Chapter 19: Viruses

Chapter 19: VirusesChapter 19We will only cover sections 19.1 and 19.2 in Chapter 19: Viruses.

Complete the Concept Check questions for section 19.1 (pg. 384) and 19.2 (pg. 390) and Self Quiz (pg. 395) #1-3Overview: A Borrowed LifeViruses called bacteriophages can infect and set in motion a genetic takeover of bacteria, such as Escherichia coliViruses lead a kind of borrowed life between life-forms and chemicalsThe origins of molecular biology lie in early studies of viruses that infect bacteria

Fig. 19-10.5 m4Figure 19.1 Are the tiny viruses infecting this E. coli cell alive?

Concept 19.1: A virus consists of a nucleic acid surrounded by a protein coatViruses were detected indirectly long before they were actually seen

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 diseaseIn 1935, Wendell Stanley confirmed this hypothesis by crystallizing the infectious particle, now known as tobacco mosaic virus (TMV)

Fig. 19-2RESULTS123Extracted sapfrom tobaccoplant withtobaccomosaic diseasePassed sapthrough aporcelain filter knownto trapbacteriaRubbed filteredsap on healthytobacco plants4Healthy plantsbecame infected8Figure 19.2 What causes tobacco mosaic disease?

The Structure of VirusesViruses are not cellsViruses are 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 of eitherDouble- or single-stranded DNA, orDouble- or single-stranded RNADepending on its type of nucleic acid, a virus is called a DNA virus or an RNA virus

Capsids and EnvelopesA capsid is the protein shell that encloses the viral genomeCapsids are built from protein subunits called capsomeresA capsid can have various structures

Fig. 19-3a(a) Tobacco mosaic virus20 nm18 250 nmCapsomereof capsidRNA12Figure 19.3 Viral structure

Fig. 19-3bDNACapsomereGlycoprotein7090 nm (diameter)50 nm(b) Adenoviruses13Figure 19.3 Viral structure

Fig. 19-3cMembranousenvelopeRNACapsidGlycoproteins80200 nm (diameter)50 nm(c) Influenza viruses14Figure 19.3 Viral structure

Fig. 19-3dHeadDNATailsheathTailfiber80 225 nm50 nm(d) Bacteriophage T415Figure 19.3 Viral structureCapsids and EnvelopesSome viruses have membranous envelopes that help them infect hosts These viral envelopes surround the capsids of influenza viruses and many other viruses found in animalsViral envelopes, which are derived from the host cells membrane, contain a combination of viral and host cell moleculesBacteriophagesBacteriophages, 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 tail piece attaches the phage to the host and injects the phage DNA inside

Concept 19.2: Viruses reproduce only in host cellsViruses are obligate intracellular parasites, which means they can reproduce only within a host cellEach virus has a host range, a limited number of host cells that it can infect

General Features of Viral Reproductive CyclesOnce a viral genome has entered a cell, the cell begins to manufacture viral proteinsThe virus makes use of host enzymes, ribosomes, tRNAs, amino acids, ATP, and other moleculesViral nucleic acid molecules and capsomeres spontaneously self-assemble into new viruses

Transcriptionand manufactureof capsid proteins Self-assembly of new virus particles and their exit from the cell Entry anduncoatingFig. 19-4VIRUS123DNACapsid4ReplicationHOST CELLViral DNAmRNACapsidproteinsViral DNA21Figure 19.4 A simplified viral reproductive cycle

Reproductive Cycles of PhagesPhages are the best understood of all virusesPhages have two reproductive mechanisms: the lytic cycle and the lysogenic cycle

The Lytic Cycle of PhagesThe lytic cycle is a phage reproductive cycle that culminates in the death of the host cellThe lytic cycle produces new phages and digests the hosts 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

Fig. 19-5-1Attachment124Figure 19.5 The lytic cycle of phage T4, a virulent phage

Fig. 19-5-2Entry of phageDNA anddegradation ofhost DNAAttachment1225Figure 19.5 The lytic cycle of phage T4, a virulent phage

Fig. 19-5-3Synthesis of viralgenomes andproteinsEntry of phageDNA anddegradation ofhost DNAAttachment12326Figure 19.5 The lytic cycle of phage T4, a virulent phage

Fig. 19-5-4Phage assemblyAssemblySynthesis of viralgenomes andproteinsEntry of phageDNA anddegradation ofhost DNAAttachment124HeadTailTail fibers327Figure 19.5 The lytic cycle of phage T4, a virulent phage

