1 BOTANYVEMLVEML......BiologyVirusesVIRUSESViruses are a unique group of "Biological entities"known to infect every type of cell, including thoseofbacteria,algae,fungi,Protozoa, plantsandanimals; Rememberthat youhave alreadygonethroughan introductory account of virusesin yourfirst year course.Viruses arevery different from otherorganisms.They are not composed of cells and cannot be seenunder a light microscope. A virus particle containsa single type of nucleic acid, either DNA or RNA.The nucleic acid is enclosed in a protein coat andthe coat is sometimes covered in an envelope oflipids,proteins and carbohydrates. Viruses do notexhibit mostof thelife processesofa cell, butmaintain genetic continuity through multiplicationand undergo mutations. Thus, they are certainly morethaninert lifeless molecules.Viruses arebestdescribed as infectious particles.Viruses areobligate intracellular parasitesi.e.they cannot multiply unless they invade a specifichostcell andinstruct itsgenetic andmetabolicmachinery to makeand release daughter or progenyviruses. In this process, they destroy the host cellscausing seriousdamage anddiseases inhumans,plants and animals. The study of viruses is knownas Virology.2.1DiscoveryViruseshavebeenvictimizingmankindfromancient times, causing many diseases inhumans andalso in economically useful plants and animals. Anidentifiable agent responsible for thesediseaseswas not known, even after theproposition of thegerm theory of disease. In 1892 for the first time,the Russian pathologist Dmitri Iwanowski, whileChapter 8 VIRUSESChapter Outline2.1 Discovery2.2 Classification of Viruses2.3 Structure of VirusesCHAPTER 2studyingtobacco mosaicdisease, filtered thesap of diseased tobacco leaf through filter whichwas designedtoretainbacteria. Howeverthe'infectious agent' passed through the pores of thefilter. After injecting the filtered sap into a healthyplant, he foundthe development of symptoms ofmosaicdiseaseinit.Unabletoseeanymicroorganism in the clear sap. Iwanowski reportedthat afilterable agentwas responsible forthedisease.Later,Martinus BeijerinckrepeatedIwanowski's experiments and concluded that thedisease causing agent was a 'contagious living fluid'(contagium vivumfluidum)ratherthanadiscrete entity.W.M. Stanley(1935)purifiedthesapandannounced that the virus causing mosaic diseasein tobacco couldbe crystallized.It was namedTobacco Mosaic Virus(TMV) Franekel Conrat(1956) confirmed that the geneticmaterial of theTMVisRNA.Utilizingthetechniquesofultracentrifugation,X-raycrystallographyandelectronmicroscopy, a number of new viruses werereported and their ultrastructure was elucidated.Living properties of viruses- They are obligate intracellular parasites, i.e, theycannot exist outside the host cell.- They canmultiply inside the living cell like anyother organisms- They contain genetic material namely DNA orRNA- Genetic material undergoes mutationsNon - living properties of viruses- They can be crystallized and stored in bottles- They do not have cells (non cellular)2.4 MultiplicationofBacteriophages2.5 Viral diseases in Plants2.6 Viral diseases in Humans2 BOTANYVEMLVEML......BiologyViruses- They do not show any metabolic activities- They cannot multiply outside the host (obligateparasites)Viruses are insensitive toantibiotics due tothe absence of any inherent metabolism.2.2 Classification of VirusesAtpresenttheInternational CommitteeonTaxonomy of Viruses (ICTV) regulates the normsof classification and nomenclature of viruses. TheICTV schemehas only three hierarchial levels - theFamily (including somesubfamilies)GenusandSpecies.The familynames endwith thesuffix'viridae' while thegenus names with 'virus' and thespecies names are common English expressionsdescribingtheir nature. Viruses are namedafterthe diseasethey cause (e.g. polio virus) Using theICTVsystem, thevirusthatcausesAcquiredimmune Deficiency Syndrome (AIDS) in humanbeings is classified as : Family : Retroviridae, Genus:lentivrius,Species: Human ImmunedeficiencyVirus (HIV)Eg :Retro viruses such as HIV possess a singlestranded RNA genome but differ from other RNAviruses in thatthey firstsynthesize DNA beforetranscribingm - RNA and replicate their genome.The transformation of RNA into DNA takes placeby RNA dependent DNA polymerase enzyme(reverse transcriptase).ss RNA ss DNA ds DNA mRNA2.3 Structure of VirusesViruses range in size from 300 nanometers (nm) asin TMV to 20 nm as inparvoviruses. At the upperend of the