Organization and evolution of an alpha satellite DNA subset shared by human chromosomes 13 and 21

The structure of the alpha satellite DNA higher-order repeat (HOR) unit from a subset shared by human chromosomes 13 and 21 (D13Z1 and D21Z1) has been examined in detail. By using a panel of hybrids possessing either a chromosome 13 or a chromosome 21, different HOR unit genotypes on chromosomes 13 and 21 have been distinguished. We have also determined the basis for a variant HOR unit structure found on approximately 8% of chromosomes 13 but not at all on chromosomes 21. Genomic restriction maps of the HOR units found on the two chromosome 13 genotypes and on the chromosome 21 genotype are constructed and compared. The nucleotide sequence of a predominant 1.9-kilobasepair HOR unit from the D13Z1/D21Z1 subset has been determined. The DNA sequences of different alpha satellite monomers comprising the HOR are compared, and the data are used to develop a model, based on unequal crossing-over, for the evolution of the current HOR unit found at the centromeres of both these chromosomes.