Transposable elements and the evolution of eukaryotic genomes

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   Fig. 1.

   Structural features and classification of eukaryotic TEs. Elements are divided into two classes, according to whether their transposition intermediate is RNA (class 1) (b and c) or DNA (class 2) (a). Class 1 elements are further divided into two groups on the basis of transposition mechanism and structure: LTR retrotransposons (b) and non-LTR retrotransposons (c). Each class contains autonomous and nonautonomous elements. Autonomous elements encode proteins required for transposition (gag, capsid-like protein; pol, reverse transcriptase; ORF1, a gag-like protein; en, endonuclease; rt, reverse transcriptase). Nonautonomous elements do not encode these proteins but retain the cis sequences necessary for transposition. TSDs are the black arrowheads flanking each element, and the inverted repeats at the termini of class 2 elements (a) and the direct repeats at the ends of LTR retrotransposons (b) are represented by large gray arrowheads. A color version of this figure originally appeared in box 1 of ref. 1.