Post on 14-Jan-2017
CAMPBELL BIOLOGY IN FOCUS
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Urry • Cain • Wasserman • Minorsky • Jackson • Reece
Lecture Presentations by Kathleen Fitzpatrick and Nicole Tunbridge
17Viruses
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Overview: A Borrowed Life
A virus is an infectious particle consisting of little more than genes packaged into a protein coat
Viruses lead “a kind of borrowed life,” existing in a shady area between life-forms and chemicals
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Figure 17.1
0.25 m
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Concept 17.1: A virus consists of a nucleic acid surrounded by a protein coat
Even the largest known virus is barely visible under the light microscope
Some viruses can be crystalized Viruses are not cells but are a nucleic acid enclosed
in a protein coat
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Viral Genomes
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
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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
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Figure 17.2
RNA
Capsomereof capsid
Glycoproteins
Capsomere Membranousenvelope RNA
Capsid HeadDNA
DNA
Tailsheath
Tailfiber
Glycoprotein80 225 nm80–200 nm (diameter)70–90 nm (diameter)18 250 nm
(a) Tobacco mosaicvirus
20 nm(b) Adenoviruses
50 nm(c) Influenza viruses
50 nm(d) Bacteriophage T4
50 nm
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Figure 17.2a
RNA
Capsomereof capsid
CapsomereDNA
Glycoprotein70–90 nm (diameter)18 250 nm
(a) Tobacco mosaicvirus
20 nm(b) Adenoviruses
50 nm
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Figure 17.2aa
(a) Tobacco mosaic virus20 nm
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Figure 17.2ab
(b) Adenoviruses50 nm
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Figure 17.2b
Membranousenvelope RNA
CapsidHead
DNA
Tailsheath
Tailfiber
80 225 nm80–200 nm (diameter)
(c) Influenza viruses50 nm
(d) Bacteriophage T450 nm
Glycoproteins
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Figure 17.2ba
(c) Influenza viruses50 nm
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Figure 17.2bb
(d) Bacteriophage T450 nm
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Some viruses have membranous envelopes that help them infect hosts
These viral envelopes are derived from the host cell’s membrane and contain a combination of viral and host cell molecules
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Bacteriophages, also called phages, are viruses that infect bacteria
They have the most complex capsids found among viruses
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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Concept 17.2: 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
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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 self-assemble into new viruses
These exit from the host cell, usually damaging or destroying it
Animation: Simplified Viral Replicative Cycle
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Figure 17.3VIRUS
Replication
Entry anduncoating
DNA
Capsid Transcription andmanufacture ofcapsid proteins
HOSTCELL
Viral DNA
Viral DNA
mRNA
Capsidproteins
1
2
3
4 Self-assembly ofnew virus particlesand their exit fromthe cell
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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
The lytic cycle produces new phages and lyses (breaks open) 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
Animation: Phage T4 Lytic Cycle
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Figure 17.4-1
Attachment1
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Figure 17.4-2
Attachment1
Entry of phageDNA anddegradationof host DNA
2
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Figure 17.4-3
Attachment1
Entry of phageDNA anddegradationof host DNA
2
Synthesis ofviral genomesand proteins
3
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Figure 17.4-4
Attachment1
Entry of phageDNA anddegradationof host DNA
2
Synthesis ofviral genomesand proteins
3Assembly
Phage assembly
Head Tail Tailfibers
4
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Figure 17.4-5
Attachment1
Entry of phageDNA anddegradationof host DNA
2
Synthesis ofviral genomesand proteins
3Assembly
Phage assembly
Head Tail Tailfibers
4
Release5
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The Lysogenic Cycle
The lysogenic cycle replicates the phage genome without destroying the host
Phages that use both the lytic and lysogenic cycles are called temperate phages
The viral DNA molecule is incorporated into the host cell’s chromosome
This integrated viral DNA is known as a prophage
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Every time the host divides, it copies the phage DNA and passes the copies to daughter cells
A single infected cell can give rise to a large population of bacteria carrying the virus in prophage form
An environmental signal can trigger the virus genome to exit the bacterial chromosome and switch to the lytic mode
Animation: Phage Lysogenic and Lytic Cycles
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Figure 17.5
The phage injects its DNA.Daughter cellwith prophage
Many celldivisionscreate manyinfectedbacteria.
