CAMPBELL
BIOLOGY
Outline
TENTH
EDITION
Reece • Urry • Cain • Wasserman • Minorsky • Jackson
19
I. Viruses
Viruses
A.
B.
C.
D.
Structure of viruses
Common Characteristics of Viruses
Viral replication
HIV
Lecture Presentation by
Dr Burns
NVC Biol 120
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The Good the Bad and the Ugly
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Structural Features and Characteristics
 Non cellular, viral particles = virions
 Viruses – fit into the bad category
 Virions are very small
 Viruses are not like other living organisms.
 They are acellular, which means they don’t
have their own cells – they hijack other cells
and use them to reproduce themselves.
 Viruses may remain inactive or latent in the
host for many years.
 Viruses lack ribosomes
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Structural Features and Characteristics
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Viral Genomes
 Viruses usually have:
 Viral genomes may consist of either
1. Genetic material (DNA or RNA)
2. Protein coat = capsid
3. Some may have a fatty membrane = envelope
 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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Figure 19.3
capsid
RNA
DNA
Viruses vary in size, as well as in shape
Membranous
RNA
envelope
Capsid
DNA
Head
Tail
sheath
capsid
Tail
fiber
Glycoprotein
18  250 nm
(a) Tobacco
mosaic virus
20 nm
Glycoproteins
70–90 nm (diameter)
80–200 nm (diameter)
80  225 nm
50 nm
(b) Adenoviruses
50 nm
(c) Influenza viruses
50 nm
(d) Bacteriophage T4
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Bacteriophage Virus
Figure 19.1
0.5 mm
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Are Viruses Alive?
Characteristics of Living Organisms
 They are acellular
1. Contain biological molecules including:
Proteins, nucleic acids, carbohydrates and
lipids
2. Cellular
3. Reproduce
4. Acquire and use energy - Metabolism
5. Growth and Development
6. Respond to environment
7. Maintain Homeostasis
8. Populations of living organisms evolve and
have adaptive traits
 They can’t perform metabolic activity and
reproduction without a host
 Contains either DNA or RNA
 Do not contain ribosomes, lack the enzymes
needed to transcribe DNA
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Types of viruses - Retrovirus
 Retroviruses – contain RNA and an
enzyme reverse transcriptase.
Types of viruses - Bacteriophages
 Bacteriophages – viruses that infect
bacteria
 Reverse transcriptase turns RNA into DNA
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DNA VIRUS
Figure 19.4
1 Entry and
uncoating
3 Transcription
and manufacture of
capsid proteins
Capsid
2 Replication
HOST
CELL
Viral DNA
mRNA
Viral
DNA
4 Self-assembly of
new virus particles
and their exit from
the cell
Animation: Simplified Viral Reproductive Cycle
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Capsid
proteins
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Replicative Cycles of Phages
The Lytic Cycle
 Phages have two reproductive mechanisms: the
lytic cycle and the lysogenic 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
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Figure 19.5-1
1 Attachment
Animation: Phage T4 Lytic Cycle
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Figure 19.5-2
Figure 19.5-3
1 Attachment
1 Attachment
2 Entry of phage
DNA and
degradation
of host DNA
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
Figure 19.5-5
1 Attachment
1 Attachment
2 Entry of phage
DNA and
degradation
of host DNA
Phage assembly
Tail
Release
5
Phage assembly
4 Assembly
Head
2 Entry of phage
DNA and
degradation
of host DNA
Tail
fibers
3 Synthesis of
viral genomes
and proteins
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4 Assembly
Head
Tail
3 Synthesis of
viral genomes
and proteins
Tail
fibers
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Lysogenic cycle
 Some viruses don’t immediately kill their hosts
 They integrate their DNA into the host DNA
 When the viral DNA is integrated into the host
DNA, the genetic material it is called a
prophage, the infected cell is the lysogen
 Induction: The virus will switch to the lytic
phase
0.25 µm
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Daughter cell
with prophage
Figure 19.6b
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
lysogenic cycle
lytic cycle
or
Prophage
is entered
is induced
Animation: Phage Lambda Lysogenic and Lytic Cycles
Phage DNA integrates into
the bacterial chromosome,
becoming a prophage.
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The bacterium reproduces,
copying the prophage and
transmitting it to daughter
cells.
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Figure 19.6a
Phage
DNA
Replicative Cycles of Animal Viruses
The phage
injects its DNA.
Phage DNA
circularizes.
Phage
Bacterial
chromosome
 There are two key variables used to classify
viruses that infect animals
 DNA or RNA?
 Single-stranded or double-stranded?
Lytic cycle
The cell lyses, releasing phages.
Certain factors
determine whether
lysogenic cycle
lytic cycle
or
is entered
is induced
New phage DNA and proteins
are synthesized and assembled
into phages.
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2009 Pearson
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Figure 19.8a
HIV - Retrovirus
Glycoprotein
Viral envelope
Capsid
RNA (two
identical
strands)
Reverse
transcriptase
HOST
CELL
HIV
 Retroviruses have RNA which gets
changed to a a double stranded DNA
Viral RNA
Reverse
transcriptase
RNA-DNA
hybrid
DNA
 Retroviruses have an enzyme called
reverse transcriptase to do this
Chromosomal
DNA
RNA genome
for the
next viral
generation
 HIV is a retrovirus, HIV uses the lysogenic
cycle then switches to the lytic cycle
NUCLEUS
Provirus
mRNA
New virus
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Figure 19.8b
HIV
Membrane
of white
blood cell
0.25 m
HIV entering a cell
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Animation: HIV Reproductive Cycle
New HIV leaving a cell
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Steps in HIV Virus Replication
1. Attach to a host cell = helper T Cell (CD4
cells)


