The profusion of eukaryotic genomes continues to amaze
geneticists: as low as a few percents of eukaryotic
genome length correspond to protein-coding sequences.
Eukaryotic genes are commonly separated by long
regions, and their coding sequences (exons) are intervened
by non-coding ones (introns), which run to tens of
kilobases. Extensive chromosomal regions free from
genes, intergenic regions and introns contain great
numbers of repetitive DNA sequences, most of which
are mobile genetic elements or transposable elements
(TEs). TEs are divided into two major classes: DNA
transposons and retrotransposons. DNA transposons
encode a transposase enzyme catalyzing the transposon
DNA excision and its integration into a new genomic
location (‘cut and paste’ mechanism). Similar to all other
TEs, DNA transposons are transmitted vertically from
parent to offspring; however, their horizontal transmission
between species (sometimes phylogenetically
distant) is not uncommon. Unlike other TEs, DNA
transposons are found in both eukaryotes and prokaryotes
(for review see Feschotte and Pritham, 2007).
Retrotransposons is the most abundant class of TEs.
The transposition of all such elements involves the ‘copy
and paste’ mechanism including transcription of the TE
gene, reverse transcription of the RNA, and integration
of the resulting DNA into a new genomic location. Long
terminal repeat (LTR) elements represent the beststudied
subclass of retrotransposons. They have a very
wide distribution among eukaryotes, from yeast to human.
Structurally, LTR elements resemble retroviral genomic
copies.