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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 18 The Genetics of Viruses and Bacteria
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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
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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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Bacteria are prokaryotes w ith cells much smaller and more simply organized than those of eukaryotes Viruses a re smaller and simpler than bacteria
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LE 18-2 Virus Bacterium Animal cell Animal cell nucleus 0.25 µm
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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
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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)
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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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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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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
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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Capsids and Envelopes A capsid i s 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 Tobacco mosaic virus 20 nm
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LE 18-4b Capsomere Glycoprotein 70–90 nm (diameter) DNA Adenoviruses 50 nm
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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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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LE 18-4c Glycoprotein 80–200 nm (diameter) RNA Capsid Influenza viruses 50 nm Membranous envelope
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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
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LE 18-4d 80 225 nm DNA Head Tail sheath Tail fiber Bacteriophage T4 50 nm
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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
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LE 18-5 DNA VIRUS Capsid HOST CELL Viral DNA Replication Entry into cell and uncoating of DNA Transcription Viral DNA mRNA Capsid proteins Self-assembly of new virus particles and their exit from cell
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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Reproductive Cycles of Phages Phages are the best understood of all viruses Phages have two reproductive mechanisms: the lytic cycle and the lysogenic cycle
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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
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LE 18-6 Attachment Entry of phage DNA and degradation of host DNA Synthesis of viral genomes and proteins Assembly Release Phage assembly Head Tails Tail fibers
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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
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LE 18-7 Phage DNA The phage attaches to a host cell and injects its DNA. Phage DNA circularizes Bacterial chromosome Lytic cycle The cell lyses, releasing phages. Lytic cycle is induced or Lysogenic cycle is entered Certain factors determine whether Lysogenic cycle Occasionally, a prophage exits the bacterial chromosome, initiating a lytic cycle. The bacterium reproduces normally, copying the prophage and transmitting it to daughter cells. Prophage Many cell divisions produce a large population of bacteria infected with the prophage. Daughter cell with prophage Phage DNA integrates into the bacterial chromosomes, becoming a prophage. New phage DNA and proteins are synthesized and assembled into phages.
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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?
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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Class/FamilyEnvelopeExamples/Disease I. Double-stranded DNA (dsDNA) AdenovirusNo Respiratory diseases, animal tumors PapovavirusNo Papillomavirus (warts, cervical cancer): polyomavirus (animal tumors) HerpesvirusYes Herpes simplex I and II (cold sores, genital sores); varicella zoster (shingles, chicken pox); Epstein-Barr virus (mononucleosis, Burkitt’s lymphoma) PoxvirusYes Smallpox virus, cowpox virus
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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Class/FamilyEnvelopeExamples/Disease II. Single-stranded DNA (ssDNA) ParvovirusNoB19 parvovirus (mild rash) III. Double-stranded RNA (dsRNA) ReovirusNoRotavirus (diarrhea), Colorado tick fever virus
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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Class/FamilyEnvelopeExamples/Disease IV. Single-stranded RNA (ssRNA); serves as mRNA PicornavirusNoRhinovirus (common cold); poliovirus, hepatitis A virus, and other enteric (intestinal) viruses CoronavirusYesSevere acute respiratory syndrome (SARS) FlavivirusYesYellow fever virus, West Nile virus, hepatitis C virus TogavirusYesRubella virus, equine encephalitis viruses
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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Class/FamilyEnvelopeExamples/Disease V. ssRNA; template for mRNA synthesis FilovirusYes Ebola virus (hemorrhagic fever) OrthomyxovirusYes Influenza virus ParamyxovirusYes Measles virus; mumps virus RhabdovirusYes Rabies virus VI. ssRNA; template for DNA synthesis RetrovirusYes HIV (AIDS); RNA tumor viruses (leukemia)
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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
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LE 18-8 RNA ER Capsid HOST CELL Viral genome (RNA) mRNA Capsid proteins Envelope (with glycoproteins) Glyco- proteins Copy of genome (RNA) Capsid and viral genome enter cell New virus Template
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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
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LE 18-9 Capsid Viral envelope Glycoprotein Reverse transcriptase RNA (two identical strands)
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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
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LE 18-10 HOST CELL Reverse transcription Viral RNA RNA-DNA hybrid DNA NUCLEUS Chromosomal DNA Provirus RNA genome for the next viral generation mRNA New HIV leaving a cell HIV entering a cell 0.25 µm HIV Membrane of white blood cell
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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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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
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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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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
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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
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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.
