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Replication of the viral genome

Replication of the viral genome always occurs in the host cell. The exact method of replication depends on the type of viral genome.

DNA viruses replicate their genome in the nucleus of eukaryotes and in the cytoplasm of prokaryotes. The viral DNA is replicated using the host's own system.

Replication of RNA viruses is more complicated; no host cell contains enzymes that allow RNA to be replicated (RNA is always transcribed from DNA).

As a result, RNA viruses carry enzymes that enable RNA to be replicated in the host.

Depending on the type of virus this enzyme is either RNA-dependent RNA polymerase or RNA-dependent DNA polymerase.

Replication of RNA viruses usually occurs in the cytoplasm.

RNA viruses cannot use enzymes present in the host cell to replicate the genome because no host cells replicate RNA under normal conditions.

RNA viruses either carry or code for enzymes that allow their RNA to be replicated in the host cell.

RNA-dependent RNA polymerase (RdRp) allows RNA to be replicated directly.

  • Positive strand RNA encodes RdRp in the genome. It is then translated (the enzyme is synthesised) once inside the host, allowing replication of the viral genome.
  • Negative strand RNA viruses carry pre-made RdRp which is used to replicate the RNA.

Retroviruses (such as HIV) convert their RNA to DNA before replicating their genome.

HIV is a retrovirus.

Instead of carrying RNA-dependent RNA polymerase (RdRp), retroviruses carry reverse transcriptase. This is an RNA-dependent DNA polymerase (RdDp). Reverse transcriptase transcribes a DNA complement from the RNA strand in the host cell.

The viral DNA is integrated into the host genome using integrase. It is then replicated along with the host.