FIGURE 1. Reverse transcription.

Like other reverse transcriptase enzymes, that of HIV mediates transcription of the virus's RNA genome into double-stranded DNA. a, To form the first DNA strand (the minus strand) the enzyme uses a transfer RNA as a primer and interacts with the tRNA 3 end in a polymerase (P) binding mode. b, As the complementary minus-DNA sequence is being synthesized, the enzyme cleaves the RNA template (but leaves the PPT sequences of the RNA intact) by binding to it in an RNase H (H) mode. c, Finally, to initiate the synthesis of the second (plus) DNA strand, the reverse transcriptase uses the PPT sequence as a primer, once again binding in the polymerase mode.