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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 10/29/08 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. 10/29/08 1 10/29/08 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 10/29/08 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 10/29/08 α α α Ascus with 4 spores α a α a α a α a Sporulation Mat1-alf ura2-1 Mat1-aaa leu1-1 2 10/29/08 “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 10/29/08 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 10/29/08 alpha-specific genes alpha2 OFF diploid cell 3 10/29/08 Topic of Pathfinder: Phases of the Cell Cycle 10/29/08 “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. 10/29/08 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. 4