Retrotransposons are the dominant class of transposable element found in eukaryotes. Two types of true
retrotransposon are known, LTR retrotransposonsand non-LTR retrotransposons, more popularly
known as long interspersed elements (LINEs).^65 Dispersed pseudogenes are defective derivatives of cellular
genes that have never encoded the proteins needed for successful transposition, but which are probably
moved to new chromosomal locations by the transposition machinery of retroviruses or retrotransposons.
They are thus genetic ‘hitch-hikers’.
The best-known dispersed pseudogenes are the short interspersed elements (SINEs) and the prototype
example of these in humans is the Alu family.^66 Both SINEs and LINEs have also been classified as ‘retro-
posons’ (Class I elements which use a common transposition mechanism called target-primed reverse
transcription). Unfortunately, the retroposon classification, although well established, conflicts with the
retrotransposon classification, because LINE elements are both retrotransposons and retroposons.
LINEs are probably the most primitive retrotransposon group and may be ancestral to the other groups.
They contain a gene encoding the protein component of a capsid-like intracellular structure (gag) required
for the transposition process. Some non-LTR retrotransposons also encode an endonuclease, which is
involved in transposition. Finally, all LINEs contain a reverse transcriptase gene, encoding the enzyme
that converts the element’s RNA into DNA.
Genes and Genomes 247
Figure 6.46 The control of transposition of the Drosophila P element. mRNA splicing limits the synthesis of active
transposase to the germ cells
Figure 6.47 Classification of Class I transposable elements. Large outlined boxes indicate overlapping classifications
for retrotransposons, retroposons and dispersed pseudogenes. Smaller shaded and hatched boxes indicate
gene conservation among different types of Class I elements