Unequal sister chromatid exchange in the rDNA array of Saccharomyces cerevisiae

In the yeast Saccharomyces cerevisiae the nucleolar organiser region (NOR) is located on chromosome XII. It contains 100-200 copies of rDNA--a minimum of 20 rDNA genes in tandem--and is termed the RDN locus. Yeast cells may exist in either haploid or diploid form. There are two forms of life cycle: haploid and diploid cells double by mitosis, and diploid cells are reduced to the haploid state by meiosis. Diploid cells have two homologous chromosomes for each of the 16 chromosomes. They are usually of the same size. However, in this study it is shown that homologous chromosomes XII can become different in size due to unequal sister chromatid exchange during mitosis in 'old' cells.