Long + Short Interspersed Nuclear Elements (LINEs + SINEs) make up approximately 21% + 11% of the human genome, respectively. They are non-long terminal repeat (non-LTR) retrotransposons, which means they are transposons that reintegrate themselves within genomes via RNA intermediates (hence, retro), + they are not flanked by long terminal repeats (hence, non-LTR). Note that, in contrast to DNA transposons, retrotransposons use a copy + paste mechanism, thereby extending the genome + repeating within it. LINE's + SINE's only occur in eukaryotic genomes + are hypothesised to have co-evolved.
Less than 700bp long
Are known as non-autonomous transposons
They contain RNA polymerase III promoters to facilitate their transcription; for their reverse-transcription, they rely on the reverse transcriptase of a LINE element.
The most abundant type is the Alu element
Read about LINEs in my previous blog post here + check out my next blog post about Alu elements.
Hancks, D.C. and Kazazian, H.H., 2016. Roles for retrotransposon insertions in human disease. Mobile DNA, 7(1), p.9. [Highly recommended.]
Singer, M.F., 1982. SINEs and LINEs: highly repeated short and long interspersed sequences in mammalian genomes. Cell, 28(3), pp.433-434.