Figure 1. Mechanism of expansion of the three classes of eukaryotic transposable elements. (a) Excision and integration of the transposon is carried out by the element-encoded transposase. The cleaved donor site can be religated by DNA repair, which frequently generates small deletions. Alternatively the homologous chromosome can serve as template to fill in the gap, which results in resynthesis of the element at the donor site. (b) Long terminal repeat (LTR) retrotransposons are transcribed and the RNA used as template by the element-encoded reverse transcriptase to generate a double-stranded DNA intermediate. Integration of the element is carried out by the transposase-like enzyme integrase. (c) Reverse transcription of the RNA transcript of a long interspersed nuclear element (LINE; non-LTR retrotransposon) occurs directly onto the cleaved site of the chromosome. Most subsequent steps are assumed to be carried out by the host cellular repair machinery.

Figure 2. Possible origin of eukaryotic transposable elements. Eukaryotic transposons are similar in sequence and mechanism of insertion to the transposons of bacteria and presumably evolved with little change directly from these prokaryotic elements. LINE elements are most similar in sequence and mechanism of insertion to prokaryotic group II introns. Group II introns are speculated to have been lost in eukaryotes, with aspects of their integration machinery surviving as LINEs. No equivalent of an LTR retrotransposon has been found in prokaryotes. Their mechanism of expansion suggests that they resulted from the fusion of a transposon and an LINE.