Chapter 1: Molecules, Cells, and Model Organisms

Yeasts Are Used to Study Fundamental Aspects of Eukaryotic Cell Structure and Function

One group of single-celled eukaryotes, the yeasts, has proven exceptionally useful in molecular and genetic analysis of eukaryotic cell formation and function. Yeasts and their multicellular cousins, the molds, which collectively constitute the fungi, have an important ecological role in breaking down plant and animal remains for reuse. They also make numerous antibiotics and are used in the manufacture of bread, beer, and wine.

The common yeast used to make bread and beer, Saccharomyces cerevisiae, appears frequently in this book because it has proved to be an extremely useful experimental organism. Homologs of many of the approximately 6000 different proteins expressed in an S. cerevisiae cell (see Table 1-2) are found in most, if not all, eukaryotes and are important for cell division or for the functioning of individual eukaryotic organelles. Much of what we know of the proteins in the endoplasmic reticulum and Golgi complex that promote protein secretion was elucidated first in yeasts (see Chapter 14). Yeasts were also essential for the identification of many proteins that regulate the cell cycle and catalyze DNA replication and transcription. S. cerevisiae (Figure 1-23a; see also Figure 1-22a) and other yeasts offer many advantages to molecular and cellular biologists:

Vast numbers of yeast cells can be grown easily and cheaply in culture from a single cell; the cells in such clones are genetically identical and have the same biochemical properties. Individual proteins or multiprotein complexes can be purified from large amounts of cells and then studied in detail.
Yeast cells may be either haploid (containing one copy of each chromosome) or diploid (containing two copies of each chromosome), and both forms can divide by mitosis; this ability makes isolating and characterizing mutations in genes encoding essential yeast cell proteins relatively straightforward.
Yeasts, like many organisms, have a sexual cycle that allows exchange of genes between cells. Under starvation conditions, diploid cells undergo meiosis (see Chapter 19) to form haploid daughter cells, which are of two types, a and α cells. If haploid a and α cells encounter each other, they can fuse, forming an a/α diploid cell that contains two copies of each chromosome, one from each parent cell (Figure 1-23b).

FIGURE 1-23 The yeast Saccharomyces cerevisiae can be haploid or diploid and can reproduce sexually or asexually. (a) Scanning electron micrograph of the budding yeast Saccharomyces cerevisiae. These cells grow by an unusual type of mitosis termed mitotic budding. One daughter nucleus remains in the “mother” cell; the other daughter nucleus is transported into the bud, which grows in size and soon is released as a new cell. After each bud cell breaks free, a scar is left at the budding site, so the number of previous buds on the parent cell can be counted. The orange-colored cells are bacteria. (b) Haploid yeast cells can have different mating types, called a (blue) and α (orange). Both types contain a single copy of each yeast chromosome, half the usual number, and grow by mitotic budding. Two haploid cells that differ in mating type, one a and one α, can fuse together to form an a/α diploid cell that contains two copies of each chromosome; diploid cells can multiply by mitotic budding. Under starvation conditions, a diploid cell can undergo meiosis, a special type of cell division, to form four haploid ascospores. Rupture of an ascus releases four haploid spores, which can germinate into haploid a and α cells. These cells can also multiply asexually.

[Part (a) SCIMAT/Science Source.]

With the use of a single species such as S. cerevisiae as a model organism, results from studies carried out by tens of thousands of scientists worldwide, using multiple experimental techniques, can be combined to yield a deeper level of understanding of a single type of cell. As we will see many times in this book, conclusions based on studies of S. cerevisiae have often proved true for all eukaryotes and have formed the basis for exploring the evolution of more complex processes in multicellular animals and plants.