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T4 bacteriophage infecting an E. coli
cell
0.5 m
Science as a Process
Research into TMV led to
the conclusion that the
pathogen was smaller
than a bacterial cell
 The pathogen was named
virus

Comparing
the size of
a virus,
a
bacterium,
and an
animal cell
Virus
Bacterium
Animal
cell
Animal cell nucleus
0.25 m
Infection by Tobacco Mosaic Virus
Figure 18.4 Viral Structure
Capsomere
of capsid
RNA
Capsomere
Membranous
envelope
DNA
Head
Capsid Tail
sheath
RNA
DNA
Tail
fiber
Glycoprotein
18  250 mm
20 nm
(a) Tobacco mosaic virus
Glycoprotein
70–90 nm (diameter)
80–200 nm (diameter)
50 nm
50 nm
(b) Adenoviruses
(c) Influenza viruses
80  225 nm
50 nm
(d) Bacteriophage T4
A virus has either DNA or RNA for its
genome, but not both and has no
mechanism for protein synthesis
(ribosomes) or metabolism
(mitochondria)
It also has a protein coat called a
capsid. Some have a coat made of
glycoproteins.
A virus can reproduce only within a
host and not on its own.
Capsids and Envelopes
Capsid = protein coat that
surrounds the viral genome
 viral envelope = derived
from host cell or nuclear
membranes; it helps the
virus invade the host cell

Figure 18.02x2 Phages
Figure 18.9 Viral infection of plants
Viral Genome
Double stranded DNA
 Single Stranded DNA
 Double stranded RNA
 Single stranded RNA
 A virus has only one of
these types of nucleic
acids

Table 18.1 Classes of Animal Viruses, Grouped by Type of Nucleic Acid
Figure 18.6 The reproductive cycle of an enveloped virus
Viral Replication
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What are the possible patterns
of viral replication?
DNA --> DNA
RNA --> RNA, where viral
genes code for RNA replicase
RNA --> DNA --> RNA; where
viral gene uses reverse
transcriptase
Bacterial Viruses
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Which scientists used
bacteriophages to prove that
DNA was the hereditary
material?
Hershey and Chase
What are the two mechanisms
of phage infection?
Lytic and Lysogenic cycles
Figure 18.4 The lytic cycle of phage T4
Figure 18.5 The lysogenic and lytic reproductive cycles of phage , a temperate
phage
Bacterial Defense
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What defense do bacteria have
against phage infection?
Restriction enzymes
What do restriction enzymes
do?
They cut up DNA The bacterial
DNA is modified to protect it
from the restriction
endonucleases.
Animal Viruses
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What is the viral envelope?
An outer membrane that helps
the virus to invade the animal
cell.
The invasion of the virus has
the following stages ...
1. Attachment
2. Entry
3. Uncoating
4. RNA and protein
synthesis
5. Assembly and release
Herpesvirus

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Consists of double stranded DNA
Envelope derived from host cell
nuclear envelope not from plasma
membrane
It, therefore, reproduces within the
nucleus
May integrate its DNA as a provirus
Tends to recur throughout lifetime
of infected individual.
Figure 18.x6 Herpes
The structure of HIV, the retrovirus that
causes AIDS
Glycoprotein
Viral envelope
Capsid
Reverse
transcriptase
RNA
(two identical
strands)
Figure 18.7x1 HIV infection
HIV
The reproductive cycle of HIV, a
retrovirus
Membrane of
white blood cell
1 The virus fuses with the
cell’s plasma membrane.
The capsid proteins are
removed, releasing the
viral proteins and RNA.
2 Reverse transcriptase
catalyzes the synthesis of a
DNA strand complementary
to the viral RNA.
HOST CELL
3 Reverse transcriptase
catalyzes the synthesis of
a second DNA strand
complementary to the first.
Reverse
transcriptase
Viral RNA
RNA-DNA
hybrid
4 The double-stranded
DNA is incorporated
as a provirus into the cell’s
DNA.
0.25 µm
HIV entering a cell
DNA
NUCLEUS
Chromosomal
DNA
Provirus
5 Proviral genes are
transcribed into RNA
molecules, which serve as
genomes for the next viral
generation and as mRNAs for
translation into viral proteins.
RNA genome
for the next
viral generation
mRNA
6 The viral proteins include capsid
proteins and reverse transcriptase
(made in the cytosol) and envelope
glycoproteins (made in the ER).
New HIV leaving a cell
9 New viruses bud
off from the host cell.
8 Capsids are
assembled around
viral genomes and
reverse transcriptase
molecules.
7 Vesicles transport the
glycoproteins from the ER to
the cell’s plasma membrane.
Viral Evolution
What is the current hypothesis
concerning how viruses evolved?
Viral Disease
Some viruses have toxic
components and cause
infected cells to release
enzymes from lysosomes
 Recovery involves ability to
repair damaged region of
the body. Ex: polio may
permanently damage nerve
cells.

Figure 18.x3 Polio
Vaccines / Drugs
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What are vaccines and how do
they work?
Introduce body to harmless or
weakened strain of the virus,
so that your immune system
learns to recognize the virus
prior to invasion
Few drugs around to fight
viruses, most interfere with
DNA, RNA, or protein synthesis
Smallpox
Emerging Viruses
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HIV
Ebola
Influenza
From where do these viruses
emerge?
From mutated versions of
current viruses
Jump from current host to new
host
Move from a previously isolated
region of the world
Deer Mouse – Vector of Hantavirus
SARS (severe acute
respiratory syndrome), a
recently emerging viral disease
(a) Young ballet students in Hong Kong
wear face masks to protect themselves
from the virus causing SARS.
(b) The SARS-causing agent is a coronavirus
like this one (colorized TEM), so named for the
“corona” of glycoprotein spikes protruding from
the envelope.
Viroids and Prions
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Viroids are naked circular RNA that
infect plants
Prions are proteins that infect cells
Examples of prions seen in
Scrapies in sheep, mad-cow
disease, and Creutzfeldt-Jakob
disease in humans
How can a prion spread infection?
Altered versions of proteins that
can alter other proteins
Model for how prions
propagate
Prion
Original
prion
Many prions
Normal
protein
New
prion
Replication of the Bacterial Chromosome
E. coli
E. coli Dividing
Transformation of Bacteria - Griffith
Bacterium releasing DNA with plasmids
Plasmids
Transduction
Bacterial
Mating
“Male” with
the F factor
Conjugation and recombination in E. coli
Viruses and Cancer
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Hepatitus B virus can cause liver
cancer
Some viral genes can trigger
cancerous genetic conditions
Oncogenes = viral genes that
trigger cancerous characteristics
proto-oncogenes = genes already
found in normal cells, usually
regulate growth factors
Insertion sequences, the simplest
transposons
Insertion of a transposon and creation of
direct repeats
Anatomy of a Composite Transposon