Figure 2.

Why so few Alu elements are active. Out of the more than 1 million Alu elements in the human genome, very few are capable of making copies, although many make transcripts. Upon insertion in a new locus, the factors that make a very active Alu element are the flanking sequences influencing the promoter, creating a short unique region. Active elements match the consensus Alu element fairly closely and they have a long and fairly perfect A-tail. Active elements degrade rapidly on an evolutionary time scale by A-tail shortening, heterogeneous base interruptions accumulating in the A-tail, and eventually by the accumulation of random mutations in the Alu element. At least some of these changes alter Alu activity through disruption of the various proteins binding to the RNA in the ribonucleoprotein (Figure 1b).