Long interspersed nuclear elements (LINEs)

Long interspersed repeat sequences (LINEs) are
mammalian retrotransposons that in contrast
to retroviruses lack long terminal repeats
(LTRs). They account for up to 70% of the human
genome by weight. They consist of repetitive
sequences up to 6500 bp long that are adeninerich
at their 3! ends. They may contain one or
two open reading frames (ORF), although they
are usually shorter and contain no ORF. At the 5!
end and at the 3! end they have an untranslated
region (5! UTR and 3! UTR). LINE elements are
thought to have arisen by transposition (see
p. 76). Mammalian genomes contain 20000–
60000 copies of LINE sequences. The major
human LINE element is the L1 sequence, a segment
that spans up to 6.4 kb. Approximately
100 000 L1 elements are dispersed throughout
the human genome. This can result in genetic
disease if one is inserted into a gene