Table 1.

Classes of transposable elements and their mobility mechanisms

Class of TE	Structural features	Replication mechanism	Variant forms	Active examples
DNA transposons	TIRs Transposase	Transposase-mediated excision of donor dsDNA followed by insertion into the target site	Some DNA transposons also mobilize via replicative mechanisms ssDNA transposons lack TIRs: donor ssDNA is inserted into target-site ssDNA, such as for IS608 of Helicobacter pylori	Tn7 in Escherichia coli P elements in Drosophila melanogaster Tc1 elements in Caenorhabditis elegans
LTR retrotransposons	LTRs Gag, protease, reverse transcriptase and integrase	Within virus-like particles, reverse transcriptase copies the mRNA of the TE into a full-length cDNA; integrase inserts the cDNA into target sites	Solo LTRs are commonly found in genomes and are a result of LTR–LTR recombination	Ty1, Ty3 and Ty5 in Saccharomyces cerevisiae Tf1 and Tf2 in Schizosaccharomyces pombe Tnt1 in tobacco
Non-LTR retrotransposons	One or two ORFs 5′ truncations and inversion/deletion (for mammalian L1 elements) Some end in poly(A) tails (for example, L1s); others do not (for example, R2)	An element-encoded endonuclease mediates TPRT. The endonuclease nicks the DNA at the target site and uses the 3′ nicked end for the primer as it reverse transcribes TE mRNA	Non-autonomous, non-LTR retrotransposons (for example, Alu and SVA elements, as well as other eukaryotic SINEs) rely on the endonuclease and reverse transcriptase of an autonomous non-LTR retrotransposon to mediate retrotransposition The L1 retrotransposition machinery can also mobilize mRNAs (to generate processed pseudogenes) and certain non-coding RNAs (for example, the U6 snRNA)	L1 in human, mouse, and other mammals I factor in D. melanogaster Zorro3 in Candida albicans R1 and R2 in insects