Practice question

Mating type switching requires precise double-strand break initiation at active MAT locus. HO endonuclease is highly specific endonuclease expressed transiently in mother cells during late G1. It recognizes long degenerate 24 base-pair site spanning Ya/Z1 region at MAT, cleaving to generate 4 nucleotide 3' overhangs. Following cleavage, resection proceeds and strand invades silent donor cassette. HML and HMR themselves resist HO cleavage because heterochromatin and Sir proteins protect and because cleavage site degraded after donor copying. Thus HO activity is restricted to MAT enabling controlled conversion.