The nucleolar and centromeric DNAs of the fission yeast Schizosaccharomyces pombe were visualized in the nucleus by fluorescence in situ hybridization using repetitive ribosomal and centromeric DNAs as the probes. The rDNAs were seen in the nuclear domain previously assigned as nucleolar, that is, the region into which the rod-like chromatin protrudes from the hemispherical chromosomal domain. Using mitotically-arrested cells containing condensed chromosomes, it was demonstrated that the rDNAs were present on the smallest chromosome III, consistent with genetic data. Using a centromeric repetitive element as the hybridization probe, the centromere of chromosome III, cen3, which contains the largest number of the repetitive elements, was visualized. The centromere in interphase cells is located near the periphery of the nucleus as a single dot. Early in mitosis, however, it divides into two and is situated in the middle of the short mitotic spindle. After spindle extension in anaphase, the centromeric DNA is present at both ends of the spindle, that is, near the spindle pole bodies. The movement of cen3 during mitosis (anaphase A and B) is discussed in relation to spindle dynamics and chromosome separation.

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