Chapter 18:Viruses and Bacteria

Overview: Microbial Model SystemsViruses called bacteriophagesCan infect and set in motion a genetic takeover of bacteria, such as Escherichia coli

E. coli and its virusesAre called model systems because of their frequent use by researchers in studies that reveal broad biological principlesBeyond their value as model systemsViruses and bacteria have unique genetic mechanisms that are interesting in their own right

Recall that bacteria are prokaryotesWith cells much smaller and more simply organized than those of eukaryotesVirusesAre smaller and simpler still

Concept 18.1: A virus has a genome but can reproduce only within a host cellScientists were able to detect viruses indirectlyLong before they were actually able to see them

The Discovery of Viruses: Scientific InquiryTobacco mosaic diseaseStunts the growth of tobacco plants and gives their leaves a mosaic coloration

In the late 1800sResearchers hypothesized that a particle smaller than bacteria caused tobacco mosaic diseaseIn 1935, Wendell StanleyConfirmed this hypothesis when he crystallized the infectious particle, now known as tobacco mosaic virus (TMV)

Structure of VirusesVirusesAre very small infectious particles consisting of nucleic acid enclosed in a protein coat and, in some cases, a membranous envelope

Viral GenomesViral genomes may consist ofDouble- or single-stranded DNADouble- or single-stranded RNA

Capsids and EnvelopesA capsidIs the protein shell that encloses the viral genomeCan have various structures

Some viruses have envelopesWhich are membranous coverings derived from the membrane of the host cell

Bacteriophages, also called phagesHave the most complex capsids found among viruses

General Features of Viral Reproductive CyclesViruses are obligate intracellular parasitesThey can reproduce only within a host cellEach virus has a host rangeA limited number of host cells that it can infect

Viruses use enzymes, ribosomes, and small molecules of host cellsTo synthesize progeny viruses

Reproductive Cycles of PhagesPhagesAre the best understood of all virusesGo through two alternative reproductive mechanisms: the lytic cycle and the lysogenic cycle

The Lytic CycleThe lytic cycleIs a phage reproductive cycle that culminates in the death of the hostProduces new phages and digests the hosts cell wall, releasing the progeny viruses

The lytic cycle of phage T4, a virulent phage

The Lysogenic CycleThe lysogenic cycleReplicates the phage genome without destroying the hostTemperate phagesAre capable of using both the lytic and lysogenic cycles of reproduction

The lytic and lysogenic cycles of phage , a temperate phage

Reproductive Cycles of Animal VirusesThe nature of the genomeIs the basis for the common classification of animal viruses

Classes of animal viruses

Viral EnvelopesMany animal virusesHave a membranous envelopeViral glycoproteins on the envelopeBind to specific receptor molecules on the surface of a host cell

The reproductive cycle of an enveloped RNA virus

RNA as Viral Genetic MaterialThe broadest variety of RNA genomesIs found among the viruses that infect animals

Retroviruses, such as HIV, use the enzyme reverse transcriptase To copy their RNA genome into DNA, which can then be integrated into the host genome as a provirus

The reproductive cycle of HIV, a retrovirus

Evolution of VirusesViruses do not really fit our definition of living organismsSince viruses can reproduce only within cellsThey probably evolved after the first cells appeared, perhaps packaged as fragments of cellular nucleic acid

Concept 18.2: Viruses, viroids, and prions are formidable pathogens in animals and plantsDiseases caused by viral infectionsAffect humans, agricultural crops, and livestock worldwide

Viral Diseases in AnimalsViruses may damage or kill cellsBy causing the release of hydrolytic enzymes from lysosomesSome viruses cause infected cellsTo produce toxins that lead to disease symptoms

VaccinesAre harmless derivatives of pathogenic microbes that stimulate the immune system to mount defenses against the actual pathogenCan prevent certain viral illnesses

Emerging VirusesEmerging virusesAre those that appear suddenly or suddenly come to the attention of medical scientists

Severe acute respiratory syndrome (SARS)Recently appeared in China

Outbreaks of new viral diseases in humansAre usually caused by existing viruses that expand their host territory

Viral Diseases in PlantsMore than 2,000 types of viral diseases of plants are knownCommon symptoms of viral infection includeSpots on leaves and fruits, stunted growth, and damaged flowers or roots

