A model proposing the formation and movement of novel inverted repeat elements.

<p>(A) TIRs of a transposable element are shown in filled boxes. The boxed sequences are the 7bp duplicated half-sites. Transposase molecules bind at the TIRs, and (B) make pairs of staggered nicks on each flank (triangles). Occasionally, the DNA may slip (C) or the transposase may bind to cryptic sites on adjacent host DNA, and make a paired nick, which leads to excision (D). (E) Short repair synthesis followed by trans-esterification leads to a hairpin at the host segment attached to the TIR, which is nicked again near the TIR. Helix unwinding, followed by microhomology-mediated ligation to a new host site inserts the transposon, and single strand gap filling by repair synthesis forms a novel inverted repeat derived from host sequences adjacent to the original location (F). The transposon may repeat the cycle of precise (B) or imprecise (C) excision, and may be dissociated from novel host-derived inverted repeat (G). If the host-derived sequence had a cryptic transposase-binding site, the site is now duplicated on the novel inverted repeat. This may lead to independent movement of the inverted repeat sequence as a novel mobile element so long as a transposase is available.