Presentation on theme: "Overview: Microbial Model Systems (This is Ch. 18 in our book, but Ch"— Presentation transcript:
1 Overview: Microbial Model Systems (This is Ch. 18 in our book, but Ch Overview: Microbial Model Systems (This is Ch. 18 in our book, but Ch. 19 according to F & T Guide.)Viruses called bacteriophages can infect and set in motion a genetic takeover of bacteria, such as Escherichia coliE. coli and its viruses are called model systems because of their frequent use by researchers in studies that reveal broad biological principlesBeyond their value as model systems, viruses and bacteria have unique genetic mechanisms that are interesting in their own right
3 Bacteria are prokaryotes with cells much smaller and more simply organized than those of eukaryotes Viruses are smaller and simpler than bacteria
4 Comparison of animal cell, bacterial cell, and virus BacteriumAnimalcellAnimal cell nucleus0.25 µm
5 1. 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 themThe story of how viruses were discovered begins in the late 1800s
6 1—2. The Discovery of Viruses: Scientific Inquiry Tobacco mosaic disease stunts growth of tobacco plants and gives their leaves a mosaic colorationIn the late 1800s, researchers hypothesized that a particle smaller than bacteria caused the disease1883—Adolf Mayer1893—Dimitri IvanowskyIn 1935, Wendell Stanley confirmed this hypothesis by crystallizing the infectious particle, now known as tobacco mosaic virus (TMV)
8 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
9 3. Viral Genomes Viral genomes may consist of Double- stranded DNA Single-stranded DNADouble- stranded RNASingle-stranded RNADepending on its type of nucleic acid, a virus is called a DNA virus or an RNA virus
10 4. Capsids and EnvelopesA capsid is the protein shell that encloses the viral genomeCapsomeres are the protein subunits that make up the capsidA capsid can have various structures
13 4. Polyhedral head and tail apparatus DNATailsheathTailfiber80 225 nm50 nmBacteriophage T4
14 5. Viral envelopesSome viral structures have membranous envelopes that help them infect hostsThese viral envelopes surround the capsids of influenza viruses and many other viruses found in animalsViral envelopes, which are derived from the host cell’s membrane, contain a combination of viral and host cell moleculesThey contain host phospholipids and membrane proteins and viral proteins and glycoproteins
18 7.Bacteriophages, also called phages, are viruses that infect bacteriaThey have the most complex capsids found among virusesPhages have an elongated capsid head that encloses their DNAA protein tailpiece attaches the phage to the host and injects the phage DNA inside
19 8—9. General Features of Viral Reproductive Cycles Viruses identify their host cells by a “lock-and-key” fit between proteins on the outside of the virus and specific receptor molecules on the surface of cellsViruses are obligate intracellular parasites, which means they can reproduce only within a host cellAnimation: Simplified Viral Reproductive Cycle
20 10—11.Each virus has a host range, a limited number of host cells that it can infectBroad host range: West Nile virus (mosquitoes, birds, and humans)Narrow host range: measles virus and poliovirus can only infect humansRabies virus has a broad host range (mammals such as dogs, raccoons, bats, humans), but a human cold virus can only infect humans
21 12.Viruses use enzymes, ribosomes, and small host molecules (nucleotides, amino acids) to synthesize progeny viruses
22 12, 13, 15. Entry into cell and uncoating of DNA VIRUS DNA Capsid ReplicationTranscriptionHOST CELLViral DNAmRNAViral DNACapsidproteinsSelf-assembly ofnew virus particlesand their exit from cell
24 Reproductive Cycles of Phages Phages are the best understood of all virusesPhages have two reproductive mechanisms: the lytic cycle and the lysogenic cycle
25 16, 18. The Lytic CycleThe lytic cycle is a phage reproductive cycle that culminates in the death of the host cellThe lytic cycle produces new phages and digests the host’s cell wall, releasing the progeny virusesA phage that reproduces only by the lytic cycle is called a virulent phageBacteria have defenses against phages, including restriction enzymes that recognize and cut up certain phage DNA
26 The Lytic Cycle Attachment Entry of phage DNA and degradation of host DNAPhage assemblyReleaseHeadTailsTail fibersAssemblySynthesis of viralgenomes and proteins
27 17.The _______ of the phage enters the cell, leaving the empty _________ outside.This is accomplished when the sheath of the __________ contracts.
