ingredient information
Yeast A
Mating in yeast, as in most organisms, is especially dramatic. Organisms which reproduce sexually have haploid and diploid stages. In flowering plants and animals it is only the gametes that are haploid and the phenotype can be observed only in this diploid stage. When Mendel made dihybrid crosses to study the inheritance of two different traits, such as seed color and seed shape, he could only observe these traits in the diploid cells of parents and offspring. Mendel had to use statistical probability in order to calculate what the genotype of the gametes was likely to be. The phenotypes of some traits can be seen in the haploid-cell colonies of Saccharomyces cerevisiae. Yeast cells may occur as either haploid or diploid. Haploid cells occur as either mating-type a or mating-type a (alpha). When cells of these opposite type come in contact, they secrete hormonelike substances called mating pheromones, which causes these cells to develop into gametes. These gametes then fuse and form a diploid zygote. This is similar to fertilization in animals, but here both parents contribute cytoplasm and nuclei. The resulting yeast zygotes reproduce asexually by budding. Diploid yeast cells do not mate, but in times of stress, such as a lack of certain nutrients, may sporulate and form spores. These spores may be found together looking like ball-bearings in a transparent sac called an ascus. Source: