Mating Type Mating And The Sexual Life Cycle

In nature most strains of Saccharomyces are diploid and carry the functional allele of the HO gene, homothalic diploids. Laboratory research strains carry mutant ho and can be grown as stable haploid cells. Haploids occur in two mating types, the a mating type and the a mating type, and these differ from one another at a single locus called the MAT locus. The two alleles of this locus are referred to as MA 7a and MATa. Stable a or a strains divide mitotically to produce genetically identical clones of cells. The existence of a stable haploid stage in the life cycle of Saccharomyces is attractive to the geneticist because strains carrying recessive mutations can be isolated and identified in the haploid cell type and it is not necessary to inbreed mutagenized cells to obtain a homozygous mutant diploid.

MA 7a strains mate with MA Ta strains by a complex process of cytoplasmic and nuclear fusion that results in a diploid cell (described in Chapter 3). This diploid cell is also stable and divides by mitosis to produce a genetically identical diploid clone. The existence of the stable diploid cell type is also extremely useful for the geneticist. It allows one to determine if a mutant allele is dominant or recessive and it provides a simple means for carrying out a complementation test. Complementation analysis is described in Chapter 5 and is used to determine if different mutations map to the same or different genes.

Haploid Life Cycle Saccharomyces

Figure 1.1 Life cycle of Saccharomyces cerevisiae. Diploid (a/a) and haploid, a- or a-mating type, Saccharomyces cerevisiae can reproduce by mitosis to form clones of genetically identical cells. Haploid cells of opposite mating type can fuse with one another to form an a/ a diploid. When subjected to nutrient starvation conditions, diploid cells undergo meiosis producing four haploid meiotic products called ascospores all contained in a single structure called an ascus. If restored to nutrient sufficient conditions, each of these four ascospores will germinate and reproduce as haploid cells as follows: two a-mating type cells and two a-mating type cells. From The Cell Cycle: An Introduction by Andrew Murray and Tim Hunt, copyright © 1993 by Oxford University Press, Inc. Used by permission of Oxford University Press, Inc.

Figure 1.1 Life cycle of Saccharomyces cerevisiae. Diploid (a/a) and haploid, a- or a-mating type, Saccharomyces cerevisiae can reproduce by mitosis to form clones of genetically identical cells. Haploid cells of opposite mating type can fuse with one another to form an a/ a diploid. When subjected to nutrient starvation conditions, diploid cells undergo meiosis producing four haploid meiotic products called ascospores all contained in a single structure called an ascus. If restored to nutrient sufficient conditions, each of these four ascospores will germinate and reproduce as haploid cells as follows: two a-mating type cells and two a-mating type cells. From The Cell Cycle: An Introduction by Andrew Murray and Tim Hunt, copyright © 1993 by Oxford University Press, Inc. Used by permission of Oxford University Press, Inc.

In starvation conditions, the a/a diploid undergoes meiotic division and produces four haploid cells that mature into ascospores. The four haploid products of a single diploid cell are contained in a sack called an ascus that is designed for survival under difficult conditions. Using a microdissection device mounted on a microscope stage, one can separate the individual haploid ascospores and germinate them in nonstarvation media. These will divide to produce genetically identical haploid clones. This process is shown in Figure 1.1. The simplicity by which the reseacher can manipulate the sexual life cycle of Saccharomyces is a tremendous advantage for genetic analysis.

SACCHAROMYCES CEREVISIAE AS A GENETIC MODEL ORGANISM 7

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  • Scolastica
    What are the six haploid forms cells called?
    7 years ago

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