Viruses consist of either DNA or RNA surrounded by a protein coat. They can only replicate inside of host cells by using the host's cellular machinery. Viruses have a variety of structures depending on whether they have an envelope derived from the host cell membrane and the structure of their protein coat. They use different replication cycles like the lytic cycle which destroys the host cell or the lysogenic cycle where the viral DNA is incorporated into the host genome. Retroviruses like HIV replicate through reverse transcribing their RNA into DNA which then integrates into the host cell genome.

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

3. © 2014 Pearson Education, Inc.
Figure 17.1
0.25 µm

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 nm80–200 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 nm80–200 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

21. © 2014 Pearson Education, Inc.
Figure 17.4-1
Attachment1

22. © 2014 Pearson Education, Inc.
Figure 17.4-2
Attachment1
Entry of phage
DNA and
degradation
of host DNA
2

23. © 2014 Pearson Education, Inc.
Figure 17.4-3
Attachment1
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
Attachment1
Entry of phage
DNA and
degradation
of host DNA
2
Synthesis of
viral genomes
and proteins
3Assembly
Phage assembly
Head Tail Tail
fibers
4

25. © 2014 Pearson Education, Inc.
Figure 17.4-5
Attachment1
Entry of phage
DNA and
degradation
of host DNA
2
Synthesis of
viral genomes
and proteins
3Assembly
Phage assembly
Head Tail Tail
fibers
4
Release5

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

59. © 2014 Pearson Education, Inc.
Figure 17.UN01

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 3Interwave
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
Numberofviralisolates
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 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 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
TimeTime
Numberofbacteria
Numberofviruses