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CAMPBELL
BIOLOGY
TENTH
EDITION
Reece • Urry • Cain • Wasserman • Minorsky • Jackson
19
Viruses
Lecture Presentation by
Nicole Tunbridge and
Kathleen Fitzpatrick
© 2014 Pearson Education, Inc.
Figure 19.1 Are the viruses (red) budding from this cell alive?
Overview: A Borrowed Life
• Viruses called bacteriophages can infect
and set in motion a genetic takeover of
bacteria, such as Escherichia coli
• Viruses lead “a kind of borrowed life”
between life-forms and chemicals
• The origins of molecular biology lie in
early studies of viruses that infect
bacteria
• Viruses were detected indirectly long
before they were actually seen
The Discovery of Viruses: Scientific
Inquiry
• Tobacco mosaic disease stunts growth of
tobacco plants and gives their leaves a
mosaic coloration
• In the late 1800s, researchers
hypothesized that a particle smaller than
bacteria caused the disease
• In 1935, Wendell Stanley confirmed this
hypothesis by crystallizing the infectious
particle, now known as tobacco mosaic
virus (TMV)
Experiment
Figure 19.2
1 Extracted sap
from tobacco
plant with
tobacco mosaic
disease
2 Passed sap
3 Rubbed filtered
through a
porcelain filter
known to trap
bacteria
4 Healthy plants
became infected
sap on healthy
tobacco plants
Structure of Viruses
• Viruses are the simplest biological systems.
• Viruses are not cells.
• Viruses are very small infectious particles
consisting of nucleic acid enclosed in a protein
coat and, in some cases, a membranous
envelope
– Are viruses living or nonliving?
• $ --- Viruses cannot reproduce 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.”
© 2014 Pearson Education, Inc.
Viral Genomes
Viruses genome of may consist of either
double-stranded DNA, or single-stranded DNA,
or double-stranded RNA,or single-stranded
RNA, depending on the kind of virus.
• A virus is called a DNA virus or an RNA virus,
according to the kind of nucleic acid that makes
up its genome.
– The viral genome is usually organized as a
single linear or circular molecule of nucleic
acid.
– Viruses have between three and several
thousand genes in their genome
© 2014 Pearson Education, Inc.
Capsids and Envelopes
• A capsid is the protein shell that encloses the viral
genome.
• Capsids are built from protein subunits called capsomeres.
• A capsid can have various structures.
• Viruses with Rod-shaped capsidsare called helical
viruses.
• Adenoviruses have 252 identical proteins arranged into a
polyhedral capsid with 20 triangular facts—an icosahedron.
• Viruses that infect bacteria -bacteriophages or phages
have the most complex capsids. Phages have an
elongated capsid head that encloses their DNA and protein
tail piece attaches the phage to the host and injects the
phage DNA inside.
• Some viruses have accessory structures viral envelopes
to help them infect their hosts. Viral envelope contain host
cell phospholipids and membrane proteins as well as
proteins and glycoproteins of viral origin
– A membranous envelope surrounds the capsids of
flu viruses.
RNA
Capsomere
DNA
Membranous RNA
Head
envelope
Capsid
Capsomere
of capsid
DNA
Tail
sheath
Tail
fiber
Glycoprotein
18 × 250 nm
Glycoproteins
70–90 nm (diameter) 80–200 nm (diameter)
80 × 225 nm
20 nm
50 nm
50 nm
50 nm
(a) Tobacco mosaic (b) Adenoviruses
(c) Influenza viruses (d) Bacteriophage T4
virus
Figure 19.3 Viral structure
Viruses replicate only in host cells
• Viruses can replicate only within a host cell.
• An isolated virus is unable to reproduce—or do
anything else, except infect an appropriate host.
• An isolated virus is merely a packaged set of
genes in transit from one host cell to another.
– Each type of virus can infect and parasitize only a
limited range of host cells, called its host range.
© 2014 Pearson Education, Inc.
General Features of Viral Replicative
Cycles
• Once a viral genome enters a cell (the host),
the host 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 self-assemble
into new viruses
© 2014 Pearson Education, Inc.
