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Chapter 1

Towards a definition of a virus

Viruses occur universally, but they can only be detected indirectly. Viruses are obligate intracellular parasitesthat require a host within which they replicate. Although they are well known for causing disease, mostviruses coexist peacefully with their hosts.

Chapter 1 Outline

1.1 Discovery of viruses1.2 Multiplication of viruses1.3 The virus multiplication cycle1.4 Viruses can be defined in chemical terms1.5 Multiplication of bacterial and animal

viruses is fundamentally similar1.6 Viruses can be manipulated genetically1.7 Properties of viruses1.8 Origin of viruses

Viruses are arguably the most ubiquitous andwidespread group of organisms on the planet,with every animal, plant and protist speciessusceptible to infection. The efficiency ofreplication demonstrated by viruses is such thatthe infection of a single host can generate morenew viruses than there are individuals in thehost population. For example, a single humaninfected with influenza virus can shed sufficientvirus particles to be theoretically capable ofinfecting the entire human population. Whilenot every species has been examined for thepresence of viruses, those that have been testedhave all yielded up new virus isolates. Further,

not only do viruses occur universally but eachspecies has its own specific range of viruses that,by and large, infects only that species. In recentyears, the application of new nucleic acidsequencing techniques has demonstrated that avast array of previously unknown virusesremains to be studied.

Current estimates of the number ofindividual viruses on earth suggest that theyconsiderably exceed the total number of stars inthe known universe, i.e. more than 1023 (100sextillion). This vast number raises questions asto what the viruses are doing there, and whatselective advantage, if any, they afford to thespecies that host them. The answer to the first ofthese is the same as if the question was posedabout any organism – it is simply occupying aparticular environmental niche which, in thecase of a virus, is another species. The answer towhether or not any benefit accrues for hosting avirus is usually not known, though the adverseeffects of virus infections are all too wellknown. However, it is clear that, despite theiradverse effects and the dramatic depictions ofviruses in popular media and cinema, viruseshave not made their hosts extinct.

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Introduction to Modern Virology, Seventh Edition. N. J. Dimmock, A. J. Easton and K. N. Leppard.� 2016 JohnWiley & Sons, Ltd. Published 2016 by JohnWiley & Sons, Ltd.

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1.1 Discovery of virusesAlthough much is known about viruses(Box 1.1), it is instructive and interesting toconsider how this knowledge came about. Itwas only just over 100 years ago, at the end ofthe 19th century, that the germ theory ofdisease was formulated, and pathologists werethen confident that a causative micro-organismwould be found for each infectious disease.Further, they believed that these agents ofdisease could be seen with the aid of amicroscope, could be cultivated on a nutrientmedium, and could be retained by filters. Therewere, admittedly, a few organisms which wereso fastidious that they could not be cultivated inthe laboratory but the other two criteria weresatisfied. However, in 1892, Dmitri Iwanowskiwas able to show that the causal agent of amosaic disease of tobacco plants, manifesting asa discoloration of the leaf, passed through abacteria-proof filter, and could not be seen orcultivated. Iwanowski was unimpressed by hisdiscovery, but Beijerinck repeated theexperiments in 1898, and became convincedthat this represented a new form of infectiousagent which he termed contagium vivum fluidum,

what we now know as a virus. In the same year,Loeffler and Frosch came to the sameconclusion regarding the cause of foot-and-mouth disease. Furthermore, because foot-and-mouth disease could be passed from animal toanimal, with great dilution at each passage, thecausative agent had to be reproducing and thuscould not be a bacterial toxin. Viruses of otheranimals were soon discovered. Ellerman andBang reported the cell-free transmission ofchicken leukaemia in 1908, and in 1911 Rousdiscovered that solid tumours of chickens couldbe transmitted by cell-free filtrates. These werethe first indications that some viruses can causecancer (see Chapter 25).

Finally, bacterial viruses were discovered. In1915, Twort published an account of a glassytransformation of micrococci. He had beentrying to culture the smallpox agent on agarplates but the only growth obtained was that ofsome contaminating micrococci. Followingprolonged incubation, some of the colonies tookon a glassy appearance, and once this occurredno bacteria could be subcultured from theaffected colonies. If some of the glassy materialwas added to normal colonies, they too took ona similar appearance, even if the glassy materialwas first passed through very fine filters to

Box 1.1

Properties common to all viruses

• Viruses have a nucleic acid genome of either DNA or RNA.• Compared with a cell genome, viral genomes are small, but genomes of different virusesrange in size by over 100-fold (ca 3000 nt to 1,200,000 bp)

• Small genomes make small particles – again with a 100-fold size range.• Viral genomes are associated with protein that at its simplest forms the virus particle, but insome viruses this nucleoprotein is surrounded by further protein or a lipid bilayer.