28 18, 19, 20. Bacterial defenses against phages: 1. Natural selection favors bacterial mutants with receptor sites that are no longer recognized by phages.2. As soon as phage DNA enters the bacterial host cell, it fights back with restriction enzymes, which hydrolyze the viral DNA. (The bacterial cell’s own DNA is chemically modified to protect it from its own restriction enzymes.)3. Not all phages kill their hosts—some use the lysogenic cycle
29 Animation: Phage Lambda Lysogenic and Lytic Cycles 16, 21. The Lysogenic CycleThe lysogenic cycle replicates the phage genome without destroying the hostThe viral DNA molecule is incorporated by genetic recombination into the host cell’s chromosomeThis integrated viral DNA is known as a prophageEvery time the host divides, it copies the phage DNA and passes the copies to daughter cellsPhages that use both the lytic and lysogenic cycles are called temperate phagesAnimation: Phage Lambda Lysogenic and Lytic Cycles
30 23, 24. Phage DNA The phage attaches to a host cell and injects its DNA.Daughter cellwith prophageMany cell divisionsproduce a largepopulation ofbacteria infected withthe prophage.Phage DNAcircularizesPhageBacterialchromosomeOccasionally, a prophageexits the bacterial chromosome,initiating a lytic cycle.Lytic cycleLysogenic cycleCertain factorsdetermine whetherThe bacterium reproducesnormally, copying the prophageand transmitting it to daughter cells.The cell lyses, releasing phages.Lytic cycleis inducedorLysogenic cycleis enteredProphageNew phage DNA and proteins aresynthesized and assembled into phages.Phage DNA integrates into thebacterial chromosomes, becoming aprophage.
31 22.The Trojan horse was full of Greek soldiers who, once they were within the walls of Troy, leapt out and destroyed the city.Prophages “hide out” within the host cell and are replicated along with the host’s DNA.They might be triggered to “go lytic” by an environmental signal, such as radiation or the presence of certain chemicals.
32 25. Few bacteriophages have an envelope or RNA genome, but… Animal viruses often have an envelope derived from the host cell’s membrane or nuclear envelopeAnimal viruses often have an RNA genome
33 Classifying Animal Viruses Two key variables in classifying viruses that infect animals:DNA or RNA?Single-stranded or double-stranded?
34 I. Double-stranded DNA (dsDNA) Adenovirus No Papovavirus Class/FamilyEnvelopeExamples/DiseaseI. Double-stranded DNA (dsDNA)AdenovirusNoRespiratory diseases, animal tumorsPapovavirusPapillomavirus (warts, cervical cancer): polyomavirus (animal tumors)HerpesvirusYesHerpes simplex I and II (cold sores, genital sores); varicella zoster (shingles, chicken pox); Epstein-Barr virus (mononucleosis, Burkitt’s lymphoma)PoxvirusSmallpox virus, cowpox virus[NOTE TO PRODUCTION: PLEASE DO NOT include a jpeg of Table For better legibility, the table contents have been input as table text in slides 25, 26, 27, and 28.]
35 II. Single-stranded DNA (ssDNA) Parvovirus No Class/FamilyEnvelopeExamples/DiseaseII. Single-stranded DNA (ssDNA)ParvovirusNoB19 parvovirus (mild rash)III. Double-stranded RNA (dsRNA)ReovirusRotavirus (diarrhea), Colorado tick fever virus[NOTE TO PRODUCTION: PLEASE DO NOT include a jpeg of Table For better legibility, the table contents have been input as table text in slides 25, 26, 27, and 28.]
36 IV. Single-stranded RNA (ssRNA); serves as mRNA Picornavirus No Class/FamilyEnvelopeExamples/DiseaseIV. Single-stranded RNA (ssRNA); serves as mRNAPicornavirusNoRhinovirus (common cold); poliovirus, hepatitis A virus, and other enteric (intestinal) virusesCoronavirusYesSevere acute respiratory syndrome (SARS)FlavivirusYellow fever virus, West Nile virus, hepatitis C virusTogavirusRubella virus, equine encephalitis viruses[NOTE TO PRODUCTION: PLEASE DO NOT include a jpeg of Table For better legibility, the table contents have been input as table text in slides 25, 26, 27, and 28.]
37 V. ssRNA; template for mRNA synthesis Filovirus Yes Orthomyxovirus Class/FamilyEnvelopeExamples/DiseaseV. ssRNA; template for mRNA synthesisFilovirusYesEbola virus (hemorrhagic fever)OrthomyxovirusInfluenza virusParamyxovirusMeasles virus; mumps virusRhabdovirusRabies virusVI. ssRNA; template for DNA synthesisRetrovirusHIV (AIDS); RNA tumor viruses (leukemia)[NOTE TO PRODUCTION: PLEASE DO NOT include a jpeg of Table For better legibility, the table contents have been input as table text in slides 25, 26, 27, and 28.]
