Nov 15, 2009
Yeasts are single-celled fungi. Yeast species inhabit diverse habitats, including skin, marine water, leaves, and flowers.
Some yeast are beneficial, being used to produce bread or allow the fermentation of sugars to ethanol that occurs during beer and wine production (e.g., Saccharomyces cerevisiae). Other species of yeasts are detrimental to human health. An example is Candida albicans, the cause of vaginal infections, diaper rash in infants, and thrush in the mouth and throat. The latter infection is fairly common in those whose immune system is compromised by another infection such as acquired immunodeficiency syndrome.
The economic benefits of yeast have been known for centuries. Saccharomyces carlsbergensis, the yeast used in the production of various types of beer that result from "bottom fermentation," was isolated in 1888 by Dr. Christian Hansen at the Carlsberg Brewery in Copenhagen. During fermentation, some species of yeast are active at the top of the brew while others sink to the bottom. In contrast to Saccharomyces carlsbergensis, Saccharomyces cerevisiae produces ales by "top fermentation." In many cases, the genetic manipulation of yeast has eliminated the need for the different yeast strains to produce beer or ale. In baking, the fermentation of sugars by the bread yeast Ascomycetes produces bubbles in the dough that makes the bread dough rise.
Yeasts are a source of B vitamins. This can be advantageous in diets that are low in meat. In the era of molecular biology, yeasts have proved to be extremely useful research tools. In particular, Saccharomyces cerevisiae has been a model system for studies of genetic regulation of cell division, metabolism, and the incorporation of genetic material between organisms. This is because the underlying molecular mechanisms are preserved in more complicated eukaryotes, including humans, and because the yeast cells are so easy to grow and manipulate. As well, Ascomycetes are popular for genetics research because the genetic information contained in the spores they produce result from meiosis. Thus, the four spores that are produced can contain different combinations of genetic material. This makes the study of genetic inheritance easy to do.
Another feature of yeast that makes them attractive as models of study is the ease by which their genetic state can be manipulated. At different times in the cell cycle yeast cells will contain one copy of the genetic material, while at other times two copies will be present. Conditions can be selected that maintain either the single or double-copy state. Furthermore, a myriad of yeast mutants have been isolated or created that are defective in various aspects of the cell division cycle. These mutants have allowed the division cycle to be deduced in great detail.
The division process in yeast occurs in several different ways, depending upon the species. Some yeast cells multiply by the formation of a small bud that grows to be the size of the parent cell. This process is referred to as budding. Saccharomyces reproduces by budding. The budding process is a sexual process, meaning that the genetic material of two yeast cells is combined in the offspring. The division process involves the formation of spores.
Other yeasts divide by duplicating all the cellular components and then splitting into two new daughter cells. This process, called binary fission, is akin to the division process in bacteria. The yeast genus Schizosaccharomyces replicates in this manner. This strain of yeast is used as a teaching tool because the division process is so easy to observe using an inexpensive light microscope.
The growth behavior of yeast is also similar to bacteria. Yeast cells display a lag phase prior to an explosive period of division. As some nutrient becomes depleted, the increase in cell number slows and then stops. If refrigerated in this stationary phase, cells can remain alive for months. Also like bacteria, yeast are capable of growth in the presence and the absence of oxygen.
The life cycle of yeast includes a step called meiosis. In meiosis pairs of chromosomes separate and the new combinations that form can give rise to new genetic traits in the daughter yeast cells. Meiosis is also a sexual feature of genetic replication that is common to all higher eukaryotes as well.
Another feature of the sexual reproduction process in yeast is the production of pheromones by the cells. Yeast cells respond to the presence of the chemicals by changing their shape. The peanut-like shape they adopt has been dubbed "shmoos," after a character in the "Li'l Abner" comic strip. This shape allows two cells to associate very closely together.
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