• The outermost proteins of the virus particle allow the virus to recognise the correct host celland gain entry.

• Viruses can only reproduce in living cells: they are obligate parasites.

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exclude all but the smallest material. Amongthe suggestions that Twort put forward toexplain the phenomenon were either theexistence of a bacterial virus or the secretion bythe bacteria of an enzyme which could lyse theproducing cells. This idea of self-destruction bysecreted enzymes was to prove a controversialtopic over the next decade. In 1917, d’Hérelleobserved a similar phenomenon in dysenterybacilli. He observed clear spots on lawns of suchcells, and resolved to find an explanation forthem. Upon noting the lysis of broth cultures ofpure dysentery bacilli by filtered emulsions offaeces, he immediately realized he was dealingwith a bacterial virus. Since this virus wasincapable of multiplying except at the expenseof living bacteria, he called his virus abacteriophage (literally a bacterium eater), orphage for short.

Thus the first definition of these new agents,the viruses, was presented entirely in negativeterms: they could not be seen, could not becultivated in the absence of cells and, mostimportant of all, were not retained by bacteria-proof filters. However, these features define keycharacteristics of viruses: they are small parasitesthat require a host in which they replicate.

1.2 Multiplication ofvirusesEarly studies focused on establishing the natureof viruses. d’Hérelle believed that the infectingphage particle multiplied within the bacteriumand that its progeny were liberated upon lysis ofthe host cell, whereas others believed thatphage-induced dissolution of bacterial cultureswas merely the consequence of a stimulation oflytic enzymes endogenous to the bacteria. Yetanother school of thought was that phagescould pass freely in and out of bacterial cells andthat lysis of bacteria was a secondaryphenomenon not necessarily concerned with

the growth of a phage. It was Delbruck whoended the controversy by pointing out that twophenomena were involved: lysis from withinand lysis from without. The type of lysisobserved was dependent on the ratio ofinfecting phages to bacteria (referred to as themultiplicity of infection). At a low multiplicity ofinfection (with the ratio of phages to bacteria nogreater than 2:1), then the phages infect thecells, multiply and lyse the cells from within.When the multiplicity of infection is high, i.e.many hundreds of phages per bacterium, thecells are lysed directly, and rather than anincrease in phage titre there is a decrease. Lysisis due to weakening of the cell wall when largenumbers of phages are attached.

Convincing support for d’Hérelle’shypothesis was provided by the one-stepgrowth experiment of Ellis and Delbruck(1939). A phage preparation such asbacteriophage λ (lambda) is mixed with asuspension of the bacterium Escherichia coli at amultiplicity of infection of 10 infectious phageparticles per cell, ensuring that virtually all cellsare infected. Then, after allowing 5 minutes forthe phage to attach, the culture is centrifuged topellet the cells and attached phage. Mediumcontaining unattached phage is discarded. Thecells are then resuspended in fresh medium.Samples of the culture are withdrawn at regularintervals, cells and medium are separated andassayed for infectious phage. The resultsobtained are shown in Fig. 1.1. After a latentperiod of 17 minutes during which no phageincrease is detected in cell-free medium, there isa sudden rise in the detection of infectiousphage in the medium. This ‘burst’ sizerepresents the average of many different burstsfrom individual cells, and can be calculatedfrom the total virus yield/number of cellinfected. The entire growth cycle here takesaround 30minutes, although this will vary withdifferent viruses and cells. The amount of cell-associated virus is determined by taking thecells pelleted from the medium, disrupting

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them and assaying for virus infectivity as before.The fact that virus appears inside the cellsbefore it appears in the medium demonstratesthe intracellular nature of phage replication. Itcan be seen also that the kinetics of appearanceof intracellular phage particles are linear, notexponential. This is consistent with particlesbeing produced by assembly from componentparts rather than by binary fission.

