Genomic Organization of Human Centromeric Alpha Satellite DNA: Characterization of a Chromosome 17 Alpha Satellite Sequence

We characterized a recombinant clone E7 containing a 1.6-kb Eco RI insert of human alpha satellite DNA (αDNA) which hybridized in situ predominantly to the centromere of chromosome 17. Three thousand copies of this sequence were detected on chromosome 17, although a lesser number of copies were also found on the centromeres of chromosomes 11, X, and the other human chromosomes, except Y. In the human genome, sequences homologous to E7 were organized principally as five major polymorphic (Pst I) forms of tandem αDNA repeats with molecular weights between 2.0 and 2.7 kb. We studied the higher-order organization of these major forms using a series of 12 cosmid clones. Close linkage of the different polymorphic forms was demonstrated, with no two cosmids showing an identical linkage pattern. Six of the cosmid clones carried a considerable amount (20–25%) of nonhomologous (non-α) DNA, indicating that the repeat arrays are relatively frequently interrupted by other genomic DNA. In none of the cosmid inserts were the repeat arrays bound on both sides by non-αDNA, suggesting that short arrays are not common. However, some of the intervening non-αDNA sequences were relatively short, and vary in size from 6 to 24 kb. Our results suggest an irregular and complex pattern of organization of αDNA in the human genome.