1 viruses, viroids, and prions. 2 are viruses living or non-living? they have some properties of...

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Viruses, Viroids, and Prions

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

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).

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What are What are Viruses?Viruses?

A virus is a non-cellular particle made up of genetic material and protein that can invade living cells.

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Viral Viral HistoryHistory

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Discovery of VirusesDiscovery of VirusesBeijerinck (1897)

coined the Latin name “virus” meaning poison

He studied filtered plant juices & found they caused healthy plants to become sick

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Tobacco Mosaic VirusTobacco Mosaic Virus

Wendell Stanley (1935) crystallized sap from sick tobacco plants

He discovered viruses were made of nucleic acid and protein

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SmallpoxSmallpoxEdward Jenner

(1796) developed a smallpox vaccine using milder cowpox viruses

Deadly viruses are said to be virulent

Smallpox has been eradicated in the world today

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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 structuresNoncellularContain a protein coat called

the capsidHave a nucleic acid core

containing DNA or RNACapable 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

CAPSID

ENVELOPE

DNA

SPIKES

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CharacteristicsCharacteristics

Viral capsids (coats) are made of individual protein subunits

Individual subunits are called capsomeres

CAPSOMERES

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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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Viral ShapesViral Shapes

Viruses come in a variety of shapes

Some may be helical shape like the Ebola virus

Some may be polyhedral shapes like the influenza virus

Others have more complex shapes like bacteriophages

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Helical VirusesHelical Viruses

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Polyhedral VirusesPolyhedral Viruses

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Complex VirusesComplex Viruses

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Taxonomy of Taxonomy of VirusesViruses

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Viral TaxonomyViral Taxonomy

Family names end in -viridaeGenus names end in -virus Viral species: A group of viruses

sharing the same genetic information and ecological niche (host). Common names are used for speciesSubspecies are designated by a

number

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Viral Taxonomy Viral Taxonomy ExamplesExamples

HerpesviridaeHerpesvirusHuman herpes virus 1, HHV 2, HHV 3

RetroviridaeLentivirusHuman Immunodeficiency Virus 1, HIV 2

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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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RNA or DNA VirusDo or do NOT have an

envelopeCapsid shapeHOST they infect

Used for Virus Used for Virus IdentificationIdentification

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Bacteriophages

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PhagesPhages

Viruses that attack bacteria are called bacteriophage or just phage

T-phages are a specific class of bacteriophages with icosahedral heads, double-stranded DNA, and tails

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T-phagesT-phages

The most commonly studied T-phages are T4 and T7

They infect E. coli , an intestinal bacteria

Six small spikes at the base of a contractile tail are used to attach to the host cell

Inject viral DNA into cell

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Escherichia Coli Bacterium

T - EVEN PHAGES ATTACK THIS BACTERIUM

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T-Even Bacteriophages

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Diagram of T-4 Diagram of T-4 BacteriophageBacteriophage

Head with 20 triangular surfaces

Capsid contains DNA

Head & tail fibers made of protein

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RetrovirusesRetroviruses

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Characteristics of Characteristics of RetrovirusesRetroviruses

Contain RNA, not DNAFamily RetroviridaeContain 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), which causes synthesis of a complementary DNA molecule (cDNA) using virus RNA as a template

RTase

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RetrovirusesRetroviruses

HIV, the AIDS virus, is a retrovirus

Feline Leukemia Virus is also a retrovirus

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Viroids & PrionsViroids & Prions

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ViroidsViroidsSmall, circular

RNA molecules without a protein coat

Infect plantsPotato famine

in IrelandResemble

introns cut out of eukaryotic

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PrionsPrionsPrions are

“infectious proteins” They are normal

body proteins that get converted into an alternate configuration by contact with other prion proteins

They have no DNA or RNA

The main protein involved in human and mammalian prion diseases is called “PrP”

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Prion Prion DiseasesDiseases

Prions form Prions form insoluble deposits in insoluble deposits in the brainthe brain

Causes neurons to Causes neurons to rapidly degeneration.rapidly degeneration.

