Practice question

Saccharomyces cerevisiae haploids maintain mating type via single active MAT locus on chromosome III encoding regulators for a or alpha phenotype. Switching involves regulated replacement of MAT sequences. HML and HMR loci contain silent copies of both mating types embedded in heterochromatin near telomeres and not expressed. HO endonuclease induced double-strand break at MAT triggers recombination using HML/HMR as donor template copying opposite mating information into MAT. Therefore actual site of switching event where expression changes occurs at MAT, while silent cassettes serve merely as information reservoirs.