Chapter 18

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


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


• Bacteria are prokaryotes with cells much smaller and more simply organized than those of eukaryotes

• Viruses are smaller and simpler than bacteria

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Virus

Bacterium

Animalcell

Animal cell nucleus0.25 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


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)


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


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


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


• 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

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Glycoprotein

80–200 nm (diameter)

RNA

Capsid

Influenza viruses50 nm

Membranousenvelope


• 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

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80 225 nm

DNAHead

TailsheathTailfiber

Bacteriophage T450 nm


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

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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


Reproductive Cycles of Phages

• Phages are the best understood of all viruses

• Phages have two 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 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

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Attachment

Entry of phage DNAand degradation of host DNA

Synthesis of viralgenomes and proteins

Assembly

ReleasePhage assembly

Head Tails Tail fibers


Animation: Phage T4 Lytic Cycle


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

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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.


Reproductive Cycles of Animal Viruses

• Two key variables in classifying viruses that infect animals:

– DNA or RNA?

– Single-stranded or double-stranded?


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



II. Single-stranded DNA (ssDNA)

Parvovirus No B19 parvovirus (mild rash)

III. Double-stranded RNA (dsRNA)

Reovirus No Rotavirus (diarrhea), Colorado tick fever virus



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



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)


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

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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


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

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Capsid

Viral envelopeGlycoprotein

Reversetranscriptase

RNA(two identicalstrands)


• 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

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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


Animation: HIV Reproductive Cycle


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


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


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

• Vaccines can prevent certain viral illnesses


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

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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.


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


• 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

• Prions propagate by converting normal proteins into the prion version

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Normalprotein

New prion

Prion Original 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 well-studied intestinal bacterium Escherichia coli (E. coli) is “the laboratory rat of molecular biology”


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

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Origin ofreplication

Replication fork

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

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Mutantstrain

arg+ trp–

Mutantstrain

arg+ trp–

Mixture

Mixture

Nocolonies(control)

Nocolonies(control)

Coloniesgrew

Mutantstrain

arg– trp+

Mutantstrain

arg– trp+


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

• For example, harmless Streptococcus pneumoniae bacteria can be transformed to pneumonia-causing cells


Transduction

• In the process known as transduction, phages carry bacterial genes from one host cell to another

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A+

Phage DNA

A+

Donorcell

B+

A+

B+

Crossingover

A+

A– B–

Recipientcell

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


• “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

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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

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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

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F+ cellMatingbridge

F+ cell


F– cell


F+ cell Hfr cell

F factor

LE 18-18_3LE 18-18_3


F+ cellMatingbridge

F+ cell


F– cell


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– cell


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


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


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


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

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Insertion sequence

Transposase gene

53

Invertedrepeat

35

Invertedrepeat


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

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53

35

Transposon

Insertion sequence

Insertion sequence

Antibioticresistance gene

Transposase geneInverted repeat


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

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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


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

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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

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DNA

Protein

Tryptophan(corepressor)

Tryptophan present, repressor active, operon off

mRNA

Activerepressor

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DNA

Protein

Tryptophan(corepressor)

Tryptophan present, repressor active, operon off

mRNA

Activerepressor

No RNA made


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

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DNA lacl

Regulatorygene

mRNA

5

3

RNApolymerase

ProteinActiverepressor

NoRNAmade

lacZ

Promoter

Operator

Lactose absent, repressor active, operon off

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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


• 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


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

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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

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DNA lacl

CAP-binding site

Promoter

RNApolymerasecan’t bind

Operator

lacZ

Inactive lacrepressor

InactiveCAP

Lactose present, glucose present (cAMP level low): little lacmRNA synthesized

Chapter 18

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