VISUALIZING THE SPATIAL RELATIONSHIPS BETWEEN DEFINED DNA-SEQUENCES AND THE AXIAL REGION OF EXTRACTED METAPHASE CHROMOSOMES

Using fluorescence in situ hybridization to extracted metaphase chromo somes, we present visual evidence that specific human DNA sequences oc cupy distinctive positions with respect to the axial region of chromos omes and that the DNA is organized into loops emanating from this regi on. In a stretch of unique DNA on chromosome 11, large loops of DNA ca n be traced and one specific region associated with the axial region o f the chromosome. Within rDNA, nontranscribed spacer sequences are mor e closely apposed to the chromosome axis than are rRNA genes. Heteroch romatic and euchromatic DNAs appear to be organized into loops of simi lar size. We could not detect loops at centromeres; most alphoid DNA a ppears to remain close to the axial region.