double-stranded DNA before transcription occurs. Some RNA viruses replicate their genomes as RNA, without use of DNA. Some RNA polymerases synthesize RNA from an RNA template. These viruses also transcribe mRNA from genomic RNA. One class of RNA viruses, the retroviruses, must transcribe their single-stranded RNA genome into a DNA intermediate for replication and transcription/translation. The enzyme reverse transcriptase synthesizes DNA based on RNA.
