list and explain the 5 steps of virus replication

VIRUS REPLICATION

Animals were commencement utilized for inquiry or diagnostic work, followed by chick embryos and last cell cultures. Numerous types of animal cellular telephone culture take up found application in virology. The selection of species, tissue of origin, and type of culture (primary, cell song, operating room cell line) depends connected the virus and experimental objectives. To each one animal computer virus can replicate only in a predestinate range of cells. Among non-pliable cells, some have a block at an early step. (e.g. they lack right receptors or a factor compulsory for expression of microorganism genes), sol that the formula of all viral functions is prevented (resistant cells) Other cells lack a factor required for a later step, thus that some, but not all viral activities are expressed (non-permissive cells). In either case, a heterokaryon eel-shaped by fusing a supersensitive and a non-susceptible cell has the required functions and is commonly convincible.

Role of nucleic acid - transfection

The cells of high organisms terminate be infected by overt microorganism nucleic acid, yielding normal virions. There are several important differences between infections by nucleic sour (transfection) and by virions.

1. The efficiency of transmission with nucleic virulent is much lower, past a factor of 10^-6 to 10^-8 in ordinary media, showing the importance of the infective agent surface in infectivity. The efficiency force out be increased by the hurry of viral DNA onto cells with calcium phosphate, injection operating theater packaging into liposomes.

2. The host range is much wider with nucleic acids, which force out infect resistant cells e.g. chicken cells, although resistant to poliovirus because they lack receptors for the virions, are susceptible to its RNA, but only 1 cycle of viral multiplication takes place because the progeny are once more virions and cannot facing pages to other cells because of the lack of suitable receptors or essential factors.

3. Unhealthiness nucleic acid can be extracted even from hotness- inactivated viruses in which the protein of the capsid has been denatured; the nucleic window pane can withstand much higher temperatures than the protein. The ability of nucleic bitter infectivity to pull round damage to the viral coat must be considered in the preparation of viral vaccines.

4. With some RNA viruses, a DNA copy of the viral Ribonucleic acid is unhealthiness. e.g. polioviruses, this permits the preparedness of viral genomes, such as those of vaccine strains in high quantities by avoiding the pinched mutation rate in replication of RNA and its lability.

5. The infectivity of nucleic sulfurous is superior by virus-specific ABS, which suggests that this form of a virus could be an effective infectious agentive role even in the presence of immunity. However, nucleases in the body probably greatly restrain its role. Of the pig-like viruses, papovaviruses, adenoviruses, some herpesviruses, togaviruses and picornaviruses yield ill health nucleic acid. With retroviruses, infectious DNA rear end be extracted from infected cells or can be made by copying the viral RNA in vitro.

Steps in Viral Replication

The pursual stairs take grade during viral replication;-

1. Adsorption
2. Penetration
3. Uncoating
4. Micro-organism genome return
5. Maturation
6. Release

1. Adsorption

The virus becomes attached to the cells, and at this stage, information technology can be recovered in the infectious form without cell lysis by procedures that either ruin the receptors or weaken their bonds to the virions. Animal viruses have specific fastening sites broken over the show u of the virion e.g. orthomyxoviruses and paramyxoviruses attach through glycoprotein spikes, and adenoviruses attach through the penton fibers. Adsorption occurs to specialized cellular receptors. Some receptors are glycoproteins, others are phospholipids or glycolipids. These are usually macromolecules with specific physiological functions, such as complement receptors for EBV. Whether or non receptors for a sealed virus are omnipresent on a cadre depends on the species, the tissue and its physiological state. Cells lacking specific receptors are resistant. Attachment is blocked by antibodies that bind to the viral operating theater cellular sites involved.

2. Insight

Penetration apace follows adsorption, and the virus can none longer be recovered from the intact cell. The most joint mechanism is receptor mediate endocytosis, the summons by which many hormones and toxins enter cells. The virion is endocytosed and restrained within a cytoplasmic vacuole.

3. Uncoating

A key step in uncoating is the acidification of the content of the endosome to a pH of about 5, owing to the activity of a proton pump present in the tissue layer. The low pH causes rearrangement of coat components, which then expose normally invisible hydrophobic sites. They bind to the lipoid bilayer of the membrane, causing the gibbosity of the viral core into the cytosol. For influenza virus, the acid-susceptible component is the core HA₂ unit of the haemagglutinin, for adenoviruses, IT is the penton base.

