Download 18- virusbacteria

GENETICS OF
BACTERIA AND
VIRUSES
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
• 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
Figure 18.1
–
Viruses and bacteria have
unique genetic mechanisms
that are interesting in their
own right
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
0.5 m
• Recall that bacteria
are prokaryotes
– With cells much
smaller and
more simply
organized than
those of
eukaryotes
Virus
Animal
cell
• Viruses
– Are smaller and
simpler still
Figure 18.2
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Animal cell nucleus
0.25 m
Structure of Viruses
• Viruses
– Are very small infectious
particles consisting of
nucleic acid enclosed in a
protein coat
– Viral glycoproteins on the
envelope bind to specific
receptor molecules on
the surface of a host cell
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Viral Genomes
• Viral genomes may consist of
– Double- or single-stranded DNA
– Double- or single-stranded RNA
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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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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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Viruses use enzymes, ribosomes, and small
molecules of host cells
– To synthesize “baby” 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 “baby” 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
– Incoporates 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
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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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RNA as Viral Genetic Material
• The broadest variety
of RNA genomes
– Is found among the
viruses that infect
animals
Glycoprotein
Viral envelope
Capsid
• 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
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Education, Inc. publishing as Benjamin Cummings
Reverse
transcriptase
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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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
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• Vaccines
– Are harmless derivatives of pathogenic
microbes that stimulate the immune system to
mount defenses against the actual pathogen
– Can prevent certain viral illnesses
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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
• Plant viruses spread disease in two
major modes
–
Horizontal transmission, entering
through damaged cell walls
–
Vertical transmission, inheriting the
virus from a parent
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DO NOW
– What is the
difference between
the lytic and
lysogenic cycle?
– Describe the
structure of a virus.
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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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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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• Further genetic diversity
– Can arise by recombination of the DNA from
two different bacterial cells
EXPERIMENT
Researchers had two mutant strains, one that could make arginine but not
tryptophan
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.
(arg+
Mixture
Mutant
strain
arg+ trp–
Figure 18.15
Mutant
strain
arg trp+
RESULTS
Only the samples from the mixed culture, contained cells that gave rise to colonies on
minimal medium, which lacks amino acids.
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Mixture
Mutant
strain
arg+ trp–
Mutant
strain
arg– trp+
No
colonies
(control)
CONCLUSION
Colonies
grew
No
colonies
(control)
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.
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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
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
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
R plasmids and Antibiotic Resistance
• R plasmids
– Confer resistance to various antibiotics
– This can be dangerous as we humans over
use 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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What are antibiotics?
• Drugs used to kill bacteria
• How do they kill bacteria
– The produce selective poisons. I
– It has been chosen so that it will kill the desired
bacteria, but not the cells in your body.
– Each different type of antibiotic affects different
bacteria in different ways.
• an antibiotic might inhibit a bacterium's ability to
turn glucose into energy, or its ability to construct its
cell wall.
• When this happens, the bacterium dies instead of
reproducing..
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