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Term yeast
- jargon
for unicellular fungus that grows
reproductively by some type of budding or
fission.
Like molds, yeasts are
heterogeneous and distributed among a number
of taxa.
1.
Ascomycetous (Hemiascomycetous)
yeasts - holocarpic forms capable of forming
endogenous microspores
2.
Basidiomycetous (Heterobasidiomycetous)
yeasts - yeast phases of Uredinales and
Ustilaginales
3.
Imperfect yeasts - those yeasts with
no known or observed sexual states*
4.
Yeast forms of sexual and/or asexual
hyphal fungi
"dimorphic fungi"
*blastomycetes
205/90a
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S.
cereviseae (review of some main points of last lecture)
1.
Most studied eucaryotic microbe
2.
Small organism
6-8 x 4-5 µm length x width for
haploid
3.
Cell wall = mannan-glucan type;
chitin restricted to septum region
4.
Multipolar = does not bud through old
bud scars
5.
17 centromere-associated linkage
groups/haploid cell
6.
DNA content
3 to 4x that of E.
coli
23 femptograms/haploid cell
150 kbp to 2500 kbp for chromosomes
0.5 x 107
kbp total
7.
Mitosis intranuclear membrane event
8.
Asymetric cell division - thus,
mothers and daughters can be
distinguished.
228
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Salient ultrastructural
events of mitosis in S. cerevisiae
a.
Vacuole fission
b.
Satellite formation on SPB half
bridge
c.
SPB duplication
d.
Vesicle-mediated bud emergence and
SPB separation (buds more axial in haploids,
more distal in diploids)
e.
Bud enlargement; spindle formation
f.
Nuclear migration to mother cell-bud
juncture
g.
Nuclear division (intranuclear
membrane mitosis)
h.
Primary and secondary septum
formation (cytokinesis)
i.
Asymetric cell separation -> bud
& birth scars
229
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Meiosis
in S.
cerevisiae = Ascosporogenesis
1.
Ascosporogenesis -> 4 meiotic
products (ascospores/tetrads)
1.
Induction in laboratory by transfer
of diploids to nonfermentable substrate
(usually acetate-containing media which
promotes aerobic respiration)
3.
High sugar inhibits ascosporogenesis
(insures high population densities) and
promotes fermentation by glucose repression
4.
As in mitosis, SPB satellite
formation announces start of meiosis.
230b
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