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Saccharomyces_A - har06584_refA_001-020 17:23pm Page 1...

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1 Saccharomyces cerevisiae, commonly known as baker’s yeast, has been a preferred organism for genetic research since the mid–twentieth century ( Fig. A.1 ). The experimental value of this single-celled eukaryotic species lies in its simple life cycle, alternating haploid and diploid phases, short generation time, and easy-to-identify meiotic products. Recent techno- logical advances enable yeast biologists to use genetic analy- sis to gain a deep understanding of the organization and regulation of eukaryotic cells. Like all eukaryotic cells, yeast cells make decisions that determine their fate. They decide, for example, whether they are a or cells, and whether to mate, enter the meiotic path- way, grow and divide by mitosis, or bud. Researchers can use genetic techniques to analyze the molecular events that influ- ence these fate-determining decisions; and knowledge of these events, which include the activation and inactivation of transcription factors, can contribute to an understanding of how the cells of multicellular organisms differentiate. Three main themes unify our genetic portrait of yeast. First, to identify the genes and proteins influencing develop- ment, researchers isolate and study mutants defective for a specific process. Second, master regulatory genes in yeast control the expression of other genes, whose activ- ities determine the phenotype of a cell. Third, cascades of proteins called signal transduction systems (described in Chapter 19 of the main textbook) transmit the signals responsible for shifting a cell from one phenotype to another. In examining yeast as a model organism for understanding development, we present: An overview of yeast in the laboratory: significant details of the yeast life cycle, current knowledge of the yeast genome, and basic tools for looking at yeast. Cell differentiation: molecular mechanisms for determining cell type. Mating: how cell-to-cell communication through pheromones promotes the conversion of haploid cells to diploid cells. Yeast colonies, each containing about 10 7 cells, grow from single cells spread on solid media. Saccharomyces cerevisiae: Genetic Portrait of a Yeast Reference A A.1 An Overview of Yeast in the Laboratory Yeast geneticists have gathered enough information on S. cerevisiae to state with confidence that single-celled eukaryotes such as yeast express genes, organize themselves, perform biological functions, and differentiate using varia- tions of the same processes found in multicellular eukaryotes. Yeast has proven to be an excellent model for the study of cell cycle control and for the dissection of eukaryotic signal transduction pathways. Using standard genetic analyses and newly developed genomic tools, yeast re- searchers continue to provide critical insights into basic cellular processes.
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The Yeast Life Cycle Yeast cells alternate between haploid (1 n ) and diploid (2 n ) phases in which new daughter cells arise mitotically as buds that grow in size and eventually split from the mother cell ( Fig. A.2 ). Haploid cells occur in two mating types,
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