Ch18 virusbacteriagenetics

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
PowerPoint Lectures for
Biology, Seventh Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero
Chapter 18
The Genetics of Viruses
and Bacteria

• 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
0.5 µm

• E. coli and its viruses
– Are called model systems because of their
frequent use by researchers in studies that
reveal broad biological principles
• Beyond their value as model systems
– Viruses and bacteria have unique genetic
mechanisms that are interesting in their own
right

• Recall that bacteria are prokaryotes
– With cells much smaller and more simply
organized than those of eukaryotes
• Viruses
– Are smaller and simpler still
Figure 18.2
0.25 µm
Virus
Animal
cell
Bacterium
Animal cell nucleus

• 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

The Discovery of Viruses: Scientific Inquiry
• Tobacco mosaic disease
– Stunts the growth of tobacco plants and gives
their leaves a mosaic coloration
Figure 18.3

• In the late 1800s
– Researchers hypothesized that a particle
smaller than bacteria caused tobacco mosaic
disease
• In 1935, Wendell Stanley
– Confirmed this hypothesis when he crystallized
the infectious particle, now known as tobacco
mosaic virus (TMV)

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

Viral Genomes
• Viral genomes may consist of
– Double- or single-stranded DNA
– Double- or single-stranded RNA

Figure 18.4a, b
18 × 250 mm 70–90 nm (diameter)
20 nm 50 nm
(a) Tobacco mosaic virus (b) Adenoviruses
RNA
DNACapsomere
Glycoprotein
Capsomere
of capsid
Capsids and Envelopes
• A capsid
– Is the protein shell that encloses the viral genome
– Can have various structures

• Some viruses have envelopes
– Which are membranous coverings derived
from the membrane of the host cell
Figure 18.4c
80–200 nm (diameter)
50 nm
(c) Influenza viruses
RNA
Glycoprotein
Membranous
envelope
Capsid

• Bacteriophages, also called phages
– Have the most complex capsids found among
viruses
Figure 18.4d
80 × 225 nm
50 nm
(d) Bacteriophage T4
DNA
Head
Tail
fiber
Tail
sheath

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

• Viruses use enzymes, ribosomes, and small
molecules of host cells
– To synthesize progeny viruses
VIRUS
Capsid
proteins
mRNA
Viral DNA
HOST CELL
Viral DNA
DNA
Capsid
Figure 18.5
Entry into cell and
uncoating of DNA
Replication
Transcription
Self-assembly of
new
virus particles and
their exit from cell

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

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

• The lytic cycle of phage T4, a virulent phage
Phage assembly
Head Tails Tail fibersFigure 18.6
Attachment. The T4 phage uses
its tail fibers to bind to specific
receptor sites on the outer
surface of an E. coli cell.
1
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.
2
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.
3Assembly. 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.
4
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.
5

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

• The lytic and lysogenic cycles of phage λ, a
temperate phage
Many cell divisions
produce a large
population of bacteria
infected with the
prophage.
The bacterium reproduces
normally, copying the prophage
and transmitting it to daughter cells.
Phage DNA integrates into
the bacterial chromosome,
becoming a prophage.
New phage DNA and
proteins are synthesized
and assembled into phages.
Occasionally, a prophage
exits the bacterial chromosome,
initiating a lytic cycle.
Certain factors
determine whether
The phage attaches to a
host cell and injects its DNA.
Phage DNA
circularizes
The cell lyses, releasing phages.
Lytic cycle
is induced
Lysogenic cycle
is entered
Lysogenic cycleLytic cycle
or Prophage
Bacterial
chromosome
Phage
Phage
DNA
Figure 18.7

Reproductive Cycles of Animal Viruses
• The nature of the genome
– Is the basis for the common classification of
animal viruses

