CAMPBELL BIOLOGY IN FOCUS
URRY • CAIN • WASSERMAN • MINORSKY • REECE
17
Viruses
Lecture Presentations by
Kathleen Fitzpatrick and
Nicole Tunbridge,
Simon Fraser University
© 2016 Pearson Education, Inc.
SECOND EDITION
Structure of Viruses
 Even the largest known virus is barely visible under the light
microscope
 Viruses are not composed of cells
 A virus is an infectious particle consisting of little more than
genes packaged into a protein coat and, in some cases, a
membranous envelope
 Viruses lead “a kind of borrowed life,” existing in a shady area
between life-forms and chemicals
Chapter 17 Viruses
Key Concepts
 17.1 A virus consists of a nucleic acid surrounded
by a protein coat
 17.2 Viruses replicate only in host cells
 17.3 Viruses and prions are formidable pathogens
in animals and plants
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Are the tiny viruses leaving this E. coli cell alive?
0.5 mm
Are Viruses “alive”
 Viruses cannot replicate or carry out metabolic
activities outside of a host cell.
 Most virologists would probably agree that viruses
are not alive but lead “a kind of borrowed life.”
Components of virus: 1. Genome
 Viral genomes may consist of either
 Double- or single-stranded DNA, or
 Double- or single-stranded RNA
 Depending on its type of nucleic acid, a virus is
called a DNA virus or an RNA virus
Components of virus: 2. Capsid
 A capsid is the protein shell that encloses the viral
genome
 Capsids are built from protein subunits called
capsomeres
 The number of different kinds of proteins making up the
capsid is usually small.
 A capsid can have various structures
Components of virus: 3. Envelope
 Some viruses have membranous envelopes
 An animal virus with an envelope uses it to enter the host cell
 The envelope is derived from the plasma membrane of a host
cell, although some of the molecules on the envelope are
specified by the genome of the virus
Viral structure
RNA
Capsomere
DNA
Membranous
envelope
RNA
Capsid Head
DNA
Tail
sheath
Capsomere
of capsid
Tail
fiber
18  250 nm
80 nm
(a)Tobacco mosaic
virus
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Glycoprotein
Glycoproteins
70–90 nm (diameter)
80–200 nm (diameter)
50 nm
(b) Adenoviruses
50 nm
(c) Influenza viruses
80  225 nm
50 nm
(d) Bacteriophage T4
Viruses replicate only in host cells
 Viruses are obligate intracellular parasites, which
means they can replicate only within a host cell
 Each virus has a host range, a limited number of
host cells that it can infect
 Bacteriophages, also called phages, are viruses that infect
bacteria
 Phages have an elongated capsid head that encloses their DNA
 A protein tail piece attaches the phage to the host and injects the
phage DNA inside
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General Features of Viral Replicative Cycles
 Once a viral genome has entered a cell, the cell begins to
manufacture viral proteins
 The virus makes use of host enzymes, ribosomes, tRNAs, amino
acids, ATP, and other molecules
 Viral nucleic acid molecules and capsomeres spontaneously selfassemble into new viruses
 These exit from the host cell, usually damaging or destroying it
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Animation: Simplified Viral Replicative Cycle
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Figure 17.3
VIRUS
DNA
Entry and
uncoating
Replication
Transcription and
manufacture of
capsid proteins
Capsid
HOST
CELL
Viral DNA
mRNA
Viral DNA
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Capsid
proteins
Self-assembly of
new virus particles
and their exit from
the cell
Replicative Cycles of Phages
 Phages are the best understood of all viruses
 Phages have two reproductive mechanisms: the lytic cycle and
the lysogenic cycle
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The Lytic Cycle
 The lytic cycle is a phage replicative cycle that culminates
in the death of the host cell
 New viruses are produced using host cell
components
 The host cell lyses (breaks open) , and viruses
are released
 A phage that reproduces only by the lytic cycle is called a
virulent phage
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Animation: Phage T4 Lytic Cycle
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The lytic cycle
Attachment
Release
Entry of phage
DNA and
degradation
of host DNA
Phage assembly
Self-assembly
Head Tail
Tail
fibers
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Synthesis of
viral genomes
and proteins
The Lysogenic Cycle
 The lysogenic cycle replicates the phage genome without
destroying the host
 Viral DNA is inserted into the host chromosome
 Viral DNA is duplicated along with the host
chromosome during each cell division
 The inserted phage DNA is called a prophage
 Most prophage genes are inactive
 Environmental signals can cause a switch to the
lytic cycle
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Animation: Phage  Lysogenic and Lytic Cycles
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The lytic and lysogenic cycles of phage λ, a temperate phage
The phage attaches to a
host cell and injects its DNA.
Phage DNA
Prophage
Bacterial
chromosome
Lytic cycle
• Virulent or temperate phage
• Destruction of host DNA
• Production of new phages
• Lysis of host cell causes release
of progeny phages
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Lysogenic cycle
• Temperate phage only
• Genome integrates into bacterial
chromosome as prophage, which
(1) is replicated and passed on to
daughter cells and
(2) can be induced to leave the chromosome and initiate a lytic cycle
Bacterial Defenses Against Phages
 Natural selection favors mutant bacteria with surface proteins that
are not recognized by a particular phage
 Phage DNA is often identified as foreign and cut up by restriction
enzymes
 The bacterium’s own DNA is methylated in a way that prevents
attack by its own restriction enzymes
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 The CRISPR-Cas system is also used by bacteria and its offspring
to protect against future infection by the same type of phage
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RNA as Viral Genetic Material
 Retroviruses use reverse transcriptase to copy their RNA
genome into DNA
 HIV (human immunodeficiency virus) is the retrovirus that
causes AIDS (acquired immunodeficiency syndrome)
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 Viral DNA that is integrated into the host genome is called a
provirus
 A provirus is a permanent resident of the host cell
 The host’s RNA polymerase transcribes the proviral DNA into
RNA molecules
 The RNA molecules function both as mRNA for synthesis of viral
proteins and as genomes for new viruses released from the cell
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Animation: HIV Replicative Cycle
© 2016 Pearson Education, Inc.
