Figure 1

Major classes of transposable elements. (a) A non-LTR transposon, which is characterized by a poly(A) tail at its 3' end undergoes replication via transcription, translation and ribonucleoprotein (RNP) assembly followed by cDNA synthesis by target-site-primed reverse transcription. (b) An LTR retrotransposon containing directly repeated LTRs at its ends is replicated via reverse transcription into cDNA in the retroviral particle and is then integrated into a new chromosomal site. (c) A DNA transposon, which has short inverted repeats at each end, can be excised (leaving a DNA break) by its transposase, which has been translated in the cytoplasm. The transposon is then integrated into a new chromosomal site. (d) A mitochondrial type II intron is replicated via a cycle involving reverse splicing of the intron into the top strand of the mitochondrial DNA. This step is followed by endo cleavage of the bottom strand and cDNA synthesis by reverse transcriptase. Key features of the different elements are indicated; gray boxes indicate open reading frames for proteins.