PHYSICAL RELATIONSHIP BETWEEN SATELLITE-I AND SATELLITE-II DNA IN CENTROMERIC REGIONS OF SHEEP CHROMOSOMES

Fluorescence in situ hybridization (FISH) with probes representing she ep satellite I and satellite II DNAs shows a different distribution of the two repetitive DNA families in the centromeric region of most chr omosomes. The single signal per chromosome produced by the satellite I probe suggests close proximity of this DNA family to the primary cons triction. Satellite II produces two separate signals on the sister chr omatids, and large blocks of satellite II DNA constitute most of the s hort arm of all acrocentric chromosomes. We have isolated and sequence d a phage clone containing a junction between discrete blocks of satel lite I and satellite II sequences. The junction is characterized by an abrupt juxtaposition of arrays of the two satellites. The possibility that the peculiar structural features of this junction could have a f unctional significance is discussed.