spectrum, viruses approximate the sizeof the smallest bacterial cells such as mycoplasmasandat thelower end, they haveabout thesamediameter asthat ofa 'ribosome'. Viruses may beclassified into several differentmorphologicaltypes. Helical viruses resemble long rods that maybe rigidor flexible like rabiesvirus andtobaccomosaicvirus. Manyanimal ,plant andbacterialviruses arepolyhedral (many-sided) Herpessimplexand polio viruses exist in this form. The capsid ofsome viruses is covered by an envelope and suchenveloped virusesare roughly spherical.Influenzavirusis an enveloped virus.Bacteriophages, theviruseswhich infectbacteria, havecomplicatedstructuresand are calledcomplex viruses. Theyhave 'polyhedral symmetry'in the'head'andhelical symmetry' in the 'tails sheath'. In some virusessuch as themeasles virus, the envelopecontains'glycoprotein projectionsknownas spikes. Thespikes function in attachingthe virus to receptorson susceptible host cells.All viruses consists of two basic components a coreofnucleicacid that formsthe genome and thesurroundings coat of protein known as capsid. The3 BOTANYVEMLVEML......BiologyVirusescapsid gives shapeto the virus and provides aprotective covering for the genome. It is made upofprotein subunits calledcapsomeres.Thenumber of capsomeresin characteristic for eachtypes of virus.A Virus contains its genetic information in either adouble stranded (ds ) DNA,or single stranded (ss)DNA, ie, dsDNA or ssDNA. In general, virusesthat infect animals have dsDNA. Bacteriophagesare usually dsDNA viruses.Viral nucleicacidmolecules are either circular or linear. Mostviruseshave a single nucleicacid molecule.but a few havemore thanone (e.gHIV which has two identicalmoleculesofRNA,representingitsgenomiccopies.Let us look at the detailed structure of two typicalviruses - the TMV and 4Tphage..The tobacco mosaic virus is about 300 nm long and18 nmin diameter,with amolecular weightof639 10 Daltons.The capsid is madeof 2.130protein subunits of identical size, called capsomeres.The capsomeres are arranged in a helical mannerarounda central hollow space of 4 nm 040A| | |\ ..Eachproteinsubunitismadeupofsinglepolypeptide chain which possess 158 amino acids.Inside the protein capsidthere is a single strandedspirally coiledRNA moleculeconsisting of 6,500nucleotides.The body of a typical bacteriophage such as 4Tcanbe distinguishedinto headand tailregionsjoinedby a collar.The tail region includes a tailsheath, a base plate, pins and tail fibreswhich helpthe virus attach to hostcell. The tail sheathaidsin injecting viral DNA into the host cell.2.4 Multiplicaiton of BacteriophagesFor a virus to multiply or reproduce or replicate, itmust invade a host celland take over the host'smetabolic machinery. Thoughthe methodbywhich a virus enters and exits a host cell may vary,the basicmechanism of viralmultiplicaitonissimilar for all viruses. The best understood viral4 BOTANYVEMLVEML......BiologyViruseslike cycles are those of the bacteriophages. Phagescan multiply by two alternativemechanisms: thelyitc cycle and the lysogenic cycle. The lytic cycleends withthe 'lysis' or death ofthe hostcell,whereas the host cells remains alive in the lysogeniccycle.T-even phages that attack the bacterium E.colicauselysis of the cells and are called virulent phages.They show lytic cycle which is a five -step processinvolving attachment, penetration, biosynthesis,maturation andrelease.Aftera chance contact between phage particles andbacteria , attachment or adsorption occurs. Thephagesusetailfibresforattachmenttothecomplementary receptor sites on the bacterial cellwall. Attachmentsis followedby the process ofpenetration, duringwhich the tail sheathof phagecontracts.and the tail core is driven in through thebacterialcell wall. When the tip ofthe core reachestheplasmamembrane,theDNAfromthebacteriophage headpasses through the tail corethrough the plasma membrane andenters thebacterialcell.Thecapsidremainsoutsidethebacterial cell and is referred to as ghost. Thus, thephage particle functions like a hypodermic syringeandinjects its DNA into the bacterial cell.Once the phage DNA reaches the cytoplasm of thehostcell, many copies of phageDNA, enzymesand capsid proteins are synthesized, using the cellularmachinery ofthe host cell. For severalminutesfollowinginfection, completephages cannot befound in the host cell but the individual DNA andprotein components can bedetected. The next inthesequenceofeventsistheprocessof'maturation'. In