Yeast

Yeast have many advantages as a genetic research tool. First, yeast are nonpathogenic (they do not cause diseases) and are therefore easy and safe to grow. Yeast can divide by simple fission (mitosis) or by budding and, like bacteria, they can be rapidly grown on solid agar plates or in liquid media. After just a few days in culture, a single yeast cell can produce millions of identical copies of itself, giving scientist a large supply of a genetically pure research tool.

Second, yeast grow as either haploids (having only one set of chromosomes) or diploids (with two chromosome sets). Thus, genetically recessive mutations can be readily identified by phenotypic (visually observable) changes in the haploid strain. In addition, complementation can be performed by simply mating two haploid strains, where one does not contain the mutation. The resulting diploid strain contains both the functional and nonfunctional version of a gene responsible for a phenotype. The addition As shown by a colorized scanning electron micrograph of a yeast cell magnified 3,025 times, this single-celled eukaryotic fungus multiplies by budding. of the functional gene complements for the defect caused by the nonfunctional gene in the haploid strain. Diploid strains can be induced to undergo meiosis, a process in which the cell divides and passes one-half of its chromosomes to each of the resulting cells. After two such divisions, reproductive structures called asci are produced that contain four haploid offspring, called ascospores. The asci can be dissected and each of the ascospores isolated. In this way, scientists can easily mate different yeast strains and obtain new haploid genotypes through sexual reproduction and meiosis.

Third, the genome of yeast is small, about 3.5 times larger than that of bacteria and 200 times smaller than that of mammals. The yeast genome is arranged in 16 linear chromosomes that range from 200 to 2,200 kilobases in length. Unlike mammals, the yeast genome is very compact, with only 12 million base pairs, very few introns, and very little spacer DNA between functional genes. As a result, in 1996 baker's yeast was the first eukaryotic organism to have its entire genome sequenced.