Mad cow diseaseMad cow disease (bovine spongiform (bovine spongiform encephalitis: BSE) is encephalitis: BSE) is an examplean example

People in New People in New Guinea used to suffer Guinea used to suffer from from kurukuru, which they , which they got from eating the got from eating the brains of their enemiesbrains of their enemies

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Viral Viral ReplicationReplication

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Viral AttackViral Attack

Viruses are very specific as to which species they attackHOST specificHumans rarely share viral

diseases with other animalsEukaryotic viruses usually

have protective envelopes made from the host cell membrane

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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 (Biosynthesis) of

new viral proteins and nucleic acids4. Assembly (Maturation) of the

new viruses 5. Release of the new viruses

into the environment (cell lyses)

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BacteriophageBacteriophage ReplicationReplication

Bacteriophage inject their nucleic acid

They lyse (break open) the bacterial cell when replication is finished

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Attachment Phage attaches by tail fibers to

host cellPenetration Phage lysozyme opens cell wall,

tail sheath contracts to force tail core and DNA into cellBiosynthesis Production of phage DNA

and proteinsMaturation Assembly of phage particlesRelease Phage lysozyme breaks cell wall

Lytic Cycle ReviewLytic Cycle Review

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Attachment:Phage attaches to host cell.

Penetration:Phage pnetrates host cell and injects its DNA.

Merozoites released into bloodsteam from liver may infect new red blood cells

1

2

3

Bacterial cell wall

Bacterial chromosome

Capsid

DNA

Capsid

SheathTail fiberBase platePin

Cell wall

Tail

Plasma membrane

Sheath contracted

Tail core

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4 Maturation:Viral components are assembled into virions.

Tail

5 Release:Host cell lyses and new virions are released.

DNA

Capsid

Tail fibers

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One-step Growth One-step Growth CurveCurve

If all 500 viruses released attach to new cells, how many viruses could there be in the body at the end of

the next 40 minute cycle?

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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 (stress, illness, etc.)

HIV, Herpes, and chickenpox (causes shingles) viruses are examples

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Lysogenic CycleLysogenic CyclePhage DNA

injected into host cell

Viral DNA joins host DNA forming a prophage

When an activation signal occurs, the phage DNA starts replicating

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Lysogenic CycleLysogenic Cycle

Viral DNA (part of prophage) may stay inactive in host cell for long periods of time

Replicated during each binary fission

Over time, many cells form containing

the prophages

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

INACTIVE STAGEACTIVESTAGE

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Virulent VirusesVirulent Viruses

HOST CELL

LYSES & DIES

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The Lysogenic CycleThe Lysogenic Cycle

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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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Latency in EukaryotesLatency in EukaryotesHerpes viruses also

become latent in the nervous system

A herpes infection lasts for a person’s lifetime

Genital herpes (Herpes Simplex 2)

Cold sores or fever blisters (Herpes Simplex 1)

SKIN TO SKIN CONTACT

PASSED AT BIRTH TO BABY

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VirulenceVirulence

VIRUS DESTROYING HOST CELL

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Lytic and Lysogenic Lytic and Lysogenic CyclesCycles

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Body’s Defense Body’s Defense Against Disease Against Disease Causing AgentsCausing Agents

First Line of Defense

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Skin, mucous, cilia, stomach acid, and saliva

Think of the human body as a hollow plastic tube…

The food is digested within the hole in the tube, but it never actually enters into the solid plastic

material.Tube inner surface ~Digestive System~

Tube inner surface ~Digestive System~

Tube inner surface ~Digestive System~

Second Line of Defense~White Blood Cells~

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-If invaders actually get within the body, then your white blood cells (WBCs) begin their attack

-WBCs normally circulate throughout the blood, but will enter the body’s tissues if invaders are detected

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White Blood Cells~Phagocytes~

These white blood cells are responsible for eating foreign particles by engulfing them

Once engulfed, the phagocyte breaks the foreign particles apart in organelles called Lysosomes

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The Second Line of Defense~Interferon~

-Virus-infected body cells release interferon when an invasion occurs

- Interferon – chemical that interferes with the ability to viruses to attack other body cells

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White Blood Cells~T-Cells~

T-Cells, often called “natural killer” cells, recognize infected human cells and cancer cellsT-cells will attack these infected cells, quickly kill them, and then continue to search for more cells to kill

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The Second Line of Defense~The Inflammatory Response~

-Injured body cells release chemicals called histamines, which begin inflammatory response

-Capillaries dilate

-Pyrogens released, reach hypothalamus, and temperature rises

-Pain receptors activate

-WBCs flock to infected area like sharks to blood

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Two Divisions of the Immune System

-The efforts of the WBCs known as phagocytes and T-cells is called the cell-mediated immune system.

