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Unformatted text preview: ll (Fig. A.2). Haploid cells occur in two mating types, a
and . Both mating types can reproduce mitotically as
stable haploid cells. Or they can engage in sexual reproduction, in which cells of opposite mating types communi- Ascus containing four
α a Germination Sporulation
(haploid) α a
a α Meiosis Vegetative life
(haploid) Budding Budding a a/α α cate with one another by proteins known as pheromones.
The pheromones induce the dissimilar cells to undergo cell
fusion followed by nuclear fusion. The zygote produced in
this way has a single diploid nucleus and buds to produce
diploid progeny. Diploid yeast cells also propagate as stable diploid cells by mitotic division. Starvation, however,
induces them to undergo meiosis and sporulation. The ability to undergo meiosis allows the yeast cells to “reshufﬂe”
their genes when conditions are poor, perhaps enabling
them to ﬁnd a combination more suitable for survival in the
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This note was uploaded on 10/03/2013 for the course BI 206 taught by Professor Celenza during the Spring '08 term at BU.
- Spring '08