Figure 1

Models for retrotransposition. Five models have been proposed for three types of retrotransposition and all are variations of target-primed reverse transcription (TPRT); the first two steps are common in all models. (A) bottom strand cleavage; (B) target-primed reverse transcription; (C) top strand cleavage; (D) completion of first strand reverse transcription; (E) template jumping; (F) second strand cDNA synthesis by either RT or cellular DNA polymerase; (G) rejoining junctions; (H) annealing between top strand and first strand cDNA; (I) target-primed second strand cDNA synthesis with the usage of short sequence complementarity by either RT (double TPRT model), DNA polymerases in microhomolgy-mediated end-joining (MMEJ) pathway (MMEJ model), or DNA polymerases in non-homlogous end-joining (NHEJ) pathway (NHEJ model); (J) rejoining junctions; (K) annealing between top strand and template RNA; (L) target-primed reverse transcription from the top strand cleavage site; (M) RNA degradation and annealing between two cDNA strands; (N) DNA synthesis and rejoining junctions by unknown mechanisms.