right © 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
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 18Chapter 18
The Genetics of Virusesand Bacteria
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 b/c of their frequent use by researchers in studies that reveal broad biological principles
• Beyond their value as model systems, viruses & bacteria have unique genetic mechanisms that are interesting in their own right
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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Bacteria are prok’s with cells much smaller and more simply organized than those of euk’s
• Viruses are smaller & simpler than bacteria
LE 18-2LE 18-2
Virus
Bacterium
Animalcell
Animal cell nucleus0.25 µm
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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 & 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)
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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Structure of Viruses
• Viruses are not cells
• Viruses- very small infectious particles consisting of nucleic acid enclosed in a protein coat &, in some cases, a membranous envelope
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Viral Genomes
• Viral genomes may consist of
– ds or ss DNA
– ds or ss 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
LE 18-4aLE 18-4aCapsomereof capsid
RNA
18 250 mm
Tobacco mosaic virus20 nm
LE 18-4bLE 18-4bCapsomere
Glycoprotein
70–90 nm (diameter)
DNA
Adenoviruses50 nm
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• Some viruses have membranous envelopes that help them infect hosts
• These viral envelopes surround the capsids of influenza viruses & 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
LE 18-4cLE 18-4c
Glycoprotein
80–200 nm (diameter)
RNA
Capsid
Influenza viruses50 nm
Membranousenvelope
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• Bacteriophages, also called phages, are viruses that infect bacteria
• Phages- 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
LE 18-4dLE 18-4d
80 225 nm
DNAHead
TailsheathTailfiber
Bacteriophage T450 nm
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General Features of Viral Reproductive Cycles
• Viruses-- 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
LE 18-5LE 18-5
DNAVIRUS
Capsid
HOST CELL
Viral DNA
Replication
Entry into cell anduncoating of DNA
Transcription
Viral DNA
mRNA
Capsidproteins
Self-assembly ofnew virus particlesand their exit from cell
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Reproductive Cycles of Phages
• Phages are the best understood of all viruses
• Phages--2 reproductive mechanisms:
• lytic cycle &
• lysogenic cycle
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The Lytic Cycle
• lytic cycle- phage reproductive cycle that ends in the death of the host cell
• makes new phages & digests the host’s cell wall, releasing baby viruses
• Virulent phage- reproduces only by the lytic cycle
• Bacteria defenses against phages- restriction enzymes recognize & cut up certain phage DNA
LE 18-6LE 18-6
Attachment
Entry of phage DNAand degradation of host DNA
Synthesis of viralgenomes and proteins
Assembly
ReleasePhage assembly
Head Tails Tail fibers
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Animation: Phage T4 Lytic Cycle
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The Lysogenic Cycle
• lysogenic cycle- replicates phage genome w/o destroying the host
• Prophage– the integrated viral DNA incorporated by recombination into the host cell’s chromo
• Every time the host divides, it copies the phage DNA & passes it to daughter cells
• temperate phages-- use both the lytic & lysogenic cycles Animation: Phage Lambda Lysogenic and Lytic Cycles
LE 18-7LE 18-7
Phage
Phage DNA
The phage attaches to ahost cell and injects its DNA.
Phage DNAcircularizes
Bacterial chromosome
Lytic cycle
The cell lyses, releasing phages.Lytic cycleis induced
or Lysogenic cycleis entered
Certain factorsdetermine whether
Lysogenic cycle
Occasionally, a prophageexits the bacterial chromosome,initiating a lytic cycle.
The bacterium reproducesnormally, copying the prophageand transmitting it to daughter cells.
Prophage
Many cell divisionsproduce a large population of bacteria infected withthe prophage.
Daughter cellwith prophage
Phage DNA integrates into thebacterial chromosomes, becoming aprophage.
New phage DNA and proteins aresynthesized and assembled into phages.
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Reproductive Cycles of Animal Viruses
• 2 key things in classifying viruses that infect animals:
– DNA or RNA?
– ss or ds?
