Retrotransposition. (a) In extrachromosomally primed (EP) retrotransposition, a tRNA or other RNA that can anneal to the transposon RNA is used as a primer for reverse transcription of the RNA to create a double-stranded DNA fragment, in two steps. This DNA fragment inserts itself into a DNA target site, as in cut-and-paste transposition. (b) In target-primed (TP) retrotransposition, the RNA migrates to the target site, where a phosphodiester bond is cleaved hydrolytically. The exposed 3′ end is used to prime reverse transcription of the RNA, which is again converted to double-stranded DNA in two steps. The insertion is completed by ligation.