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Overview: Microbial Model Systems. Viruses called bacteriophages can infect and set in motion a genetic takeover of bacteria, such as Escherichia coli - PowerPoint PPT Presentation
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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
LE 18-2LE 18-2
Virus
Bacterium
Animalcell
Animal cell nucleus0.25 µm
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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
• A capsid is the protein shell that encloses the viral genome
• A capsid can have various structures
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• 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
LE 18-4dLE 18-4d
80 225 nm
DNAHead
TailsheathTailfiber
Bacteriophage T450 nm
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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
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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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)
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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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)
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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)
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• 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
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
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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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
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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• 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
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.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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
• 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
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
LE 18-14LE 18-14
Origin ofreplication
Replication fork
Termination of replication
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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
LE 18-15LE 18-15
Mutantstrainarg+ trp–
Mutantstrainarg+ trp–
Mixture
Mixture
Nocolonies(control)
Nocolonies(control)
Coloniesgrew
Mutantstrainarg– trp+
Mutantstrainarg– trp+
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Mechanisms of Gene Transfer and Genetic Recombination in Bacteria
• Three processes bring bacterial DNA from different individuals together:
– Transformation
– Transduction
– Conjugation
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Transduction
• In the process known as transduction, phages carry bacterial genes from one 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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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
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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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
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
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
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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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
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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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
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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• 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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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 18-23aLE 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
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