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Chapter 18
The Genetics of Viruses
and Bacteria
PowerPoint Lectures for
Biology, Seventh Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Overview: Microbial Model Systems
• Viruses called bacteriophages
– Can infect and set in motion a genetic takeover
of bacteria, such as Escherichia coli
Figure 18.1
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0.5 m
• Recall that bacteria are prokaryotes
– With cells much smaller and more simply
organized than those of eukaryotes
• Viruses
– Are smaller and simpler still
Virus
Bacterium
Animal
cell
Animal cell nucleus
0.25 m
Figure 18.2
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• Concept 18.1: A virus has a genome but can
reproduce only within a host cell
• Scientists were able to detect viruses indirectly
– Long before they were actually able to see
them
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The Discovery of Viruses
• Tobacco mosaic disease
– Stunts the growth of tobacco plants and gives
their leaves a mosaic colorationIn 1935,
– Wendell Stanley - Confirmed this hypothesis
when he crystallized the infectious particle,
now known as tobacco mosaic virus (TMV)
Figure 18.3
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Structure of Viruses
• Viruses
– Are very small infectious particles consisting of
nucleic acid enclosed in a protein coat and, in
some cases, a membranous envelope
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Viral Genomes
• Viral genomes may consist of
– Double- or single-stranded DNA
– Double- or single-stranded RNA (4 kinds)
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Capsids and Envelopes
• A capsid
– Is the protein shell that encloses the viral genome
– Can have various structures
Capsomere
of capsid
RNA
Capsomere
DNA
Glycoprotein
70–90 nm (diameter)
18  250 mm
20 nm
50 nm
Figure 18.4a, b (a) Tobacco mosaic virus (b) Adenoviruses
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• Some viruses have envelopes
– Which are membranous coverings derived
from the membrane of the host cell
Membranous
envelope
Capsid
RNA
Glycoprotein
80–200 nm (diameter)
Figure 18.4c
50 nm
(c) Influenza viruses
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• Bacteriophages, also called phages
– Have the most complex capsids found among
viruses
Head
Tail
sheath
DNA
Tail
fiber
80  225 nm
Figure 18.4d
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50 nm
(d) Bacteriophage T4
General Features of Viral Reproductive Cycles
• Viruses are obligate intracellular parasites
– They can reproduce only within a host cell
• Each virus has a host range
– A limited number of host cells that it can infect
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• Viruses use enzymes, ribosomes, and small
molecules of host cells
– To synthesize progeny viruses
Entry into cell and
uncoating of DNA
DNA
Capsid
VIRUS
Transcription
Replication
HOST CELL
Viral DNA
mRNA
Viral DNA
Capsid
proteins
Self-assembly of
new
virus particles and
their exit from cell
Figure 18.5
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Reproductive Cycles of Phages
• Phages
– Are the best understood of all viruses
– Go through two alternative reproductive
mechanisms: the lytic cycle and the lysogenic
cycle
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The Lytic Cycle
• The lytic cycle
– Is a phage reproductive cycle that culminates
in the death of the host
– Produces new phages and digests the host’s
cell wall, releasing the progeny viruses
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• The lytic cycle of phage T4, a virulent phage
1 Attachment. The T4 phage uses
its tail fibers to bind to specific
receptor sites on the outer
surface of an E. coli cell.
5 Release. The phage directs production
of an enzyme that damages the bacterial
cell wall, allowing fluid to enter. The cell
swells and finally bursts, releasing 100
to 200 phage particles.
2 Entry of phage DNA
and degradation of host DNA.
The sheath of the tail contracts,
injecting the phage DNA into
the cell and leaving an empty
capsid outside. The cell’s
DNA is hydrolyzed.
Phage assembly
4 Assembly. Three separate sets of proteins
self-assemble to form phage heads, tails,
and tail fibers. The phage genome is
packaged inside the capsid as the head forms.
Figure 18.6
Head
Tails
Tail fibers
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3 Synthesis of viral genomes
and proteins. The phage DNA
directs production of phage
proteins and copies of the phage
genome by host enzymes, using
components within the cell.
The Lysogenic Cycle
• The lysogenic cycle
– Replicates the phage genome without
destroying the host
• Temperate phages
– Are capable of using both the lytic and
lysogenic cycles of reproduction
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• The lytic and lysogenic cycles of phage , a
temperate phage
Phage
DNA
The phage attaches to a
host cell and injects its DNA.
Phage DNA
circularizes
Phage
Occasionally, a prophage
exits the bacterial chromosome,
initiating a lytic cycle.