-Protective factor = living cells

-Phagocytes – eat invaders

-T-cells – kill invaders

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Two Divisions of the Immune System

The other half of the immune system is called antibody-mediated immunity, meaning it is controlled by antibodiesThis represents the third line

of defense in the immune system

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The Third Line of Defense~Antibodies~

-Most infections never make it past the first and second levels of defense

-Those that do trigger the production and release of antibodies

-Proteins that latch onto, damage, clump, and slow foreign particles

-Each antibody binds only to one specific binding site, known as an antigen

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Antibody Production

-WBCs gobble up invading particles and break them up

-They show the particle pieces to T-cells, who identify the pieces and find specific B-cells to help

-B-cells produce antibodies that are equipped to find that specific piece on a new particle and attach

What is immunity?

-Resistance to a disease causing organism or harmful substance

-Two types

-Active Immunity

-Passive Immunity

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Immunity

-New particles take longer to identify, and a person remains ill until a new antibody can be crafted

-Old particles are quickly recognized, and a person may never become ill from that invader again. This person is now immune.

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Active Immunity

-You produce the antibodies

-Your body has been exposed to the antigen in the past either through:

-Exposure to the actual disease causing antigen – You fought it, you won, you remember it

-Planned exposure to a form of the antigen that has been killed or weakened – You detected it, eliminated it, and remember it

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VaccinesVaccines

An attenuated virus is a weakened, less vigorous virus“Attenuate" refers to procedures that

weaken an agent of disease (heating)A vaccine against a viral disease can

be made from an attenuated, less virulent strain of the virusAttenuated virus is capable of

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

VaccineAntigens are deliberately introduced into the immune system to produce immunityBecause the bacteria has been killed or weakened, minimal symptoms occurHave eradicated or severely limited several diseases from the face of the Earth, such as polio and smallpox

How long does active immunity last?

It depends on the antigenSome disease-causing agents multiply into new forms that our body doesn’t recognize, requiring annual vaccinations, like the flu shotBooster shot - reminds the immune system of the antigenOthers last for a lifetime, such as chicken pox

Think the flu is no big deal?

-Think again…

-In 1918, a particularly deadly strain of flu, called the Spanish Influenza, spread across the globe

-It infected 20% of the human population and killed 5%, which came out to be about 100 million people

Do we get all the possible vaccines we can?

Although the Center for Disease Control (CDC) recommends certain vaccines, many individuals go without themThose especially susceptible include travelers and studentsConsider the vaccine for meningitis, which is recommended for all college students and infects 3,000 people in the U.S., killing 300 annually

Passive Immunity

You don’t produce the antibodies

A mother will pass immunities on to her baby during pregnancy - through what organ?These antibodies will protect the baby for a short period of time following birth while its immune system develops. What endocrine gland is responsible for this?Lasts until antibodies die Why doesn’t the mother just

pass on the WBCs that “remember” the antigens?

Thymus

Placenta

Acquired Immune Deficiency Syndrome

Caused by the Human Immunodeficiency VirusDiscovered in 1983Specifically targets and kills T-cellsBecause normal body cells are unaffected, immune response is not launched

AIDS~The Modern Plague~

-The HIV virus doesn’t kill you – it cripples your immune system

-With your immune system shut down, common diseases that your immune system normally could defeat become life-threatening

-Can show no effects for several months all the way up to 10 years

AIDS~The Silent Spread~

Transmitted by sexual contact, blood transfusions, contaminated needlesAs of 2007, it affects an estimated 33.2 million people

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Other Viral TreatmentsOther Viral Treatments

Interferon are naturally occurring proteins made by cells to fight viruses

Genetic altering of viruses (attenuated viruses)

Antiviral drugs (AZT)Protease inhibitors –

prevent capsid formation