Chapter 6
An Introduction
to Viruses
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
The Great Fever video
1918 Flu
• 1918 Flu Virus Video
(12min)http://www.pbs.org/wgbh/nova/bod
y/1918-flu.html
– Reviving the virus wks
6.1 The Search for elusive viruses
• Leeuwenhoek’s microscope
– But there was a limit to magnification
• Louis Pasteur had an idea that rabies was
caused by a “living thing” that was even
smaller than bacteria
– Pasteur coined the term “virus”—means
poison
6.1 The Search for Elusive Viruses
• 1890s—first virus discovered
• D. Ivanovski and M. Beijernick showed
that a disease in tobacco plants was
caused by a virus
– Tobacco mosaic virus
6.1 The Search for the Elusive
Virus
• Frederich Loeffler and Paul Frosch
– Virus was the causative agent in foot-andmouth disease in cattle
– Filtered fluids from host—infectious particles
were small enough to pass through
• Smaller than bacteria
6.2 The Position of Viruses in the
Biological Spectrum
• Viruses are a unique
group of biological
entities known to infect
every type of cell,
including bacteria,
fungi, protozoa, plants,
and animals
• Viruses have been
around ever since cells
have on this planet
6.2 The Position of Viruses in the
Biological Spectrum
• Viruses are considered the most abundant
microbes on earth
• There are 10x more viruses than bacteria!
6.2 The Position of Viruses in the
Biological Spectrum
• Viral Terminology
– Referred to as infectious particles as opposed
to “organisms”
– Refer to them as active or inactive instead of
“alive” or “dead”
– Obligate Intracellular Parasites— cannot
multiply unless it invades a specific host cell
and instructs its genetic and metabolic
machinery to make and release quantities of
new viruses
6.3 The General Structure of
Viruses
• Viral Components
– Viruses are made up of many repeating units
and can be purified into crystals (Stanley)
• Further proof that they are non-living
PROCESS BOX 6-1
• If viruses are not considered to be living,
why do you think they are studied in
microbiology?
TWO LINE MINIMUM
6.3 The General Structure of
Viruses
• Size Range
– Viruses are the smallest infectious
particles
• Called ultramicroscopic
• Less than 0.2 µm
– Actual range is 20nm-450 nm
– Need to be seen with an electron
microscope
• Specimens are also usually stained
– Shadowcasting
6.3 The General Structure of
Viruses
• Size Range
Virus
Covering
Capsid
Envelope
(not in all
viruses)
Central
Core
DNA or RNA
Matrix
Proteins
6.3 The General Structure of Viruses
• Viral Components
– Protein capsid or shell that surrounds the
nucleic acid
• Nucleic acid + capsid = nucleocapsid
• Naked viruses—only has nucleocapsid (no
envelope)
• Enveloped viruses– envelope is present
6.3 The General Structure of
Viruses
• Viral Components
– The Viral Capsid
• Capsids are made of monomers called
capsomers
• Can self-assemble
• Two main shapes
– Helical
– Icosahedral
6.3 The General Structure of
Viruses
• The Viral Capsid
– Helical--continuous helix of capsomers
forming a cylindrical nucleocapsid
– Naked Helical Ex: Tobacco mosaic virus
– Enveloped Helical Ex: influenza, measles,
rabies
6.3 The General Structure of
Viruses
• The Viral Capsid
– Icosahedron 20-sided with 12 corners
– Naked ex: rotavirus
– Enveloped ex: herpes simplex
6.3 The General Structure of
Viruses
• Nucleic Acids: At the Core of a Viruses
– Viral genome – either DNA or RNA but never
both
– Unusual nucleic acids
• Single-stranded DNA (parvoviruses)
• Double-stranded RNA (reoviruses)
PROCESS BOX 6-2
• What shape and what envelope
combination would best describe this
virus?