Bacterial
chromosome
Lytic cycle
The cell lyses, releasing phages.
Lysogenic cycle
Certain factors
determine whether
Lytic cycle
is induced
Figure 18.7
Many cell divisions
produce a large
population of bacteria
infected with the
prophage.
or
New phage DNA and
proteins are synthesized
and assembled into phages.
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Lysogenic cycle
is entered
Prophage
The bacterium reproduces
normally, copying the prophage
and transmitting it to daughter cells.
Phage DNA integrates into
the bacterial chromosome,
becoming a prophage.
Reproductive Cycles of Animal Viruses
• The nature of the genome
– Is the basis for the common classification of
animal viruses
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• Classes of animal viruses
Table 18.1
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• The genome of viruses may consist of:
– double-stranded DNA, ex. Flu, herpes
– single-stranded DNA, ex. Parvovirus (rash)
– double-stranded RNA, ex. rotavirus (diarrhea)
– single-stranded RNA, ex. Hepatitis
– ssRNA; template for mRNA, ex. Eboloa
– ssRNA; template for DNA, ex. HIV
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Viral Envelopes
• Many animal viruses
– Have a membranous envelope
• Viral glycoproteins on the envelope
– Bind to specific receptor molecules on the
surface of a host cell
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• The reproductive cycle of an enveloped RNA virus
1 Glycoproteins on the viral envelope
bind to specific receptor molecules
(not shown) on the host cell,
promoting viral entry into the cell.
Capsid
RNA
Envelope (with
glycoproteins)
2 Capsid and viral genome
enter cell
HOST CELL
Viral genome (RNA)
Template
5 Complementary RNA
strands also function as mRNA,
which is translated into both
capsid proteins (in the cytosol)
and glycoproteins for the viral
envelope (in the ER).
3 The viral genome (red)
functions as a template for
synthesis of complementary
RNA strands (pink) by a viral
enzyme.
mRNA
Capsid
proteins
ER
Glycoproteins
Copy of
genome (RNA)
4 New copies of viral
genome RNA are made
using complementary RNA
strands as templates.
6 Vesicles transport
envelope glycoproteins to
the plasma membrane.
8 New virus
7 A capsid assembles
Figure 18.8
around each viral
genome molecule.
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RNA as Viral Genetic Material
• The broadest variety of RNA genomes
– Is found among the viruses that infect animals
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• Retroviruses, such as HIV, use the enzyme
reverse transcriptase
– To copy their RNA genome into DNA, which
can then be integrated into the host genome
as a provirus
Glycoprotein
Viral envelope
Capsid
Reverse
transcriptase
Figure 18.9
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RNA
(two identical
strands)
• The reproductive cycle of HIV, a retrovirus
HIV
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
RNA genome
for the next
viral generation
Provirus
mRNA
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.
6 The viral proteins include
capsid proteins and reverse
transcriptase (made in the cytosol)
and envelope glycoproteins
(made in the ER).
Figure 18.10
New HIV leaving a cell
9 New viruses bud
off from the host cell.
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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.
Evolution of Viruses
• Viruses do not really fit our definition of living
organisms
• Since viruses can reproduce only within cells
– They probably evolved after the first cells
appeared, perhaps packaged as fragments of
cellular nucleic acid
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• Concept 18.3: Rapid reproduction, mutation,
and genetic recombination contribute to the
genetic diversity of bacteria
• Bacteria allow researchers
– To investigate molecular genetics in the
simplest true organisms
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The Bacterial Genome and Its Replication
• The bacterial chromosome
– Is usually a circular DNA molecule with few
associated proteins
• In addition to the chromosome
– Many bacteria have plasmids, smaller circular
DNA molecules that can replicate
independently of the bacterial chromosome
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• Bacterial cells divide by binary fission
– Which is preceded by replication of the
bacterial chromosome
Replication
fork
Origin of
replication
Termination
of replication
Figure 18.14
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Mutation and Genetic Recombination as Sources
of Genetic Variation
• Since bacteria can reproduce rapidly
– New mutations can quickly increase a
population’s genetic diversity
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Mechanisms of Gene Transfer and Genetic
Recombination in Bacteria
• Three processes bring bacterial DNA from
different individuals together
– Transformation
– Transduction
– Conjugation
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Transformation
• Transformation
– Is the alteration of a bacterial cell’s genotype
and phenotype by the uptake of naked, foreign
DNA from the surrounding environment
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Transduction
• In the process known as transduction
– Phages carry bacterial genes from one host
cell to another
Phage DNA
A+ B+
1 Phage infects bacterial cell that has alleles A+ and B+
A+ B+
2 Host DNA (brown) is fragmented, and phage DNA
and proteins are made. This is the donor cell.