1.3 The virusmultiplication cycleWe now know a great deal about the processeswhich occur during the multiplication of viruseswithin single cells. The precise details vary forindividual viruses but have in common a seriesof events marking specific phases in themultiplication cycle. These phases aresummarized in Fig. 1.2 and are considered indetail in Part II of this book. The first stage isthat of attachment when the virus attaches tothe potential host cell. The interaction isspecific, with the virus attachment protein(s)binding to target receptor molecules on thesurface of the cell. The initial contact between avirus and host cell is a dynamic, reversible oneoften involving weak electrostatic interactions.However, the contacts quickly become muchstronger with more stable interactions which insome cases are essentially irreversible. Theattachment phase determines the specificity ofthe virus for a particular type of cell or hostspecies. Having attached to the surface of thecell, the virus must effect entry to be able toreplicate in a process called penetration orentry. Once inside the cell, the genome of thevirus must become available. This is achieved bythe loss of many, or all, of the proteins thatmake up the particle in a process referred to asuncoating. For some viruses, the entry anduncoating phases are combined in a singleprocess. Typically, these first three phases donot require the expenditure of energy in theform of ATP hydrolysis. Having made the virusgenome available it is now used in thebiosynthesis phase when genome replication,transcription of mRNA and translation of themRNA into protein occur. The process oftranslation uses ribosomes provided by the hostcell and it is this requirement for the translationmachinery, as well as the need for molecules forbiosynthesis, that makes viruses obligateintracellular parasites. The newly-synthesized

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Fig. 1.1 A one-step growth curve ofbacteriophage λ following infection of susceptiblebacteria (Escherichia coli). During the eclipse phase,the infectivity of the cell-associated, infecting virusis lost as it uncoats; during the maturation phaseinfectious virus is assembled inside cells (cell-associated virus), but not yet released; and thelatent phasemeasures the period before infectiousvirus is released from cells into the medium. Totalvirus is the sum of cell-associated virus + releasedvirus. Cell-associated virus decreases as cells arelysed. This classic experiment shows that phagesdevelop intracellularly. A consideration of themethods used to determine the yield of viruses isgiven in Chapter 5.

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genomes may then be used as templates forfurther rounds of replication and as templatesfor transcription of more virus mRNA in anamplification process which increases the yieldof virus from the infected cells. When the newgenomes are produced, they come togetherwith the newly-synthesized virus proteins toform progeny virus particles in a process calledassembly. Finally, the particles must leave thecell in a release phase after which they seek outnew potential host cells to begin the processagain. The particles produced within the cellmay require further processing to becomeinfectious and thismaturation phase mayoccur before or after release.

Combining the consideration of the stepswhich make up a virus multiplication cycle withthe information in the graph of the results of asingle step growth curve, it can be seen thatduring the eclipse phase the virus is undergoingthe processes of attachment, entry, uncoatingand biosynthesis. At this time, the cells containall of the elements necessary to produce virusesbut the original infecting virus has beendismantled and no new infectious particles haveyet been produced. It is only after the assemblystep that we see virus particles inside the cellbefore they are released and appear in themedium.

1.4 Viruses can be definedin chemical termsThe first virus was purified in 1933 bySchlessinger using differential centrifugation.Chemical analysis of the purified bacteriophageshowed that it consisted of approximately equalproportions of protein and deoxyribonucleicacid (DNA). A few years later, in 1935, Stanleyisolated tobacco mosaic virus in paracrystallineform, and this crystallization of a biologicalmaterial thought to be alive raised manyphilosophical questions about the nature of life.In 1937, Bawden and Pirie extensively purifiedtobacco mosaic virus and showed it to benucleoprotein containing ribonucleic acid(RNA). Thus virus particles may contain eitherDNA or RNA. However, at this time it was notknown that nucleic acid constituted geneticmaterial.

The importance of viral nucleic acid

In 1949, Markham and Smith found thatpreparations of turnip yellow mosaic viruscomprised two types of identically sizedspherical particles, only one of which contained

AssemblyRelease

Attachment

Penetration Uncoating

Biosynthesis

Fig. 1.2 A diagrammatic representation of the six phases common to all virus multiplication cycles. Seetext for details.

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nucleic acid. Significantly, only the particlescontaining nucleic acid were infectious. A fewyears later, in 1952, Hershey and Chasedemonstrated the independent functions ofviral protein and nucleic acid using the head-tailvirus, bacteriophage T2 (Box 1.2).

