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You are watching: A strand of hereditary material surrounded by a protein coating

Lodish H, Berk A, Zipursky SL, et al. Molecular cabinet Biology. Fourth edition. New York: W. H. Freeman; 2000.


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A virus is a little parasite the cannotreproduce by itself. As soon as it infects a at risk cell, however, a virus have the right to directthe cabinet machinery come produce much more viruses. Many viruses have either RNA or DNA astheir genetic material. The main point acid may be single- or double-stranded. Theentire infectious virus particle, dubbed a virion, consists of the main point acid and an outer shell of protein. Thesimplest viruses save on computer only sufficient RNA or DNA to encode 4 proteins. The mostcomplex can encode 100 – 200 proteins.

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The examine of plant viruses influenced someof the very first experiments in molecule biology. In 1935, Wendell Stanley purified andpartly crystallized tobacco mosaic virus (TMV); other plant viruses werecrystallized quickly thereafter. Pure proteins had been crystallized just a quick timebefore Stanley’s work, and it was considered very surprising in ~ the timethat a replicating organism can be crystallized.

A riches of subsequent research with bacterial viruses and also animal viruses hasprovided in-depth understanding of famous structure, and also virus-infected cells haveproved extremely beneficial as design systems because that the research of simple aspects the cellbiology. In numerous cases, DNA viruses make use of cellular enzymes because that synthesis of theirDNA genomes and also mRNAs; every viruses utilize normal cellular ribosomes, tRNAs, andtranslation components for synthetic of their proteins. Many viruses comman-deer thecellular machinery for macromolecular synthesis throughout the so late phase that infection,directing it come synthesize large amounts of a small variety of viral mRNAs andproteins instead of the countless normal cellular macromolecules. Because that instance,animal cells infected by influenza or vesicular stomatitis virus synthesize only oneor two types of glycoproteins, which space encoded by viral genes, vice versa, uninfectedcells produce hundreds that glycoproteins. Such virus-infected cells have been usedextensively in research studies on synthesis of cell-surface glycoproteins. Similarly, muchinformation around the system of DNA replication has actually come from researches withbacterial cells and animal cell infected with basic DNA viruses, since theseviruses depend virtually entirely top top cellular proteins to replicate their DNA. Virusesalso regularly express proteins the modify host-cell processes so as to maximize viralreplication. Because that example, the functions of particular cellular factors in initiation ofprotein synthesis to be revealed since viral proteins interrupt your action.Finally, when certain genes lugged by cancer-causing viruses integrate intochromosomes the a normal animal cell, the normal cell can be convert to a cancercell.

Since plenty of viruses deserve to infect a big number of different cell types, geneticallymodified viruses regularly are supplied to lug foreign DNA into a cell. This approachprovides the basis for a growing list of experimental gene treatment treatments.Because of the considerable use that viruses in cabinet biology research and their potentialas therapeutic agents, we describe the an easy aspects of famous structure and functionin this section.


Viral Capsids Are consistent Arrays the One or a few Types that Protein

The nucleic acid of a virion is enclosed in ~ a protein coat, or capsid, created of multiple copiesof one protein or a couple of different proteins, each of i beg your pardon is encoded by a singleviral gene. Because of this structure, a virus is able come encode all theinformation for making a relatively huge capsid in a small number of genes.This reliable use of hereditary information is important, because only a limitedamount that RNA or DNA, and also therefore a limited number the genes, have the right to fit into avirion capsid. A capsid to add the fastened nucleic acid is dubbed a nucleocapsid.

Nature has uncovered two straightforward ways of arranging the multiple capsid protein subunitsand the viral genome right into a nucleocapsid. The easier structure is a proteinhelix through the RNA or DNA protected within. Tobacco mosaic virus (TMV) is aclassic example of the helical nucleocapsid. In TMV the protein subunits formbroken disklike structures, prefer lock washers, which form the helical covering of follow me rodlike virus once stacked with each other (Figure 6-11a).



