ORGANIZATION AND EVOLUTION OF AN ALPHA-SATELLITE DNA SUBSET SHARED BYHUMAN CHROMOSOME-13 AND CHROMOSOME-21

The structure of the alpha satellite DNA higher-order repeat (HOR) uni t 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 genotype s on chromosomes 13 and 21 have been distinguished. We have also deter mined the basis for a variant HOR unit structure found on approximatel y 8% of chromosomes 13 but not at all on chromosomes 21. Genomic restr iction maps of the HOR units found on the two chromosome 13 genotypes and on the chromosome 21 genotype are constructed and compared. The nu cleotide 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 dat a 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 th ese chromosomes.