In another classic experiment, Fraenkel-Conrat and Singer (1957) were able to confirmby a different means the hereditary role of viralRNA. Their experiment was based on the earlierdiscovery that particles of tobacco mosaic viruscan be dissociated into their protein and RNAcomponents, and then reassembled to give

Box 1.2

Evidence that DNA is the genetic material of bacteriophage T2. The Hershey-Chaseexperiment

Bacteriophage T2 was grown in E. coli in thepresence of 35S (as sulphate) to label theprotein moiety, or 32P (as phosphate) to mainlylabel the nucleic acid. Purified, labelled phageswere allowed to attach to sensitive host cellsand then given time for the infection tocommence. The phages, still on the outside ofthe cell, were then subjected to the shearingforces of a Waring blender. Such treatmentremoves any phage components attached tothe outside of the cell but does not affect cellviability. Moreover, the cells are still able toproduce infectious progeny virus. When thecells were separated from the medium, it wasobserved that 75% of the 35S (i.e. phageprotein) had been removed from the cells byblending but only 15% of the 32P (i.e. phagenucleic acid) had been removed. Thus, afterinfection, the bulk of the phage proteinappeared to have no further function and thissuggested (but does not prove – that had toawait more rigorous experiments with purifiednucleic acid genomes) that the nucleic acid isthe carrier of viral heredity. The transfer of thephage nucleic acid from its protein coat to thebacterial cell upon infection also accounts forthe existence of the eclipse period during theearly stages of intracellular virus development,since the nucleic acid on its own cannotnormally infect a cell (Fig. 1.3).

Blend inWaringblender

Supernatant (phage)75% ofradioactivity

Pellet (cells)25% ofradioactivity

Centrifuge

Blend inWaringblender

Supernatant (phage)15% ofradioactivity

Pellet (cells)85% ofradioactivity

Centrifuge

Phage labelledwith 35S

Phage labelledwith 32P

Mix withbacteria

Mix withbacteria

Fig. 1.3 The Hershey-Chase experimentproving that DNA (labelled with 32P) is thegenetic material of bacteriophage T2.

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particles which are morphologically mature andfully infectious (see Chapter 12). Whenparticles of two different strains (differing in thesymptoms produced in the host plant) wereeach disassociated and the RNA of onereassociated with the protein of the other, andvice versa, the properties of the virus which waspropagated when the resulting ‘hybrid’ particleswere used to infect host plants were alwaysthose of the parent virus from which the RNAwas derived (Fig. 1.4).

The ultimate proof that viral nucleic acid isthe genetic material came from numerousobservations that, under special circumstances,purified viral nucleic acid is capable of initiatinginfection, albeit with a reduced efficiency. Forexample, in 1956 Gierer and Schramm, andFraenkel-Conrat independently showed thatthe purified RNA of tobacco mosaic virus can beinfectious, provided precautions are taken toprotect it from inactivation by ribonuclease. Anextreme example is the causative agent ofpotato spindle tuber disease which lacks anyprotein component and consists solely of RNA.Because such agents have no protein coat, theycannot be called viruses and are referred to asviroids.

Synthesis of macromolecules ininfected cells

Following introduction of the virus geneticmaterial into the cell, the next phase of thereplication cycle is the synthesis of newmacromolecules that play a role in thereplication process and/or find their way intothe next generation of virus particles. Thediscovery in 1953, by Wyatt and Cohen, thatthe DNA of the T-even bacteriophages T2, T4and T6 contains hydroxymethylcytosine (HMC)instead of cytosine made it possible for Hershey,Dixon and Chase to examine infected bacteriafor the presence of phage-specific DNA atvarious stages of intracellular growth. DNA was

extracted from T2-infected E. coli at differenttimes after the onset of phage growth, andanalyzed for its content of HMC. This providedan estimate of the number of phage equivalentsof HMC-containing DNA present at any time,based on the total nucleic acid and relativeHMC content of the intact T2 phage particle.The results showed that, with T2, synthesis ofphage DNA commences about 6 minutes after

Strain A virus Strain B virus

Strain B virus Strain A virus

Treatment with7M urea

Treatment with7M urea

Make hybrid virus

Infect plants

Harvest virus

Infect plants

Harvest virus

Proteinsubunits

ProteinsubunitsRNA RNA

Fig. 1.4 The experiment of Fraenkel-Conrat andSinger which proved that RNA is the geneticmaterial of tobacco mosaic virus.

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infection and the amount present then risessharply, so that by the time the first infectiousparticles begin to appear 6 minutes later thereare 50–80 phage equivalents of HMC.Thereafter, the numbers of phage equivalents ofDNA and of infectious particles increase linearlyand at the same rate up until lysis, and continueto rise even if lysis is delayed beyond the normalburst time.