this process, bacteriophage DNA andcapsids are assembledinto complete virions . Thisperiod of time between the infection by a virusandthe appearance of the maturevirus within the cellis called the eclipseperiod.The final stage of viral multiplication is the lysisphase of thehost cell and the release of virionsfrom the host cell. The plasma membrane of thehost cell gets dissolved or lysed due to theviralenzyme, called lysozyme, an enzyme synthesizedwithin thecell and the bacterial cell wall breaksreleasing the newly producedphage particles /virions.The number of newly synthesized phage particlesreleased from a single cell is referred to as the burstsize". and it usually ranges from about 50 to 200.The released phage particles infect other susceptiblecellsin the vicinity and the multiplication cycle isrepeated.5 BOTANYVEMLVEML......BiologyViruses2.4.1The Lysogenic CycleSome bacteriophages such as (Lambda) phagesdo not cause lysis and death of the host cell whentheymultiply. Instead,the phageDNAuponpenetration into an E.coli gets integrated in to thecircularbacterial DNA,becomes partof itandremainslatent (inactive) such phages arecalledtemperate phages. The inserted phage DNA is nowcalled prophage. Every time the bacterial geneticmaterial replicates,the prophage also undergoesreplication. The prophage remains latent within theprogeny cells. However, in some rare spontaneousevents or when the host cell its exposed to UV lightor some chemicals, the phage DNA separates fromthebacterialgenetic material,leadingtotheinitiation to the lytic cycle.Whichof thetwo -lytic orlysogenic-cyclesfacilitates the phenomenon of transduction?2.5 ViralDiseases in PlantsViruses, being obligate parasites, cause many plantdiseases while growing inside them. Plant diseasescausedby virusesare mostlysystemic innature(affectthewholeplant).Theleavesgenerallyexhibitcharacteristicsymptoms.Theusualsymptoms of viral disease) chlorosis (e.gpeachyellowing disease) aremosaic (e.g, tobacco mosaicdisease), veinclearing (e.g. bhendi veinclearing)malformaiton (e.g.swollen shoot of cocoa)andbreaking of flowers (e.g. tulip mosaic break) etc.Usuallyvirionsareinfectiousandcausediseases. A virion is a completely assembled virusoutside its host cell. Unusually a tiny fragment ofnucleicacid with 300-400 nucleotides and withouta protein coat, called viroid, cancause plant diseaseson a variety of economically important plants (e.gtomatos, potatos , cucumbers , citrus)2.6 Viraldiseases in HumansViruses cause widespread diseases in humanssuchascommoncold,hepatitis,chickenpox,influenza herpes, warts, polio etc. Although mostviral infections do not result in death, some such asRabies, AIDS and Ebola have highmortality rate,others such aspolio andneonatal rubella can lead6 BOTANYVEMLVEML......BiologyVirusesTable2.6.1DifferencesbetweenLysogenicphaseandLyticphaseLysogenic phase Lytic phase 1 The viralgenome or its complementary DNA gets integrated with the host DNA. It is called prophage or provirus The viral genome does not integrate with host DNA.2 The host DNA is not hydrolysed during lysogenic phaseThe host DNA is often hydrolysed in the lytic phase. 3 The prophage or provirus replicates only once along with the replication of host genome so that a single particle is transferredto a daughter cell. The viral genome replicates repeatedly and forms a number of copies in the same host cell.4 The cellular machinery of the hostis only slightlydistrubed.The cellular machinery of the host is completely taken over by the viral genome5 The virus is non virulent or temperate The virusis virulent6 The host cell does not get lysed The host cell undergoes lysis7 Virus particles are liberated only rarely. A number of virus particules are liberated when the host cell becomes lysed to long term debility (physical weakness)Do your know, some viruses such as Epstein-Barrvirus and Human Papilloma virus cause cancers inanimals and humans!.Chronic hepatitis B (casued by Hepatitis B virus)may leadto cancer. Such cancer casuing virusesare called Oncoviruses. Not only viruses, but also"proteinaceous infectious particles"calledprions, causesome serious animaldiseases suchasmadcowdisease(BovineSpongiformEncephalitis) in cowsand scrapiedisease in sheep.The 'mad cow diseases such as mad cow diseases(Bovine SpongiformEncephalitis) incowsandscrapiediseases in sheep. The'mad cowdisease'causing prionmay reach man through beef and cause'Creutzfeldt -Jakob diseases in him.