Vegetative cell division of yeast characteristically occurs by budding, in which a daughter is initiated as an out growth from the mother cell, followed by nuclear division, cell-wall formation, and finally cell separation. The sizes of haploid and diploid cells vary with the phase of growth and from strain to strain. Typically, diploid cells are 5 x 6 mm ellipsoids and haploid cells are 4 mm diameter spheroids. The volumes and gross composition of yeast cells are listed in Table 1. During exponential growth, haploid cultures tend to have higher numbers of cells per cluster compared to diploid cultures. Also haploid cells have buds that appear adjacent to the previous one; whereas diploid cells have buds that appear at the opposite pole. Each mother cell usually forms no more than 20-30 buds, and it age can be determined by the number of bud scars left on the cell wall.

"Normal" laboratory haploid strains have a doubling time of approximately 90 min. in complete YPD (1% yeast extract, 2% peptone, and 2% glucose) medium and approximately 140 min. in synthetic media during the exponential phase of growth at the optimum temperature of 30°C. However, strains with greatly reduced growth rates in synthetic media are often encountered. Usually strains reach a maximum density of 2 x10⁸ cells/ml in YPD medium. Titers 10 times this value can be achieved with special conditions, such as pH control, continuous additions of balanced nutrients, filtered-sterilized media and extreme aeration that can be delivered in fermenters.

S. cerevisiae can be stably maintained as either heterothallic or homothallic strains, as illustrated in Figure 4.1. Both heterothallic and homothallic diploid strains sporulate under conditions of nutrient deficiency, and especially in special media, such as potassium acetate medium. During sporulation, the diploid cell undergoes meiosis yielding four progeny haploid cells, which become encapsulated as spores (or ascospores) within a sac-like structure called an ascus (plural asci). The percent sporulation varies with the particular strain, ranging from no or little sporulation to nearly 100%. Many laboratory strains sporulate to over 50%. The majority of asci contains four haploid ascospores, although varying proportions asci with three or less spores are also observed.

Figure 4.1. Life cycles of heterothallic and homothallic strains of S. cerevisiae. Heterothallic strains can be stably maintained as diploids and haploids, whereas homothallic strains are stable only as diploids, because the transient haploid cells switch their mating type, and mate.

Because the a and a mating types are under control of a pair of MATa/MATa heterozygous alleles, each ascus contains two MATa and two MATa haploid cells. Upon exposure to nutrient condition, the spores germinate, vegetative growth commences and mating of the MATa and MATa can occur. However, if the haploid spores are mechanically separated by micromanipulation, the haplophase of heterothallic strains can be stably maintained, thus allowing the preparation of haploid strains. In contrast, the presence of the HO allele in homothallic strains causes switching of the mating type in growing haploid cells, such that MATa cells produce MATa buds and MATa cells produce MATa buds. As a consequence, mating occurs and there is only a transient haplophase in homothallic strains (Figure 4.1).

Controlled crosses of MATa and MATa haploid strains are simply carried out by mixing approximately equal amounts of each strain on a complete medium and incubating the mixture at 30°C for at least 6 hr. Prototrophic diploid colonies can then be selected on appropriate synthetic media if the haploid strains contain complementing auxotrophic markers. If the diploid strain cannot be selected, zygotes can be separated from the mating mixture with a micromanipulator. Zygotes are identified by a characteristic thick zygotic neck, and are best isolated 4 to 6 hr after incubating the mixture when the mating process has just been completed.