Major classes of transposable elements
The two major classes of transposable elements are defined by the intermediates in the transposition process. One class moves by DNA intermediates, using transposases and DNA polymerases to catalyze transposition. The other class moves by RNA intermediates, using RNA polymerase, endonucleases and reverse transcriptase to catalyze the process. Both classes are abundant in many species, but some groups of organisms have a preponderance of one or the other. For instance, bacteria have mainly the DNA intermediate class of transposable elements, whereas the predominant transposable elements in mammalian genomes move by RNA intermediates.