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Chapter 17 - Viruses.pdf
- 1. CAMPBELL BIOLOGY IN FOCUS
© 2014 Pearson Education, Inc.
Urry • Cain • Wasserman • Minorsky • Jackson • Reece
Lecture Presentations by
Kathleen Fitzpatrick and Nicole Tunbridge
17
Viruses
- 2. © 2014 Pearson Education, Inc.
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
- 4. © 2014 Pearson Education, Inc.
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
- 5. © 2014 Pearson Education, Inc.
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
- 6. © 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
- 7. © 2014 Pearson Education, Inc.
Figure 17.2
RNA
Capsomere
of capsid
Glycoproteins
Capsomere Membranous
envelope
RNA
Capsid Head
DNA
DNA
Tail
sheath
Tail
fiber
Glycoprotein
80 × 225 nm
80–200 nm (diameter)
70–90 nm (diameter)
18 × 250 nm
a) Tobacco mosaic
virus
20 nm
(b) Adenoviruses
50 nm
(c) Influenza viruses
50 nm
(d) Bacteriophage T4
50 nm
- 8. © 2014 Pearson Education, Inc.
Figure 17.2a
RNA
Capsomere
of capsid
Capsomere
DNA
Glycoprotein
70–90 nm (diameter)
18 × 250 nm
(a) Tobacco mosaic
virus
20 nm
(b) Adenoviruses
50 nm
- 9. © 2014 Pearson Education, Inc.
Figure 17.2aa
(a) Tobacco mosaic virus
20 nm
- 10. © 2014 Pearson Education, Inc.
Figure 17.2ab
(b) Adenoviruses
50 nm
- 11. © 2014 Pearson Education, Inc.
Figure 17.2b
Membranous
envelope
RNA
Capsid
Head
DNA
Tail
sheath
Tail
fiber
80 × 225 nm
80–200 nm (diameter)
(c) Influenza viruses
50 nm
(d) Bacteriophage T4
50 nm
Glycoproteins
- 12. © 2014 Pearson Education, Inc.
Figure 17.2ba
(c) Influenza viruses
50 nm
- 13. © 2014 Pearson Education, Inc.
Figure 17.2bb
(d) Bacteriophage T4
50 nm
- 14. © 2014 Pearson Education, Inc.
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
- 15. © 2014 Pearson Education, Inc.
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
- 16. © 2014 Pearson Education, Inc.
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
- 17. © 2014 Pearson Education, Inc.
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
- 18. © 2014 Pearson Education, Inc.
Figure 17.3
VIRUS
Replication
Entry and
uncoating
DNA
Capsid
Transcription and
manufacture of
capsid proteins
HOST
CELL
Viral DNA
Viral DNA
mRNA
Capsid
proteins
1
2
3
4 Self-assembly of
new virus particles
and their exit from
the cell
- 19. © 2014 Pearson Education, Inc.
Replicative Cycles of Phages
Phages are the best understood of all viruses
Phages have two reproductive mechanisms: the
lytic cycle and the lysogenic cycle
- 20. © 2014 Pearson Education, Inc.
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
- 22. © 2014 Pearson Education, Inc.
Figure 17.4-2
Attachment
1
Entry of phage
DNA and
degradation
of host DNA
2
- 23. © 2014 Pearson Education, Inc.
Figure 17.4-3
Attachment
1
Entry of phage
DNA and
degradation
of host DNA
2
Synthesis of
viral genomes
and proteins
3
- 24. © 2014 Pearson Education, Inc.
Figure 17.4-4
Attachment
1
Entry of phage
DNA and
degradation
of host DNA
2
Synthesis of
viral genomes
and proteins
3
Assembly
Phage assembly
Head Tail Tail
fibers
4
- 25. © 2014 Pearson Education, Inc.
Figure 17.4-5
Attachment
1
Entry of phage
DNA and
degradation
of host DNA
2
Synthesis of
viral genomes
and proteins
3
Assembly
Phage assembly
Head Tail Tail
fibers
4
Release
5
- 26. © 2014 Pearson Education, Inc.
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
- 27. © 2014 Pearson Education, Inc.
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 λ Lysogenic and Lytic Cycles
- 28. © 2014 Pearson Education, Inc.
Figure 17.5
The phage injects its DNA.
Daughter cell
with prophage
Many cell
divisions
create many
infected
bacteria.
Prophage is copied
with bacterial
chromosome.
