Fig. 1. Process of reverse transcription of the HIV-1 genome. (A) Minus strand DNA synthesis (DNA strand in red) is initiated using a cellular tRNA annealed to the PBS. The RNA strand of the RNA:DNA duplex is degraded by RNase H of HIV-1 RT. (B) First strand transfer allows annealing of the newly formed DNA to the 3′ end of the viral genome. Transfer is mediated by identical repeated (R) sequences. (C) Minus strand DNA synthesis resumes, accompanied by RNase H digestion of all template RNA except PPT. (D) PPT is used as a primer for second strand DNA synthesis. (E) RNase H removes the tRNA and the PPT. In HIV-1, a single RNA nucleotide (from tRNA) is left by RNase H at the RNA/DNA PBS junction. (F) During second strand transfer (not shown) the newly formed PBS DNA (second strand) anneals to the PBS DNA from the first strand. Completion of second strand synthesis results in a linear DNA duplex with LTRs at both ends.