Figure 6-11

Two simple geometric forms of viruses. (a) In part viruses, the protein subunits type helical arrays aroundan RNA or DNA molecule (red), which runs in a helical groove withinthe enclosing protein tube. The electron micrograph to the best isof tobacco (more...)


The other significant structural class of viruses, referred to as icosahedralor quasi-spherical viruses, is based on the icosahedron, asolid object developed of 20 identical faces, every of which is an equilateraltriangle. In the simplest type of icosahedral virion every of the 20 triangularfaces is constructed of three identical capsid protein subunits, making a totalof 60 subunits every capsid. At every of the 12 vertices, 5 subunits makecontact symmetrically (Figure 6-11b).Thus every protein subunits are in equivalent contact with oneanother. Tobacco satellite necrosis virus has actually such a straightforward icosahedralstructure. However, many quasi-spherical viruses space larger, request theassembly of more than 3 subunits per confront of the icosahedron. This proteinsform shells who subunits room in quasi-equivalent contact.Here, the protein at the icosahedral vertices stay arranged in a fivefoldsymmetry, but added subunits covering the surfaces between in a pattern ofsixfold the contrary (Figure 6-11c).

The atomic frameworks of a variety of icosahedral viruses have been determined byx-ray crystallography (Figure 6-12a).The first three together viruses to beanalyzed — tomato bushy stunt virus, poliovirus,and rhinovirus (the usual cold virus) — exhibita remarkably comparable design, in regards to the rules of icosahedral symmetry aswell as in the details the their surface ar proteins. In every virus, in ~ atomicresolution, clefts (“canyons”) space observed encircling eachof the vertices that the icosahedral structure. Interaction of this clefts withcell-surface receptors attaches the virus to a organize cell, the first step inviral infection (Figure 6-12b).Neutralizing antibodies specific for a certain virus additionally interact with theseclefts, in order to inhibiting attachment of the virus to the hold cell.



Figure 6-12

Structure the picornaviruses. This icosahedral viruses incorporate poliovirus and also the rhinoviruses,which cause the usual cold. (a) The picornavirus capsid is composedof 4 proteins (VP1, VP2, VP3, and VP4; VP4 is located in theinterior). This design (more...)


In part viruses, the symmetrically arranged nucleocapsid is extended by anexternal membrane, or envelope, which consists mainly the aphospholipid bilayer but likewise contains one or two species of virus-encodedglycoproteins (Figure 6-13). Thephospholipids in the viral envelope are comparable to those in the plasma membraneof an infected hold cell. The famous envelope is, in fact, acquired by buddingfrom the membrane, however contains mainly viral glycoproteins.



Figure 6-13

Electron micrograph that a negative stained influenza virusvirion. The virion is surrounded by a phospholipid bilayer; the big spikesprotruding external from the membrane are composed that trimers ofhemagglutinin protein and tetramers of neuraminidase (more...)


The contents of simple viruses such together TMV, which is composed of a solitary RNAmolecule and also one protein species, experience self-assembly if lock are combined insolution. More complicated viruses containing a dozen or more protein varieties do notspontaneously rally in vitro. The multiple contents of together virusesassemble in ~ infected cells in stages, first into subviral particles and theninto perfect virions. The genomes the these facility viruses encode proteinsthat aid in the assembly that the virion, but the assembly proteins room notthemselves contents of the perfect virion.


Most Viral organize Ranges are Narrow

The truth that the hold range — the group of celltypes the a virus have the right to infect — is generallyrestricted serves as a basis because that classifying viruses. A virus that infects onlybacteria is referred to as a bacteriophage,or merely a phage. Viruses thatinfect pet or plant cells are referred to generally as animalviruses or tree viruses. A couple of viruses can growin both plants and the insects that feed top top them. The very mobile insectsserve as vectors for delivering such viruses between susceptible tree hosts.An instance is potato yellow dwarf virus, which can grow in leafhoppers (insectsthat feed on potato tree leaves) and in potato plants. Wide host rangesare characteristics of some strictly animal viruses, such together vesicular stomatitisvirus, which grows in insects and also in numerous different types of mammalian cells.Most animal viruses, however, carry out not overcome phyla, and some (e.g., poliovirus)infect only carefully related species such together primates. The host-cell selection ofsome animal viruses is further restricted to a limited number of cabinet typesbecause just these cell have ideal surface receptors to i m sorry the virionscan attach.