Donor
cell
3 A bacterial DNA fragment (in this case a fragment with
the A+ allele) may be packaged in a phage capsid.
A+
4 Phage with the A+ allele from the donor cell infects
a recipient A–B– cell, and crossing over (recombination)
between donor DNA (brown) and recipient DNA
(green) occurs at two places (dotted lines).
Crossing
over
A+
A– B–
Recipient
cell
5 The genotype of the resulting recombinant cell (A+B–)
Figure 18.16
differs from the genotypes of both the donor (A+B+) and
the recipient (A–B–).
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A+ B–
Recombinant cell
Conjugation and Plasmids
• Conjugation
– Is the direct transfer of genetic material between
bacterial cells that are temporarily joined
Figure 18.17
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Sex pilus
1 m
• Conjugation and transfer of an F plasmid from
an F+ donor to an F recipient
F Plasmid
Bacterial chromosome
F+ cell
F+ cell
Mating
bridge
F– cell
Bacterial
chromosome
1 A cell carrying an F plasmid
(an F+ cell) can form a
mating bridge with an F– cell
and transfer its F plasmid.
2 A single strand of the
F plasmid breaks at a
specific point (tip of blue
arrowhead) and begins to
move into the recipient cell.
As transfer continues, the
donor plasmid rotates
(red arrow).
F+ cell
3 DNA replication occurs in 4
both donor and recipient
cells, using the single
parental strands of the
F plasmid as templates
to synthesize complementary
strands.
Figure 18.18a
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The plasmid in the
recipient cell
circularizes. Transfer
and replication result
in a compete F plasmid
in each cell. Thus, both
cells are now F+.
(a) Conjugation and transfer of an
F plasmid from an F+ donor to
an F– recipient
• Chromosomal genes can be transferred during
conjugation
– When the donor cell’s F factor is integrated into the
chromosome
• A cell with the F factor built into its chromosome
– Is called an Hfr cell
• The F factor of an Hfr cell
– Brings some chromosomal DNA along with it when it
is transferred to an F– cell
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• Conjugation and transfer of part of the
bacterial chromosome from an Hfr donor
to an F– recipient, resulting in recombination
Hfr cell
F+ cell
F factor
The circular F plasmid in an F + cell
can be integrated into the circular
chromosome by a single crossover
event (dotted line).
1
Hfr cell
A+
B+
C+
The resulting cell is called an Hfr cell
(for High frequency of recombination).
D+
C+
B+
D+
2
D+ C+
A+
B+
A+
D+ C+
B+
B+
A+
B–
A+
A+
F– cell
3
B–
C–
A–
B+
D–
Since an Hfr cell has all
the F-factor genes, it can
form a mating bridge with
an F– cell and transfer DNA.
B–
A–
B+
A+
B–
C–
A–
A+
D–
4 A single strand of the F factor
breaks and begins to move
through the bridge. DNA
replication occurs in both donor
and recipient cells, resulting in
double-stranded DNA
Temporary
partial
diploid
7
C–
B–
C– D–
A–
5 The location and orientation
of the F factor in the donor
chromosome determine
the sequence of gene transfer
during conjugation. In this
example, the transfer sequence
for four genes is A-B-C-D.
B–
D–
Two crossovers can result
in the exchange of similar
(homologous) genes between
the transferred chromosome fragment
(brown) and the recipient cell’s
chromosome (green).
Figure 18.18b
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A+
B+
C–
A–
D–
6
C–
A–
D–
The mating bridge
usually breaks well
before the entire
chromosome and
the rest of the
F factor are transferred.
Recombinant F–
bacterium
8 The piece of DNA ending up outside the
bacterial chromosome will eventually be
degraded by the cell’s enzymes. The recipient
cell now contains a new combination of genes
but no F factor; it is a recombinant F – cell.