HIV envelope has gp120 fits CD4 receptor on T
cell
Then binds with coreceptor ie CCR5
2. Penetrate host cell using endocytosis
3. Viral RNA is turned into viral DNA

Using the enzyme reverse transcriptase
Steps in HIV Virus Replication
5. Transcribe the integrated viral DNA to make
viral mRNA
6. Protein synthesis
7. Assemble new viral particles

Using the enzyme protease
8. Release = new particles bud off
4. Integrate the viral DNA into the host

Using the enzyme integrase
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Fig. 27.6
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HIV
 Human Immunodeficiency Virus: Cause AIDS
 HIV is a particularly devastating virus because
it attacks the hosts immune system
 The human immune system has T cells to
protect us against foreign bodies like viruses
and bacteria.
 T cells have receptors on their surface that
recognize foreign vs our own cells
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HIV Infection
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HIV Infection
 HIV receptors on the surface of the envelope
bind with the receptors CD4 and coreceptor
CCR5) on the T cells, penetrated host cell
 Reverse transcriptase does the opposite of
RNA polymerase: It turns RNA into a double
stranded DNA molecule
 Protein coat (capsid) dissolves
 Integrase puts this viral DNA into the T cell’s
DNA
 HIV has three main enzymes:
 Integrase, reverse transcriptase and
protease
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 For a period of time the DNA may not
produce any protein but if the host cell
replicates then the viral DNA is also
replicated
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HIV Infection
 Eventually the viral DNA in the host cell will
begin to make proteins needed to make new
HIV
 Proteases help package the new virus
components into a new envelope using the
host cell membrane
 The new viruses are released from the host
to infect other cells
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AZT anti-viral drug
 AZT is a reverse transcriptase inhibitor.
 It also inhibits DNA polymerase but has 100 –
300 X greater affinity for reverse
transcriptase
 AZT is a thymidine analog
HIV Treatments
 There is no cure. These treatments can slow
the spread of the virus in the body but not
completely get rid of it
 There are four main targets to fight HIV:
1.
2.
3.
4.
Reverse Transcriptase Inhibitors
Protease Inhibitors
Integrase Inhibitors
Fusion Inhibitors - prevent HIV entry into cells.
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Influenza Virus
 In 1918 – 1919 influenza killed 20 - 50 million
people, more than the numbers of soldiers in
WWI
 The type of influenza that causes the most
problem infects both bird, swine and human
hosts
 Influenza virus capsid covered with envelope
with protein spikes. Different strains have
different types of proteins.
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Ebola virus
 Ebola virus causes severe hemorrhagic fever
 Natural host unknown – most likely fruit bats
 Death rates 25 – 90% of those infected
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Viruses can cause cancer
 Hepatitis B infection – liver cancer
 Papilloma virus – cervical cancer
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Important Concepts
Important Concepts
 Know the vocabulary in the lecture
 What three enzymes does HIV have, what are their
functions
 Structural features and characteristics of viruses,
what are common shapes of the caspids
 What cells are the host cell of HIV
 lysogenic cycle and lytic cycle
 What are the main targets of HIV drugs
 Be able to describe in detail the steps of HIV
infection of T Cells
 Examples of viruses given in class
 What are retroviruses and what enzyme do they
have
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 Know the examples of viruses that cause cancer,
and what cancers they cause.
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9