Simple insertions and cointegrate formation. Simple insertions and cointegrate formation. (A) Strand transfer and replication leading to simple insertions and cointegrates. The IS DNA is shown as a shaded cylinder. Liberated transposon 3′OH groups are shown as small shaded circles, and those of the donor backbone (bold lines) are shown as filled circles. The 5′ phosphates are indicated by bars. Strand polarity is indicated. Target DNA is shown as open boxes. The left panel shows an example of an IS which undergoes double-strand cleavage prior to strand transfer. The right panel shows an element which undergoes single-strand cleavage at its ends. After strand transfer, this can evolve into a cointegrate molecule by replication or a simple insertion by second-strand cleavage. (B) Replicative and nonreplicative transposition as mechanisms leading to cointegrates. Three “cointegrate” pathways are illustrated: (I) by replicative transposition, (II) by simple insertion from a dimeric form of the donor molecule, and (III) by simple insertion from a donor carrying tandem copies of the transposable element. Transposon DNA is indicated by a heavy line, and the terminal repeats are indicated by small open circles. The relative orientation is indicated by an open arrowhead. Square and oval symbols represent compatible origins of replication and are included to visually distinguish the different replicons. Arrows show which transposon ends are involved in each reaction. Jacques Mahillon, and Michael Chandler Microbiol. Mol. Biol. Rev. 1998; doi: