Transposable elements ( Alu s, LTRs , LINEs , SINEs )

Transposable elements play a key role in the evolution of most genomes. Class 1 transposable elements, which include Alu elements, long terminal repeats (LTRs), long interspersed elements (LINEs), and short interspersed elements (SINEs), have risen to a high copy number in the genomes of primates and other mammals. In humans, an estimated 42 percent of the genome is composed of LINEs, SINEs, and LTR transposable elements. Propagation of transposable elements is, in general, a parasitic process that is detrimental to the host, and host mechanisms have evolved to limit the activity of retrotransposons. Previous and ongoing retrotransposition has produced genetic differences within and among species, giving rise to genetic markers that are particularly useful for phylogenetic analyses.