1 Entry and
uncoating
DNA
VIRUS
3 Transcription
and manufacture of
capsid proteins
Capsid
2 Replication
HOST
CELL
Viral DNA
mRNA
Viral
DNA
Figure 19.4
Capsid
proteins
4 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
© 2014 Pearson Education, Inc.
The Lytic Cycle
• The lytic cycle is a phage reproductive cycle
that culminates in the death of the host cell
• The lytic cycle produces new phages and
digests the host’s cell wall, releasing the
progeny viruses
• A phage that reproduces only by the lytic cycle
is called a virulent phage
• Bacteria have defenses against phages,
including restriction enzymes that recognize
and cut up certain phage DNA
© 2014 Pearson Education, Inc.
Fig. 19.5
1 Attachment: T4 uses its tail fibers
The Lytic Cycle
2 Entry of phage
5
5 Release: Phage enzymes damage the cell
Wall, fluid enters the bacteria and it bursts.
DNA and
degradation of
host DNA
Phage assembly
4
Assembly: 3 separate sets
of proteins form phage heads,
tails, and tail fibers.
The phage genome is packaged into
the capsid as it forms.
Head
Tail Tail fibers
3 Synthesis of viral
genomes and
proteins using host
machinery.
The Lysogenic Cycle
• The lysogenic cycle replicates the phage genome
without destroying the host
• The viral DNA molecule is incorporated into the host
cell’s chromosome
• This integrated viral DNA is known as a prophage
• Every time the host divides, it copies the phage DNA
and passes the copies to daughter cells
• An environmental signal can trigger the virus genome
to exit the bacterial chromosome and switch to the
lytic mode
• Phages that use both the lytic and lysogenic cycles are
called temperate phages
© 2014 Pearson Education, Inc.
Figure 19.6 The lytic and lysogenic cycles of phage λ, a temperate phage
Phage
DNA
The phage injects its DNA.
Daughter cell
with prophage
Many cell
divisions
create many
infected
bacteria.
Phage DNA
circularizes.
Tail fiber
Phage
Bacterial
chromosome
Lytic
cycle
The cell lyses,
releasing
phages.
Prophage exits
chromosome.
Lysogenic
cycle
Prophage
Phage DNA and proteins
are synthesized and
assembled.
Prophage is copied
with bacterial
chromosome.
Phage DNA integrates
into bacterial
chromosome.
The lytic and lysogenic cycles
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
releases of progeny phages
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
Replicative Cycles of Animal Viruses
• There are two key variables used to
classify viruses that infect animals
– An RNA or DNA genome
– A single-stranded or double-stranded genome
• Whereas few bacteriophages have an
envelope or an RNA genome, many
animal viruses have both
Table 19.1
Replitive Cycles of Animal Envelope
Viruses
• Most animal viruses with RNA genomes have
an envelope.
• Viruses equipped with an outer glycoprotein
envelope use the envelope to enter the host cell.
– The envelope fuses with the host’s membrane, transporting the
capsid and the viral genome inside.
– In the reproductive cycle of an enveloped virus with an RNA
genome, viral glycoproteins for new envelopes are made by
ribosomes bound to the ER of the host cell, then modified by the
host’s Golgi apparatus.
– These glycoproteins are transported to the cell surface, where
they wrap themselves in membrane as they bud from the cell.
– The viral envelope is thus derived from the host’s plasma
membrane, although viral genes specify some of the molecules
in the membrane.
© 2014 Pearson Education, Inc.
Capsid
RNA
HOST CELL
Envelope (with
glycoproteins)
Template
Viral genome
(RNA)
mRNA
Capsid
proteins
ER
Glycoproteins
Figure 19.7 The replicative cycle of an
enveloped RNA virus
Copy of
genome
(RNA)
New virus
RNA as Viral Genetic Material
• In some viruses with single-stranded RNA (class IV), the
genome acts as mRNA and is translated into viral protein
immediately after infection.
• In others (class V), the RNA genome serves as a
template for complementary RNA strands, which function
both as mRNA and as templates for the synthesis of
additional copies of genome RNA.
