Download The Genetics of Viruses and Bacteria Chapter 18 PowerPoint Lectures for

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
• 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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Bacteria are prokaryotes with cells much smaller
and more simply organized than those of eukaryotes
• Viruses are smaller and simpler than bacteria
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 18-2
Virus
Bacterium
Animal
cell
Animal cell nucleus
0.25 µm
Concept 18.1: A virus has a genome but can
reproduce only within a host cell
• Scientists detected viruses indirectly long before
they could see them
• The story of how viruses were discovered begins
in the late 1800s
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The Discovery of Viruses: Scientific Inquiry
• Tobacco mosaic disease stunts growth of tobacco
plants and gives their leaves a mosaic coloration
• In the late 1800s, researchers hypothesized that a
particle smaller than bacteria caused the disease
• In 1935, Wendell Stanley confirmed this
hypothesis by crystallizing the infectious particle,
now known as tobacco mosaic virus (TMV)
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Structure of Viruses
• Viruses are not cells
• 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
• Depending on its type of nucleic acid, a virus is
called a DNA virus or an RNA virus
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Capsids and Envelopes
• A capsid is the protein shell that encloses the viral
genome
• A capsid can have various structures
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LE 18-4a
Capsomere
of capsid
RNA
18  250 mm
20 nm
Tobacco mosaic virus
LE 18-4b
Capsomere
DNA
Glycoprotein
70–90 nm (diameter)
50 nm
Adenoviruses
• Some viruses have structures have membranous
envelopes that help them infect hosts
• These viral envelopes surround the capsids of
influenza viruses and many other viruses found in
animals
• Viral envelopes, which are derived from the host
cell’s membrane, contain a combination of viral
and host cell molecules
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 18-4c
Membranous
envelope
Capsid
RNA
Glycoprotein
80–200 nm (diameter)
50 nm
Influenza viruses
• Bacteriophages, also called phages, are viruses
that infect bacteria
• They have the most complex capsids found
among viruses
• Phages have an elongated capsid head that
encloses their DNA
• A protein tailpiece attaches the phage to the host
and injects the phage DNA inside
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 18-4d
Head
Tail
sheath
Tail
fiber
DNA
80  225 nm
50 nm
Bacteriophage T4
General Features of Viral Reproductive Cycles
• Viruses are obligate intracellular parasites, which
means 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 host
molecules to synthesize progeny viruses
Animation: Simplified Viral Reproductive Cycle
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 18-5
Entry into cell and
uncoating of DNA
VIRUS
DNA
Capsid
Transcription
Replication
HOST CELL
Viral DNA
mRNA
Viral DNA
Capsid
proteins
Self-assembly of
new virus particles
and their exit from cell
Reproductive Cycles of Phages
• Phages are the best understood of all viruses
• Phages have two reproductive mechanisms: the
lytic cycle and the lysogenic cycle
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The Lytic Cycle
• The lytic cycle is a phage reproductive cycle that
culminates in the death of the host cell
• The lytic cycle produces new phages and digests
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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 18-6
Attachment
Phage assembly
Head
Tails
Release
Entry of phage DNA
and degradation of
host DNA
Tail fibers
Assembly
Synthesis of viral
genomes and proteins
Animation: Phage T4 Lytic Cycle
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The Lysogenic Cycle
• The lysogenic cycle replicates the phage genome
without destroying the host
• The viral DNA molecule is incorporated by genetic
recombination into the host cell’s chromosome
• This integrated viral DNA is known as a prophage
• Every time the host divides, it copies the phage
DNA and passes the copies to daughter cells
• Phages that use both the lytic and lysogenic
cycles are called temperate phages
Animation: Phage Lambda Lysogenic and Lytic Cycles
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 18-7
Phage
DNA
The phage attaches to a
host cell and injects its DNA.
Daughter cell
with prophage
Many cell divisions
produce a large
population of
bacteria infected with
the prophage.
Phage DNA
circularizes
Phage
Bacterial
chromosome
Lytic cycle
The cell lyses, releasing phages.
Occasionally, a prophage
exits the bacterial chromosome,
initiating a lytic cycle.
Lysogenic cycle
Certain factors
determine whether
Lytic cycle or Lysogenic cycle
is induced
is entered
New phage DNA and proteins are
synthesized and assembled into phages.
The bacterium reproduces
normally, copying the prophage
and transmitting it to daughter cells.