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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
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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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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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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
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LE 18-13 Normal protein New prion Prion Original prion Many prions
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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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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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
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LE 18-14 Origin of replication Replication fork Termination of replication
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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
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LE 18-15 Mutant strain arg + trp – Mutant strain arg + trp – Mixture No colonies (control) No colonies (control) Colonies grew Mutant strain arg – trp + Mutant strain arg – trp +
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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
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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
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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
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LE 18-16 A+A+ Phage DNA A+A+ Donor cell B+B+ A+A+ B+B+ Crossing over A+A+ A–A– B–B– Recipient cell A+A+ B–B– Recombinant cell
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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
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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
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LE 18-17 Sex pilus 5 µm
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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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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LE 18-18_1 F plasmidBacterial chromosome F + cell Mating bridge F + cell Bacterial chromosome F – cell Conjunction and transfer of an F plasmid from and F + donor to an F – recipient
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LE 18-18_2 F plasmidBacterial chromosome F + cell Mating bridge F + cell Bacterial chromosome F – cell Conjunction and transfer of an F plasmid from and F + donor to an F – recipient F + cell Hfr cell F factor
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LE 18-18_3 F plasmidBacterial chromosome F + cell Mating bridge F + cell Bacterial 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
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LE 18-18_4 F plasmid Bacterial chromosome F + cell Mating bridge F + cell Bacterial 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 Temporary partial diploid Recombinant F – bacterium Conjugation and transfer of part of the bacterial chromosome from an Hfr donor to an F – recipient, resulting in recombiination
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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
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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
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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
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LE 18-19a Insertion sequence Transposase gene 5 3 Inverted repeat 3 5 Inverted repeat
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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
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LE 18-19b 5 3 3 5 Transposon Insertion sequence Insertion sequence Antibiotic resistance gene Transposase gene Inverted repeat
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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
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LE 18-20 Regulation of enzyme activity Regulation of enzyme production 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 Feedback inhibition
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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
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LE 18-21a Promoter DNA trpR Regulatory gene RNA polymerase mRNA 3 5 Protein Inactive repressor Tryptophan absent, repressor inactive, operon on mRNA 5 trpE trpD trpC trpBtrpA Operator Start codon Stop codon trp operon Genes of operon E Polypeptides that make up enzymes for tryptophan synthesis D C B A
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LE 18-21b_1 DNA Protein Tryptophan (corepressor) Tryptophan present, repressor active, operon off mRNA Active repressor
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LE 18-21b_2 DNA Protein Tryptophan (corepressor) Tryptophan present, repressor active, operon off mRNA Active repressor No RNA made
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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
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LE 18-22a DNA lacl Regulatory gene mRNA 5 3 RNA polymerase Protein Active repressor No RNA made lacZ Promoter Operator Lactose absent, repressor active, operon off
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LE 18-22b DNAlacl mRNA 5 3 lac operon Lactose present, repressor inactive, operon on lacZ lacYlacA RNA polymerase mRNA 5 Protein Allolactose (inducer) Inactive repressor -Galactosidase Permease Transacetylase
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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
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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
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LE 18-23a DNA cAMP lacl CAP-binding site Promoter Active CAP Inactive CAP RNA polymerase can bind and transcribe Operator lacZ Inactive lac repressor Lactose present, glucose scarce (cAMP level high): abundant lac mRNA synthesized
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LE 18-23b DNA lacl CAP-binding site Promoter RNA polymerase can’t bind Operator lacZ Inactive lac repressor Inactive CAP Lactose present, glucose present (cAMP level low): little lac mRNA synthesized
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