Long interspersed nuclear elements[8] are a group of genetic elements that are found in large numbers in eukaryotic genomes. They are transcribed (or are the evolutionary remains of what was once transcribed) to an RNA using an RNA polymerase II promoter that resides inside the LINE. LINEs code for the enzyme reverse transcriptase, and many LINEs also code for an endonuclease (e.g. RNase H). The reverse transcriptase has a higher specificity for the LINE RNA than other RNA, and makes a DNA copy of the RNA that can be integrated into the genome at a new site.
The 5' UTR contains the promoter sequence, while the 3' UTR contains a polyadenylation signal (AATAAA) and a poly-A tail.[10] Because LINEs move by copying themselves (instead of moving, like transposons do), they enlarge the genome. The human genome, for example, contains about 20,000-40,000 LINEs, which is roughly 21% of the genome.
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