Prophage
Phage DNA integrates into the
bacterial chromosomes, becoming a
prophage.
Reproductive Cycles of Animal Viruses
• Two key variables in classifying viruses that infect
animals:
– DNA or RNA?
– Single-stranded or double-stranded?
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Class/Family Envelope Examples/Disease
I. Double-stranded DNA (dsDNA)
No
Respiratory diseases, animal tumors
Papovavirus No
Papillomavirus (warts, cervical
cancer): polyomavirus (animal
tumors)
Herpes simplex I and II (cold sores,
genital sores); varicella zoster
(shingles, chicken pox); Epstein-Barr
virus (mononucleosis, Burkitt’s
lymphoma)
Smallpox virus, cowpox virus
Adenovirus
Herpesvirus
Yes
Poxvirus
Yes
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Class/Family
Envelope Examples/Disease
II. Single-stranded DNA (ssDNA)
Parvovirus
No
B19 parvovirus (mild rash)
III. Double-stranded RNA (dsRNA)
Reovirus
No
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Rotavirus (diarrhea), Colorado
tick fever virus
Class/Family
Envelope Examples/Disease
IV. Single-stranded RNA (ssRNA); serves as mRNA
Picornavirus
No
Coronavirus
Yes
Flavivirus
Yes
Togavirus
Yes
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Rhinovirus (common cold);
poliovirus, hepatitis A virus, and
other enteric (intestinal) viruses
Severe acute respiratory
syndrome (SARS)
Yellow fever virus, West Nile
virus, hepatitis C virus
Rubella virus, equine
encephalitis viruses
Class/Family
Envelope Examples/Disease
V. ssRNA; template for mRNA synthesis
Filovirus
Yes
Ebola virus (hemorrhagic fever)
Orthomyxovirus Yes
Influenza virus
Paramyxovirus
Yes
Measles virus; mumps virus
Rhabdovirus
Yes
Rabies virus
VI. ssRNA; template for DNA synthesis
Retrovirus
Yes
HIV (AIDS); RNA tumor viruses
(leukemia)
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Viral Envelopes
• Many viruses that infect animals have a
membranous envelope
• Viral glycoproteins on the envelope bind to specific
receptor molecules on the surface of a host cell
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 18-8
Capsid
Capsid and viral genome
enter cell
RNA
HOST CELL
Envelope (with
glycoproteins)
Viral genome (RNA)
Template
mRNA
ER
Glycoproteins
Capsid
proteins
Copy of
genome (RNA)
New virus
RNA as Viral Genetic Material
• The broadest variety of RNA genomes is found in
viruses that infect animals
• Retroviruses use reverse transcriptase to copy
their RNA genome into DNA
• HIV is the retrovirus that causes AIDS
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 18-9
Glycoprotein
Viral envelope
Capsid
Reverse
transcriptase
RNA
(two identical
strands)
• The viral DNA that is integrated into the host
genome is called a provirus
• Unlike a prophage, a provirus remains a
permanent resident of the host cell
• The host’s RNA polymerase transcribes the
proviral DNA into RNA molecules
• The RNA molecules function both as mRNA for
synthesis of viral proteins and as genomes for
new virus particles released from the cell
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 18-10
HIV
Membrane of
white blood cell
HOST CELL
Reverse
transcription
Viral RNA
0.25 µm
HIV entering a cell
RNA-DNA
hybrid
DNA
NUCLEUS
Provirus
Chromosomal
DNA
RNA genome
for the
next viral
generation
New HIV leaving a cell
mRNA
Animation: HIV Reproductive Cycle
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Evolution of Viruses
• Viruses do not fit our definition of living organisms
• Since viruses can reproduce only within cells, they
probably evolved as bits of cellular nucleic acid
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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
• Smaller, less complex entities called viroids and
prions also cause disease in plants and animals
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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
• Vaccines can prevent certain viral illnesses
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Emerging Viruses
• Emerging viruses are those that appear suddenly
or suddenly come to the attention of scientists
• Severe acute respiratory syndrome (SARS)
recently appeared in China
• Outbreaks of “new” viral diseases in humans are
usually caused by existing viruses that expand
their host territory
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 18-11
Young ballet students in Hong
Kong wear face masks to
protect themselves from the
virus causing SARS.
The SARS-causing agent is a
coronavirus like this one
(colorized TEM), so named for
the “corona” of glyco-protein
spikes protruding form the
envelope.
