Viral RNA replication cycle. Computer artwork showing the three stages of the replication cycle that forms double-stranded DNA (deoxyribonucleic acid, lower left) from a single strand of RNA (ribonucleic acid, upper right) from a virus. The DNA strands are yellow, while the viral RNA strand is orange. The enzymes carrying out the nucleic acid replication are the coloured symbols down centre. In the first stage, the RNA strand (upper right) is copied by reverse transcriptase to form the double-stranded c-DNA (complementary-DNA, upper left and centre right). In the second stage (centre), ribonuclease H destroys the viral RNA strand. In the third stage (bottom), reverse transcriptase turns the single DNA strand into a double-stranded DNA molecule. This cycle occurs in many viral infections.