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Abstract Yeasts are a group of lower eukaryotic microorganisms showing biological and biochemical diversity. In common usage, the term ”yeasts” is used to describe strains of Saccharomyces cerevisiae that have a great commercial value in baking, brewing and effectively able to produce ethanol from molasses. Alcoholic fermentation is considered as the most important example of microbial production of ethanol. Ethanol is widely used in both medical and pharmaceutical preparations. Furthermore, in addition to its use as a fuel, ethanol has considerable value in chemical industries resulting in the production of ”Alcochemicals” as opposed today’s ”petrochemicals” (Stewart 1981). Ethanol is an ideal fuel and also has the basis for countless chemicals transformations. However, optimal conversion of carbohydrates to ethanol requires yeast cells that are tolerant to high concentrations of the substrate (carbohydrates); end product (ethanol) and at the same time able to produce efficiently ethanol at relatively high temperature. On the other hand, industrial yeast strains have been reported to be polyploid or aneuploid (Spencer et aL, 1983), suggesting that an mcrease m chromosome number may be considered as an advantageous in processes involving the use of such strains. |