Visual based explanation of Genomic Replication of Viruses
The formation of biological viruses during the infection process in the target host cells is termed as viral replication. The type of replication strategy a virus applies greatly depends upon the sort of virus it is. Baltimore classification system devised by David Baltimore allows for the classification of viruses based on their genome types and this makes it fairly easy to predict the broad sort of strategy that a virus with a given genome will employ in order to get replicated. The feature covers the VII groups of virus according to the Baltimore classification and explains them with their respective animations.Image Courtesy: http://www.answers.com
Group I: Double-stranded DNA viruses (dsDNA)Image Courtesy: http://www.mcb.uct.ac.za
In double stranded DNA virus, the first step in replication is the primary transcription of the host enzymes which is followed by translation of proteins. Then the viral genomic replication is carried out usually by the host enzymes after which late transcription takes place. This is followed by synthesis of late proteins after which the structural protein and DNA is assembled into virions. The animation shows a schematic replication of the double-stranded DNA virus.
Class II: Single-stranded DNA viruses (ssDNA)Image Courtesy: http://www.mcb.uct.ac.za
These single stranded DNA viruses usually replicate within the nucleus. The replication starts with the conversion of ssDNA into dsDNA followed by early transcription and translation of proteins. The rolling cycle ssDNA replication takes place before the late transcription which is usually carried out by the viral proteins. The synthesis of late proteins and sequestering of viral genomic ssDNA happens before the assemblage into virions. The animation shows the replication process of the single stranded DNA viruses.
Class III: Double-stranded RNA viruses (dsRNA)Image Courtesy: http://www.mcb.uct.ac.za
Viruses with DNA genomes rely more on host polymerase as compared to the double stranded RNA viruses. IN the dsRNA replication process, all genomes replicate by a conservative mechanism, wherein ds input RNA is transcribed by viral enzyme to mRNA, which both codes for protein, and then acts as template for second strand synthesis.
Class IV: Single-stranded RNA viruses - Positive-senseImage Courtesy: http://www.mcb.uct.ac.za
The ssRNA positive sense viruses have their genome directly utilized as if it were mRNA. It is then translated into a single protein using the host ribosomes to form the various proteins needed to carry out the process of replication. One of the resultant proteins is RNA polymerase, which copies the viral RNA to form a double-stranded replicative form. In turn this leads to the formation of new virions.
Class V: Single-stranded RNA viruses - Negative-senseImage Courtesy: http://www.mcb.uct.ac.za
The negative sense RNA viruses want their genome copied by an RNA-dependent RNA polymerase to form positive-sense RNA for which they require the RNA replicase enzyme along with the virus. The positive-sense RNA molecule then acts as viral mRNA, which is translated into proteins by the host ribosomes. The resultant protein goes on to direct the synthesis of new virions, such as capsid proteins and RNA replicase, which is used to produce new negative-sense RNA molecules.
Class VI: Positive-sense single-stranded RNA viruses that replicate through a DNA intermediateImage Courtesy: http://www.mcb.uct.ac.za
Retroviruses are among the most frequently studied family of this class. The replication process in this class begins with the reverse transcription in cytoplasm, using tRNA primer, of virion ss(+)RNA by virion-associated reverse transcriptase (RT), into intermediate RNA/DNA complex followed by the conversion of RNA/DNA complex into linear and circular proviral dsDNA forms with long terminal repeats (LTRs) by RT. Using the integrase function of the of the reverse transcriptase, linear proviral DNA is integrated in host cell DNA. This host DNA then undergoes replication and transcription with the modification of transcription by viral products. After this translation and accumulation of structural proteins, positive sense RNA is assembled into viral nucleoprotein thus budding through membrane containing viral envelope proteins. The animation shows the schematic replication of the virus.
Class VII: Double-stranded DNA viruses that replicate through a single-stranded RNA intermediateImage Courtesy: http://www.mcb.uct.ac.za
This group of viruses also relies on reverse transcription, but unlike the Retroviruses, this occurs inside the virus particle on maturation. On infection of a new cell, the first event to occur is repair of the gapped genome, followed by transcription. The pregenome RNA serves as template for the viral reverse transcriptase and for production of the DNA genome.