Retroviruses are viruses whose genetic material is RNA. HIV and the virus of SARS (severe acute respiratory syndrome) are examples of retrovirus. These viruses inoculate their RNA into the host cell and within the cell the viral RNA is reversely transcripted into DNA. DNA made from the viral RNA then commands the synthesis of viral proteins for the assemblage of new viruses and the breaking of the host cell to liberate them outside. The enzyme reverse transcriptase is the catalyst of the reverse transcription of RNA into DNA. The enzyme is part of the virus and it is also inoculated into the host cell.   _{Your Comment}