The replicative cycle of HIV, the retrovirus that causes AIDS
Glycoprotein
Viral envelope
Membrane of
HIV white blood cell
Capsid
RNA (two
identical
strands)
HOST
CELL
Reverse HIV
transcriptase
Reverse
transcriptase
Viral RNA
RNA-DNA
hybrid
0.25 𝛍m
HIV entering a cell
DNA
NUCLEUS
Chromosomal
DNA
RNA genome
for the
progeny
viruses
Provirus
mRNA
New virus
New HIV leaving a cell
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Viral Diseases in Animals
 Viruses may damage or kill cells by causing the release of
hydrolytic enzymes from lysosomes
 Some viruses cause infected cells to produce toxins that lead to
disease symptoms
 Others have molecular components such as envelope proteins
that are toxic
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 Vaccines are the major medical tool for preventing viral infections
 A vaccine is a harmless derivative of a pathogen
that stimulates the immune system to mount defenses against the
harmful pathogen
 Antibiotics are powerless against viruses
 Antiviral drugs can help to treat, though not cure,
viral infections
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 Receptor proteins on the surfaces of cells are important in
treatment or prevention of viral infection
 Some people have been found to be resistant to HIV infection
 These people have an unusual form of the CCR5 protein, one of
the proteins to which HIV must bind in order to infect most cells
 A drug that masks the CCR5 protein is being tested currently
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Figure 17.9
HIV
Receptor
(CD4)
Co-receptor
(CCR5)
(a)
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Receptor (CD4)
Plasma
but no CCR5
membrane
(b)
Influenza
 Influenza viruses infect the epithelial lining of the upper respiratory
system
 Cause the “flu”, a highly contagious respiratory illness
 Influenza viruses are RNA viruses and have a high rate of mutation
Source: www.cdc.gov/flu/about/viruses/change.htm
influenza
 Seasonal flu vaccines are directed against surface proteins (H & N) of the
common circulating influenza strains
 Primes the immune system so it can quickly mount an attack if the influenza
virus is encountered (involves “memory” T-lymphocytes and antibodies).
Genetic Drift
Emerging Viruses
Genetic Shift
Image credit: jbiol.com/content/figures/jbiol147-1-l.jpg
Emerging Viruses
 Viruses that suddenly become apparent are called emerging
viruses
 HIV is a classic example
 The Ebola virus, recognized initially in 1976 in central Africa,
causes hemorrhagic fever, an often fatal syndrome
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Ebola viruses infecting a monkey cell
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 In 2009 a general outbreak, or epidemic, of a flu-like illness
occurred in Mexico and the United States; the virus responsible
was named H1N1
 H1N1 spread rapidly, causing a pandemic, or global epidemic
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Influenza in humans
1 𝛍m
(a) 2009 pandemic H1N1
influenza A virus
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(b) 2009 pandemic screening
 Three processes contribute to the emergence of viral diseases
 The mutation of existing viruses, which is especially high in
RNA viruses
 Dissemination of a viral disease from a small, isolated
human population, allowing the disease to go unnoticed before
it begins to spread
 Spread of existing viruses from animal populations; about
three-quarters of new human diseases originate this way
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Evolution of Viruses
 Viruses do not fit our definition of living organisms
 Since viruses can replicate only within cells, they probably evolved
after the first cells appeared
 Candidates for the source of viral genomes are plasmids (circular
DNA in bacteria and yeasts) and transposons (small mobile DNA
segments)
 Plasmids, transposons, and viruses are all mobile genetic
elements
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Viral Diseases in Plants
 More than 2,000 types of viral diseases of plants are
known, accounting for an annual loss of $15 billion
worldwide.
 Damage includes spots on leaves and fruits, stunted
growth, and damaged flowers or roots
 Most plant viruses are RNA viruses
Viral infection of plants.
Viruses and prions are formidable pathogens in
animals and plants
 Diseases caused by viral infections afflict humans, agricultural
crops, and livestock worldwide
 Smaller, less complex entities known as prions also cause disease
in animals
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Prions: Proteins as Infectious Agents
 There are proteins, called prions, that are known to be infectious
 These appear to cause a number of degenerative brain diseases
in various animal species
 Prions can be transmitted in food
 Prions act very slowly, with an incubation period of at least 10
years
 Prions are also virtually indestructable
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 To date, there is no known cure for prion diseases
 Prions, according to the leading model, are misfolded forms of
proteins normally found in the brain
 The prion somehow converts the normal proteins to the incorrectly
folded form
 The incorrectly folded form of the proteins forms aggregates that
interfere with normal cell functions
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Model for how prions propagate
Prion
New
prion
Original
prion
Normal
protein
Aggregates
of prions
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You should now be able to:
1.
2.
3.
4.
5.
6.
Describe what the virus is made of?
Explain why viruses are obligate parasites?
Distinguish between the Lytic and Lysogenic cycle
Define: a temperate phage and a virulent phage
Explain the reproductive cycle of RNA virus
Explain the function of reverse transcriptase