Viruses can Be Cloned and Counted in Plaque Assays

The number of infectious famous particles in a sample deserve to be quantified by a plaque assay. This assay is performedby culturing a dilute sample of viral particles on a plate extended with hostcells and also then counting the number of local lesions, calledplaques, that construct (Figure 6-14). A plaque establishes on the plate wherever a singlevirion originally infects a single cell. The virus replicates in this initialhost cell and then lyses the cell, releasing numerous progeny virions the infectthe surrounding cells on the plate. After a few such cycles that infection, enoughcells space lysed to produce a clearly shows plaque in the class of staying uninfectedcells.


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Figure 6-14

Plaque assay because that determining number of infectious corpuscle in aviral suspension. (a) each lesion, or plaque, which develops where a single virioninitially infected a solitary cell, constitutes a pure viral clone.(b) bowl illuminated indigenous behind shows (more...)


Since all the progeny virions in a plaque are acquired from a single parentalvirus, castle constitute a virus clone. This type of plaque assay is in standarduse for bacterial and animal viruses. Tree viruses deserve to be assayed likewise bycounting neighborhood lesions ~ above plant leaves inoculated with viruses. Analysis ofviral mutants, which are frequently isolated through plaque assays, has actually contributedextensively to present understanding of molecule cellular processes. The plaqueassay also is vital in isolating λ bacter clones carryingsegments of to move DNA, as discussed in thing 7.


Viral expansion Cycles space Classified as Lytic or Lysogenic

The surface of viruses contains many duplicates of one form of protein the binds, oradsorbs, specifically to multiple duplicates of a receptor protein on a host cell.This communication determines the host range of a virus and also begins the infectionprocess (Figure 6-15). Then, in one ofvarious ways, the viral DNA or RNA the cross the plasma membrane into thecytoplasm. The entering hereditary material may still be accompanied by within viralproteins, although in the instance of countless bacteriophages, every capsid proteinsremain outside an infected cell. The genome of many DNA-containing viruses thatinfect eukaryotic bio cells is transported (with some linked proteins) right into thecell nucleus, where the to move DNA is, of course, additionally found. Once inside thecell, the viral DNA interacts through the host’s machinery fortranscribing DNA into mRNA. The viral mRNA that is developed then is translatedinto famous proteins by host-cell ribosomes, tRNA, and translation factors.


Figure 6-15

Electron micrograph of a T4 bacteriophage adsorbed top top an E.coli cell. When viral surface ar proteins interact with receptors on the organize cell,the viral DNA is injected into the cell. (more...)


Most viral protein assets fall into one of 3 categories: one-of-a-kind enzymesneeded because that viral replication; inhibitory determinants that protect against host-cell DNA, RNA,and protein synthesis; and structural proteins provided in the building of newvirions. These critical proteins typically are made in lot larger quantities than theother two types. ~ the synthesis of hundreds to thousands of brand-new virions hasbeen completed, most infected bacter cells and some infected plant and animalcells rupture, or lyse, releasing all the virions in ~ once. In countless plant andanimal famous infections, however, no discrete lytic event occurs; rather, thedead host cell release the virions as it progressively disintegrates.

These events — adsorption, penetration,replication, and release — describe the revolt cycle of famous replication. Theoutcome is the production of a new round of viral particles and also death the thecell. Number 6-16 illustrates the riot cycle for T4 bacteriophage. Adsorption and release that enveloped animal virusesare somewhat more facility processes. In this case, the virions“bud” from the organize cell, thereby gaining their outerphospholipid envelope, which consists of mostly famous glycoproteins.