(b) Conjugation and transfer of part
of the bacterial chromosome from
an Hfr donor to an F– recipient,
resulting in recombination
R plasmids and Antibiotic Resistance
• R plasmids
– Confer resistance to various antibiotics
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Transposition of Genetic Elements
• Transposable elements
– Can move around within a cell’s genome
– Are often called “jumping genes”
– Contribute to genetic shuffling in bacteria
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Insertion Sequences
• An insertion sequence contains a single gene
for transposase
– An enzyme that catalyzes movement of the
insertion sequence from one site to another
within the genome
Insertion sequence
3
A T C C G G T…
A C C G G A T…
3
5
TAG G C CA…
TG G C CTA…
5
Transposase gene
Inverted
Inverted
repeat
repeat
(a) Insertion sequences, the simplest transposable elements in bacteria, contain a single gene that
encodes transposase, which catalyzes movement within the genome. The inverted repeats are
backward, upside-down versions of each other; only a portion is shown. The inverted repeat
sequence varies from one type of insertion sequence to another.
Figure 18.19a
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Transposons
• Bacterial transposons
– Also move about within the bacterial genome
– Have additional genes, such as those for
antibiotic resistance
Transposon
Insertion
sequence
Antibiotic
resistance gene
Insertion
sequence
5
5
3
3
Inverted repeats
Transposase gene
(b) Transposons contain one or more genes in addition to the transposase gene. In the transposon
shown here, a gene for resistance to an antibiotic is located between twin insertion sequences.
The gene for antibiotic resistance is carried along as part of the transposon when the transposon
is inserted at a new site in the genome.
Figure 18.19b
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• Concept 18.4: Individual bacteria respond to
environmental change by regulating their gene
expression
• E. coli, a type of bacteria that lives in the
human colon
– Can tune its metabolism to the changing
environment and food sources
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• This metabolic control occurs on two levels
– Adjusting the activity of metabolic enzymes
already present
– Regulating the genes encoding the metabolic
enzymes
(a) Regulation of enzyme
activity
Precursor
Feedback
inhibition
(b) Regulation of enzyme
production
Enzyme 1 Gene 1
Enzyme 2 Gene 2
Regulation
of gene
expression
Enzyme 3 Gene 3
–
Enzyme 4 Gene 4
–
Enzyme 5 Gene 5
Tryptophan
Figure 18.20a, b
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Operons: The Basic Concept
• In bacteria, genes are often clustered into
operons, composed of
– An operator, an “on-off” switch
– A promoter
– Genes for metabolic enzymes
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• An operon
– Is usually turned “on”
– Can be switched off by a protein called a
repressor
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• The trp operon: regulated synthesis of
repressible enzymes
trp operon
Promoter
DNA
Promoter
Genes of operon
trpD
trpC
trpE
trpR
trpB
trpA
Operator
Regulatory
gene
mRNA
5
3
RNA
polymerase
Start codon
Stop codon
mRNA 5
E
Protein
Inactive
repressor
D
C
B
A
Polypeptides that make up
enzymes for tryptophan synthesis
(a) Tryptophan absent, repressor inactive, operon on. RNA polymerase attaches to the DNA at the
promoter and transcribes the operon’s genes.
Figure 18.21a
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DNA
No RNA made
mRNA
Protein
Active
repressor
Tryptophan
(corepressor)
(b) Tryptophan present, repressor active, operon off. As tryptophan
accumulates, it inhibits its own production by activating the repressor protein.
Figure 18.21b
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Repressible and Inducible Operons: Two Types of
Negative Gene Regulation
• In a repressible operon
– Binding of a specific repressor protein to the
operator shuts off transcription
• In an inducible operon
– Binding of an inducer to an innately inactive
repressor inactivates the repressor and turns
on transcription
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• The lac operon: regulated synthesis of
inducible enzymes
Promoter
Regulatory
gene
DNA
Operator
lacl
lacZ
3
mRNA
Protein
No
RNA
made
RNA
polymerase
5
Active
repressor
(a) Lactose absent, repressor active, operon off. The lac repressor is innately active, and in
the absence of lactose it switches off the operon by binding to the operator.
Figure 18.22a
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lac operon
DNA
lacl
lacz
3
mRNA
5
lacA
RNA
polymerase
mRNA 5'
5
mRNA
-Galactosidase
Protein
Allolactose
(inducer)
lacY
Permease
Transacetylase
Inactive
repressor
(b) Lactose present, repressor inactive, operon on. Allolactose, an isomer of lactose, derepresses
the operon by inactivating the repressor. In this way, the enzymes for lactose utilization are induced.
Figure 18.22b
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• Inducible enzymes
– Usually function in catabolic pathways
• Repressible enzymes
– Usually function in anabolic pathways
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• Regulation of both the trp and lac operons
– Involves the negative control of genes,
because the operons are switched off by the
active form of the repressor protein
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