Alu transposons are found only in primate genomes and have accumulated in large numbers since primates diverged from other mammals. Human chromosomes contain more than one million Alu copies, equaling about 10% of the genome by mass. This accumulation was made possible by a transposition mechanism that reverse transcribes Alu mRNAs into mobile DNA copies. Another transposon, the long interspersed element (LINE) L1, supplies a specialized reverse transcriptase enzyme needed for Alu to jump. Hence, Alu and L1 exist in a sort of molecular symbiosis. At any point in evolutionary time, only one or several Alu “masters”were capable of transposing. Although the rate of transposition was once much higher, a new.
