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T

ypical

Virus Shapes

RODS SPHERES

CUBES

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The Structure of Viruses

Tobacco

Mosaic

Virus

HIV

Bacteriophage

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Helical VirusesHelical Viruses

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Polyhedral VirusesPolyhedral Viruses

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More Virus Shapes

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Complex VirusesComplex Viruses

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Taxonomy ofTaxonomy of

VirusesViruses

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ira Taxonomyira TaxonomyExamplesExamples

Herpesviridae

Herpesvirus

Human herpes virus 1, HHV 2, HHV 3

Retroviridae

Lentivirus Human Immunodeficiency Virus 1, HIV 2

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Herpes VirusHerpes Virus

SIMPLEXI and II

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AdenovirusAdenovirus

COMMONCOLD

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Influenza VirusInfluenza Virus

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PapillomavirusPapillomavirus Warts!Warts!

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RNA or DNA Virus

Do or do NOT have an envelope

Capsid shape

HOST they infect

Used for VirusUsed for Virus

IdentificationIdentification

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Bacteriophages

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PhagesPhages

Viruses that attackbacteria are calledbacteriophage or just

phageT-phages are aspecific class of

bacteriophages withicosahedral heads,double-stranded DNA,and tails

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TT--phagesphages

The most commonly studiedT-phages are T4 and T7They infect E. coli , an

intestinal bacteriaSix small spikes at the baseof a contractile tail are usedto attach to the host cellInject viral DNA into cell

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EscherichiaColi

Bacterium

T - EVENPHAGESATTACKTHISBACTERIUM

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E. Coli and the

Bacteriophage

What it looks like in real

life

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T-Even Bacteriophages

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Diagram of TDiagram of T--44BacteriophageBacteriophage

Head with 20triangular

surfacesCapsidcontains DNA

Head & tailfibers madeof protein

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Composition ofT-Even

Bacteriophage

Capsid: brains of virus,

tightly-wound protein

protecting nucleic acids Body: attached to

capsid head, rod-like

structure w/retractible

sheath, hollow core

Tail: at end of core is a

spiked plate carrying 6

slender tail fibers,

anchor virus to its host

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R

et

rovirusesR

et

roviruses

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