• All viruses that require RNA  RNA synthesis to make
mRNA use a viral enzyme that is packaged with the
genome inside the capsid.
• Retroviruses (class VI) have the most complicated
life cycles.
• Retroviruses carry an enzyme called reverse
transcriptase that transcribes DNA from an RNA
template.
• This provides RNA  DNA information flow.
© 2014 Pearson Education, Inc.
HIV is a Retrovirus
HIV (human immunodeficiency virus) is the retrovirus
that causes AIDS (acquired immunodeficiency
syndrome)
HIV and other retroviruses are enveloped viruses that
contain two identical molecules of single-stranded
RNA and two molecules of reverse transcriptase.
The reproductive cycle of HIV illustrates the
pattern of infection and replication in a retrovirus.
– After HIV enters the host cell, reverse transcriptase
molecules are released into the cytoplasm and
catalyze the synthesis of viral DNA.
– The newly made viral DNA enters the cell’s
nucleus and is inserted as a permanent provirus
into a chromosome.
– The host’s RNA polymerase transcribes the proviral
DNA into RNA molecules that can function both as
mRNA for the synthesis of viral proteins and as
genomes for new virus particles released from the
cell.
Glycoprotein
Life Cycle of HIV
Viral envelope
HIV
Capsid
Reverse
transcriptase HIV
RNA (two
identical
strands)
Membrane
of white
blood cell
HOST
CELL
Reverse
transcriptase
Viral RNA
RNA-DNA
hybrid
0.25 µm
DNA
HIV entering a cell
NUCLEUS
Provirus
Chromosomal
DNA
RNA genome
for the
next viral
generation
mRNA
New virus
New HIV leaving a cell
Figure 19.8
The viral DNA that is integrated into the host genome is called a provirus.
Unlike a prophage, a provirus remains 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 Avirus particles released from the cell
© 2014 Pearson Education, Inc.
Evolution of Viruses
• Viruses do not fit our definition of living organisms
• Since viruses can reproduce only within cells, they
probably evolved as bits of cellular nucleic acid
• 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
• Mimivirus, a double-stranded DNA virus, is the
largest virus yet discovered
• There is controversy about whether this virus
evolved before or after cells
© 2014 Pearson Education, Inc.
Viruses, viroids, and prions are formidable
pathogens in animals and plants
• Diseases caused by viral infections affect humans,
agricultural crops, and livestock worldwide.
• Smaller, less complex entities called viroids and prions
also cause disease in plants and animals, respectively
• Some viruses damage or kill cells by triggering the
release of hydrolytic enzymes from lysosomes.
• Some viruses cause the infected cell to produce toxins
that lead to disease symptoms.
• Others viruses have molecular components, such as
envelope proteins, that are toxic.
– In some cases, viral damage is easily repaired (respiratory
epithelium after a cold), but in others, infection causes
permanent damage (nerve cells after polio).
– Many of the temporary symptoms associated with a viral
infection result from the body’s own efforts at defending itself
against infection.
© 2014 Pearson Education, Inc.
The Immune System
• The immune system is the body’s natural defense
mechanism against viral and other infections.
• Vaccines are harmless variants or derivatives of
pathogenic microbes, that stimulate the immune system
to mount defenses against the actual pathogen.
Vaccines can prevent certain viral illnesses.
• Vaccination has eradicated smallpox.
• Effective vaccines are available against polio, measles,
rubella, mumps, hepatitis B, and a number of other viral
diseases.
– Antiviral drugs can help to treat, though not cure, viral
infections.
– Viral infections cannot be treated by antibiotics.
– Most antiviral drugs resemble nucleosides and interfere with viral
nucleic acid synthesis.
© 2014 Pearson Education, Inc.
Emerging new Viruses
• The emergence of these new viral diseases is due to three
processes: mutation; the dissemination of a viral disease from a
small, isolated population; and the spread of existing viruses
from one species to another.
• RNA viruses tend to have high mutation rates because replication of
their nucleic acid lacks proofreading.