38 26. RNA as Viral Genetic Material The broadest variety of RNA genomes is found in viruses that infect animalsRetroviruses use reverse transcriptase to copy their RNA genome into DNAHIV is the retrovirus that causes AIDS
40 28.The viral DNA that is integrated into the host genome is called a provirusUnlike a prophage, a provirus remains a permanent resident of the host cellThe host’s RNA polymerase transcribes the proviral DNA into RNA moleculesThe RNA molecules function both as mRNA for synthesis of viral proteins and as genomes for new virus particles released from the cell
41 29. HOST CELL Reverse transcription Viral RNA RNA-DNA hybrid Membrane ofwhite blood cellHIVHOST CELLReversetranscriptionViral RNARNA-DNAhybrid0.25 µmHIV entering a cellDNANUCLEUSProvirusChromosomalDNARNA genomefor thenext viralgenerationmRNANew HIV leaving a cell
42 30. Evolution of VirusesViruses do not fit our definition of living organismsSince viruses can reproduce only within cells, they probably evolved from bits of cellular nucleic acid, such as…Plasmids, which are small, circular DNA molecules found in bacteria and yeast, orTransposons, which are DNA segments that can move around within a cell’s genome.
43 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 worldwideSmaller, less complex entities called viroids and prions also cause disease in plants and animals
44 31. Viral Diseases in Animals Viruses may damage or kill cells by causing the release of hydrolytic enzymes from lysosomesSome viruses cause infected cells to produce toxins that lead to disease symptomsSome have molecular components that are toxic, such as membrane proteinsCold viruses attack epithelial cells, which easily replace themselves through cell division. Poliovirus attacks nerve cells, which are irreparable.
45 32. Fighting back against viruses: Our immune system provides natural defense against virusesModern medicine has developed vaccines and drugsVaccines are harmless derivatives of pathogenic microbes that stimulate the immune system to mount defenses against the actual pathogenVaccines can prevent certain viral illnessesAntiviral drugs include-acyclovir, which interferes with viral nucleic acid synthesis-AZT, which interferes with reverse transcriptase
46 33. Emerging VirusesEmerging viruses are those that appear suddenly or suddenly come to the attention of scientistsSevere acute respiratory syndrome (SARS) recently appeared in ChinaOutbreaks of “new” viral diseases in humans are usually caused by:--Mutation of existing viruses creating new strains (ex. flu)--Spread of existing viruses from one host species to another (ex. hantavirus, bird flu, swine flu)--Cultural change (ex. people traveling more, blood transfusions, IV drug use, and sexual promiscuity all contributed to the spread of HIV)
47 Young ballet students in Hong Kong wear face masks to 33.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.
48 34. H1N1H and N stand for the two glycoproteins that are on the surface of the viral envelope of this flu virus (their scientific names are hemagglutinin and neuramidinase) and do its dirty work. There are 16 types of the H protein, numbered H1 through H16, and 9 types of the N protein, numbered N1 through N9. That makes 144 possible combinations of the virus, a constantly changing challenge for prevention or treatment.A new combination, H2N2, cause a brief swine flu epidemic in An H3N2 strain was the source of another epidemic in The bird flu virus that began in Southeast Asia a decade ago and has spread throughout the Old World is an H5N1 combination.
49 Viral Diseases in Plants More than 2,000 types of viral diseases of plants are knownSome symptoms are spots on leaves and fruits, stunted growth, and damaged flowers or roots
51 35. Plant viruses spread disease in two major modes: Horizontal transmission, entering through damaged cell wallsVertical transmission, inheriting the virus from a parent
52 36.Once inside a plant cell, the virus replicates and can spread throughout the plant body through plasmodesmataSome viral genes code for proteins that increase the diameter of plasmodesmata so the viruses can spread more easily!