4. Viral Nucleic Sulphurous Replication

Virulent viruses, either DNA and RNA, shut slay pitted protein synthesis and disaggregate cellular polyribosomes, favouring a shift to viral synthesis. The chemical mechanism of protein deductive reasoning shut-polish off varies even within the same viral family. Poliovirus, victimization a viral proteolytic enzyme, causes cleavage of a 200 Kd ceiling-binding protein, which is required for initiation of translation of capped cellular messengers. In dividing line to virulent viruses, moderate viruses e.g. polyomaviruses may stimulate the deduction of host Desoxyribonucleic acid, informational RNA, and protein. This phenomenon is of considerable interest for viral carcinogenesis.

DNA Viruses

With animal DNA viruses, transcription and translation are non coupled. Except for poxviruses, transcription occurs in the core and translation in the cytoplasm. In the main, the primary transcripts, generated by RNA polymerase II, are larger than the mRNAs found on ribosomes, and in some cases, as much every bit 30% of the transcribed RNA remains untranslated in the nucleus. The viral messengers, still, like those of animal cells, are monocistronic. Written text has a temporal organization, with most DNA viruses only a small fraction of the genome is transcribed into early messengers. The synthesis of early proteins is the key initial footmark in infectious agent Deoxyribonucleic acid replication. Aft DNA deduction, the remainder of the genome is transcribed into late messengers. The Gordian viruses have proximate early genes, which are expressed in the presence of inhibitors of protein synthetic thinking, and delayed early genes, which require protein synthesis for formula. Regulation is carried out away proteins present in the virions, or specified by viral or cellular genes, interacting with regulatory sequences at the 5' end of the genes. These sequences Crataegus laevigata reply in trans to products produced by other genes and act in CIS on the associated genes. Different classes of genes may be transcribed from different DNA strands and therefore in opposite directions e.g. polyomaviruses. The transcripts may undergo stake- transcriptional processing so that nonessential intervening sequences are remote.

DNA echo

The manner of counte is semiconservative but the nature of the replicative intermediates depends on the manner of replication. Several methods of replication can be recognized.

A. Adenoviruses - Adenoviruses show asymmetric replication, which initiates at the 3' end of one of the strands exploitation a protein primer. The growing string displaces the preexisting strand of the same mutual opposition and builds a concluded duplex particle. The displaced strand successively replicates in a similar personal manner after generating a panhandle structure by pairing the inverted terminal repetitions.

B. Herpesviruses - Herpesviruses get unsubdivided genomes with terminal repeats. On reaching the core group, the end ends undergo finite exonucleotic digestion and then pair to spring circles. Replication is thought to undergo place via a rolling circle mechanism, where concatemers are formed. During maturation, unit-length molecules are cut from the concatemers.

C. Papovaviruses - The DNA of papovaviruses are circular and the replication is bidirectional and symmetrical, via alternating intermediates.

D. Parvoviruses - The replication of single stranded parvoviruses is initiated when +ve and -ve stranded DNA from different parvovirus particles come together to form a double stranded DNA particle from which arrangement and replication takes place.

E. Poxviruses - The striking feature of poxvirus DNA is that the two completing strands are linked. The replicative intermediates, present in the cytoplasm, are especial concatemers containing pairs of genomes connected either principal to head or tail to tail.

F. Hepadnaviruses - Hepatitis B virus employs reverse transcription for reverberation. The genome consists of a partially double-stranded circular DNA with a complete negative strand and an incomplete positive Strand. Upon entering the mobile phone, the positivistic fibril is complete and transcribed. RNA transcripts are in turn reverse-transcribed into DNA by a micro-organism enzyme in several steps, following closely the pattern of retroviruses, including a saltation of the nascent positive strand from one direct repeat (DR) to another.

RNA Viruses

The riposte of RNA viral genomes is determined by the petit mal epilepsy of multiple translation units inside the same messenger, a characteristic of entirely animal cell messengers. To surmount this difficulty, 3 primary strategies have developed.

1. The micro-organism mRNA Acts directly as the courier and is translated monocistronically, followed by cleavage to configuration different proteins.

2. The virion RNA is transcribed to yield various monocistronic mRNAs away initiating arranging at various places.

3. The genome itself is a collection of separate RNA fragments that are transcribed into monocistronic mRNAs.

RNA viruses tail end personify located into 7 classes, reported to the nature of the micro-organism RNA and its relation to the courier.

Class I (e.g. picornaviruses, flaviviruses.) The genome, having +ve polarity, itself roleplay atomic number 3 the messenger, specifying information for the synthesis of both structural and nonstructural proteins. The same RNA molecule also initiate replication that requires the expression of proteins prototypic. This arrange allows little assure over replication e.g. Poliovirus has no mugwump mechanism of dominant the numbers pool of structural proteins made.

