Visualizing the spatial relationships between defined DNA sequences and the axial region of extracted metaphase chromosomes.

Using fluorescence in situ hybridization to extracted metaphase chromosomes, we present visual evidence that specific human DNA sequences occupy distinctive positions with respect to the axial region of chromosomes and that the DNA is organized into loops emanating from this region. In a stretch of unique DNA on chromosome 11, large loops of DNA can be traced and one specific region associated with the axial region of the chromosome. Within rDNA, nontranscribed spacer sequences are more closely apposed to the chromosome axis than are rRNA genes. Heterochromatic and euchromatic DNAs appear to be organized into loops of similar size. We could not detect loops at centromeres; most alphoid DNA appears to remain close to the axial region.