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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
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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