• Classes of animal viruses
Table 18.1

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

RNA
Capsid
Envelope (with
glycoproteins)
HOST CELL
Viral genome (RNA)
Template
Capsid
proteins
Glyco-
proteins
mRNA
Copy of
genome (RNA)
ER
Figure 18.8
• The reproductive cycle of an enveloped RNA virus
Glycoproteins on the viral envelope
bind to specific receptor molecules
(not shown) on the host cell,
promoting viral entry into the cell.
1
Capsid and viral genome
enter cell
2
The viral genome (red)
functions as a template for
synthesis of complementary
RNA strands (pink) by a viral
enzyme.
3
New copies of viral
genome RNA are made
using complementary RNA
strands as templates.
4
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).
5
Vesicles transport
envelope glycoproteins to
the plasma membrane.
6
A capsid assembles
around each viral
genome molecule.
7
New virus8

RNA as Viral Genetic Material
• The broadest variety of RNA genomes
– Is found among the viruses that infect animals

• 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
Figure 18.9
Reverse
transcriptase
Viral envelope
Capsid
Glycoprotein
RNA
(two identical
strands)

• The reproductive cycle of HIV, a retrovirus
Figure 18.10
mRNA
RNA genome
for the next
viral generation
Viral RNA
RNA-DNA
hybrid
DNA
Chromosomal
DNA
NUCLEUS
Provirus
HOST CELL
Reverse
transcriptase
New HIV leaving a cell
HIV entering a cell
0.25 µm
HIV Membrane of
white blood cell
The virus fuses with the
cell’s plasma membrane.
The capsid proteins are
removed, releasing the
viral proteins and RNA.
1
Reverse transcriptase
catalyzes the synthesis of a
DNA strand complementary
to the viral RNA.
2
Reverse transcriptase
catalyzes the synthesis of
a second DNA strand
complementary to the first.
3
The double-stranded
DNA is incorporated
as a provirus into the
cell’s DNA.
4
Proviral genes are
transcribed into RNA
molecules, which serve as
genomes for the next viral
generation and as mRNAs
for translation into viral
proteins.
5
The viral proteins include
capsid proteins and reverse
transcriptase (made in the
cytosol) and envelope
glycoproteins (made in the ER).
6
Vesicles transport the
glycoproteins from the ER to
the cell’s plasma membrane.
7Capsids are
assembled around
viral genomes and
reverse transcriptase
molecules.
8
New viruses bud
off from the host cell.
9

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

• Concept 18.2: Viruses, viroids, and prions are
formidable pathogens in animals and plants
• Diseases caused by viral infections
– Affect humans, agricultural crops, and
livestock worldwide

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

• Vaccines
– Are harmless derivatives of pathogenic
microbes that stimulate the immune system to
mount defenses against the actual pathogen
– Can prevent certain viral illnesses

Emerging Viruses
• Emerging viruses
– Are those that appear suddenly or suddenly
come to the attention of medical scientists

• Severe acute respiratory syndrome (SARS)
– Recently appeared in China
Figure 18.11 A, B
(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.

• Outbreaks of “new” viral diseases in humans
– Are usually caused by existing viruses that
expand their host territory

Viral Diseases in Plants
• More than 2,000 types of viral diseases of
plants are known
• Common symptoms of viral infection include
– Spots on leaves and fruits, stunted growth, and
damaged flowers or roots
Figure 18.12

• Plant viruses spread disease in two major
modes
– Horizontal transmission, entering through
damaged cell walls
– Vertical transmission, inheriting the virus from
a parent

Viroids and Prions: The Simplest Infectious Agents
• Viroids
– Are circular RNA molecules that infect plants
and disrupt their growth

• Prions
– Are slow-acting, virtually indestructible
infectious proteins that cause brain diseases in
mammals
– Propagate by converting normal proteins into
the prion version
Figure 18.13
Prion
Normal
protein
Original
prion
New
prion
Many prions

• 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

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

• 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

Mutation and Genetic Recombination as Sources of
Genetic Variation
• Since bacteria can reproduce rapidly
– New mutations can quickly increase a
population’s genetic diversity

• Further genetic diversity
– Can arise by recombination of the DNA from
two different bacterial cells
Mutant
strain
arg−
trp+
EXPERIMENT
Figure 18.15
Only the samples from the mixed culture, contained cells that gave rise to colonies on
minimal medium, which lacks amino acids.
RESULTS
Researchers had two mutant strains, one that could make arginine but not
tryptophan (arg+
trp–
) and one that could make tryptophan but not arginine (arg−
trp+
). Each
mutant strain and a mixture of both strains were grown in a liquid medium containing all the
required amino acids. Samples from each liquid culture were spread on plates containing a
solution of glucose and inorganic salts (minimal medium), solidified with agar.
Mutant
strain
arg+
trp–
Mixture

