Two general rules for ho mediated switching have

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Unformatted text preview: second generation, the spore gives birth to the second daughter (D2), and the first daughter gives birth to its first daughter (D1–1). Two general rules for HO-mediated switching have emerged from observations of many generations. First, cells always switch in pairs; if a mother cell switches, its immediate daughter will have the same switched mating type (Fig. A.12b). Switching occurs in pairs because the HO-generated cut at the MAT locus occurs in G1, before chromosomes have replicated. Second, cells must gain competence to switch before actually switching; and they gain competence through the experience of undergoing at least one cell division. Thus, an “inexperienced” spore never switches at the first cell division. In addition, a daughter cell will not switch when it produces its first daughter cell. Once a cell has gained experience through cell division, however, it becomes competent to switch in subsequent generations (Fig. A.12c). What aspect of cell division triggers competence to switch? The answer relates to the transcriptional activation of t...
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