To analyze the internal workings of the eukaryotic cell without the additional problems of multicellular development, it makes sense to use a species that is unicellular and as simple as possible. The popular choice for this role of minimal model eukaryote has been the yeast Saccharomyces cerevisiae (Figure 1–36)—the same species that is used by brewers of beer and bakers of bread.
S. cerevisiae is a small, single-celled member of the kingdom of fungi and thus, according to modern views, is at least as closely related to animals as it is to plants. It is robust and easy to grow in a simple nutrient medium. Like other fungi, it has a tough cell wall, is relatively immobile, and possesses mitochondria but not chloroplasts. When nutrients are plentiful, it grows and divides almost as rapidly as a bacterium. It can reproduce either vegetatively (that is, by simple cell division), or sexually: two yeast cells that are haploid (possessing a single copy of the genome) can fuse to create a cell that is diploid (containing a double genome); and the diploid cell can undergo meiosis (a reduction division) to produce cells that are once again haploid (Figure 1–37). In contrast with higher plants and animals, the yeast can divide indefinitely in either the haploid or the diploid state, and the process leading from one state to the other can be induced at will by changing the growth conditions.
In addition to these features, the yeast has a further property that makes it a convenient organism for genetic studies: its genome, by eukaryotic standards, is exceptionally small. Nevertheless, it suffices for all the basic tasks that every eukaryotic cell must perform. Mutants are available for essentially every gene,
Figure 1–35 An assortment of protozoa: a small sample of an extremely diverse class of organisms. The drawings are done to different scales, but in each case the scale bar represents 10 μm. The organisms in (A), (c), and (G) are ciliates; is a heliozoan; (d) is an amoeba; is a dinoflagellate; and (F) is a euglenoid. (From m.A. sleigh, Biology of Protozoa. cambridge, uk: cambridge university Press, 1973.)
Figure 1–36 The yeast Saccharomyces cerevisiae. (A) A scanning electron micrograph of a cluster of the cells. This species is also known as budding yeast; it proliferates by forming a protrusion or bud that enlarges and then separates from the rest of the original cell. many cells with buds are visible in this micrograph.
(B) A transmission electron micrograph of a cross section of a yeast cell, showing its nucleus, mitochondrion, and thick cell wall. (A, courtesy of Ira herskowitz and eric schabatach.)
Figure 1–37 The reproductive cycles of the yeast S. cerevisiae.
depending on environmental conditions and on details of the genotype, cells of this species can exist in either a diploid (2n) state, with a double chromosome set, or a haploid (n) state, with a single chromosome set. The diploid form can either proliferate by ordinary cell-division cycles or undergo meiosis to produce haploid cells. The haploid form can either proliferate by ordinary cell-division cycles or undergo sexual fusion with another haploid cell to become diploid. meiosis is triggered by starvation and gives rise to spores—haploid cells in a dormant state, resistant to harsh environmental conditions.