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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
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- Spring '08