Viral Diseases in Plants
• More than 2,000 types of viral diseases of plants
are known
• Some symptoms are spots on leaves and fruits,
stunted growth, and damaged flowers or roots
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Plant viruses spread disease in two major modes:
– Horizontal transmission, entering through
damaged cell walls
– Vertical transmission, inheriting the virus from
a parent
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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
• Prions propagate by converting normal proteins
into the prion version
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 18-13
Prion
Original
prion
Many prions
Normal
protein
New
prion
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 well-studied intestinal bacterium Escherichia
coli (E. coli) is “the laboratory rat of molecular
biology”
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
• Many bacteria also have plasmids, smaller circular
DNA molecules that can replicate independently of
the chromosome
• Bacterial cells divide by binary fission, which is
preceded by replication of the chromosome
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 18-14
Replication fork
Origin of
replication
Termination
of replication
Mutation and Genetic Recombination as Sources
of Genetic Variation
• Since bacteria can reproduce rapidly, new
mutations quickly increase genetic diversity
• More genetic diversity arises by recombination of
DNA from two different bacterial cells
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 18-15
Mixture
Mutant
strain
arg+ trp–
Mutant
strain
arg– trp+
Mixture
Mutant
strain
arg+ trp–
Mutant
strain
arg– trp+
No
colonies
(control)
Colonies
grew
No
colonies
(control)
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
• For example, harmless Streptococcus
pneumoniae bacteria can be transformed to
pneumonia-causing cells
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Transduction
• In the process known as transduction, phages
carry bacterial genes from one host cell to another
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 18-16
Phage DNA
A+ B+
A+ B+
Donor
cell
A+
Crossing
over
A+
A– B–
Recipient
cell
A+ B–
Recombinant cell
Conjugation and Plasmids
• Conjugation is the direct transfer of genetic
material between bacterial cells that are
temporarily joined
• The transfer is one-way: One cell (“male”)
donates DNA, and its “mate” (“female”) receives
the genes
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• “Maleness,” the ability to form a sex pilus and
donate DNA, results from an F (for fertility) factor
as part of the chromosome or as a plasmid
• Plasmids, including the F plasmid, are small,
circular, self-replicating DNA molecules
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 18-17
Sex pilus
5 µm
The F Plasmid and Conjugation
• Cells containing the F plasmid, designated F+
cells, function as DNA donors during conjugation
• F+ cells transfer DNA to an F recipient cell
• Chromosomal genes can be transferred during
conjugation when the donor cell’s F factor is
integrated into the chromosome
• A cell with a built-in F factor is called an Hfr cell
• The F factor of an Hfr cell brings some
chromosomal DNA along when transferred to an
F– cell
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 18-18_1
F plasmid
F+ cell
Mating
bridge
F– cell
Bacterial chromosome
F+ cell
F+ cell
Bacterial
chromosome
Conjunction and transfer of an F plasmid from and F+ donor to an F– recipient
LE 18-18_2
F plasmid
Bacterial chromosome
F+ cell
Mating
bridge
F– cell
F+ cell
F+ cell
Bacterial
chromosome
Conjunction and transfer of an F plasmid from and F+ donor to an F– recipient
Hfr cell
F+ cell
F factor
LE 18-18_3
F plasmid
Bacterial chromosome
F+ cell
Mating
bridge
F– cell
F+ cell
F+ cell
Bacterial
chromosome
Conjunction and transfer of an F plasmid from and F+ donor to an F– recipient
Hfr cell
F+ cell
F factor
Hfr cell
F– cell
LE 18-18_4
F plasmid
Bacterial chromosome
F+ cell
Mating
bridge
F– cell
F+ cell
F+ cell
Bacterial
chromosome
Conjunction and transfer of an F plasmid from and F+ donor to an F– recipient
Hfr cell
F+ cell
F factor
Hfr cell
F– cell
Temporary
Recombinant F–
partial
bacterium
diploid
Conjugation and transfer of part of the bacterial chromosome from an
Hfr donor to an F– recipient, resulting in recombiination
R plasmids and Antibiotic Resistance
• R plasmids confer resistance to various antibiotics
• When a bacterial population is exposed to an
antibiotic, individuals with the R plasmid will
survive and increase in the overall population
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Transposition of Genetic Elements