Prophage is copiedwith bacterialchromosome.
Phage DNA integrates intobacterial chromosome.
Phage DNA and proteins aresynthesized and assembled.
The cell lyses, releasing phages.
Lytic cycle Lysogenic cycle
Prophage exitschromosome.
Phage DNAcircularizes.
PhageDNA
Phage
Bacterialchromosome
Prophage
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Figure 17.5a
The phage injects its DNA.
Phage DNA and proteins aresynthesized and assembled.
The cell lyses, releasing phages.
Lytic cycle
Phage DNAcircularizes.
PhageDNA
PhageBacterialchromosome
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Figure 17.5bDaughter cellwith prophage
Many celldivisionscreate manyinfectedbacteria.
Prophage is copiedwith bacterialchromosome.
Phage DNA integrates intobacterial chromosome.
Lysogenic cycle
Prophage exitschromosome.
Prophage
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Replicative Cycles of Animal Viruses
There are two key variables used to classify viruses that infect animals The nature of the viral genome (single- or double-
stranded DNA or RNA) The presence or absence of an envelope
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Viral 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
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Figure 17.6
Capsid
RNA
Envelope (withglycoproteins)
HOST CELL
(RNA)
New virus
Copy of genome
Viral genome(RNA)Template
mRNA
ER
Glycoproteins
Capsidproteins
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RNA as Viral Genetic Material
The broadest variety of RNA genomes is found in viruses that infect animals
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
Unlike a prophage, 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
Animation: HIV Replicative Cycle
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Figure 17.7
Reversetranscriptase
HIV
GlycoproteinViral envelope
Capsid
RNA (twoidenticalstrands)
HOSTCELL
ReversetranscriptaseViral RNA
RNA-DNAhybrid
DNA
NUCLEUS
ChromosomalDNA
RNA genomefor the nextviral generation mRNA
HIVMembrane ofwhite blood cell
HIV entering a cell0.25 m
Provirus
New virusNew HIV leaving a cell
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Figure 17.7a
Reversetranscriptase
HIV
GlycoproteinViral envelope
Capsid
RNA (twoidenticalstrands)
HOSTCELL
ReversetranscriptaseViral RNA
RNA-DNAhybrid
DNA
NUCLEUS
ChromosomalDNA
RNA genomefor the nextviral generation mRNA
Provirus
New virus
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Figure 17.7aa
Reversetranscriptase
HIV
GlycoproteinViral envelope
Capsid
RNA (twoidenticalstrands)
HOSTCELL
ReversetranscriptaseViral RNA
RNA-DNAhybrid
DNA
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Figure 17.7ab
NUCLEUS
ChromosomalDNA
RNA genomefor the nextviral generation mRNA
Provirus
New virus
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Figure 17.7b
HIVMembrane ofwhite blood cell
HIV entering a cell0.25 m
New HIV leaving a cell
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Figure 17.7ba
HIVMembrane ofwhite blood cell
HIV entering a cell0.25 m
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Figure 17.7bb
HIV entering a cell0.25 m
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Figure 17.7bc
New HIV leaving a cell0.25 m
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Figure 17.7bd
New HIV leaving a cell0.25 m
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Figure 17.7be
New HIV leaving a cell0.25 m
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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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Concept 17.3: Viruses are formidable pathogens in animals and plants
Diseases caused by viral infections afflict humans, agricultural crops, and livestock worldwide
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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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A vaccine is a harmless derivative of a pathogen that stimulates the immune system to mount defenses against the harmful pathogen
Vaccines can prevent certain viral illnesses Viral infections cannot be treated by antibiotics Antiviral drugs can help to treat, though not cure,
viral infections
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Emerging Viruses
Viruses that suddenly become apparent are called emerging viruses
HIV is a classic example The West Nile virus appeared in North America