Figure 6-16

The measures in the riot replication bicycle of a nonenveloped virusare portrayed for E. Coli bacteriophage T4, which has adouble-stranded DNA genome. During adsorption (step 1), famous coat proteins (at thetip that the tail in T4) communicate with particular (more...)


We illustrate the revolt cycle of sheathe viruses with the rabies virus, whosenucleocapsid is composed of a single-stranded RNA genome surrounded by multiplecopies of nucleocapsid protein (Figure6-17, upper left). In ~ the nucleocapsid that rabiesvirions are viral enzyme for manufacturing viral mRNA and also replicating the viralgenome. The envelope around the nucleocapsid is a phospholipid bilayercontaining multiple copies of a famous transmembrane glycoprotein. Thisreceptor-binding, or “attachment,” protein has actually a largeexternal folded domain top top the exterior of the famous envelope, anα-helical transmembrane domain the spans the viral envelope, and ashort interior domain. The internal domain interacts v the viral matrixprotein, which attributes as a bridge between the transmembrane glycoprotein andnucleocapsid protein. Figure 6-17outlines the events affiliated in adsorption that a rabies virion, assembly ofprogeny nucleocapsids, and also release of progeny virions through budding from thehost-cell plasma membrane. Budding virions are plainly visible in electronmicrographs, as shown by Figure6-18.


Figure 6-17

The actions in the riot replication bicycle of an sheathe virusare shown for rabies virus, which has a single-stranded RNAgenome. The structural contents of this virus are portrayed at thetop. Note that the nucleocapsid of this virusis helical (more...)


Figure 6-18

Transmission electron micrograph of measles virus budding fromthe surface of an infected cell.


In some cases, ~ a bacteriophage DNA molecule start a bacterial cell, itbecomes combined into the host-cell chromosome, whereby it stays quiescent andis replicated as part of the cell’s DNA indigenous one generation come thenext. This combination is dubbed lysogeny, and the integrated phage DNA is described as aprophage (Figure6-19). Under particular conditions, the prophage DNA is activated,leading to its excision from the host-cell chromosome and also entrance right into thelytic cycle. Bacter viruses the this form are dubbed temperatephages. The genomes the a number of animal viruses also canintegrate right into the host-cell genome. Most likely the most necessary are theretroviruses, explained briefly later in this chapter.


Figure 6-19

λ bacteriophage undergoes one of two people lytic replication orlysogeny following infection that E. Coli. The direct double-stranded DNA is converted to a circular formimmediately ~ infection. (Left) If thenutritional state that the organize cell is favorable, (more...)


A couple of phages and animal viruses can infect a cell and also cause brand-new virion productionwithout death the cell or becoming integrated.


Four types of bacter Viruses space Widely offered in Biochemical and GeneticResearch

Bacterial viruses have played a crucial role in the advance of molecular cellbiology. Thousands of different bacteriophages have actually been isolated; many of theseare particularly well suitable for research studies of particular biochemical or geneticevents. Here, we briefly explain four types of bacteriophages, all of whichinfect E. Coli, that have actually been especially helpful in molecularbiology research.


DNA Phages that the T Series

The T phages the E. Coli are huge lytic phages the contain a single moleculeof double-stranded DNA. This molecule is about2 × 105 basic pairs long inT2, T4, and also T6 viruses and about4 × 104 base pairs lengthy inT1, T3, T5, and also T7 viruses. T-phage virions covers a helical protein“tail” attached come an icosahedral“head” filled with the famous DNA. ~ the reminder of aT-phage tail adsorbs to receptor on the surface of one E. Coli cell, the DNAin the head start the cell with the tail (see figure 6-16). The phage DNA then directs a regimen ofevents that produces approximately 100 new phage corpuscle in about 20minutes, in ~ which time the infected cell lyses and also releases the new phages.The initial discovery of the function of messenger RNA in protein synthesis wasbased on studies of E. Coli cell infected through bacteriophage T2. Through 20minutes ~ infection, infected cell synthesize T2 protein only. Thefinding that the RNA synthesized right now had the exact same base compositionas T2 DNA (not E. Coli DNA) implied that mRNA copies of T2 DNA weresynthesized and also used to direct cellular ribosomes to synthesize T2proteins.


