Sequence definition and organization of a human repeated DNA

DNA sequence data for a DNA repeated sequence, found largely in centromeres of specific human chromosomes is presented. The sequence consists of two tandem 169 and 171 base-pair units that show 27% base variation with each other. In contrast the dimer is more faithfully copied in longer tandem repeats, such as the sequenced 680 base-pair tetramer. In the major sequence of the tetramer, base variation of the order of only 1%, in comparison to the complete dimer is seen. A minimum of two steps in the formation of this sequence is proposed, consisting of evolution of a tandem dimer of two 170 base-pair variant units of a related family within the human genome, and later saltation or amplification of this dimer. No evidence that these sequences contained or evolved from a simpler 6 to 20 base-pair repeat was found, and no homology with known simpler human satellites could be discerned. In reviewing and comparing the literature on repeated DNAs it appears that overall length and tandem repetition are the critical features, rather than individual unit repeat length or secondary structural potential, in defining these sequences as a class and their special centromeric functions and higher chromosome order. The possibility that such sequences arise from a reservoir of interspersed sequences that are common to at least several species is discussed.