الفهرس 
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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.