Today, scientists know that there are many different types of TEs, as well as a number of ways to categorize them. One of the more common divisions is between those TEs that require reverse transcription (i.e., the transcription of RNA into DNA) in order to transpose and those that do not. The former elements are known as retrotransposons or class 1 TEs, whereas the latter are known as DNA transposons or class 2 TEs. The Ac/Ds system that McClintock discovered falls in the latter category. Different classes of transposable elements are found in the genomes of different eukaryotic organisms (Figure 1).
Figure 1: The relative amount of retrotransposons and DNA transposons in diverse eukaryotic genomesThis graph shows the contribution of DNA transposons and retrotransposons in percentage relative to the total number of transposable elements in each species. (Sc: Saccharomyces cerevisiae; Sp: Schizosaccharomyces pombe; Hs: Homo sapiens; Mm: Mus musculus; Os: Oryza sativa; Ce: Caenorhabditis elegans; Dm: Drosophila melanogaster; Ag: Anopheles gambiae, malaria mosquito; Aa: Aedes aegypti, yellow fever mosquito; Eh: Entamoeba histolytica; Ei: Entamoeba invadens; Tv: Trichomonas vaginalis.)© 2007 Annual Reviews Feschotte, C. & Pritham, E. J. DNA transposons and the evolution of eukaryotic genomes. Annual Reviews in Genetics 41, 331–348. All rights reserved.
All complete or "autonomous" class 2 TEs encode the protein transposase , which they require for insertion and excision (Figure 2). Some of these TEs also encode other proteins. Note that DNA transposons never use RNA intermediaries—they always move on their own, inserting or excising themselves from the genome by means of a so-called "cut and paste" mechanism.