Retroviruses and other reverse-transcribing viruses carry an RNA genome, but they do not use it directly as mRNA. Instead, the virion packages an enzyme called reverse transcriptase, which copies the RNA genome into a complementary DNA (cDNA) strand. Reverse transcriptase has three activities: it acts as an RNA-dependent DNA polymerase to make the first DNA strand, as an RNase H to degrade the original RNA strand, and as a DNA-dependent DNA polymerase to make the second DNA strand. The result is a double-stranded DNA copy of the viral genome.
That DNA copy is then transported into the nucleus, where another viral enzyme, integrase, inserts it into a host chromosome. Once integrated, the viral DNA is called a provirus. The provirus is replicated along with the host DNA every time the cell divides, and it is transcribed by host RNA polymerase II into viral mRNAs and new genomic RNA. This strategy explains why retroviral infections can persist for the lifetime of the host: the viral genome becomes a permanent part of the host's genetic material. Integration also means that the virus cannot be cleared simply by eliminating free virions; the provirus remains as a reservoir.