Temperate Phages

Bacteriophage λ, i beg your pardon infects E. Coli, typifies the temperatephages. This phage has one that the most studied genomes and is usedextensively in DNA cloning (Chapter7). On start an E. Coli cell, the double-stranded λDNA suspect a one form, which can get in either the riot cycle (as Tphages do) or the lysogenic cycle (see figure 6-19). In the last case, protein expressed from theviral DNA bind a particular sequence top top the circular famous DNA to a similarspecific sequence on the circular bacter DNA. The famous proteins thenbreak both circular molecules of DNA and also rejoin the broken ends, so that theviral DNA becomes put into the host DNA. The very closely controlledaction that viral genes maintains λ DNA as component of the hostchromosome by repressing the lytic features of the phage. Under appropriatestimulation, the λ prophage is activated and undergoes lyticreplication.


Small DNA Phages

The genome of part bacteriophages encodes only10 – 12 proteins, roughly5 – 10 percent the the number encoded through Tphages. These little DNA phages space typified through the ΦΧ174and the filamentous M13 phages. These were the first organisms in which theentire DNA succession of a genome was determined, permitting extensiveunderstanding of the viral life cycle. The viruses in this team are sosimple the they execute not encode most of the proteins compelled for replicationof their DNA however depend on to move proteins for this purpose. For thisreason, they have actually been an especially useful in identify and examining thecellular proteins involved in DNA replication (Chapter 12).


RNA Phages

Some E. Coli bacteriophages save a genome composed of RNA rather of DNA.Because castle are simple to flourish in big amounts and because your RNA genomesalso serve as your mRNA, these phages space a ready source of a pure speciesof mRNA. In among the more quickly demonstrations the cell-free proteinsynthesis have the right to be mediated by mRNA, RNA from these phages was displayed to directthe synthesis of viral coat protein when included to one extract of E. Colicells containing all the other materials needed for protein synthesis.Also, the first long mRNA molecule to be sequenced was the genome of an RNAphage. This viruses, among the smallest known, encode only four proteins:an RNA polymerase because that replication the the viral RNA, 2 capsid proteins, andan enzyme the dissolves the bacter cell wall and permits release that theintracellular virus particles right into the medium.


Animal Viruses room Classified by Genome kind and mRNA SynthesisPathway

Animal viruses come in a selection of shapes, sizes, and genetic strategies. Inthis book, we are involved with viruses the exhibit at the very least one that twofeatures: they utilize crucial cellular pathways to kind their molecules,thereby carefully mimicking a regular cellular function, or they deserve to integratetheir genomes into those of regular cells.

The name of countless viruses room based onthe name of the illness they cause or that the pets or tree they infect.Common examples include poliovirus, which reasons poliomyelitis; tobacco mosaicvirus, which causes a mottling condition of tobacco leaves; and humanimmunodeficiency virus (HIV), which reasons acquired immunodeficiency syndrome(AIDS). However, countless different kinds of viruses often develop the same symptomsor the exact same apparent an illness states; because that example, number of dozen differentviruses can cause the red eyes, runny nose, and sneezing described as thecommon cold. Clearly, any attempt to classify viruses on the basis of thesymptoms they produce or your hosts obscures countless important distinctions intheir structures and also life cycles.

What are central to the life cycle of a virus space the types ofnucleic mountain formed during its replication and the pathway whereby mRNA isproduced. The relation in between the famous mRNA and the nucleic acid of theinfectious fragment is the basis of a simple means of classifying viruses. Inthis system, a famous mRNA is designated together a to add strand andits security sequence, i m sorry cannot role as an mRNA, is a minusstrand. A strand of DNA complementary come a famous mRNA is additionally aminus strand. Manufacturing of a to add strand that mRNA calls for that a minus strandof RNA or DNA be used as a template. Making use of this system, 6 classes the animalviruses are recognized. Bacteriophages and plant viruses likewise can it is in classifiedin this way, but the system has actually been used most widely in animal virology becauserepresentatives that all 6 classes have actually been identified.

