A chromosome breakage assay to monitor mitotic forces in budding yeast.

During the eukaryotic cell cycle, genetic material must be accurately duplicated and faithfully segregated to each daughter cell. Segregation of chromosomes is dependent on the centromere, a region of the chromosome which interacts with mitotic spindle microtubules during cell division. Centromere function in the budding yeast, Saccharomyces cerevisiae, can be regulated by placing an inducible promotor adjacent to centromere DNA. This conditional centromere can be integrated into chromosome III to generate a conditionally functional dicentric chromosome. Activation of the dicentric chromosome results in a transient mitotic delay followed by the generation of monocentric derivatives. The propagation of viable cells containing these monocentric derivative chromosomes is dependent upon the DNA repair gene RAD52, indicating that double-strand DNA breaks are structural intermediates in the dicentric repair pathway. We have used these conditionally dicentric chromosomes to monitor the exertion of mitotic forces during cell division. Analysis of synchronized cells reveal that lethality in dicentric, rad52 mutant cells occurs during G2/M phase and is concomitant with the transient mitotic delay. the delay is largely dependent upon the cell cycle checkpoint gene RAD9, which is involved in monitoring DNA damage. These data demonstrate that DNA lesions resulting from dicentric activation are responsible for signalling the mitotic delay. Since the delay precedes the decline of p34cdc28 kinase activity, mitotic forces sufficient to result in dicentric chromosome breakage are generated prior to spindle elongation and anaphase onset in yeast.