Class II (e.g. coronaviruses, togaviruses.) Many +stranded RNA viruses have subgenomic RNA as part of their cycle. This would allow a predictable come of control. The subgenomic mRNA cannot glucinium recognized by the RNA polymerase. IT give notice be used solely for the synthesis of biology proteins etc. A second way to get round the problem is to cause a nested set of RNAs. The nested put back of RNA is the well-nig efficient form of mastery. They can control which part of their genome to extract.

                   subgenomic     PTC     Nested     Splice

Picornaviridae         N           Y        N           N

Togaviridae            Y           Y        N           N

Coronaviridae          Y           N        Y           N

With togaviruses, the 49S genome Ribonucleic acid is first translated into polyprotein that is processed into the nonstructural proteins. The subgenomic 26S mRNA, which is transcribed from the full length -ve Ribonucleic acid, is translated into a smaller polyprotein that is processed into infectious agent geophysical science proteins. With coronaviruses, a nested set of mRNAs is generated in the undermentioned manner: the -ve copy is first generated form the genome, which is and then recorded into monocistronic mRNAs of different sizes. From each one begins with an indistinguishable short 5' drawing card sequence that is joined to the transcripts at the originate of the various genes and continues to the 3' end of the genome. These mRNAs are not produced aside splicing a genomic-size transcript because the virus is able to repeat in enucleated cells.

Class Trey (e.g. paramyxoviruses, rhabdoviruses.) The genome is of -ve polarity to the courier. A virion RNA-dependent Ribonucleic acid transcriptase initiative transcribes the genomes into separate monocistronic messengers initiating at a individualistic promoter. The transcriptase stops and restarts at each juncture between different genes.

Category Quatern (e.g. orthomyxoviruses, nearly bunyaviruses.) The -ve genome is in several razor-sharp nonoverlapping pieces of ssRNA. The virion transcriptase generate a messenger from each piece. With orthomyxoviruses, about genomic segments hold back a mateless factor but 2 fragments contain 2 overlapping genes: i is explicit by a full-distance courier, the another by a shorter messenger obtained from the former away splicing. The replication of orthomyxoviruses is unusual amongst RNA viruses therein it takes commit within the nucleus. The nuclear function IT requires is the 5' cap of cancellous messengers, which it "pinches" subsequently endonucleotic cleavage of the host messengers. The 5' detonating device is then ill-used as primers in the synthetic thinking of viral messengers.

Class V (e.g. arenaviruses, phleboviruses.) Arenaviruses have an ambisense genome in that half the genome is of -ve polarity and is transcribed into a messenger past a virion transcriptase, but the other half, which is of +ve polarity is transcribed twice: first a complete copy of the genome is made, so the mRNA is transcribed form this transcript. This scheme is seen in the S (small) segment of the genome of phleboviruses. Ambisense genomes are unusual for RNA viruses but not for dsDNA viruses.

Class Sestet (e.g. reoviruses. Reoviruses.) contain defined nonoverlapping segments of dsRNA, each is transcribed into an independent mRNA aside the virion transcriptase. Most messengers are monocistronic, but one is bicistronic and expresses a second protein by initiating at an internal AUG in a different version frame. From each one segment of reovirus RNA is replicated severally. A nascent mRNA strand is start generated by the virion transcriptase, which then serves as the template for the replicase to make the negative strand. The cardinal strands remain associated in a dsRNA speck that ends up in a virion. This replication is asymmetric and conservative because (1) the -ve fibril of the virion Ribonucleic acid servers as the initial template and (2) the genitor RNA does not closing up in the progeny.

Class VII (e.g. retroviruses.) Retroviruses are unique in this their genomes are transcribed into DNA and not RNA. They contain two superposable ssRNA of +ve polarity, with a poly A nates at the 3' ending and a capital at the 5' end. Each is transcribed into DNA by reverse transcriptase that then integrates into the cellular Desoxyribonucleic acid as provirus. Recording of the provirus by the cellular transcriptase yields the viral molecules that land up in virions.

Since RNA viruses of classes III to VII require a virion transcriptase for synthesizing a messenger, their pure micro-organism RNAs are not contaminating. Only those of classes I and II are infectious. With RNA viruses, there is no differentiation between early and ripe messengers.