Hershey and his co-workers also studied thesynthesis of phage protein, which can bedistinguished from bacterial protein by itsinteraction with specific antibodies. Duringinfection of E. coli by T2 phage, protein can bedetected about 9 minutes after the onset of thelatent period, i.e. after DNA synthesis beginsand before infectious particles appear. A fewminutes later there are approximately 30–40phages inside the cell. Whereas the synthesis ofviral protein starts about 9 minutes after theonset of the latent period, it was shown bymeans of pulse–chase experiments that theuptake of 35S into intracellular protein isconstant from the start of infection. A smallquantity (a pulse) of 35S (as sulphate) wasadded to the medium at different times afterinfection and was followed shortly after by avast excess of unlabelled sulphate (chase) tostop any further incorporation of label. Whenthe pulse was made from the 9th minuteonward, the label could be chased into materialidentifiable by its reaction with antibody (i.e.serologically) as phage coat protein. However, ifthe pulse was made before 9 minutes ofinfection, although it could still be chased intoprotein and was non-bacterial, it did not reactwith antibodies to phage structural proteins.This early protein comprises mainly virus-specified enzymes that are concerned withphage replication but are not incorporated intophage particles. The concept of early and late,non-structural and structural viral proteins isdiscussed in Chapter 10.

These classical experiments are typical onlyof head-tail phages (see Section 2.5) infecting

E. coli under optimum growth conditions. E. coliis normally found in the anaerobicenvironment of the intestinal tract, and it isdoubtful that it grows with its optimal doublingtime of 20 minutes under natural conditions.Other bacterial cells grow more slowly than E.coli and their viruses have longer multiplicationcycles.

1.5 Multiplication ofbacterial and animal virusesis fundamentally similarThe growth curves and other experimentsdescribed above have been repeated with manyanimal viruses with essentially similar results.Both bacterial and animal viruses attach to theirtarget cell through specific interactions with cellsurface molecules. Like the T4 bacteriophage,the genomes of some animal viruses (e.g. HIV-1) enter the cell and leave their coat proteins onthe outside. However, with most animalviruses, some viral protein, usually from insidethe particle, enters the cell in association withthe viral genome. In fact, it is now known thatsome phage protein enters the bacterial cellswith the phage genome. Such proteins areessential for genome replication. Many otheranimal viruses behave slightly differently, andafter attachment are engulfed by the cellmembrane and taken into the cell inside avesicle. However, strictly speaking this virus hasnot yet entered the cell cytoplasm, and is stilloutside the cell. The virus genome gains entryto the cytoplasm through the wall of the vesicle,when the particle is stimulated to uncoat.Again, the outer virion proteins stay in thevesicle – i.e. outside the cell. Animal viruses gothrough the same stages of eclipse, and virusassembly from constituent viral componentswith linear kinetics, as bacterial viruses.Release of progeny virions may happen bycell lysis (although this is not an enzymatic

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process as it is with some bacterial viruses),but frequently virus is released withoutmajor cell damage. The cell may die later, butdeath of the cell does not necessarilyaccompany the multiplication of all animalviruses. One major difference in themultiplication of bacterial and animal virus isthat of time scale – animal virus growthcycles take in the region of 5–15 hours forcompletion.

1.6 Viruses can bemanipulated geneticallyOne of the easiest ways to understand the stepsinvolved in a particular reaction within anorganism is to isolate mutants which are unableto carry out that reaction. Like all otherorganisms, viruses sport mutants in the courseof their growth, and these mutations can affectall properties including the type of plaqueformed, the range of hosts which the virus caninfect, and the physico-chemical properties ofthe virus. One obvious caveat is that manymutations will be lethal to the virus and remainundetected. This problem was overcome in1963 by Epstein and Edgar and theircollaborators with the discovery of conditionallethal mutants. One class of these mutants, thetemperature-sensitive mutants, was able to grow ata lower temperature than normal, the permissivetemperature, but not at a higher, restrictivetemperature at which normal virus could grow.Another class of conditional lethal mutants wasthe ambermutant. In these mutants a geneticlesion converts a codon within transcribed RNAinto a triplet which terminates proteinsynthesis. They can only grow on a permissivehost cell, which has an amber-suppressortransfer RNA (tRNA) that can insert an aminoacid at the mutation site during translation.