TWO LINE MINIMUM
Human Viruses & Viral Diseases
25
26
Viral Replication
• https://www.youtube.com/watch?v=Rpj0e
mEGShQ
6.5 Modes of Viral Replication
• Multiplication Cycles in Animal Viruses
– General phases are
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Adsorption
Penetration
Uncoating
Synthesis
Assembly
Release
6.5 Modes of Viral Multiplication
• Adsorption: The virus attaches to the host
cell by specific binding of the spikes to cell
receptors
– Spectrum of cells a virus can infect – host
range
• EX: Hepatitis B only infects human liver cells
• EX: polio virus infects intestinal and nerve cells of
primates
6.5 Modes of Viral Multiplication
• Penetration: The virus is engulfed into a
vesicle
• Uncoating: The envelope of the cell is
uncoated, which frees the viral genetic
material into the cytoplasm
• **These two steps basically happen at the
same time**
Variety in Penetration and Uncoating
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Host cell
membrane
Free
RNA
Receptors
Uncoating of
nucleic acid
Receptor-spike
complex
Membrane
fusion
Irreversible
attachment
(a)
Entry of
nucleocapsid
Uncoating step
Host cell membrane
Virus in
vesicle
Specific
attachment
(b)
Vesicle, envelope and
capsid break down
Free
DNA
Engulfment
Capsid
RNA
Nucleic
acid
34
Receptor
(c)
Adhesion of virus to host receptors
Engulfment into vesicle
Viral RNA is released from vesicle
6.5 Modes of Viral Multiplication
• Synthesis: Replication and Protein
Production
– Under the control of viral genes, the cell
synthesizes the basic components of new
viruses, RNA molecules, capsomers, and
spikes
6.5 Modes of Viral Multiplication
• Assembly: Viral spike proteins are inserted
into the cell membrane for the viral
envelope; nucleocapsid is formed from
RNA and capsomere
6.5 Modes of Viral Multiplication
• Release: Enveloped viruses bud off of the
membrane (exocytosis), carrying away an
envelope with the spikes. This complete
virus or virion is ready to infect another
cell.
• Naked viruses cause the cell to erupt or
lyse
6.5 Modes of Viral Multiplication
• Virion—fully formed, extracellular virus
particle that is virulent
– Number releases varies from virus to virus but
is typically high
• Poliovirus—100,000 virions!!
Virus Cycle Animation
Putting the steps together
PROCESS BOX 6-3
• In your own words, sum up the animal viral
multiplication cycle.
THREE LINE MINIMUM
6.5 Modes of Viral Multiplication
• Damage to the Host Cell and Persistent
Infections
– Cytopathic effects—virus-induced damage
to the cell that alters its microscopeic
appearance
6.5 Modes of Viral Multiplication
• Damage to Host Cell and Persistent
Infection
– Productive Responses – viruses are produced
in host cells
• Lysis – host cell bursts, releasing virions. Dies
• Non-lysis – the host cell doesn’t burst. It slowly
leaks the new virons.
– Latent State – Viral nucleic acid becomes
integrated into host’s chromosomes,
replicating as part of the host genome.
– Persistent – Usually bud off from cell & does
not damage cell – Herpes
6.5 Modes of Viral Multiplication
• Some viruses change host DNA (can lead
to cancer)
– Oncoviruses
– Transduction is process of DNA change
Transduction
• DNA transferred from 1 bacterial cell to
another by way of a phage
– Generalized – Phages that can transfer any
bacterial gene. Transferred to new bacterial cell
once phage is incorporated. Bacterial genes
(DNA) are integrated into recipient’s DNA
– Specialized – only a few specific genes are
transferred to recipient bacteria
– Episomes – pieces of a chromosome that can
replicate as part of a bacterial chromosome (as a
prophage) or independently of it (virulent phage).
Such as temperate bacteriophages.
Generalized Transduction
Episome
PROCESS BOX 6-4
• How do generalized and specialized
transduction compare (include a similarity
and a difference)?
THREE LINE MINIMUM
6.5 Modes of Viral Multiplication
• Bacteriophage multiplication cycles
– Viruses that target bacteria
– Double-stranded DNA
– T2 & T4 bacteriophages target E. coli
– Stages overview
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Adsorption
Penetration (of nucleic acid only)
Synthesis of viral parts
Assembly
Release
6.5 Modes of Viral Multiplication
• Lysogeny: The Silent Virus Infection
– Virus enters cell but is not replicated and
released right away
– Viral DNA gets replicated every time with host
• Allows DNA to spread without killing host
– Eventually, viral DNA will be triggered
(induction), and viral components will be
replicated and assembled
Lytic Cycle
Release/Lysis
Attachment at Receptor site
Entry/Penetration
Assembly/Maturation
Replication
The Lytic Cycle of Virus infection
Chance meeting
& attachment
Lysozyme degrades
hole & DNA injected in.