Fig. 19-5-5Phage assemblyHeadTailTail fibersAssemblyReleaseSynthesis of viralgenomes andproteinsEntry of phageDNA anddegradation ofhost DNAAttachment1245328Figure 19.5 The lytic cycle of phage T4, a virulent phage

The Lysogenic Cycle of PhagesThe lysogenic cycle replicates the phage genome without destroying the hostThe viral DNA molecule is incorporated into the host cells chromosomeThis integrated viral DNA is known as a prophageEvery time the host divides, it copies the phage DNA and passes the copies to daughter cellsTemperate PhagesAn environmental signal can trigger the virus genome to exit the bacterial chromosome and switch to the lytic mode Phages that use both the lytic and lysogenic cycles are called temperate phages

Fig. 19-6PhageDNAPhageThe phage injects its DNA.Bacterialchromosome Phage DNAcircularizes.Daughter cellwith prophageOccasionally, a prophageexits the bacterialchromosome,initiating a lytic cycle.Cell divisionsproducepopulation ofbacteria infectedwith the prophage.The cell lyses, releasing phages.Lytic cycleLytic cycleis inducedor Lysogenic cycleis enteredLysogenic cycleProphageThe bacterium reproduces,copying the prophage andtransmitting it to daughter cells.Phage DNA integrates intothe bacterial chromosome,becoming a prophage.New phage DNA and proteinsare synthesized andassembled into phages.31Figure 19.6 The lytic and lysogenic cycles of phage , a temperate phage

Reproductive Cycles of Animal VirusesThere are two key variables used to classify viruses that infect animals:DNA or RNA?Single-stranded or double-stranded?Viral EnvelopesMany viruses that infect animals have a membranous envelopeViral glycoproteins on the envelope bind to specific receptor molecules on the surface of a host cellSome viral envelopes are formed from the host cells plasma membrane as the viral capsids exit

Viral EnvelopesOther viral membranes form from the hosts nuclear envelope and are then replaced by an envelope made from Golgi apparatus membrane

Fig. 19-7CapsidRNAEnvelope (withglycoproteins)Capsid and viral genomeenter the cellHOST CELLViral genome (RNA)TemplatemRNAERGlyco-proteinsCapsidproteinsCopy ofgenome (RNA)New virus35Figure 19.7 The reproductive cycle of an enveloped RNA virus

RNA as Viral Genetic MaterialThe broadest variety of RNA genomes is found in viruses that infect animalsRetroviruses use reverse transcriptase to copy their RNA genome into DNAHIV (human immunodeficiency virus) is the retrovirus that causes AIDS (acquired immunodeficiency syndrome)

Fig. 19-8GlycoproteinViral envelopeCapsidRNA (twoidenticalstrands)ReversetranscriptaseHIVHIVMembrane ofwhite blood cellHIV entering a cell0.25 mViral RNARNA-DNAhybridHOST CELLReversetranscriptaseDNANUCLEUSProvirusChromosomalDNARNA genomefor the next viralgenerationmRNANew virusNew HIV leaving a cell37Figure 19.8 The reproductive cycle of HIV, the retrovirus that causes AIDS

Fig. 19-8aGlycoproteinReversetranscriptaseHIVRNA (twoidenticalstrands) CapsidViral envelopeHOST CELLReversetranscriptaseViral RNARNA-DNAhybridDNANUCLEUSProvirusChromosomalDNARNA genomefor thenext viralgenerationmRNANew virus38Figure 19.8 The reproductive cycle of HIV, the retrovirus that causes AIDS

Fig. 19-8bHIVMembrane ofwhite blood cellHIV entering a cell0.25 mNew HIV leaving a cell39Figure 19.8 The reproductive cycle of HIV, the retrovirus that causes AIDS

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 hosts 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 cellEvolution of VirusesViruses do not fit our definition of living organismsSince viruses can reproduce only within cells, they probably evolved as bits of cellular nucleic acidCandidates for the source of viral genomes are plasmids, circular DNA in bacteria and yeasts, and transposons, small mobile DNA segmentsPlasmids, transposons, and viruses are all mobile genetic elementsMimivirus, a double-stranded DNA virus, is the largest virus yet discoveredThere is controversy about whether this virus evolved before or after cells