Viruses, Prions and Viroids Infectious Agents of Animals and Plants Chapter 14.
Viruses, Viroids, and Prions
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Transcript of Viruses, Viroids, and Prions
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VIRUSES, VIROIDS, AND PRIONS
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Are Viruses Living or Non-living?
Viruses are both and neither They have some properties of
life but not others For example, viruses can be
killed, even crystallized like table salt
However, they can’t maintain a constant internal state (homeostasis).
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What are 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 HISTORY
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Discovery of VirusesBeijerinck (1897) coined the Latin name “virus” meaning poisonHe studied filtered plant juices & found they caused healthy plants to become sick
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Tobacco Mosaic VirusWendell Stanley (1935) crystallized sap from sick tobacco plantsHe discovered viruses were made of nucleic acid and protein
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SmallpoxEdward Jenner (1796) developed a smallpox vaccine using milder cowpox virusesDeadly viruses are said to be virulentSmallpox has been eradicated in the world today
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Viewing VirusesViruses are smaller than the smallest cellMeasured in nanometersViruses couldn’t be seen until the electron microscope was invented in the 20th century
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Size of Viruses
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VIRAL STRUCTURE
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Characteristics
Non living structures Noncellular Contain a protein coat called the
capsid Have a nucleic acid core
containing DNA or RNA Capable of reproducing only when
inside a HOST cell
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Characteristics
Some viruses are enclosed in an protective envelope
Some viruses may have spikes to help attach to the host cell
Most viruses infect only SPECIFIC host cells
CAPSID
ENVELOPE
DNA
SPIKES
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CharacteristicsViral capsids (coats) are made of individual protein subunitsIndividual subunits are called capsomeres
CAPSOMERES
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CharacteristicsOutside of host cells, viruses are inactiveLack ribosomes and enzymes needed for metabolismUse the raw materials and enzymes of the host cell to be able to reproduce
EBOLA VIRUS
HIV VIRUS
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CharacteristicsSome viruses cause diseaseSmallpox, measles, mononucleosis, influenza, colds, warts, AIDS, EbolaSome viruses may cause some cancers like leukemiaVirus-free cells are rare
MEASLES
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Viral ShapesViruses come in a variety of shapesSome may be helical shape like the Ebola virusSome may be polyhedral shapes like the influenza virusOthers have more complex shapes like bacteriophages
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Helical Viruses
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Polyhedral Viruses
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Complex Viruses
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TAXONOMY OF VIRUSES
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Viral Taxonomy
Family names end in -viridae Genus names end in -virus Viral species: A group of viruses
sharing the same genetic information and ecological niche (host).
Common names are used for species
Subspecies are designated by a number
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Viral Taxonomy Examples
Herpesviridae Herpesvirus Human herpes virus 1, HHV 2, HHV 3
Retroviridae Lentivirus Human Immunodeficiency Virus 1, HIV 2
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Herpes Virus
SIMPLEX I and II
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Adenovirus
COMMON COLD
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Influenza Virus
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Chickenpox Virus
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Papillomavirus – Warts!
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Used for Virus Identification
RNA or DNA Virus Do or do NOT have an envelope Capsid shape HOST they infect
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Bacteriophages
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PhagesViruses that attack bacteria are called bacteriophage or just phageT-phages are a specific class of bacteriophages with icosahedral heads, double-stranded DNA, and tails
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T-phagesThe most commonly studied T-phages are T4 and T7They infect E. coli , an intestinal bacteriaSix 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 Bacteriophage
Head with 20 triangular surfacesCapsid contains DNAHead & tail fibers made of protein
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Retroviruses
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Characteristics of Retroviruses
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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RetrovirusesThe 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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RetrovirusesHIV, the AIDS virus, is a retrovirusFeline Leukemia Virus is also a retrovirus
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Viroids & Prions
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ViroidsSmall, circular RNA molecules without a protein coatInfect plantsPotato famine in IrelandResemble introns cut out of eukaryotic
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PrionsPrions 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 RNAThe main protein involved in human and mammalian prion diseases is called “PrP”
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Prion Diseases
Prions form insoluble deposits in the brainCauses neurons to rapidly degeneration.Mad cow disease (bovine spongiform encephalitis: BSE) is an examplePeople in New Guinea used to suffer from kuru, which they got from eating the brains of their enemies
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Viral Replication
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Viral Attack Viruses are very specific as to
which species they attack HOST specific Humans rarely share viral
diseases with other animals Eukaryotic viruses usually have
protective envelopes made from the host cell membrane
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5 Steps of Lytic Cycle
1. Attachment to the cell 2. Penetration (injection) of viral
DNA or RNA 3. Replication (Biosynthesis) of
new viral proteins and nucleic acids
4. Assembly (Maturation) of the new viruses
5. Release of the new viruses into the environment (cell lyses)
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Bacteriophage Replication
Bacteriophage inject their nucleic acidThey lyse (break open) the bacterial cell when replication is finished
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Lytic Cycle Review Attachment Phage attaches by tail fibers
to host cell
Penetration Phage lysozyme opens cell wall,
tail sheath contracts to force tail core and DNA into cell
Biosynthesis Production of phage DNA and proteins
Maturation Assembly of phage particles Release Phage lysozyme breaks cell
wall
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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 platePinCell 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 Curve
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Viral 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 signalHIV and Herpes viruses are examples
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Lysogenic CyclePhage DNA injected into host cell Viral DNA joins host DNA forming a prophageWhen an activation signal occurs, the phage DNA starts replicating
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Lysogenic CycleViral DNA (part of prophage) may stay inactive in host cell for long periods of timeReplicated during each binary fissionOver time, many cells form containing the prophages
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Viral 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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Virulent Viruses
HOST CELL LYSES & DIES
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The Lysogenic Cycle
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Latency in EukaryotesSome eukaryotic
viruses remain dormant for many years in the nervous system tissues Chickenpox (caused by the virus Varicella zoster) is a childhood infectionIt 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 EukaryotesHerpes viruses also become latent in the nervous systemA herpes infection lasts for a person’s lifetimeGenital herpes (Herpes Simplex 2)Cold sores or fever blisters (Herpes Simplex1)
SKIN TO SKIN CONTACT
PASSED AT BIRTH TO BABY
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VIRULENCE
VIRUS DESTROYING HOST CELL
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Lytic and Lysogenic Cycles
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Treatment for Viral Disease
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Vaccines 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 virus
Attenuated virus is capable of stimulating an immune response and creating immunity, but not causing illness
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Other Viral TreatmentsInterferon are naturally occurring proteins made by cells to fight virusesGenetic altering of viruses (attenuated viruses)Antiviral drugs (AZT)Protease inhibitors – prevent capsid formation
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