Phage DNA integrates into
bacterial chromosome.
Phage DNA and proteins are
synthesized and assembled.
The cell lyses, releasing phages.
Lytic cycle Lysogenic cycle
Prophage exits
chromosome.
Phage DNA
circularizes.
Phage
DNA
Phage
Bacterial
chromosome
Prophage
- 29. © 2014 Pearson Education, Inc.
Figure 17.5a
The phage injects its DNA.
Phage DNA and proteins are
synthesized and assembled.
The cell lyses, releasing phages.
Lytic cycle
Phage DNA
circularizes.
Phage
DNA
Phage
Bacterial
chromosome
- 30. © 2014 Pearson Education, Inc.
Figure 17.5b
Daughter cell
with prophage
Many cell
divisions
create many
infected
bacteria.
Prophage is copied
with bacterial
chromosome.
Phage DNA integrates into
bacterial chromosome.
Lysogenic cycle
Prophage exits
chromosome.
Prophage
- 31. © 2014 Pearson Education, Inc.
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
- 32. © 2014 Pearson Education, Inc.
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
- 33. © 2014 Pearson Education, Inc.
Figure 17.6
Capsid
RNA
Envelope (with
glycoproteins)
HOST CELL
(RNA)
New virus
Copy of genome
Viral genome
(RNA)
Template
mRNA
ER
Glycoproteins
Capsid
proteins
- 34. © 2014 Pearson Education, Inc.
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)
- 35. © 2014 Pearson Education, Inc.
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
- 36. © 2014 Pearson Education, Inc.
Figure 17.7
Reverse
transcriptase
HIV
Glycoprotein
Viral envelope
Capsid
RNA (two
identical
strands)
HOST
CELL
Reverse
transcriptase
Viral RNA
RNA-DNA
hybrid
DNA
NUCLEUS
Chromosomal
DNA
RNA genome
for the next
viral generation mRNA
HIV
Membrane of
white blood cell
HIV entering a cell
0.25 µm
Provirus
New virus
New HIV leaving a cell
- 37. © 2014 Pearson Education, Inc.
Figure 17.7a
Reverse
transcriptase
HIV
Glycoprotein
Viral envelope
Capsid
RNA (two
identical
strands)
HOST
CELL
Reverse
transcriptase
Viral RNA
RNA-DNA
hybrid
DNA
NUCLEUS
Chromosomal
DNA
RNA genome
for the next
viral generation mRNA
Provirus
New virus
- 38. © 2014 Pearson Education, Inc.
Figure 17.7aa
Reverse
transcriptase
HIV
Glycoprotein
Viral envelope
Capsid
RNA (two
identical
strands)
HOST
CELL
Reverse
transcriptase
Viral RNA
RNA-DNA
hybrid
DNA
- 39. © 2014 Pearson Education, Inc.
Figure 17.7ab
NUCLEUS
Chromosomal
DNA
RNA genome
for the next
viral generation mRNA
Provirus
New virus
- 40. © 2014 Pearson Education, Inc.
Figure 17.7b
HIV
Membrane of
white blood cell
HIV entering a cell
0.25 µm
New HIV leaving a cell
- 41. © 2014 Pearson Education, Inc.
Figure 17.7ba
HIV
Membrane of
white blood cell
HIV entering a cell
0.25 µm
- 42. © 2014 Pearson Education, Inc.
Figure 17.7bb
HIV entering a cell
0.25 µm
- 43. © 2014 Pearson Education, Inc.
Figure 17.7bc
New HIV leaving a cell
0.25 µm
- 44. © 2014 Pearson Education, Inc.
Figure 17.7bd
New HIV leaving a cell
0.25 µm
- 45. © 2014 Pearson Education, Inc.
Figure 17.7be
New HIV leaving a cell
0.25 µm
- 46. © 2014 Pearson Education, Inc.
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
- 47. © 2014 Pearson Education, Inc.
Concept 17.3: Viruses are formidable pathogens
in animals and plants
Diseases caused by viral infections afflict humans,
agricultural crops, and livestock worldwide
- 48. © 2014 Pearson Education, Inc.
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
- 49. © 2014 Pearson Education, Inc.
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
- 50. © 2014 Pearson Education, Inc.
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
- 51. © 2014 Pearson Education, Inc.
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
- 52. © 2014 Pearson Education, Inc.
Figure 17.8
(a) 2009 pandemic H1N1
influenza A virus
(b) 2009 pandemic screening
1 µm
- 53. © 2014 Pearson Education, Inc.