The composition of the famous genome and its partnership to the famous mRNA areillustrated in figure 6-20 because that each ofthe 6 classes of virus. Table 6-3summarizes crucial properties the common animal viruses in every class and theresearch areas in i beg your pardon they have actually been commonly used. Structure models that severalvirions are presented in figure 6-21.


Figure 6-20

Classification of pet viruses based on the ingredient oftheir genomes and pathway the mRNA formation. DNA is displayed in blue; RNA, in red. The viral mRNA is designated as aplus strand, which is synthesized from a minus strand that DNA or RNA.Class vi (more...)


Figure 6-21

Structures of viruses identified by cryo-electron microscopy andimage analysis. Cowpea mosaic virus (CPMV) is a plant RNA virus, poliovirus (polio) ahuman RNA virus, nudaureila capensis β virus (NβV)an insect RNA virus, simian virus (more...)


DNA Viruses (Classes I and also II)

The genomes of both course I and also class II viruses consists DNA. Varioustypes the DNA viruses are typically used in researches on DNA replication, genomestructure, mRNA production, and oncogenic cabinet transformation.

Class ns viruses save a single molecule that double-strandedDNA (dsDNA). In the case of the most common type of course I animal virus,viral DNA enters the cabinet nucleus, whereby cellular enzymes transcribe the DNAand procedure the resulting RNA right into viral mRNA. Examples of these virusesinclude the following:


Adenoviruses, which cause infections in the upperrespiratory tract and also gastrointestinal tract in numerous animals
Herpesviruses, which cause various inflammatoryskin diseases (e.g., chickenpox) and also latent infections that recurafter lengthy intervals (e.g., cold sores and shingles)

Some varieties of HPV space passedthrough sexual contact. In some infected women, the HPV genome integratesinto the chromosome of a cervical epithelial cell. This rarely integrationevent initiates one intensively studied procedure that have the right to lead to developmentof cervical carcinoma, one of the most common species of human being cancers.Routine Pap smears performed for early on detection of cervical carcinoma aredone to determine cells in the early on stages that the revolution processinitiated by HPV integration.

The second kind of course I virus, jointly referred to aspoxviruses, replicates in the host-cell cytoplasm.Typical of class Ib viruses room variola, which causes smallpox, andvaccinia, one attenuated (weakened) poxvirus supplied in inoculations to induceimmunity come smallpox. These really large, brick-shaped viruses(0.1 × 0.1 × 0.2μm) bring their own enzymes for synthesizing viral mRNA and DNA inthe cytoplasm.

Class II viruses, dubbed parvoviruses (fromLatin parvo, “poor”), space simpleviruses the contain one molecule the single-stranded DNA (ssDNA). Someparvoviruses encapsidate (enclose) both plus and minus strands that DNA, butin separate virions; others encapsidate just the minus strand. In bothcases, the ssDNA is replicated inside the cell right into dsDNA, which is climate itselfcopied into mRNA.


RNA Viruses (Classes III – VI)

All the animal viruses belonging come classesIII – VI have actually RNA genomes. A wide selection ofanimals, native insects to human beings, room infected through viruses in each ofthese classes. These viruses have actually been an especially useful in researches onmRNA synthesis and translation (class III); glycoprotein synthesis, membraneformation, and also intracellular deliver (classes IV and also V); and celltransformation and also oncogenes (class VI).