Colonies
grew
Mutant
strain
arg+
trp–
Mutant
strain
arg–
trp+
No
colonies
(control)
No
colonies
(control)
Mixture
Because only cells that can make both arginine and tryptophan (arg+
trp+
cells) can grow into
colonies on minimal medium, the lack of colonies on the two control plates showed that no further mutations had
occurred restoring this ability to cells of the mutant strains. Thus, each cell from the mixture that formed a colony on the
minimal medium must have acquired one or more genes from a cell of the other strain by genetic recombination.
CONCLUSION

Mechanisms of Gene Transfer and Genetic
Recombination in Bacteria
• Three processes bring bacterial DNA from
different individuals together
– Transformation
– Transduction
– Conjugation

Transformation
• Transformation
– Is the alteration of a bacterial cell’s genotype
and phenotype by the uptake of naked, foreign
DNA from the surrounding environment

Transduction
• In the process known as transduction
– Phages carry bacterial genes from one host
cell to another
1
Figure 18.16
Donor
cell
Recipient
cell
A+
B+
A+
A+
B–
A–
B–
A+
Recombinant cell
Crossing
over
Phage infects bacterial cell that has alleles A+
and B+
Host DNA (brown) is fragmented, and phage DNA
and proteins are made. This is the donor cell.
A bacterial DNA fragment (in this case a fragment with
the A+
allele) may be packaged in a phage capsid.
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).
The genotype of the resulting recombinant cell (A+
B–
)
differs from the genotypes of both the donor (A+
B+
) and
the recipient (A–
B–
).
2
3
4
5
Phage DNA
A+ B+

Conjugation and Plasmids
• Conjugation
– Is the direct transfer of genetic material between
bacterial cells that are temporarily joined
Figure 18.17 Sex pilus 1 µm

The F Plasmid and Conjugation
• Cells containing the F plasmid, designated F+
cells
– Function as DNA donors during conjugation
– Transfer plasmid DNA to an F−
recipient cell

• Conjugation and transfer of an F plasmid from
an F+
donor to an F−
recipient
Figure 18.18a
A cell carrying an F plasmid
(an F+
cell) can form a
mating bridge with an F–
cell
and transfer its F plasmid.
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).
2 DNA replication occurs in
both donor and recipient
cells, using the single
parental strands of the
F plasmid as templates
to synthesize complementary
strands.
3 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+
.
4
F Plasmid Bacterial chromosome
Bacterial
chromosome
F+
cell
F+
cell
F+
cell
Mating
bridge
1
Conjugation and transfer of an
F plasmid from an F+
donor to
an F–
recipient
(a)
F–
cell

• 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

• Conjugation and transfer of part of the
bacterial chromosome from an Hfr donor
to an F–
recipient, resulting in recombination
F+
cell Hfr 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
The resulting cell is called an Hfr cell
(for High frequency of recombination).
2
Since an Hfr cell has all
the F-factor genes, it can
form a mating bridge with
an F–
cell and transfer DNA.
3 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
4 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.
5 The mating bridge
usually breaks well
before the entire
chromosome and
the rest of the
F factor are transferred.
6
Two crossovers can result
in the exchange of similar
(homologous) genes between
the transferred chromosome fragment
(brown) and the recipient cell’s
chromosome (green).
7 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.
8
Temporary
partial
diploid
Recombinant F–
bacterium
A+
B+
C+
D+
F–
cell A–
B–
C–
D–
A–
B–
C–
D– D–
A–
C–
B–
A+
B+C+D+
A+B+
D+
C+
A+
A+
B+
A–
B–
C–
D–
A–
B+
C–
D–
A+
B+
B–
A+
Hfr cell
D–
A–
C–
B–
A+
B+C+D+
A+
B+
Conjugation and transfer of part
of the bacterial chromosome from
an Hfr donor to an F–
recipient,
resulting in recombination
(b)
Figure 18.18b