The drawback to conditional lethal mutantsis that mutation is random, but the advent of

recombinant DNA technology has facilitatedcontrolled mutagenesis, known as reversegenetics, at least for those viruses for whichinfectious particles can be reconstituted fromcloned genomic DNA or cDNA (DNA that hasbeen transcribed from RNA) inserted into aplasmid vector. This process is described inSection 5.7.

1.7 Properties of virusesWith the assumption that the features of virusgrowth described above for particular virusesare true of all viruses, it is possible to compareand contrast the properties of viruses withthose of their host cells. Whereas host cellscontain both types of nucleic acid, DNA andRNA, each virus contains only one type.However, just like their host cells, viruseshave their genetic information encoded innucleic acid. Another difference is that thevirus is reproduced solely from its geneticmaterial, whereas the host cell is reproducedfrom the integrated sum of its components.Thus, the virus never arises directly from apre-existing virus, whereas the cell alwaysarises by division from a pre-existing cell. Theexperiments of Hershey and his collaboratorsshowed quite clearly that the components of avirus are synthesized independently and thenassembled into many virus particles. Incontrast, the host cell increases its constituentparts, during which the individuality of thecell is continuously maintained, and thendivides and forms two cells. Finally, virusesare incapable of synthesizing ribosomes, anddepend on pre-existing host cell ribosomesfor synthesis of viral proteins. Thesefeatures clearly separate viruses fromall other organisms, even Chlamydia species,which for many years were consideredto be intermediate between bacteriaand viruses.

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1.8 Origin of virusesThe question of the origin of viruses is afascinating topic; as so often happens whenhard evidence is scarce, discussion can beheated but often not illuminating. There aretwo popular theories: viruses are eitherdegenerate cells or vagrant genes. Just asfleas are descended from flies by loss ofwings, viruses may be derived from pro- oreukaryotic cells that have dispensed withmany of their cellular functions (degeneracy).Alternatively, some nucleic acid might havebeen transferred accidentally into a cell of adifferent species (e.g. through a wound or bysexual contact) and, instead of beingdegraded as would normally be the case,might have survived and replicated (escape).Despite decades of discussion and argumentthere are no firm indications if either, orboth, of these theories are correct. Rapidsequencing of viral and cellular genomes isnow providing data for computer analysisthat are giving an ever-better understandingof the relatedness of different viruses.However, while such analyses may identify,or more commonly infer, the progenitors of avirus, they cannot decide betweendegeneracy and escape. It is unlikely that allcurrently-known viruses have evolved from asingle progenitor. Rather, viruses haveprobably arisen numerous times in the pastby one or both of the mechanisms outlinedabove.

Key points• Viruses are obligate intracellular parasites.• It is likely that every living organism on this

planet is infected by a species-specific rangeof viruses.

• Viruses multiply by assembling manyprogeny particles from a pool of virus-specified components, whereas cells multiplyby binary fission.

• Viruses have probably originatedindependently many times.

Further readingFlint, S. J., Enquist, L. W., Racaniello, V. R., Skalka,A. M. 2008. Principles of Virology: Molecular Biology,Pathogenesis, and Control, 3rd edn. ASM Press,Washington DC.

Hull, R. 2013. Plant Virology, 5th edn. Academic Press,San Diego.

Knipe, D. M., Howley, P. M. (2007). Field’s Virology,5th edn. Lippincott Williams & Wilkins,Philadelphia.

Maclachlan,N. J., Dubovi, E. J. 2010.Fenner’s VeterinaryVirology, 4th edn.Academic Press, SanDiego.

Mahy, B. W. J., van Regenmortel, M. H. V. 2008.Encyclopedia of Virology, 3rd edn. Academic Press,San Diego.

Richman, D. D., Whitley, R. J., Hayden, F. G. 2009.Clinical Virology, 3rd edn. ASMPress,Washington DC.

Zuckerman, A. J., Banatvala, J., Griffiths, P. D.,Schoub, B., Mortimer, P. 2009. Principles andPractice of Clinical Virology, 6th edn. John Wiley &Sons, Chichester.

Part I The nature of viruses12

Chapter 1: Towards a definition of a virus1.1 Discovery of viruses1.2 Multiplication of viruses1.3 The virus multiplication cycle1.4 Viruses can be defined in chemical termsThe importance of viral nucleic acidSynthesis of macromolecules in infected cells

1.5 Multiplication of bacterial and animal viruses is fundamentally similar1.6 Viruses can be manipulated genetically1.7 Properties of viruses1.8 Origin of virusesKey pointsFurther reading