Class III viruses contain double-stranded genomic RNA(dsRNA). The minus RNA strand acts together a theme for the synthesis of plusstrands of mRNA. The virions the all class III viruses well-known to day havegenomes comprise 10 – 12 separatedouble-stranded RNA molecules, every of i beg your pardon encodes one or twopolypeptides. Consequently, this viruses are stated to have“segmented” genomes. In these viruses, the virion itselfcontains a complete collection of enzymes that can utilize the minus strand that thegenomic RNA as a design template for synthetic of mRNA in the check tube also asin the cabinet cytoplasm after infection. A variety of important studies haveused class III viruses together a source of pure mRNA.

Class IV viruses save on computer a single plus strand the genomicRNA, i beg your pardon is the same with the famous mRNA. Due to the fact that the genomic RNA encodesproteins, the is contagious by itself. During replication of class IVviruses, the genomic RNA is replicated into a minus strand, which then acts together atemplate for synthesis of much more plus strands, or mRNA. Two species of class IVviruses space known. In course IVa viruses, typified by poliovirus, viralproteins are an initial synthesized, from a single mRNA species, together a longpolypeptide chain, or polyprotein, i m sorry is then cleaved toyield the assorted functional proteins. Course IVb viruses synthesize at leasttwo species of mRNA in a organize cell. One of these mRNAs is the same size asthe virion’s genomic RNA; the other coincides to the3′ 3rd of the genomic RNA. Both mRNAs are analyzed intopolyproteins. Included in class IVb space a big number of rarely insect-borneviruses including Sindbis virus and also those causing yellow fever and also viralencephalitis in human being beings. This viruses when were calledarboviruses (arthropod-borne viruses), but now arecalled togaviruses (from Latin toga,cover) since the virions room surrounded by a lipid envelope.

Class V viruses save on computer a single negative strand the genomicRNA, whose sequence is complementary to that of the viral mRNA. The genomicRNA in the virion acts together a template for synthetic of mRNA however does notitself encode proteins. Two species of course V viruses can be distinguished.The genome in course Va viruses, which incorporate the viruses resulting in measlesand mumps, is a solitary molecule that RNA. A virus-specific RNA polymerasepresent in the virion catalyzes synthesis of number of mRNAs, each encoding asingle protein, indigenous the genomic layout strand. Class Vb viruses, typifiedby influenza virus, have segmented genomes; every segment acts as a templatefor the synthetic of a different mRNA species. In most cases, each mRNAproduced by a course Vb virus encodes a solitary protein; however, some mRNAscan be read in two different frames to productivity two distinct proteins. As withclass Va viruses, a class Vb virion has a virus-specific polymerasethat catalyzes synthetic of the famous mRNA. Thus the genomic RNA (a minusstrand) in both types of course V viruses is not contagious in the absence ofthe virus-specific polymerase. The influenza RNA polymerase initiatessynthesis of each mRNA through a distinct mechanism. In the host-cell nucleus, thepolymerase cuts off 12 – 15 nucleotides indigenous 5′ finish of a to move mRNA or mRNA precursor; thisoligonucleotide acts as a “primer” the is elongated bythe polymerase to type viral (+) mRNAs, utilizing the genomic(−) RNA as a template.

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Class using viruses space enveloped viruses whose genome consistsof two similar plus strands the RNA. These viruses are also known asretroviruses because their RNA genome directs the formationof a DNA molecule. The DNA molecule at some point acts as the theme forsynthesis of famous mRNA (Figure6-22). Initially, a famous enzyme dubbed reverse transcriptase copies the viral RNA genomeinto a solitary minus strand of DNA; the same enzyme then catalyzes synthesisof a complementary plus strand. (This facility reaction is comprehensive in chapter 9.) The result dsDNAis combined into the chromosomal DNA that the infected cell. Finally, theintegrated proviral DNA is spelling, orthography by the cell’s very own machineryinto (+) RNA, which either is translated into viral proteins or ispackaged within virion coat protein to type progeny virions, which arereleased by budding indigenous the host-cell membrane. Because most retrovirusesdo no kill their host cells, infected cells can replicate, producingdaughter cells with incorporated proviral DNA. These daughter cells continueto transcribe the proviral DNA and bud progeny virions.