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Class/Family Envelope Examples/Disease
I. Double-stranded DNA (dsDNA)
Adenovirus No Respiratory diseases, animal tumors
Papovavirus No Papillomavirus (warts, cervical cancer): polyomavirus (animal tumors)
Herpesvirus Yes Herpes simplex I and II (cold sores, genital sores); varicella zoster (shingles, chicken pox); Epstein-Barr virus (mononucleosis, Burkitt’s lymphoma)
Poxvirus Yes Smallpox virus, cowpox virus
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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 Rotavirus (diarrhea), Colorado tick fever virus
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Class/Family Envelope Examples/Disease
IV. Single-stranded RNA (ssRNA); serves as mRNA
Picornavirus No Rhinovirus (common cold); poliovirus, hepatitis A virus, and other enteric (intestinal) viruses
Coronavirus Yes Severe acute respiratory syndrome (SARS)
Flavivirus Yes Yellow fever virus, West Nile virus, hepatitis C virus
Togavirus Yes Rubella virus, equine encephalitis viruses
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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
LE 18-8LE 18-8
RNA
ER
Capsid
HOST CELL
Viral genome (RNA)
mRNA
Capsidproteins
Envelope (withglycoproteins)
Glyco-proteins Copy of
genome (RNA)
Capsid and viral genomeenter cell
New virus
Template
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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
LE 18-9LE 18-9
Capsid
Viral envelopeGlycoprotein
Reversetranscriptase
RNA(two identicalstrands)
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• Provirus-- viral DNA that is integrated into the host genome Unlike a prophage, a provirus remains a permanent resident of the host cell
• host’s RNA pol transcribes proviral DNA into RNA molecules
• RNA molecules function both as mRNA for synthesis of viral proteins & as genomes for new virus particles released from the cell
LE 18-10LE 18-10
HOST CELL
ReversetranscriptionViral RNA
RNA-DNAhybrid
DNA
NUCLEUS
ChromosomalDNA
Provirus
RNA genomefor thenext viralgeneration
mRNA
New HIV leaving a cell
HIV entering a cell
0.25 µm
HIVMembrane ofwhite blood cell
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Animation: HIV Reproductive Cycle
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Evolution of Viruses
• Viruses do not fit our definition of living organisms
• Since viruses can reproduce only w/in 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 & prions also cause disease in plants and animals
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Viral Diseases in Animals
• Viruses damage or kill cells by:
• releasing hydrolytic enzymes from lysosomes or
• making toxins that cause disease symptoms
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• Vaccines- harmless derivatives of microbes that stimulate the immune system to mount defenses against the pathogen
• Vaccines can prevent certain viral illnesses
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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
LE 18-11LE 18-11
Young ballet students in HongKong wear face masks toprotect themselves from thevirus causing SARS.
The SARS-causing agent is acoronavirus like this one(colorized TEM), so named forthe “corona” of glyco-proteinspikes protruding form theenvelope.
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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
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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Plant viruses spread disease in 2 ways:
– Horizontal transmission- through damaged cell walls
– Vertical transmission- inheriting virus from a parent
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Viroids and Prions: The Simplest Infectious Agents
• Viroids- circular RNA that infect plants & disrupts growth
• Prions- slow-acting, virtually indestructible infectious proteins that cause brain diseases in mammals
• Prions propagate by converting normal proteins into the prion version
LE 18-13LE 18-13
Normalprotein
New prion
Prion Original prion
Many prions
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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
• The well-studied intestinal bacterium Escherichia coli (E. coli) is “the laboratory rat of molecular biology”
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The Bacterial Genome and Its Replication
• Bacterial chromo- usually a circular DNA molecule w/ few associated proteins
• Many bacteria also have plasmids, smaller circular DNA molecules that can replicate independently of the chromo
• Bacterial cells divide by binary fission, which is preceded by replication
• Plasmids-- including the F plasmid, are small, circular, self-replicating DNA molecules
LE 18-14LE 18-14
Origin ofreplication
Replication fork
Termination of replication
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Mutation and Genetic Recombination as Sources of Genetic Variation
• Since bacteria can reproduce rapidly, new mutations quickly ↑ genetic diversity
• More genetic diversity arises by recombination of DNA from two different bacterial cells
LE 18-15LE 18-15
Mutantstrain
arg+ trp–
Mutantstrain
arg+ trp–
Mixture
Mixture
Nocolonies(control)
Nocolonies(control)
Coloniesgrew
Mutantstrain
arg– trp+
Mutantstrain
arg– trp+
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Mechanisms of Gene Transfer and Genetic Recombination in Bacteria
• 3 processes bring bacterial DNA from diff indiv’s together:
– Transformation
– Transduction
– Conjugation
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Transformation
• Transformation- alteration of a bacterial cell’s genotype & 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
• transduction- phages carry bacterial genes from 1 host cell to another
LE 18-16LE 18-16
A+
Phage DNA
A+
Donorcell
