CHROMOSOME CONDENSATION AND SISTER-CHROMATID FAIRING IN BUDDING YEAST

We have developed a fluorescent in situ hybridization (FISH) method to examine the structure of both natural chromosomes and small artificia l chromosomes during the mitotic cycle of budding yeast. Our results s uggest that the pairing of sister chromatids: (a) occurs near the cent romere and at multiple places along the chromosome arm as has been obs erved in other eukaryotic cells; (b) is maintained in the absence of c atenation between sister DNA molecules; and (c) is independent of larg e blocks of repetitive DNA commonly associated with heterochromatin. C ondensation of a unique region of chromosome XVI and the highly repeti tive ribosomal DNA (rDNA) cluster from chromosome XII were also examin ed in budding yeast. Interphase chromosomes were condensed 80-fold rel ative to B form DNA, similar to what has been observed in other eukary otes, suggesting that the structure of interphase chromosomes may be c onserved among eukaryotes. While additional condensation of budding ye ast chromosomes were observed during mitosis, the level of condensatio n was less than that observed for human mitotic chromosomes. At most s tages of the cell cycle, both unique and repetitive sequences were eit her condensed or decondensed. However, in cells arrested in late mitos is (M) by a cdc15 mutation, the unique DNA appeared decondensed while the repetitive rDNA region appeared condensed, suggesting that the con densation state of separate regions of the genome may be regulated dif ferently. The ability to monitor the pairing and condensation of siste r chromatids in budding yeast should facilitate the molecular analysis of these processes as well as provide two new landmarks for evaluatin g the function of important cell cycle regulators like p(34) kinases a nd cyclins. Finally our FISH method provides a new tool to analyze cen tromeres, telomeres, and gene expression in budding yeast.