Structure	Genes Encoded	Transposition	Examples
Class I (retrotransposon)	Long terminal direct repeats; short flanking direct repeats at target site	Reverse-transcriptase gene (and sometimes others)	By RNA intermediate	Ty (yeast) copia (Drosophila) Alu (human)
Class II	Short terminal inverted repeats; short flanking direct repeats at target site	Transposase gene (and sometimes others)	Through DNA (replicative or nonreplicative)	IS1 (E. coli) Tn3 (E. coli) Ac, Ds (maize) P elements (Drosophila)