R plasmids and Antibiotic Resistance
• R plasmids
– Confer resistance to various antibiotics

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

Figure 18.19a
(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.
Insertion sequence
Transposase geneInverted
repeat
Inverted
repeat
3′
5′
3′
5′
A T C C G G T…
T A G G C C A …
A C C G G A T…
T G G C C T A …
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

Transposons
• Bacterial transposons
– Also move about within the bacterial genome
– Have additional genes, such as those for
antibiotic resistance
Figure 18.19b
(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.
Inverted repeats Transposase gene
Insertion
sequence
Insertion
sequence
Antibiotic
resistance gene
Transposon
5′
3′
5′
3′

• 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

• This metabolic control occurs on two levels
– Adjusting the activity of metabolic enzymes
already present
– Regulating the genes encoding the metabolic
enzymes
Figure 18.20a, b
(a) Regulation of enzyme
activity
Enzyme 1
Enzyme 2
Enzyme 3
Enzyme 4
Enzyme 5
Regulation
of gene
expression
Feedback
inhibition
Tryptophan
Precursor
(b) Regulation of enzyme
production
Gene 2
Gene 1
Gene 3
Gene 4
Gene 5
–
–

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

• An operon
– Is usually turned “on”
– Can be switched off by a protein called a
repressor

• The trp operon: regulated synthesis of
repressible enzymes
Figure 18.21a
(a) Tryptophan absent, repressor inactive, operon on. RNA polymerase attaches to the DNA at the
promoter and transcribes the operon’s genes.
Genes of operon
Inactive
repressor
Protein
Operator
Polypeptides that make up
enzymes for tryptophan synthesis
Promoter
Regulatory
gene
RNA
polymerase
Start codon Stop codon
Promoter
trp operon
5′
3′
mRNA 5′
trpDtrpE trpC trpB trpAtrpRDNA
mRNA
E D C B A

DNA
mRNA
Protein
Tryptophan
(corepressor)
Active
repressor
No RNA made
Tryptophan present, repressor active, operon off. As tryptophan
accumulates, it inhibits its own production by activating the repressor protein.
(b)
Figure 18.21b

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

• The lac operon: regulated synthesis of
inducible enzymes
Figure 18.22a
DNA
mRNA
Protein
Active
repressor
RNA
polymerase
No
RNA
made
lacZlacl
Regulatory
gene
Operator
Promoter
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.
(a)
5′
3′

mRNA 5'
DNA
mRNA
Protein
Allolactose
(inducer)
Inactive
repressor
lacl lacz lacY lacA
RNA
polymerase
Permease Transacetylaseβ-Galactosidase
5′
3′
(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.
mRNA 5′
lac operon
Figure 18.22b

• Inducible enzymes
– Usually function in catabolic pathways
• Repressible enzymes
– Usually function in anabolic pathways

• 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

Positive Gene Regulation
• Some operons are also subject to positive
control
– Via a stimulatory activator protein, such as
catabolite activator protein (CAP)

Promoter
Lactose present, glucose scarce (cAMP level high): abundant lac mRNA synthesized.
If glucose is scarce, the high level of cAMP activates CAP, and the lac operon produces
large amounts of mRNA for the lactose pathway.
(a)
CAP-binding site OperatorRNA
polymerase
can bind
and transcribe
Inactive
CAP
Active
CAP
cAMP
DNA
Inactive lac
repressor
lacl lacZ
Figure 18.23a
• In E. coli, when glucose, a preferred food
source, is scarce
– The lac operon is activated by the binding of a
regulatory protein, catabolite activator protein
(CAP)

• When glucose levels in an E. coli cell increase
– CAP detaches from the lac operon, turning it
off
Figure 18.23b
(b)Lactose present, glucose present (cAMP level low): little lac mRNA synthesized.
When glucose is present, cAMP is scarce, and CAP is unable to stimulate transcription.
Inactive lac
repressor
Inactive
CAP
DNA
RNA
polymerase
can’t bind
Operator
lacl lacZ
CAP-binding site
Promoter

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