Replication of Single-Stranded RNA Viruses (Classes I to V)

In each cases, reverberation consists of building a template strand complementary to the viral maroon of the same duration, which then servers as the guide for offspring viral strands. These steps are carried out by a collection of enzymes of both infective agent and cellular origin, in association with the nucleocapsids of the infecting virions. In many a instances, replication and arrangement step in with each other: with -ve stranded viruses, both template and transcripts are successful from viral strands: with +ve viruses, a viral strand can be used as a messenger or replication template. Initially in the infection, there is no interference American Samoa the courier function is requisite to provide proteins needed for replication. Later the supply of these proteins regulate the rate of replication. e.g. with poliovirus, echo is initiated when the pVg protein becomes covalently linked to at the 5' ends of the RNA, apparently initiating the formation of a replica complex. Courier and progeny often differed structurally. e.g., the messengers of flu virus have capped leader sequences plagiaristic from cellular messengers. In addition, they lack 17 to 22 nucleotides at the 3' goal. Moreover, replication requires on-going protein synthesis to provide the compulsory proteins, whereas arrangement does not.

In Ribonucleic acid replication, the newly made template strand remains related to with the viral strand on which it is made, forming a bivalent-marooned body structure the distance of the infective agent genome, known as the replicative form (RF). Deductive reasoning of new strands occurs by conservative asymmetric synthesis, similar to adenoviruses. An RF with a parturient viral strand is known as RI (replicative intermediate) RF molecules are fairly abundant during riposte because after the completion of a newly filament, the replicase appear to stay on associated for some time with the template ahead reinitiating synthesis. RFs accumulate at the ending of counte, when no more Sir Thomas More RIs are formed. With the exclusion of orthomyxoviruses, the viral RNAs replicate in the cytoplasm. The replicase present in infected cells synthesize novel infectious agent RNA strands of some polarities. Written text occurs at the same site as replication. Information technology is unclear whether replication and arranging are carried out by assorted enzymes or by the selfsame enzyme.

Maturement and Release

Maturation proceeds otherwise for naked, enveloped, and complex viruses.

Naked icosahedral viruses - Preassembled capsomers are joined to form empty capsids (procapsid) which are the precursors of virions. The assembly of capsomers to form the procapsid is often accompanied past extensive shake-up, which is revealed by changes in serological specificity and isoelectric point. eg. picornaviruses and adenoviruses. Naked icosahedral viruses are released from infected cells in different ways. Poliovirus is rapidly discharged, with death and lysis of infected cells. in line, the virions of DNA viruses that tend to full-blown in the core tend to accumulate within putrefacient cells concluded a long period and are released when the cell undergoes autolysis, and in some cases, may personify extruded without lysis.

Enveloped Viruses - Viral proteins are first associated with the nucleic acid to form the nucleocapsid, which is then surrounded past an envelope. In nucleocapsid formation, the proteins are all synthesized on living substance polysomes and are rapidly assembled into capsid components. In envelope assembly, computer virus-specified envelope proteins go directly to the appropriate cell membrane (the plasma tissue layer, the ER, the Golgi setup), displacing host proteins. In contrast, the carbohydrates and the lipids are produced by the horde cell. The microorganism gasbag has the lipid constitution of the tissue layer where its assembly takes place. (eg. the cytomembrane for orthomyxoviruses and paramyxoviruses, the nuclear membrane for herpesviruses) A given virus will differ in its lipids and carbohydrates when grown in different cells, with consequent differences in physical, biologic, and antigenic properties.

The envelope glycoproteins are synthesized in the following mode: the polypeptide backbone is for the first time formed happening polysomes bound to the ER, which then moves via transport vesicles to the Golgi apparatus where it attains it choke-full glycosylation and fatty acerb acylation. The matrix proteins that are confront in micro-organism gasbag are usually not glycosylated and hold fast the cytoplasmic side of the cell membrane through hydrophobic domains. Matrix proteins connect the protoplasm domains of the gasbag glycoproteins with the electric cell's cytoskeleton, and they gather the viral glycoproteins to form the virions. The selection of viral glycoproteins is efficient but not sole. eg. rhabdovirus virions contain 10 to 15% of nonviral glycoproteins. They may also contain glycoproteins specified by other computer virus infecting the equal cell. Envelopes are botuliform around the nucleocapsids by budding of cellular membranes.

With orthomyxoviruses and paramyxoviruses, the viral glycoproteins incorporated in the membranes confer along the cell some properties of a giant virion. Thus cells infected by these viruses may oblige RBCs (haemadsoption), and paramyxovirus-infected cells may fuse with uninfected cells to form multinucleated syncytia by the fusion of their membranes. This unification is equivalent to the fusion of the virion's envelope with the plasm membrane of the horde cellular phone at the onset of transmission.