• The DNA of a cell can also undergo recombination
due to movement of transposable elements within
the cell’s genome
• Transposable elements, often called “jumping
genes,” contribute to genetic shuffling in bacteria
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Insertion Sequences
• The simplest transposable elements, called
insertion sequences, exist only in bacteria
• An insertion sequence has a single gene for
transposase, an enzyme catalyzing movement of
the insertion sequence from one site to another
within the genome
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 18-19a
Insertion sequence
5
3
3
5
Inverted
repeat
Transposase gene
Inverted
repeat
Transposons
• Transposable elements called transposons are
longer and more complex than insertion
sequences
• In addition to DNA required for transposition,
transposons have extra genes that “go along for
the ride,” such as genes for antibiotic resistance
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 18-19b
Transposon
Insertion
sequence
Antibiotic
resistance gene
Insertion
sequence
5
3
3
5
Inverted repeat
Transposase gene
Concept 18.4: Individual bacteria respond to
environmental change by regulating their gene expression
• A bacterium can tune its metabolism to the
changing environment and food sources
• This metabolic control occurs on two levels:
– Adjusting activity of metabolic enzymes
– Regulating genes that encode metabolic
enzymes
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 18-20
Regulation of enzyme
activity
Precursor
Regulation of enzyme
production
Feedback
inhibition
Enzyme 1
Gene 1
Enzyme 2
Gene 2
Regulation
of gene
expression
Enzyme 3
Gene 3
Enzyme 4
Gene 4
Enzyme 5
Tryptophan
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 can be switched off by a protein called
a repressor
• A corepressor is a small molecule that cooperates
with a repressor to switch an operon off
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 18-21a
trp operon
Promoter
Promoter
Genes of operon
DNA
Regulatory
gene
mRNA
trpE
trpR
3
trpC
trpB
trpA
C
B
A
Operator
Start codon Stop codon
RNA
polymerase
mRNA 5
5
E
Protein
trpD
Inactive
repressor
D
Polypeptides that make up
enzymes for tryptophan synthesis
Tryptophan absent, repressor inactive, operon on
LE 18-21b_1
DNA
mRNA
Active
repressor
Protein
Tryptophan
(corepressor)
Tryptophan present, repressor active, operon off
LE 18-21b_2
DNA
No RNA made
mRNA
Active
repressor
Protein
Tryptophan
(corepressor)
Tryptophan present, repressor active, operon off
Repressible and Inducible Operons: Two Types of
Negative Gene Regulation
• A repressible operon is one that is usually on;
binding of a repressor to the operator shuts off
transcription
• The trp operon is a repressible operon
• An inducible operon is one that is usually off; a
molecule called an inducer inactivates the
repressor and turns on transcription
• The classic example of an inducible operon is the
lac operon, which contains genes coding for
enzymes in hydrolysis and metabolism of lactose
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 18-22a
Promoter
Regulatory
gene
Operator
lacl
DNA
lacZ
No
RNA
made
3
mRNA
5
Protein
RNA
polymerase
Active
repressor
Lactose absent, repressor active, operon off
LE 18-22b
lac operon
DNA
lacZ
lacl
3
mRNA
5
lacA
Permease
Transacetylase
RNA
polymerase
mRNA 5
-Galactosidase
Protein
Allolactose
(inducer)
lacY
Inactive
repressor
Lactose present, repressor inactive, operon on
• Inducible enzymes usually function in catabolic
pathways
• Repressible enzymes usually function in anabolic
pathways
• Regulation of the trp and lac operons involves
negative control of genes because operons are
switched off by the active form of the repressor
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Positive Gene Regulation
• Some operons are also subject to positive control
through a stimulatory activator protein, such as
catabolite activator protein (CAP)
• When glucose (a preferred food source of E. coli )
is scarce, the lac operon is activated by the
binding of CAP
• When glucose levels increase, CAP detaches from
the lac operon, turning it off
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 18-23a
Promoter
DNA
lacl
lacZ
CAP-binding site
Active
CAP
cAMP
Inactive
CAP
RNA
Operator
polymerase
can bind
and transcribe
Inactive lac
repressor
Lactose present, glucose scarce (cAMP level high): abundant lac
mRNA synthesized
LE 18-23b
Promoter
DNA
lacl
CAP-binding site
Inactive
CAP
lacZ
Operator
RNA
polymerase
can’t bind
Inactive lac
repressor
Lactose present, glucose present (cAMP level low): little lac
mRNA synthesized