Fundamentals of Molecular Biology - V.I. Rezyapkin 2009

Genome Rearrangements
DNA Rearrangement in Yeast

Yeast are single-celled organisms whose life cycle consists of two phases: Haploid and Diploid. Haploid Cells fuse with one another to form diploid cells. In the yeast Saccharomyces cerevisiae, haploid cells occur in two mating types, designated a and α. Cells of the same mating type are unable to conjugate with each other. Conjugation occurs exclusively between Different types of haploid cells, specifically between a and α cells. A fascinating feature of Saccharomyces cerevisiae is that haploid cells periodically switch from one mating type to the other—from a to α and vice versa. This switching results from alterations in the primary DNA Structure OF THE yeast. The yeast genome contains silent (dormant) copies of both the a and α genes, which are normally not expressed. At the same time, a cells harbor an additional, actively expressed copy of the a Gene, whereas α cells harbor an expressed copy of the α gene (Fig. 9.10). Thus, the mating type of a haploid Cell depends entirely on which gene—a or α—is currently being expressed within The Cell. When an a cell converts into an α cell, the expressed a gene is excised and replaced by an α gene. Conversely, α cells switch to a cells through a similar mechanism. In other words, mating-type switching in haploid yeast cells is a reversible process (Fig. 10).

Class="center">

Fig. 9.10. Mating-type switching in haploid yeast cells