• Some mutations create new viral strains with sufficient genetic
differences from earlier strains that they can infect individuals who
had acquired immunity to these earlier strains.
• Flu epidemics are caused by new strains of influenza virus to which
people have little immunity. H1N1 virus of Influenza outbreak from
Mexico recently.
• These strains can cause pandemics, global epidemics.
• The “avian flu” is a virus that recently appeared in humans and
originated in wild birds
• Emerging viruses are those that appear suddenly or suddenly come
to the attention of scientists like Severe acute respiratory syndrome
(SARS) recently appeared in China.
© 2014 Pearson Education, Inc.
Figure 19.9
1 µm
(a) 2009 pandemic H1N1 (b) 2009 pandemic
screening
influenza A virus
(c) 1918 flu pandemic
Dissemination of viral disease
– A viral disease can spread from a small,
isolated population to become a
widespread epidemic.
• For example, AIDS went unnamed and virtually
unnoticed for decades before spreading around
the world.
• Technological and social factors, including
affordable international travel, blood transfusion
technology, sexual promiscuity, and the abuse of
intravenous drugs, allowed a previously rare
disease to become a global scourge.
© 2014 Pearson Education, Inc.
Viral spread between species
– A third source of new viral diseases is the
spread of existing viruses from one host
species to another.
• It is estimated that about three-quarters of
new human diseases originated in other
animals.
• For example, a species of bat has been
identified as the likely natural initiator of
the SARS virus.
© 2014 Pearson Education, Inc.
Flu epidemics illustrate the effects of
viruses moving between species
– There are three types of influenza virus: Types B and
C- infect only humans and have never caused an
epidemic
Type A- which infects a range of animals.
• Influenza A strains have caused three major flu epidemics
among humans in the last 100 years.
• The worst was the “Spanish flu” pandemic of 1918–1919,
which killed 40 million people.
• This strain probably originated in birds and then passed
between different species, undergoing mutations.
• In animals infected with multiple strains of flu virus, the
different strains underwent genetic recombination, leading to
the emergence of a virus capable of infecting human cells.
• Humans lacked immunity to this novel, virulent recombinant
virus.
© 2014 Pearson Education, Inc.
Viral Diseases in Plants
Figure 19.10
• More than 2,000 types of viral diseases of plants are known and
cause spots on leaves and fruits, stunted growth, and damaged
flowers or roots
• Most plant viruses have an RNA genome
• Viruses can spread from one plant cell to another through
plasmodesmata.
• Plant viruses spread disease in two major modes:
– Horizontal transmission, entering through damaged cell walls
– Vertical transmission, inheriting the virus from a parent
© 2014 Pearson Education, Inc.
Viroids and Prions: The Simplest
Infectious Agents
• Viroids are circular RNA molecules that
infect plants and disrupt their growth.
– These small RNA molecules can disrupt plant
metabolism and stunt plant growth, perhaps by
causing errors in the regulatory systems that
control plant growth.
– Viroids show that a single molecule can act as
an infectious agent to spread disease. Viroids
are small circular RNA molecules that infect
plants and disrupt their growth.
© 2014 Pearson Education, Inc.
Prions are infectious proteins that spread
disease
• Prions are slow-acting (10 year onset), virtually
indestructible infectious proteins that cause brain
diseases in mammals
• Prions propagate by converting normal proteins into
the prion version
• Scrapie in sheep, mad cow disease, and CreutzfeldtJakob disease in humans are all caused by prions
• Prions are likely transmitted in food.
– Prions are virtually indestructible. They are not
destroyed or deactivated by heating to normal
cooking temperatures.
• There is no known cure for prion diseases.
© 2014 Pearson Education, Inc.
How can a protein be a transmissible
pathogen?
Fig. 19.11
• How prions (probably) propagate:
1. Prions are just misfolded normal proteins.
2. When a prion comes in contact with a normally folded
protein, it might induce that protein to fold into the same
abnormal shape.
3. $ --- Aggregates of misfolded (prion) protiens may
just cause brain cells to be disfunctional and
eventually brain degeneration will occur.
Figure 19.UN01c
H1N1 flu vaccination