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Viruses, Viroids, and Prions

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Are Viruses Living or Are Viruses Living or Non-living?Non-living?

Most people say no!They have some properties of

life but not othersFor example, viruses can be

killed, even crystallized like table saltHowever, they can’t maintain a

constant internal state (homeostasis) and aren’t made of cells

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What are What are Viruses?Viruses?

A virus is made of genetic material and protein that can invade living cells.

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Viewing VirusesViewing VirusesViruses are Viruses are

smaller than the smaller than the smallest cellsmallest cell

Measured in Measured in nanometersnanometers

Viruses couldn’t Viruses couldn’t be seen until the be seen until the electron microscopeelectron microscope was invented in the was invented in the 2020thth century century

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Size of VirusesSize of Viruses

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Viral Viral StructureStructure

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CharacteristicsCharacteristics

Non living structuresNoncellularHave a nucleic acid core

containing DNA or RNAContain a protein coat called

the capsidCapable of reproducing only

when inside a HOST cell

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CharacteristicsCharacteristics

Some viruses are enclosed in an protective envelopeSome viruses may

have spikes to help attach to the host cellMost viruses infect

only SPECIFIC host cells (ex. Plant viruses infect plant cells)

CAPSID

ENVELOPE

DNA

SPIKES

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CharacteristicsCharacteristics

Outside of host cells, viruses are inactive

Lack ribosomes and enzymes needed for metabolism

Use the raw materials and enzymes of the host cell to be able to reproduce

EBOLA VIRUS

HIV VIRUS

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CharacteristicsCharacteristics

Some viruses cause disease

Smallpox, measles, mononucleosis, influenza, colds, warts, AIDS, Ebola

Some viruses may cause some cancers like leukemia

Virus-free cells are rare

MEASLES

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Herpes VirusHerpes Virus

SIMPLEX I and II

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AdenovirusAdenovirus

COMMON COLD

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Influenza VirusInfluenza Virus

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Chickenpox VirusChickenpox Virus

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Papillomavirus – Papillomavirus – Warts!Warts!

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Bacteriophages

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PhagesPhages

Viruses that attack bacteria are called bacteriophage or just phage

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Escherichia Coli Bacterium

T - EVEN PHAGES ATTACK THIS BACTERIUM

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RetrovirusesRetroviruses

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Characteristics of Characteristics of RetrovirusesRetroviruses

Contain RNA, not DNAContain enzyme called

Reverse TranscriptaseWhen a retrovirus infects

a cell, it injects its RNA and reverse transcriptase enzyme into the cytoplasm of that cell

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ENZYME

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RetrovirusesRetroviruses

The enzyme reverse transcriptase (or RTase), produces a DNA copy of the virus’s RNA

RTase

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RetrovirusesRetroviruses

HIV, the AIDS virus, is a retrovirus

Feline Leukemia Virus is also a retrovirus

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Viral Viral ReplicationReplication

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Viral AttackViral Attack

Humans rarely share viral diseases with other animalsEukaryotic viruses usually

have protective envelopes made from the host cell membraneOnce inside a cell, 2

processes may occur: the lytic cycle or the lysogenic cycle.

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Lytic Cycle Lysogenic Cycle

Virus enters cell Virus integrates its DNA into host’s DNA (prophage)

Eliminates host cell’s DNA

Virus replicates along with host cell’s DNA

Makes copies of itself Cell may replicate many times, giving rise to host cells with a prophage

Causes cell to burst (lyse)

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5 Steps of Lytic 5 Steps of Lytic CycleCycle

1. Attachment to the cell2. Penetration (injection) of

viral DNA or RNA3. Replication of new viral

proteins and nucleic acids4. Assembly of the new viruses 5. Release of the new viruses

into the environment (cell lyses--bursts)

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The Lytic Cycle--The Lytic Cycle--AnimationAnimation

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Lysogenic CycleLysogenic CyclePhage DNA injected into host cell Viral DNA joins host DNA forming

a prophageWhen an activation signal occurs,

the phage DNA starts replicatingViral DNA (part of prophage) may stay

inactive in host cell for long periods of time

Replicated during each binary fissionOver time, many cells form containing

the prophages

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Viral LatencyViral LatencySome viruses have the ability

to become dormant inside the cellCalled latent virusesThey may remain inactive for

long periods of time (years)Later, they activate to produce

new viruses in response to some external signal

HIV and Herpes viruses are examples

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Viral LatencyViral LatencyOnce a prophage cell is activated, host cell enters the lytic cellNew viruses form a & the cell lyses (bursts)Virus said to be virulent (deadly)

INACTIVE STAGEACTIVESTAGE

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Latency in Latency in EukaryotesEukaryotesSome eukaryotic

viruses remain dormant for many years in the nervous system tissues

Chickenpox (caused by the virus Varicella zoster) is a childhood infection

It can reappear later in life as shingles, a painful itching rash limited to small areas of the body

SHINGLES

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Treatment for Treatment for Viral DiseaseViral Disease

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VaccinesVaccines

A vaccine against a viral disease can be made from an attenuated (weakened), less virulent strain of the virusAttenuated virus is capable of

stimulating an immune response and creating immunity, but not causing illness

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SmallpoxSmallpoxEdward Jenner

(1796) developed a smallpox vaccine using milder cowpox viruses

Smallpox has been eradicated in the world today

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Other Viral TreatmentsOther Viral Treatments

Interferons are naturally occurring proteins made by cells to fight viruses

Antiviral drugs (AZT)

Protease inhibitors – prevent capsid formation

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Viroids & PrionsViroids & Prions

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ViroidsViroidsSmall, circular

RNA molecules without a protein coat

Infect plantsPotato famine

in Ireland

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PrionsPrionsPrions are

“infectious proteins” They are normal

body proteins that get converted into prions by contact with other prion proteins

They have no DNA or RNA

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Prion Prion DiseasesDiseases

Prions form Prions form insoluble insoluble deposits in the braindeposits in the brain

Damages nerve tissueDamages nerve tissueMad cow diseaseMad cow disease is an is an

exampleexamplePeople in New Guinea People in New Guinea

used to suffer from used to suffer from kurukuru, which they got , which they got from eating the brains from eating the brains of their enemiesof their enemies

Other examples:Other examples:Sheep Sheep scrapiescrapie and and Creutzfeldt-Creutzfeldt-Jacob diseaseJacob disease

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