Plant viruses spread disease in two major modesHorizontal transmission, entering through damaged cell walls Vertical transmission, inheriting the virus from a parent

Viroids and Prions: The Simplest Infectious AgentsViroidsAre circular RNA molecules that infect plants and disrupt their growth

PrionsAre slow-acting, virtually indestructible infectious proteins that cause brain diseases in mammalsPropagate by converting normal proteins into the prion version

Concept 18.3: Rapid reproduction, mutation, and genetic recombination contribute to the genetic diversity of bacteriaBacteria allow researchersTo investigate molecular genetics in the simplest true organisms

The Bacterial Genome and Its ReplicationThe bacterial chromosomeIs usually a circular DNA molecule with few associated proteinsIn addition to the chromosomeMany bacteria have plasmids, smaller circular DNA molecules that can replicate independently of the bacterial chromosome

Bacterial cells divide by binary fissionWhich is preceded by replication of the bacterial chromosome

Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Mutation and Genetic Recombination as Sources of Genetic VariationSince bacteria can reproduce rapidlyNew mutations can quickly increase a populations genetic diversity

Further genetic diversityCan arise by recombination of the DNA from two different bacterial cellsMutant strain arg trp+

Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Mechanisms of Gene Transfer and Genetic Recombination in BacteriaThree processes bring bacterial DNA from different individuals togetherTransformationTransductionConjugation

TransformationTransformationIs the alteration of a bacterial cells genotype and phenotype by the uptake of naked, foreign DNA from the surrounding environment

TransductionIn the process known as transductionPhages carry bacterial genes from one host cell to another

Conjugation and PlasmidsConjugationIs the direct transfer of genetic material between bacterial cells that are temporarily joined

The F Plasmid and ConjugationCells containing the F plasmid, designated F+ cellsFunction as DNA donors during conjugationTransfer plasmid DNA to an F recipient cell

Conjugation and transfer of an F plasmid from an F+ donor to an F recipient

Chromosomal genes can be transferred during conjugationWhen the donor cells F factor is integrated into the chromosomeA cell with the F factor built into its chromosomeIs called an Hfr cellThe F factor of an Hfr cellBrings some chromosomal DNA along with it when it is transferred to an F cell

Conjugation and transfer of part of the bacterial chromosome from an Hfr donor to an F recipient, resulting in recombinationFigure 18.18b

R plasmids and Antibiotic ResistanceR plasmidsConfer resistance to various antibiotics

Transposition of Genetic ElementsTransposable elementsCan move around within a cells genomeAre often called jumping genesContribute to genetic shuffling in bacteria

Insertion SequencesAn insertion sequence contains a single gene for transposaseAn enzyme that catalyzes movement of the insertion sequence from one site to another within the genome

TransposonsBacterial transposonsAlso move about within the bacterial genomeHave additional genes, such as those for antibiotic resistance

Concept 18.4: Individual bacteria respond to environmental change by regulating their gene expressionE. coli, a type of bacteria that lives in the human colonCan tune its metabolism to the changing environment and food sources

This metabolic control occurs on two levelsAdjusting the activity of metabolic enzymes already presentRegulating the genes encoding the metabolic enzymes

Operons: The Basic ConceptIn bacteria, genes are often clustered into operons, composed ofAn operator, an on-off switchA promoterGenes for metabolic enzymes

An operonIs usually turned onCan be switched off by a protein called a repressor

The trp operon: regulated synthesis of repressible enzymes

Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Repressible and Inducible Operons: Two Types of Negative Gene RegulationIn a repressible operonBinding of a specific repressor protein to the operator shuts off transcriptionIn an inducible operonBinding of an inducer to an innately inactive repressor inactivates the repressor and turns on transcription

The lac operon: regulated synthesis of inducible enzymes

Inducible enzymesUsually function in catabolic pathwaysRepressible enzymesUsually function in anabolic pathways

Regulation of both the trp and lac operonsInvolves the negative control of genes, because the operons are switched off by the active form of the repressor protein

Positive Gene RegulationSome operons are also subject to positive controlVia a stimulatory activator protein, such as catabolite activator protein (CAP)

In E. coli, when glucose, a preferred food source, is scarceThe lac operon is activated by the binding of a regulatory protein, catabolite activator protein (CAP)

When glucose levels in an E. coli cell increaseCAP detaches from the lac operon, turning it off

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