B+
A+
B+
Crossingover
A+
A– B–
Recipientcell
A+ B–
Recombinant cell
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Conjugation and Plasmids
• Conjugation- direct transfer of genetic material b/w bacterial cells that are temporarily joined
• The transfer is 1-way: One cell (“male”) donates DNA, & its “mate” (“female”) receives the genes
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• “Maleness,” the ability to form a sex pilus & donate DNA, results from an F (for fertility) factor as part of the chromo or as a plasmid
LE 18-17LE 18-17
Sex pilus 5 µm
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The F Plasmid and Conjugation
• Cells w/ F plasmid, designated F+ cells, are 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
LE 18-18_1LE 18-18_1
F plasmid Bacterial chromosome
F+ cellMatingbridge
F+ cell
F+ cellBacterial chromosome
F– cell
Conjunction and transfer of an F plasmid from and F+ donor to an F– recipient
LE 18-18_2LE 18-18_2
F plasmid Bacterial chromosome
F+ cellMatingbridge
F+ cell
F+ cellBacterial chromosome
F– cell
Conjunction and transfer of an F plasmid from and F+ donor to an F– recipient
F+ cell Hfr cell
F factor
LE 18-18_3LE 18-18_3
F plasmid Bacterial chromosome
F+ cellMatingbridge
F+ cell
F+ cellBacterial chromosome
F– cell
Conjunction and transfer of an F plasmid from and F+ donor to an F– recipient
F+ cell Hfr cell
F factor
Hfr cell
F– cell
LE 18-18_4LE 18-18_4F plasmid Bacterial chromosome
F+ cellMatingbridge
F+ cell
F+ cellBacterial chromosome
F– cell
Conjunction and transfer of an F plasmid from and F+ donor to an F– recipient
F+ cell Hfr cell
F factor
Hfr cell
F– cell
Temporarypartialdiploid
Recombinant F–
bacterium
Conjugation and transfer of part of the bacterial chromosome from anHfr donor to an F– recipient, resulting in recombiination
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R plasmids and Antibiotic Resistance
• R plasmids confer resistance to various antibiotics
• When a bacterial pop is exposed to an antibiotic, indiv’s w/ the R plasmid will survive & ↑ in the overall pop
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Transposition of Genetic Elements
• DNA of a cell can also undergo recomb due to mvmt of transposable elements w/in 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 mvmt of the insertion seq from 1 site to another w/in the genome
LE 18-19aLE 18-19a
Insertion sequence
Transposase gene
53
Invertedrepeat
35
Invertedrepeat
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Transposons
• Transposable elements called transposons are longer & more complex than insertion seq’s
• In addition to DNA req’d for transposition, transposons have extra genes that “go along for the ride,” such as genes for antibiotic resistance
LE 18-19bLE 18-19b
53
35
Transposon
Insertion sequence
Insertion sequence
Antibioticresistance gene
Transposase geneInverted repeat
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Concept 18.4: Individual bacteria respond to environmental change by regulating their gene expression
• A bacterium can tune its metabolism to the changing environment & food sources
• metabolic control occurs on 2 levels:
– Adjusting enzymes activity
– Regulating genes that encode metabolic enzymes
LE 18-20LE 18-20
Regulation of enzymeactivity
Regulation of enzymeproduction
Enzyme 1
Regulation of gene expression
Enzyme 2
Enzyme 3
Enzyme 4
Enzyme 5
Gene 2
Gene 1
Gene 3
Gene 4
Gene 5
Tryptophan
Precursor
Feedbackinhibition
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Operons: The Basic Concept
• In bacteria, genes are often clustered into operons:
• An operator, “on-off” switch
– A promoter
– Genes for metabolic enzymes
• An operon can be switched off by a protein called a repressor
• corepressor- small molecule that cooperates w/ a repressor to switch an operon off
LE 18-21aLE 18-21a
Promoter Promoter
DNA trpR
Regulatorygene
RNApolymerase
mRNA
3
5
Protein Inactiverepressor
Tryptophan absent, repressor inactive, operon on
mRNA 5
trpE trpD trpC trpB trpA
OperatorStart codonStop codon
trp operon
Genes of operon
E
Polypeptides that make upenzymes for tryptophan synthesis
D C B A
LE 18-21b_1LE 18-21b_1
DNA
Protein
Tryptophan(corepressor)
Tryptophan present, repressor active, operon off
mRNA
Activerepressor
LE 18-21b_2LE 18-21b_2
DNA
Protein
Tryptophan(corepressor)
Tryptophan present, repressor active, operon off
mRNA
Activerepressor
No RNA made
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Repressible and Inducible Operons: Two Types of Negative Gene Regulation
• A repressible operon is 1 that is usually on; binding of a repressor to the operator shuts off txn
• The trp operon is a repressible operon
• An inducible operon is 1 that is usually off; a molecule called an inducer inactivates the repressor & turns on txn
• The classic example of an inducible operon is the lac operon, which contains genes coding for enzymes in hydrolysis and metabolism of lactose
LE 18-22aLE 18-22a
DNA lacl
Regulatorygene
mRNA
5
3
RNApolymerase
ProteinActiverepressor
NoRNAmade
lacZ
Promoter
Operator
Lactose absent, repressor active, operon off
LE 18-22bLE 18-22b
DNA lacl
mRNA5
3
lac operon
Lactose present, repressor inactive, operon on
lacZ lacY lacA
RNApolymerase
mRNA 5
Protein
Allolactose(inducer)
Inactiverepressor
-Galactosidase Permease Transacetylase
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• 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
LE 18-23bLE 18-23b
DNA lacl
CAP-binding site
Promoter
RNApolymerasecan’t bind
Operator
lacZ
Inactive lacrepressor
InactiveCAP
Lactose present, glucose present (cAMP level low): little lacmRNA synthesized
LE 18-23aLE 18-23a
DNA
cAMP
lacl
CAP-binding site
Promoter
ActiveCAP
InactiveCAP
RNApolymerasecan bindand transcribe
Operator
lacZ
Inactive lacrepressor
Lactose present, glucose scarce (cAMP level high): abundant lacmRNA synthesized