DNA was believed to be the sole medium for genetic information storage Furthermore Watson and Cricks central dogma assumed that information flowed oneway from DNA to RNA to protein So it came as a surprise when in 1971 it was discovered that some viruses shift their genetic information from RNA to DNA Even so these viruses ultimately make proteins in the same way as higher organisms During infection the RNA code is first transcribed back to DNA — then to RNA to protein according to the accepted scheme The initial conversion of RNA to DNA — going in reverse of the central dogma — is called reverse transcription and viruses that use this mechanism are classified as retroviruses A specialized polymerase reverse transcriptase uses the RNA as a template to synthesize complementary and doublestranded DNA molecule