Fig. 3

shows the strategies of replication and protein synthesis used by different types of viruses: (1)in most dsDNA viruses (e.g., Herpesvirus, Adenovirus), viral DNA transcribes mRNA by the help of enzyme DNA-dependent RNA polymerase (RNA polymerase-II of host), mRNA translates early viral protein in the cytoplasm. DNA replication occurs with the help of host DNA polymerase in the nucleus. In poxvirus, the mRNA transcription and genomic replication occurs in cytoplasm because the virus carries its own DNA and RNA polymerases (2) In Parvovirus ssDNA of virus is first converted to dsDNA by cellular DNA polymerase, then mRNA transcription occurs. (3) In dsRNA containing viruses (e.g., Reovirus), each virion carries RNA-dependent RNA polymerases which use each strand to transcribe mRNA (4) In ssRNA viruses of positive sense, the RNA directly can act as mRNA transcript, genomic RNA is replicated via a negative sense RNA intermediate. (5) Viruses (e.g., influenza virus, rhabdoviruses) with negative sense RNA viruses carry RNA polymerases for mRNA transcription. (6) Retroviruses (e.g., HIV) have reverse transcriptase enzyme which transcribes DNA strand using each of the viral ssRNA strands (RNA-dependent DNA polymerase activity), it then lyses the parent ssRNA (ribonuclease activity) and forms complementary DNA strand taking the newly formed ssDNA as template, ultimately resulting in the dsDNA which then enters the host nucleus and binds to host DNA with the help of enzyme integrase. When the cell is actively replicating, viral mRNA is transcribed and protein synthesis occurs. (7) In Hepatitis B virus, which has partially dsDNA (circular -not shown in figure), the viral genome is first converted into covalently closed circular dsDNA (cccDNA), which serves as template for mRNA transcription, pre-genomic positive sense RNA is produce too which with the help of reverse transcriptase enzyme subsequently form the genomic partially dsDNA (not shown)