53 37-38. Viroids and Prions: The Simplest Infectious Agents Viroids are circular RNA molecules that infect plants and disrupt their growthThey teach us that even a single molecule can be an infectious agent that spreads diseaseOne viroid disease killed 10 million coconut palms in the PhilippinesPrions are slow-acting, virtually indestructible infectious proteins that cause brain diseases in mammalsPrions propagate by converting normal proteins into the prion versionPrions are transmitted in food
54 38. Prions propagating Original Prion prion Many prions New prion Normalprotein
55 39. Diseases caused by prions Scrapie (in sheep)Mad cow diseaseCreutzfeldt-Jakob diseaseKuru (laughing disease of the Fore tribe of Papau, New Guinea)
56 40, 41. Prions are scaryThe diseases they cause are all neurodegenerativeThe diseases they cause are incurableStanley B Prusiner (in 1982) claimed to have isolated prions (he also named them) and won the Nobel prize in 1997
57 Sorry for the confusion!! Concept 18.3: Rapid reproduction, mutation, and genetic recombination contribute to the genetic diversity of bacteriaNow we’re working on part of a F & T Guide (it’s their Chapter 27, but still our Chapter 18)!Sorry for the confusion!!Bacteria allow researchers to investigate molecular genetics in the simplest true organismsThe well-studied intestinal bacterium Escherichia coli (E. coli) is “the laboratory rat of molecular biology”
58 14. The Bacterial Genome and Its Replication The bacterial chromosome is usually a circular DNA molecule with few associated proteinsMany bacteria also have plasmids, smaller circular DNA molecules that can replicate independently of the chromosomeBacterial cells divide by binary fission, which is preceded by replication of the chromosome
59 Not in the Lecture Guide, but you need to know… Replication forkOrigin ofreplicationTerminationof replication
60 15, 16.See book, page 536.Endospores are dormant, non-reproductive forms of bacteria. They are formed due to environmental factors, such as when nutrient levels are low. Bacteria can survive in this dormant state for centuries.Ex: Bacillus anthracis (anthrax), Clostridium tetani (tetanus)
61 17. Mechanisms of Gene Transfer and Genetic Recombination in Bacteria Three processes bring bacterial DNA from different individuals together:TransformationTransductionConjugation
62 17, 18. TransformationTransformation is the alteration of a bacterial cell’s genotype and phenotype by the uptake of naked, foreign DNA from the surrounding environmentFor example, harmless Streptococcus pneumoniae bacteria can be transformed to pneumonia-causing cells (Frederick Griffith’s experiment)
63 17, 19, 20. TransductionIn the process known as transduction, phages carry bacterial genes from one host cell to another
65 Conjugation and Plasmids Conjugation is the direct transfer of genetic material between bacterial cells that are temporarily joinedThe transfer is one-way: One cell (“male”) donates DNA, and its “mate” (“female”) receives the genes
66 21.“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 plasmidPlasmids, including the F plasmid, are small, circular, self-replicating DNA molecules
68 22, 23, 24. The F Plasmid and Conjugation Cells containing the F plasmid, designated F+ cells, function as DNA donors during conjugationF+ cells transfer DNA to an F recipient cellChromosomal genes can be transferred during conjugation when the donor cell’s F factor is integrated into the chromosomeA cell with a built-in F factor is called an Hfr cellThe F factor of an Hfr cell brings some chromosomal DNA along when transferred to an F– cell
69 F plasmid Bacterial chromosome F+ cell F+ cell Mating bridge F– cell 24. ConjugationF plasmidBacterial chromosomeF+ cellF+ cellMatingbridgeF– cellF+ cellBacterialchromosomeConjunction and transfer of an F plasmid from and F+ donor to an F– recipient
70 F plasmid Bacterial chromosome F+ cell F+ cell Mating bridge F– cell 22, 25, 26. Episomes and Hfr cellsF plasmidBacterial chromosomeF+ cellF+ cellMatingbridgeF– cellF+ cellBacterialchromosomeConjunction and transfer of an F plasmid from and F+ donor to an F– recipient.The F Factor is incorporated into the host’s chromosome and is called an episomeF+ cellHfr cellF factor
71 F plasmid Bacterial chromosome F+ cell F+ cell Mating bridge F– cell 27. Hfr stands for “high frequency of recombination, because when these cells conjugate with other cells, there’s a good chance genetic recombination is going to occur:F plasmidBacterial chromosomeF+ cellF+ cellMatingbridgeF– cellF+ cellBacterialchromosomeConjunction and transfer of an F plasmid from and F+ donor to an F– recipientF+ cellHfr cellF factorHfr cellF– cell
72 27.F plasmidBacterial chromosomeF+ cellF+ cellMatingbridgeF– cellF+ cellBacterialchromosomeConjunction and transfer of an F plasmid from and F+ donor to an F– recipientF+ cellHfr cellF factorHfr cellF– cellTemporarypartialdiploidRecombinant F–bacteriumConjugation and transfer of part of the bacterial chromosome from anHfr donor to an F– recipient, resulting in recombiination
73 28. R plasmids and Antibiotic Resistance R plasmids confer resistance to various antibioticsWhen a bacterial population is exposed to an antibiotic, individuals with the R plasmid will survive and increase in the overall population
