Author: admin, 28.08.2014Most of us know yeast is a very helpful organism, especially with respect to baking, wine making, and brewing. Yeast are single-celled microorganisms that are classified, along with molds and mushrooms, as members of the Kingdom Fungi.
Yeast are single-celled organisms classified as eukaryotes due the presence of a nucleus that harbors their genetic information. Although yeast are single-celled organisms, they possess a cellular organization similar to that of higher organisms, including humans. Another important characteristic of yeast essential to their role as “model organisms” is the fact they are relatively easy to work with.
When yeast cells are grown in rich carbon sources such as glucose they prefer to grow by fermentation. In 1815, Joseph-Louis Gay-Lussac, a French chemist, developed methods to maintain grape juice in an unfermented state and discovered that the introduction of ‘ferment’ (which contains yeast) was required to convert unfermented wort, demonstrating the importance of yeast for alcoholic fermentation. Since that time many other researchers have carried out groundbreaking research using budding yeast. Yeast has long been considered to be the organism of choice for the production of alcoholic beverages, bread, and a large variety of industrial products.
Saccharomyces cerevisiae or baker’s yeast has long been used as a leavening agent in baking.
In addition to these traditional uses yeast has also been used for many other commercial applications.
By virtue of the high degree of similarity between yeast genes and their human counterparts, and conserved fundamental cellular biology, yeast has become a popular model system for the study of human disease genes.
Yeast is becoming the organism of choice in studies aimed at the identification of drug targets and the mode of action of various drugs. These are just a few examples of how yeast can be used both aid the study of human disease. Saccharomyces Genome Database (SGD) provides comprehensive integrated biological information for the budding yeast Saccharomyces cerevisiae along with search and analysis tools to explore these data. MIPS Comprehensive Yeast Genome Database (CYPD) presents information on the molecular structure and functional network of the entirely sequenced, well-studied model eukaryote, the budding yeast Saccharomyces cerevisiae. PomBase, a comprehensive database for the fission yeast Schizosaccharomyces pombe, providing structural and functional annotation, literature curation and access to large-scale data sets.
An extensive list of yeast-related resources on topics ranging from general yeast information to nucleic acids, genomes and proteins, expression data, localization, phenotypes and more. However, he thought that these globules were starchy particles of the grain used to make wort, the liquid extract used in brewing, rather than fermenting yeast cells. In 1835, Charles Cagniard de la Tour used a more powerful microscope to show that yeast were single celled and multiplied by budding. He discovered that yeast alternate between haploid and diploid states and that yeast are heterothallic, as two strains are required to convert haploids to diploids (conjugation). This is based on the ease with which the metabolism of yeast can be manipulated using genetic techniques, the speed with which it can be grown to high cell yields (biomass), the ease with which this biomass can be separated from products and the knowledge that it is generally recognized as safe (GRAS).
Although wine fermentation can be initiated by naturally occurring yeast present in the vineyards, many wineries choose to add a pure yeast culture to dominate and control the fermentation. Vegans often use yeast as a cheese substitute and it is often used as a topping for products such as popcorn. Several approaches have been used to learn more about human genes once a connection between a human and yeast gene is made. Studies in yeast can help researchers learn more about the underlying biology using this model system, or to help them identify drug targets or the drugs mode of action. Keeney (2014) Budding Yeast for Budding Geneticists: A Primer on the Saccharomyces cerevisiae Model System. In the 1850s Louis Pasteur discovered that fermented beverages resulted from the conversion of glucose to ethanol by yeast and defined fermentation as "respiration without air". He and his colleague Otto Laustsen devised techniques to micromanipulate yeast so they could be investigated genetically. There are two major types of brewing yeast, top-fermenting ale yeast and bottom-fermenting lager yeast. Studying misfolded yeast proteins with similar amyloid forming potential, called prions, has provided researchers with insight into these neurodegenerative diseases. Yeast-like cells life in soil and are saprotrophic, but switch to filamentous growth upon initiation of the pathogenic cycle.
Even in the presence of oxygen yeast cells prefer to grow fermentatively and this is referred to as the Crabtree Effect after the biologist who discovered this preference. Archaeological digs have uncovered evidence in the form of jars containing the remains of wine that is 7,000 years old.
Near the end of the 1800s Eduard Buchner used cell-free extracts obtained by grinding yeast cells to detect zymase, the collection of enzymes that promote or catalyze fermentation and for this he was awarded the Nobel Prize in 1907. Sourdough bread is an exception, as it is not produced using baker's yeast, but is instead made with a combination of wild yeast and bacteria. Sporulation and subsequent meiosis requires infection of the maize plant (image taken from Steinberg and Perez-Martin, 2008, Trends Cell Biol. The yeast Candida milleri is used to strengthen the gluten, and an acid-generating bacteria “Lactobacillus sanfranciscensis”, is used to ferment the maltose. Yeast is used in the food industry for the production of food additives including colorants, antioxidants, and flavor enhancers. The series of events that occur in a cell and lead to duplication and division are referred to as the cell cycle.
This allows researchers to determine whether or not the human gene is able to rescue viability, growth, or more specific defects associated with loss of the yeast gene, a method referred to as functional complementation. The cell cycle consists of four distinct phases (G1, S, G2 and M) and is regulated similar to that of the cell cycle in larger eukaryotes.
As long as adequate nutrients such as sugar, nitrogen and phosphate are present yeast cells will continue to divide asexually. The sulfur dioxide present in commercially produced wine is added just after the grapes are crushed to kill the naturally present bacteria, mold, and yeast. Upon initiation of the infection, two yeast-like cells recognize each other and fuse in order to form another type of hypha that is able to invade maize tissue.
Comparing the expression profile of yeast cells deleted for a gene to those of wild type yeast cells treated with a particular drug can also be an effective way to identify genes which may tell the researchers something about how the drug works in cells.
If a yeast gene is known to be similar in DNA sequence to a human gene, studies in yeast can provide powerful clues as to the role of the related gene(s) in humans.
These different states, budding, conjugation and sporulation together make up the yeast life cycle. However, depending on climatic conditions infections can lead to significant grain-yield loss.
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