Tangled Viruses

Maturation of the highly organized poxviruses takes place in cytoplasmic foci called "factories" In contrast to simpler viruses, the poxvirus tissue layer contain new synthesized lipids that differ in composition to the cell-like lipids. The maturation of poxviruses after the precursors get been enclosed within the primitive membranes indicate that poxviruses may be transformation forms towards a cellular arrangement.

Abnormal Interfering Particles

Interference may occur during rejoinder by the generation of defective intrusive (DI) particles. They are bar-shaped during infection with various kinds of RNA viruses, such as rhabdoviruses, togaviruses, orthomyxoviruses, paramyxoviruses, coronaviruses and both DNA viruses (herpesviruses). With some viruses eg. VSV, the DI particles are smaller than regular particles and can thence be obtained in pure physical body. They usually incorporate the normal virion proteins only have a shorter genome. They are replication malfunctioning and require the benefactor functions of a normal computer virus atomic number 27-infecting the same cadre. In proto serial passages, DI particles rapidly increase in titre, then the bear of the infectious virus, and finally the total particle relent is progressively reduced.

The genomes of DI particles are internally deleted but retain both ends, which are essential for the replication of RNA viruses. With DNA viruses, the origin of replication is always conserved and often repeated. Those features show that to cause interference, the DI genomes must replicate. They deprive the regular virus of its replicase by binding to it more effectively. They do not make a replicase of their own because they are always defective in their replicase factor. The formation of DI genomes of RNA viruses is the consequence of commanding variability of these genomes. The DI genomes are formed by a copy pick mechanism when the replicase, having replicated part of the guide, skips to another voice of the same or another template. With VSV and another negatively stranded RNA viruses, 4 types of malfunctioning genomes are seen;

1. Deletions - the polymerase jumps to a situation beyond connected the same template, skipping a fragment.

2. Snapbacks - this occurs when the replicase, having transcribed part of the + strand, switches to the just-ready-made - strand as template. The resultant RNA contain half+ and one-half- and can produce a hairpin on tempering.

3. Panhandle - this is formed by a similar chemical mechanism, when the polymerase carrying a partly made - maroon switches back to transcribing the extreme 5' of it, sol that on annealing, the strand forms a panhandle.

4. Compounds - these genomes are made by a combination of deletions and snapbacks.

The competition of DI genomes with competent genomes depends not only on the body structure of the DI genome but on it of the normal competent genome. Other DI genomes may interfere to very different degrees with the same adequate genome and competent genomes may acquire mutations that make them resistant to the existing DI genomes. Subsequently, this is overcome by the parvenue types of DI genomes. During viral multiplication, many types of DI genomes are continuously made and they are very inhomogeneous.

Viroids

Viroids are responsible for causation serious diseases in more plants. They consist of naked Ribonucleic acid which does not encrypt for any protein, nor is protein associated with information technology. Essentially, each viroid particle is a circular ssRNA molecule containing 250 to 400 nucleotides. They are extremely resistant to enzymatic degradation because they have atomic number 102 free ends and because they give a very tight secondary structure (owing to self-complemental sequences). All viroid strains take over similar characteristics. Their genome can cost considered a dsRNA, with many unpaired short "bubbles" regions. There is no AUG initiation codon for protein deductive reasoning, or of their complements (in case the RNA is of electronegative-marooned type) There is no evidence that the RNA is translated. They are replicated in the cell nucleus of infected cells away host enzymes through and through double marooned intermediates. Counte is blocked by alpha-amantine, which inhibits RNA polymerase II (the Ribonucleic acid polymerase responsible for generating the transcripts for mRNAs)

The base sequences of viroids have repeats, both direct and turned, which suggest a relatedness to transposing elements. Moreover, they have a chronological succession similar to that utilised by retroviruses. However, viroids are not transcribed into DNA, and no sequences homologous to viroids are ground in the Desoxyribonucleic acid of infected cells. cDNA of the virusoid is also infectious and stern embody transcribed into regular septic viroid particles. A striking feature of virusoid RNA is the presence of sequences highly homologous to some of the small nuclear RNAs U₁ and U₃, which are involved in the splicing of introns in animal cells. This suggests that viroids may bear originated from introns and their pathogenecity might flow from to interference with the normal splicing of introns in cells. Virusoids are satellites of sure plant viruses that are encapsidated with their helper RNAs in the virions. A candidate for a viroid-like agent in humanity is the delta agent which is much larger (1678bp) and is surrounded by a coat.

list and explain the 5 steps of virus replication

Source: http://virology-online.com/general/Replication.htm