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10/29/08
Sex in yeast
- Mating combines haploid genomes
to diploid genomes
us: female & male produce egg &
sperm
yeast: mating types “a” and “alpha”
produce spores (“dauer state”) that
produce haploid cells (active state)
- our (human) haploids are short-lived
gamets but yeast haploids are free
living organisms that are virtually
identical to diploids
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yeast exists as diploids, haploids, and haploid spores
- the two sexes of haploids (a and alpha) are virtually indistinguishable except:
+ “a” cells produce a pheromone that is highly attractive for alpha cells.
+ “alpha” cells produce a pheromone that is highly attractive for a cells.
- upon pheromone detection, cells stop cell cycle, mitosis, etc., and get ready for mating,
technically they “schmoo” toward each other.
- upon touching, cells and nuclei start fusing.
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Life cycle of yeast
a::α
Mating &
Conjugation
Mitosis
a::α
a::α a::α
a
α
diploid
“hard times”
cells “schmoo”
a::α
α haploid
α
α
α α
Mitosis
a a
a
a
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a
Ascus with
4 spores
α
a
Germination
Meiosis &
Sporulation
α
a
Genotypes!
a::α
Mating &
Conjugation
a
α
a::α
a::α a::α
diploid
Mat1-alf/aaa leu1-1/+ ura2-1
a::α
a a
a
haploid
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α
α
α
Ascus with
4 spores
α
a
α
a
α
a
α
a
Sporulation
Mat1-alf ura2-1
Mat1-aaa leu1-1
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“Sexy Yeast”
a-specific genes
ON by default
alpha-specific genes
Off by default
a1 gene
haploid-specific genes
a cell
ON by default
a-specific genes
alpha2
OFF
alpha-specific genes
ON
alpha1
haploid-specific genes
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ON by default
alpha cell
a-specific genes
a-specific genes
OFF
ON by default
alpha-specific genes
OFF by default
a1 gene
a cell
haploid-specific genes
alpha1
ON
haploid-specific genes
alpha cell
ON by default
ON
a-specific genes
alpha2
alpha1
X
a1 gene
OFF
default: on
alpha-specific genes
default: off
OFF
haploid-specific genes
default: on
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alpha-specific genes
alpha2
OFF
diploid cell
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Topic of Pathfinder: Phases of the Cell Cycle
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“Behavior” of temperature-sensitive cell cycle control
mutants: example mutation in G1
At permissive temperature, cell divide
normal and are found in all phases of the
cycle.
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At restrictive temperature, cell’s can’t divide
normal since the cell cycle will be arrested at
the mutated phase.
Consequently, all cells are found in the same
phase of the cycle.
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