This RNA to DNA conversion was the first example of a 'backwards' direction of genetic information in biological systems. It has formed the basis of the first.
A retrovirus is a virus whose genes are encoded in RNA, and, using an enzyme called reverse transcriptase, replicates itself by first reverse-coding its genes into the DNA of the cells it infects. Like other viruses, retroviruses need to use the cellular machinery of the organisms they infect to make copies of themselves. However, infection by a retrovirus requires an additional step. The retrovirus genome needs to be reverse-transcribed into DNA before it can be copied in the usual way. The enzyme that does this backward transcription is known as reverse transcriptase. Retroviruses use reverse transcriptase to transform their single-stranded RNA into double-stranded DNA.
It is DNA that stores the genome of human cells and cells from other higher life forms. Once transformed from RNA to DNA, the viral DNA can be integrated into the genome of the infected cells. When the DNA versions of the retroviral genes have been incorporated into the genome, the cell then is tricked into copying those genes as part of its normal replication process. In other words, the cell does the work of the virus for it.