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Figure 19.1
Viruses: A Borrowed Life
0.5 mm
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The Discovery of Viruses: Scientific Inquiry
• 1935, Wendell Stanley crystallized tobacco
mosaic virus (TMV)
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Figure 19.2
RESULTS
3 Rubbed filtered
1 Extracted sap 2 Passed sap
through a
sap on healthy
from tobacco
porcelain filter
tobacco plants
plant with
known to trap
tobacco mosaic
bacteria
disease
4 Healthy plants
became infected
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Figure 19.2b
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Figure 19.2c
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Structure of Viruses
• Not cells
• Consists of nucleic acid enclosed in a protein coat
and, in some cases, a membranous envelope
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Viral Genomes
– Double- or single-stranded DNA, or
– Double- or single-stranded RNA
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Figure 19.3
Capsomere
RNA
DNA
Membranous
RNA
envelope
Capsid
Head
DNA
Tail
sheath
Capsomere
of capsid
Tail
fiber
Glycoprotein
18  250 nm
20 nm
(a) Tobacco
mosaic virus
Glycoproteins
70–90 nm (diameter) 80–200 nm (diameter)
50 nm
(b) Adenoviruses
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80  225 nm
50 nm
50 nm
(c) Influenza viruses (d) Bacteriophage T4
• Some viruses have viral envelopes
• Derived from the host cell’s membrane
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• Bacteriophages (phage)  viruses that infect
bacteria
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Lytic Cycle
1
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Attachment
Figure 19.5-2
1 Attachment
2 Entry of phage
DNA and
degradation
of host DNA
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Figure 19.5-3
1 Attachment
2 Entry of phage
DNA and
degradation
of host DNA
3 Synthesis of
viral genomes
and proteins
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Figure 19.5-4
1 Attachment
2 Entry of phage
DNA and
degradation
of host DNA
Phage assembly
4 Assembly
Head
Tail
Tail
fibers
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3 Synthesis of
viral genomes
and proteins
Figure 19.5-5
1 Attachment
2 Entry of phage
DNA and
degradation
of host DNA
5 Release
Phage assembly
4 Assembly
Head
Tail
Tail
fibers
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3 Synthesis of
viral genomes
and proteins
The Lysogenic Cycle
• Replicates the phage genome without destroying
the host
Animation: Phage Lambda Lysogenic and Lytic
Cycles
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Figure 19.6
Phage
DNA
Daughter cell
with prophage
The phage
injects its DNA.
Cell divisions
produce a
population of
bacteria infected
with the prophage.
Phage DNA
circularizes.
Phage
Bacterial
chromosome
Occasionally, a prophage
exits the bacterial chromosome,
initiating a lytic cycle.
Lytic cycle
The cell lyses, releasing phages.
Lysogenic cycle
Certain factors
determine whether
lytic cycle
is induced
New phage DNA and proteins
are synthesized and assembled
into phages.
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or
lysogenic cycle
is entered
Prophage
The bacterium reproduces,
copying the prophage and
transmitting it to daughter
cells.
Phage DNA integrates into
the bacterial chromosome,
becoming a prophage.
Figure 19.6a
Phage
DNA
The phage
injects its DNA.
Phage DNA
circularizes.
Phage
Bacterial
chromosome
Lytic cycle
The cell lyses, releasing phages.
Certain factors
determine whether
lytic cycle or lysogenic cycle
is entered
is induced
New phage DNA and proteins
are synthesized and assembled
into phages.
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Figure 19.6b
Daughter cell
with prophage
Cell divisions
produce a
population of
bacteria infected
with the prophage.
Phage DNA
circularizes.
Occasionally, a prophage
exits the bacterial chromosome,
initiating a lytic cycle.
Lysogenic cycle
Certain factors
determine whether
lytic cycle or lysogenic cycle
Prophage
is entered
is induced
The bacterium reproduces,
copying the prophage and
transmitting it to daughter
cells.
Phage DNA integrates into
the bacterial chromosome,
becoming a prophage.
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Table 19.1a
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Table 19.1b
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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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Education, Inc.
Figure 19.8a
Glycoprotein
Viral envelope
Capsid
RNA (two
identical
strands)
Reverse
transcriptase
HOST
CELL
HIV
Viral RNA
Reverse
transcriptase
RNA-DNA
hybrid
DNA
Chromosomal
DNA
RNA genome
for the
next viral
generation
New virus
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NUCLEUS
Provirus
mRNA
Figure 19.8c
HIV
Membrane
of white
blood cell
HIV entering a cell
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Figure 19.8d
0.25 m
HIV entering a cell
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Figure 19.8e
New HIV leaving a cell
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Figure 19.8f
New HIV leaving a cell
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Figure 19.8g
New HIV leaving a cell
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Evolution of Viruses
• Probably evolved as bits of cellular nucleic acid
• e.g. Plasmids, transposons, and viruses are all
mobile genetic elements
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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 harmless derivatives of pathogenic
microbes that stimulate the immune system to
mount defenses against the harmful pathogen
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Emerging Viruses
• e.g. influenza virus H1N1 Flu epidemics
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• Viral diseases in a small isolated population can
emerge and become global
• Viral strains that jump species can exchange
genetic information with other viruses to which
humans have no immunity
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• Can cause pandemics, global epidemics
• The 2009 flu pandemic was likely passed to
humans from pigs; for this reason it was originally
called the “swine flu”
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Figure 19.9a
1 m
(a) 2009 pandemic H1N1
influenza A virus
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Figure 19.9b
(b) 2009 pandemic screening
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Figure 19.9c
(c) 1918 flu pandemic
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Viral Diseases in Plants
• Most plant viruses have an RNA genome
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Figure 19.10a
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Figure 19.10c
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Viroids and Prions: The Simplest Infectious Agents
• Viroids are small circular RNA molecules that
infect plants and disrupt their growth
• Prions are slow-acting, virtually indestructible
infectious proteins that cause brain diseases in
mammals
• Convert normal proteins into the prion version
• e.g. mad cow disease, and Creutzfeldt-Jakob
disease in humans are all caused by prions
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Education, Inc.
Figure 19.11
Prion
Normal
protein
Original
prion
New
prion
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Aggregates
of prions