Replication of Virus

Replication of Virus The replication strategy of agenomes virus largely depends on theandstructure and composition of its genome. Viruses with RNA genome replication transcription are often closely related, and are often carried outaccording by the sameto enzymes. Therefore, the processes of replication and transcription are discussed the virus class, namely based on the following Baltimore classification. a) Group I: Double Stranded DNA This group of viruses can be further divided into two replication sites, namely: •Replication Replicationofexclusively occurs in thedependent nucleus oronis associated with thefactors. nucleoid of prokaryotes. this virus is relatively host cell cellular In some cases, no enzymes are encoded by viruses of this class because they do not require them, whereas in more complex viruses much of the enzymatic activity may be present within the virus particle. •orReplication occursits inhost. the These cytoplasm. This virus has components that are thefactors result and of evolution obtained from components are required for transcription genome replication. Therefore, most of these groups do not depend on cellular components for DNA replication and transcription. Due to its complexity, this virus has the largest structure and most complex particles of any known containing many different enzymes. b) Group II: Single Strand DNA Replication of intermediate these viral genomes occurs intemplate the nucleus, involving theofformation of a doublestranded DNA that serves as a for the synthesis a new single-stranded genome. In general, none of these virus-encoded enzymes are packaged the viral particle because most of the functions required for replication are provided by thewithin host cell. c) Group III: Double Stranded RNA All viruses ofRNA this group have segmented genomes, and each segmentReplication is transcribed separately tocytoplasm produce that carries a single monocistronic message. occurs inthese the and is largely independent of cellular components or cellular machinery, as viral particles contain many enzymes encodedtheby processes their genome and are essential for RNA replication and transcription. This is because of replication and transcription involve copying RNA to create its partner RNA molecule which generally does not occur in cellular organisms. d) Group IV: Single-Stranded (+) RNA These viruses can be divided into two groups, namely: •represents Viruses with polycistronic mRNA such asafter Flavivirus andandPicornavirus. ThetheRNA genomeof the mRNA that will be translated infection then produce synthesis polyprotein products, which are then processed to form mature viral proteins. •translation Viruses withcycles complex transcription such as Coronavirus and Togavirus. In thisassubgroup, two are required to produce subgenomic RNA that functions mRNA. The second cycleAlthough will form a long RNA transcript and will copying form theRNA viralfrom genome in template, the next generation. replication of these viruses involves an RNA there aregenetic no enzymes these viruses encode in their genomes. Thisprior is because the ability to express information directly from the genome without transcription allows replication to occur after viral infection. e) Group V: Single Strand (-) RNA This virus genome can also be divided into two types, namely: •group, Segmented. The first step in replication of genome this virus,(-)forby example in the Orthomyxoviruses is transcription of the sense RNA the RNA-dependent virion RNA polymerase which is packaged in the virus particle to produce monocistronic mRNA and also functions as a template for subsequent genome replication. •transcriptase Not segmented. In this group, monocistronic mRNA ingenome. each viralNext, genea complete is producedcopy by ofthetheviral enzyme in the viral particle from the viral (+) strand genome is synthesized and serves as a template for the (‒) strand genome in next generation viruses for example, Paramyxoviruses and Rhabdoviruses. f) Group VI: Reverse Transcription Single Stranded RNA This retrovirus genome consists of directly an RNAbut (+) asstrand, but itsforgenome istranscription unique because it is diploid and does not provide mRNA a template reverse into DNA. Amaterial complete replication cycle involves thecanconversion of thegenome RNA form of the virus' genetic into DNA. This group of viruses integrate their into the host, for example proviruses which haveneeds their togenome integrated intothethevirion chromatin ofoutthethis hostconversion. cell. The reverse transcriptase enzyme be packaged inside to carry This is because the viral genes are only expressed from the provirus DNA and not from the RNA genome found in the retrovirus particles. g) Group VII: Double Stranded DNA with RNA Intermediary This group of viruses inrelies ongroup the process of reverse transcription, but unliketheretroviruses, reverse transcription this occurs within the virus particle during maturation process. Upon infection of a new target cell, the first event that occurs is repair ofof the fragmented genome, followed by transcription. As with group VI viruses, the activity the reverse transcriptase enzyme is carried in the virus particle, but before that the enzyme converts the viral where RNA into a DNA genome inoccurs the virus particle. This is significantly different from retroviruses reverse transcription after the RNA genome is released from the virus particle into the host cell