The Mammalian Alu Family of Dispersed Repeats

The DNAs of most eukaryotic organisms that have been examined are organized as interspersions of repetitious sequences among unique, or low-frequency, sequences. (For a compilation of DNAs that have been examined, see Lewin 1980.) Except for clustered repeats (so-called satellite DNA sequences), most of an organism's DNA has this pattern of interspersion, although the frequency of repetitious sequences varies at different chromosomal locations; i.e., some regions of DNA are devoid of interspersed, repetitious sequences, whereas other regions contain more than the average per unit of DNA length. In mammalian DNAs this average frequency is approximately 1000–3000 bp, with the repeats themselves approximately 100–300 bp in length. This organization has been termed the short-period interspersion pattern. Long repeats are also interspersed with unique sequences. Their frequency is less than that of the short repeats. The structures of the short-period dispersed repeats and how they function have been the subject of investigation...