Behaviour of nucleolus organizing regions (NORs) and nucleoli during mitotic and meiotic divisions in budding yeast.

Spatial organization and segregation behaviour in mitosis and meiosis of nucleoli and NORs of Saccharomyces cerevisiae were examined by fluorescence in situ hybridization (FISH) and immunostaining, and integrated with previous ultrastructural studies. Our observations suggest that in interphase the NOR-bearing chromosome arm forms a hairpin-shaped loop inside the nucleolus, unlike the other arms which adopt a Rabl-like orientation. Prior to mitosis and meiosis, the appearance of the NOR changes from puffed to thread-shaped. In mitosis, it is stretched between the mother and daughter nuclei and seems to be among the last regions where chromatids separate. The nucleoli remain intact and split at the end of anaphase. Similarly, during meiosis I, intact nucleoli trail behind the separating homologous NORs and are partitioned equally to the two half-nuclei. During the second meiotic division, however, the nucleolus, together with a major portion of the nucleoplasm and the nuclear pore complexes, are not included in the spores. The behaviour of nucleoli in meiotic mutants and a strain with extrachromosomal rDNA suggests that they are not actively extruded but rather are lost due to their detachment from the separating chromosomes. We discuss the possibility that the exclusion of the nucleolus from the spores serves the disposal of agents that resort to the nucleolus, and has a role in spore dormancy or rejuvenation.