Figure 17.8a
(a) 2009 pandemic H1N1
influenza A virus
1 µm
- 54. © 2014 Pearson Education, Inc.
Figure 17.8b
(b) 2009 pandemic screening
- 55. © 2014 Pearson Education, Inc.
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
- 56. © 2014 Pearson Education, Inc.
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
- 57. © 2014 Pearson Education, Inc.
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
- 58. © 2014 Pearson Education, Inc.
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
- 60. © 2014 Pearson Education, Inc.
Figure 17.UN02a
A/California/07/2009 Group 1
A/Taiwan/1164/2010 Group 3
A/Taiwan/T0724/2009
Group 6
Group 7
Group 9
A/Taiwan/1018/2011
A/Taiwan/552/2011
Group 8
Group 8-1
Group 10
Group 11
A/Taiwan/T1773/2009
A/Taiwan/T1338/2009
A/Taiwan/T1821/2009
A/Taiwan/940/2009
A/Taiwan/937/2009
A/Taiwan/T1339/2009
A/Taiwan/7418/2009
A/Taiwan/8575/2009
A/Taiwan/4909/2009
A/Taiwan/8542/2009
A/Taiwan/2826/2009
A/Taiwan/T0826/2009
A/Taiwan/1017/2009
A/Taiwan/7873/2009
A/Taiwan/11706/2009
A/Taiwan/6078/2009
A/Taiwan/6341/2009
A/Taiwan/6200/2009
A/Taiwan/5270/2010
A/Taiwan/3994/2010
A/Taiwan/2649/2011
A/Taiwan/1102/2011
A/Taiwan/4501/2011
A/Taiwan/67/2011
A/Taiwan/1749/2011
A/Taiwan/4611/2011
A/Taiwan/5506/2011
A/Taiwan/1150/2011
A/Taiwan/2883/2011
A/Taiwan/842/2010
A/Taiwan/3697/2011
- 61. © 2014 Pearson Education, Inc.
Figure 17.UN02aa
A/California/07/2009 Group 1
A/Taiwan/1164/2010 Group 3
A/Taiwan/T0724/2009
Group 6
Group 7
Group 9
A/Taiwan/1018/2011
A/Taiwan/552/2011
A/Taiwan/T1773/2009
A/Taiwan/T1338/2009
A/Taiwan/T1821/2009
A/Taiwan/940/2009
A/Taiwan/937/2009
A/Taiwan/T1339/2009
A/Taiwan/7418/2009
A/Taiwan/8575/2009
A/Taiwan/4909/2009
A/Taiwan/8542/2009
A/Taiwan/2826/2009
A/Taiwan/T0826/2009
- 62. © 2014 Pearson Education, Inc.
Figure 17.UN02ab
Group 8
Group 8-1
Group 10
Group 11
A/Taiwan/1017/2009
A/Taiwan/7873/2009
A/Taiwan/11706/2009
A/Taiwan/6078/2009
A/Taiwan/6341/2009
A/Taiwan/6200/2009
A/Taiwan/5270/2010
A/Taiwan/3994/2010
A/Taiwan/2649/2011
A/Taiwan/1102/2011
A/Taiwan/4501/2011
A/Taiwan/67/2011
A/Taiwan/1749/2011
A/Taiwan/4611/2011
A/Taiwan/5506/2011
A/Taiwan/1150/2011
A/Taiwan/2883/2011
A/Taiwan/842/2010
A/Taiwan/3697/2011
- 63. © 2014 Pearson Education, Inc.
Figure 17.UN02b
Wave 1 Wave 2 Wave 3
Interwave
Key
Groups 1, 3, 6
Group 7
Group 8
Group 8-1
Group 9
Group 10
Group 11
800
700
600
500
400
300
200
100
0
Number
of
viral
isolates
M J
2009
J A S O N D J F M A M J J A S O N D J F M A
2010 2011
- 64. © 2014 Pearson Education, Inc.
Figure 17.UN03
Phage
DNA
Prophage
Bacterial
chromosome
The phage attaches to a
host cell and injects its DNA.
Lysogenic cycle
Lytic 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 to
daughter cells and
(2) can be induced to leave the
chromosome and initiate a lytic
cycle
- 65. © 2014 Pearson Education, Inc.
Figure 17.UN04
A B
Time
Time
Number
of
bacteria
Number
of
viruses