74 29. Transposition of Genetic Elements Transposable elements are sections of DNA that can move around with a cell’s genomeThe DNA of a cell can also undergo recombination due to the movement of transposable elementsTransposable elements, often called “jumping genes,” contribute to genetic shuffling in bacteria
75 30. Insertion SequencesInsertion sequences are the simplest transposable elements and exist only in bacteriaAn insertion sequence has a single gene for transposase, an enzyme catalyzing movement of the insertion sequence from one site to another within the genome
77 31. TransposonsTransposable elements called transposons are longer and more complex than insertion sequencesIn addition to DNA required for transposition, transposons have extra genes that “go along for the ride,” such as genes for antibiotic resistance
79 This metabolic control occurs on two levels: 1, 2. Concept 18.4: Individual bacteria respond to environmental change by regulating their gene expressionNow, we’re continuing on to the Chapter 18 F & T Guide! (still our Chapter 18!)A bacterium can tune its metabolism to the changing environment and food sourcesThis metabolic control occurs on two levels:Adjusting activity of metabolic enzymesRegulating genes that encode metabolic enzymes
80 2, 3. Regulation of enzyme activity Regulation of enzyme production PrecursorFeedbackinhibitionEnzyme 1Gene 1Enzyme 2Gene 2Regulationof geneexpressionEnzyme 3Gene 3Enzyme 4Gene 4Enzyme 5Gene 5Tryptophan
81 4—7. Regulation of Gene Expression in Prokaryotes Involves Operons: In bacteria, genes are often clustered into operons, composed ofAn operator, an “on-off” switchA promoter, the place where _______ bindsGenes for metabolic enzymesAn operon can be switched off by a protein called a repressorA corepressor is a small molecule that cooperates with a repressor to switch an operon off
82 Example: the trp operon, a repressible operon PromoterPromoterGenes of operonDNAtrpRtrpEtrpDtrpCtrpBtrpAOperatorRegulatorygeneRNApolymeraseStart codonStop codon3¢mRNA 5¢mRNA5¢EDCBAProteinInactiverepressorPolypeptides that make upenzymes for tryptophan synthesisTryptophan absent, repressor inactive, operon on
83 The repressor protein is coded for by a regulatory gene (not part of the operon) in an inactive form:DNAThe repressor is activated when the corepressor, tryptophan, is presentmRNAProteinActiverepressorTryptophan(corepressor)Tryptophan present, repressor active, operon off
84 The activated repressor binds to the operator, which blocks RNA polymerase from binding. The gene is turned __________.DNANo RNA mademRNAProteinActiverepressorTryptophan(corepressor)Tryptophan present, repressor active, operon off
85 8—9.Repressor proteins bind to the operator and block RNA polymerase from binding to the promoter.This STOPS transcriptionThe regulatory gene codes for the repressor protein. The regulatory gene is NOT part of the operon and is usually located elsewhere on the bacterial chromosome
86 10. 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 transcriptionThe trp operon is a repressible operonAn inducible operon is one that is usually off; a molecule called an inducer inactivates the repressor and turns on transcriptionThe classic example of an inducible operon is the lac operon, which contains genes coding for enzymes in hydrolysis and metabolism of lactose
87 The lac operon. The repressor is coded for by the regulatory gene in an active form. PromoterOperatorDNAlacllacZNoRNAmade3¢mRNARNApolymerase5¢ActiverepressorProteinLactose absent, repressor active, operon off
89 12.Inducible enzymes usually function in catabolic pathways. Their repressors are made in an ____________ form and require an __________ before they can turn the gene _________.Repressible enzymes usually function in anabolic pathways. Their repressors are made in an ____________ form and require a __________ before they can turn the gene _________.Regulation of the BOTH trp and lac operons involves negative control of genes because, ultimately, operons are switched off by the active form of the repressor
90 13, 17. Negative regulationRemember, both repressible and inducible operons are eventually turned OFF when the repressor protein binds.Therefore, they are both examples of negative control.
91 14. 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 CAPWhen glucose levels increase, CAP detaches from the lac operon, turning it off
94 15. Positive regulationCAP is an example of positive regulation of a geneWhen CAP is bound to the CAP-binding site, it increases RNA polymerase’s affinity for the promoter, and increases transcription of the genesYou can think of the repressor protein as an on-off switch and CAP as volume control
95 16.When glucose levels are high, cAMP levels will be _____________ and active CAP levels will be _____________.When glucose levels are low, cAMP levels will be _____________ and active CAP levels will be _____________.
96 FINALLY!We’re done with OUR Chapter 18 and we’ve covered all the topics in that Chapter.Now, on to Chapter 19, even though we’re still working on the Ch. 18 F & T Guide… :\