Yeast Genetics
Torey Arvik

The environment around us is considered to be a selective place. You would not live here if there were no water, oxygen or a temperate climate. Similarly, selection in the form of specialized environments allows for extreme diversity (e.g. tube worms in oceanic thermal vents). The organisms living in specialized habitats have adapted to the selective constraints of those environments.

Yeast are eukaryotes. They were considered part of the plant kingdom for a hundred years, now they are classified as fungi. Yeast have many of the same genes we humans do, because we have a universal form of nucleic acid reproduction (central dogma). Yeast are different from bacteria in that they have a different achitechture than plants or bacteria. They have i.) a nuclear membrane that houses their DNA (and some RNA), ii.) multiple chromosomes 10-20), whereas bacteria have one or two, and iii.) very different cell wall structures than bacteria and plants. S. cereviseae has 16 chromosomes and the entire genome was recently sequenced (12 Megabases; 6,000+ genes). Yeast can be sexual or asexual. How do yeast have sex? They ³shmoo²! When yeast get ready to mate they secrete sex pheromones that cause others nearby to do the same. The cell wall changes and they look like a ³Shmoo². The cells fuse and then the nuclei are fused while the two strains trade genes (see Fig. 1 in attached pdf).

Living in grape juice/wine requires several special tools. Yeast need to be able to import sugars, export wastes (alcohol, etc.) and sense the amount of food around them to regulate their growth. Grape juice has a pH between 2.5 and 3.8 and most of that acidity is related to organic acids (e.g. tartrate, lactate, malate) -somewhat more toxic than mineral acids (e.g. HCl). Grape juice has a 10-40% sugar content and contains toxic phenolic compounds as well as high levels of potassium salts. Saccharomyces cereviseae will ferment grape juice to take advantage of the useful components therein. Competition among yeast is fierce. Some produce sulfite (SO₂), the same compound used in the beverage industry to kill unwanted yeast. Fermenting grape must has little-to-no oxygen, thus, it is a metabolic barrier that prevents growth of competing aerobic organisms.