Eukaryotes Genomes - SACCHAROMYCES CEREVISIAE

Saccharomyces cerevisiae is commonly known as "bakers yeast", "brewers yeast"or "budding yeast".

Yeast are a group of unicellular fungi a few species of which are commonly used to leaven bread and ferment alcoholic beverages. More than one-thousand species of yeasts have been described. The most commonly used yeast is Saccharomyces cerevisiae, which was domesticated for wine, bread and beer production thousands of years ago. Most yeasts belong to the division Ascomycota.

Yeast physiology can be either obligately aerobic or facultatively fermentative. There is no known obligately anaerobic yeast. In the absence of oxygen, fermentative yeasts produce their energy by converting sugars into carbon dioxide and ethanol (alcohol). In brewing, the ethanol is used, while in baking the carbon dioxide raises the bread and the ethanol evaporates.

The fermentation of beer and wine was originally caused by naturally occurring yeasts present in the environment. Some wineries still use natural yeast strains, however most modern brewers use highly cultured isolates, e.g. Saccharomyces carlsbergensis, named after the Carlsberg Brewery in Copenhagen. The bubbles in sparkling wines such as Champagne are trapped CO 2 , the result of yeast fermenting sugars in the grape juice. One yeast cell can ferment approximately its own weight of glucose per hour, giving rise to large volumes of CO 2.

Yeasts can reproduce asexually through budding or sexually through the formation of ascospores . During asexual reproduction a new bud grows out of the parent yeast when the condition is right, then after the bud reaches an adult size, it separates from the parent yeast. Under low nutrient conditions, yeasts that are capapable of sexual reproduction will form ascospores.

Saccharomyces cerevisiae are single-celled fungi which multiply by budding, or in some cases by division (fission). Yeast fermentations comprise the oldest and largest application of microbial technology. Beer brewers classify yeasts as top-fermenting and bottom-fermenting. This distinction was introduced by the Dane Emil Christian Hansen. Saccharomyces cerevisiae is a top-fermenting yeast (so-called because they float to the top of the beer) it produces higher alcohol concentrations and prefers higher temperatures, top-fermenting yeasts produce fruitier, sweeter, real ale type beers.

In pharmacy and chemistry, a large number of substances (vitamins and enzymes) are extracted from this organism. In addition, thanks to genetic engineering, some medications are now produced by manipulated yeasts, for example vaccines against hepatitis B surface antigen are produced by Saccharomyces cerevisiae utilising recombinant technology.

Aside from its industrial value, Saccharomyces cervisiae is a very useful model for scientists: it is a eukaryotic organism (its genetic material is contained in a nucleus), and is therefore similar to cells from evolved organisms. As easy to manipulate as a bacterium, yeast is a simplified model of the cells that make up human beings. It has been widely studied because the cell cycle in a simple yeast is very similar to the cell cycle in humans, and regulated by homologous proteins. Its prominent useful features are the cheap and easy cultivation, short generation times, the detailed genetic and biochemical knowledge accumulated in many years of research and the ease of the application of molecular techniques for its genetic manipulation. Therefore, this fungus provides a highly suitable system to study basic biological processes that are relevant for many other higher eukaryotes including man.

Saccharomyces cerevisiae was the first eukaryote to have its genome sequenced (published in 1996 ). The genome is composed of about 13 million bp and contains 6,275 genes . It is estimated that yeast shares about 23% of its genome with humans.

References:

http://www.mips.biochem.mpg.de/proj/yeast/info/sequence.html
http://www-micro.msb.le.ac.uk/video/Scerevisiae.html
http://www.yeastgenome.org/
http://en.wikipedia.org/wiki/Yeast
http://www.pasteur.fr/