first in 1999 and has now spread to all 48 contiguous states
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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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Figure 17.8
(a) 2009 pandemic H1N1 influenza A virus
(b) 2009 pandemic screening
1 m
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Figure 17.8a
(a) 2009 pandemic H1N1 influenza A virus
1 m
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Figure 17.8b
(b) 2009 pandemic screening
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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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Strains of influenza A are given standardized names The name H1N1 identifies forms of two viral surface
proteins, hemagglutinin (H) and neuraminidase (N) There are numerous types of hemagglutinin and
neuraminidase, identified by numbers
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Viral Diseases in Plants
More than 2,000 types of viral diseases of plants are known; these have enormous impacts on the agricultural and horticultural industries
Plant viruses have the same basic structure and mode of replication as animal viruses
Most plant viruses known thus far have an RNA genome and many have a helical capsid
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Plant viral diseases spread by two major routes Infection from an external source of virus is called
horizontal transmission Herbivores, especially insects, pose a double threat
because they can both carry a virus and help it get past the plant’s outer layer of cells
Inheritance of the virus from a parent is called vertical transmission
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Figure 17.UN01
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Figure 17.UN02a
A/California/07/2009 Group 1A/Taiwan/1164/2010 Group 3
A/Taiwan/T0724/2009
Group 6
Group 7
Group 9A/Taiwan/1018/2011A/Taiwan/552/2011
Group 8
Group 8-1
Group 10
Group 11
A/Taiwan/T1773/2009A/Taiwan/T1338/2009
A/Taiwan/T1821/2009
A/Taiwan/940/2009
A/Taiwan/937/2009A/Taiwan/T1339/2009
A/Taiwan/7418/2009A/Taiwan/8575/2009A/Taiwan/4909/2009
A/Taiwan/8542/2009
A/Taiwan/2826/2009A/Taiwan/T0826/2009
A/Taiwan/1017/2009A/Taiwan/7873/2009
A/Taiwan/11706/2009A/Taiwan/6078/2009
A/Taiwan/6341/2009A/Taiwan/6200/2009
A/Taiwan/5270/2010A/Taiwan/3994/2010
A/Taiwan/2649/2011A/Taiwan/1102/2011
A/Taiwan/4501/2011A/Taiwan/67/2011
A/Taiwan/1749/2011A/Taiwan/4611/2011
A/Taiwan/5506/2011A/Taiwan/1150/2011
A/Taiwan/2883/2011A/Taiwan/842/2010
A/Taiwan/3697/2011
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Figure 17.UN02aa
A/California/07/2009 Group 1A/Taiwan/1164/2010 Group 3
A/Taiwan/T0724/2009
Group 6
Group 7
Group 9A/Taiwan/1018/2011A/Taiwan/552/2011
A/Taiwan/T1773/2009A/Taiwan/T1338/2009
A/Taiwan/T1821/2009
A/Taiwan/940/2009
A/Taiwan/937/2009A/Taiwan/T1339/2009
A/Taiwan/7418/2009A/Taiwan/8575/2009A/Taiwan/4909/2009
A/Taiwan/8542/2009
A/Taiwan/2826/2009A/Taiwan/T0826/2009
© 2014 Pearson Education, Inc.
Figure 17.UN02ab
Group 8
Group 8-1
Group 10
Group 11
A/Taiwan/1017/2009A/Taiwan/7873/2009
A/Taiwan/11706/2009A/Taiwan/6078/2009
A/Taiwan/6341/2009A/Taiwan/6200/2009
A/Taiwan/5270/2010A/Taiwan/3994/2010
A/Taiwan/2649/2011A/Taiwan/1102/2011
A/Taiwan/4501/2011A/Taiwan/67/2011
A/Taiwan/1749/2011A/Taiwan/4611/2011
A/Taiwan/5506/2011A/Taiwan/1150/2011
A/Taiwan/2883/2011A/Taiwan/842/2010
A/Taiwan/3697/2011
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Figure 17.UN02b
Wave 1 Wave 2 Wave 3Interwave
Key
Groups 1, 3, 6Group 7Group 8Group 8-1Group 9Group 10Group 11
800700600500400300
200100
0
Num
ber o
f vira
l iso
late
s
M J2009
J A S O N D J F M A M J J A S O N D J F M A2010 2011
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Figure 17.UN03
PhageDNA
ProphageBacterialchromosome
The phage attaches to ahost cell and injects its DNA.
Lysogenic cycleLytic cycle• Virulent or temperate phage• Destruction of host DNA• Production of new phages• Lysis of host cell causes release
of progeny phages
• Temperate phage only• Genome integrates into bacterial
chromosome as prophage, which(1) is replicated and passed on todaughter cells and(2) can be induced to leave thechromosome and initiate a lyticcycle
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Figure 17.UN04
A B
TimeTime
Num
ber o
f bac
teria
Num
ber o
f viru
ses