Capsid remains outside
Assembly of virions Maturation
Nuclease destroys
host DNA, phage DNA
translated phage
structures
Lysis of cell & release
of virions ~ 200/cell
PROCESS BOX 6-5
• What differences do you notice between
the animal and bacterial viral multiplication
cycles?
TWO LINE MINIMUM
6.6 Techniques in Cultivating and
Identifying Animal Viruses
•
Primary purposes of viral cultivation
1. Isolate and identify viruses in clinical
specimens
2. To prepare viruses for vaccines
3. To do detailed research on viral structure,
multiplication cycles, genetics, and effects
on host cells
6.6 Techniques in Cultivating and
Identifying Animal Viruses
• Cell culturing
– Growing a thin
layer of cells to do
research with
– Often study viralhost interactions
– Can detect growth
by looking at a
plaque
6.6 Techniques in Cultivating and
Identifying Animal Viruses
• Using bird embryos
– Bird embryos contained within an egg
– Isolated system great for studying viral
propagation (“growth”)
Inoculation
of embryo
Inoculation
of amniotic cavity
Air sac
Inoculation of
chorioallantoic
membrane
Amnion
Shell
Allantoic
cavity
Albumin
(b)
Inoculation of
yolk sac
6.7 Medical Importance of Viruses
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AIDS
The Cold
Measles
Mumps
Rubella
Chicken pox/Shingles
Small Pox
Hepatitis
SARS
The Flu
Ebola
HPV
Bird Flu
Polio
Herpes
6.8 Detection and Treatment of
Animal Viral Infections
• How to detect a viral infection
– Symptoms
– Presence of antigens
– Amplify viral DNA using PCR (Lyme, HPV)
– Cell culturing
– Screening test for specific antibodies (HIV)
6.8 Detection and Treatment of Animal Viral
Infections
• How to prevent a viral infection
– Antiviral drugs – not a lot since viruses aren’t living.
Basically change the receptor sites & prevent
attachment
– Vaccines – either inactivated (dead viral particles) or
attenuated (weakened or altered viral particles) are
injected into organism. Body starts the production of
antibodies and memory cells to combat viral invaders
when needed.
Jonas Salk with
live Polio
vaccine
6.8 Detection and Treatment of
Animal Viral Infections
• How to treat a viral infection
• Very difficult!
– Not living
• No “antibiotics”
– Viral drugs often are not specific
• Target host cells too!
PROCESS BOX 6-6
• Compare methods of viral infection
detection, prevention, and treatment.
THREE LINE MINIMUM
6.9 Prions and Other Nonviral
Infectious Particles
• Prions—protinaceous infectious particle
– Piece of naked protein that can cause
infection
• NO DNA!!!
– Disease: transmissible spongiform
encephalopathies (TSEs)
• Spread by
– Direct contact
– Contaminated food
• Turns brain/nervous tissue into spongy matter
6.9 Prions and Other Nonviral
Infectious Agents
• Examples of TSEs (animal)
– Scrapie (sheep)
– Bovine spongiform encephalophathy—
(“mad cow disease”) (cattle)
– Wasting disease (elk, deer, mink)
• Examples of TSEs (human)
– Creutzfeldt-Jackob syndrome (CJS)
6.9 Prions and Other Nonviral
Infections Particles
• Viroids
– Very very small (1/10 normal virus size)
– Naked strands of RNA
– Only infect plants
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Tomatoes
Potatoes
Cucumbers
Citrus trees
PROCESS BOX 6-7
• Why do you think prions and viroids are
not considered true viruses?
TWO LINE MINIMUM
Chapter Overview
• Viruses are a unique group of tiny infectious particles
that are obligate parasites of cells
• Viruses do not exhibit the characteristics of life but can
regulate the function of host cells
• They infect all groups of living things and produce a
variety of diseases
• They are not cells but resemble complex molecules
composed of protein and nucleic acid
• They are encased in an outer shell or envelope and
contain either DNA or RNA as their genetic material
• Viruses are genetic parasites that take over the host
cell’s metabolism and synthetic machinery
Chapter Overview
• Viruses can instruct the cell to manufacture new
virus parts and assemble them
• They are released in a mature, infectious form,
followed by destruction of the host cell
• Viruses may persist in cells, leading to slow
progressive diseases and cancer
• They are identified by structure, host cell, type of
nucleic acid, outer coating, and type of disease
• They are among the most common infectious
agents, causing serious medical and agricultural
impact