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Yeast

YEAST

The yeast Saccharomyces cerevisiae, known popularly as bakers or brewers yeast, has been used extensively in aging research. Since 1990, it has emerged as an important model organism for the dissection of the biological aging process at the genetic and molecular levels. Its distant cousin, Schizosaccharomyces pombe, or fission yeast, was shown in 2000 to undergo a very similar aging process. This entry describes the research with S. cerevisiae, hereinafter called yeast, exclusively.

Yeast is a unicellular organism whose DNA is packaged into chromosomes that are localized in a subcellular structure called the nucleus. In addition to this organelle, yeast also possesses mitochondria, which are the power plants of the cell that generate the energy needed for cellular function. The mitochondrion also possesses its own DNA, but it is dependent on the nuclear genes for most of its biochemical functions. The yeast cell is very similar in structure and function to typical cells from higher organisms, including humans. It has been used widely to elucidate a variety of basic biological processes, because of the ease of experimentation. About 25 percent of human genes have yeast counterparts, and these human genes have frequently been shown to functionally replace the corresponding gene in the yeast cell.

Aging is not typically measured by time in yeast, but rather by the number of divisions an individual cell completes before it dies. An individual cell is easy to follow from birth to death because yeast divides asymmetrically by budding off new daughters. Unlike their mothers, the daughters start from scratch, having the potential for a full life span. Thus, individual cells are mortal, while the yeast population is immortal. The probability that a cell will continue dividing decreases exponentially as a function of the number of completed divisions. Thus, mortality rate increases exponentially with age. However, it plateaus at older ages in similarity to what has been observed in other species. Yeasts undergo a variety of changes as they age, and some of these are clearly detrimental. In view of this, it is reasonable to speak of an aging process. In practical terms, yeast life span is measured by observing individual cells periodically under a microscope and removing buds with a micro-manipulator.

As of 2000, twenty genes that determine yeast life span had been identified. This has been achieved in three ways. First, genes whose activity changes during the life span were isolated, followed by an examination of their causal role in yeast aging. Second, genes were tested for their function in longevity on the basis of hypotheses formulated regarding the aging process. Third, yeast mutants were selected on the basis of a phenotype (property) frequently associated with aging. The characterization of the isolated genes has provided a rich description of the aging process at the physiological level. The powerful tools of yeast genetics and cell biology have extended this description. Further analysis of the pathways and processes that were revealed by these genes has in some cases been refined to the biochemical and molecular levels. Methods for the preparation of age-synchronized yeast cells have facilitated biochemical and molecular studies.

There are many advantages to the study of aging in the yeast model system:

  1. The yeast cell is at the same time the yeast organism. Therefore, the study of yeast is pertinent to both cellular and organismal aging.
  2. Because yeast are microbes they divide very rapidly, in a short time generating much material for physiological, biochemical, and molecular analysis.
  3. Yeast mutants can be created and selected rapidly, again because it is a microbe producing many generations of progeny in a short time.
  4. Yeast life spans are short, and last as little as a few days.
  5. Methodologies for life span determination are in place. Several procedures for the bulk preparation of age-synchronized yeast cells are available.
  6. The basic phenomenology of yeast aging is well established.
  7. The yeast genome was the first to be completely sequenced. This has revolutionized yeast genetics. The priority of yeast in this field has resulted in rapid advances in the study of function at the whole genome level, providing a wide range of materials, tools, and concepts that are being applied to other organisms as well.
  8. Several yeast genetic databases are accessible online, which facilitates functional genome analyses. In addition, cross-referencing databases are online, allowing comparative genomic analyses.
  9. A large community of yeast researchers exists, and, consequently, there is a wealth of biological information and expertise that can be tapped.

Yeast also possesses certain disadvantages for aging research: (1) The role of cell-cell interactions and systemic mechanisms, such as endocrine function, in aging lies beyond the scope of yeast aging research; (2) the extent to which the results of studies in yeast can be extrapolated to an understanding of aging in humans has not as yet been demonstrated; and (3) the determination of yeast life spans and the preparation of age-synchronized yeast cells is tedious. The quantities of old yeast that can be obtained are relatively small.

Studies of yeast longevity have revealed the operation of four, broad physiological processes in yeast aging: metabolic control, stress resistance, gene dysregulation, and genetic stability. Interestingly, these processes appear to be important in the aging of other species as well. Two distinct metabolic control mechanisms play a role in yeast aging. One of them (retrograde response ) appears to compensate for accumulating mitochondrial dysfunction. The other (caloric restriction ) may help prevent dysfunction. Repeated bouts of stress reduce yeast life span. This can be overcome by enhancing the activity of certain longevity genes. An exposure to mild heat stress, on the other hand, appears to condition the yeast such that an extension of longevity occurs. Changes in the structure of the chromatin into which the DNA is packaged result in alterations in the normal activity of genes. This process intensifies with age. It can be prevented by manipulating certain genes, with an attendant increase in life span. Nuclear DNA can undergo rearrangements. Rearrangements that are not normally favored seem to occur with higher frequency as yeasts get older, constituting one of the causes of aging.

S. Michal Jazwinski

See also Genetics; Genetics: Gene Expression; Genetics: Gene-Environment Interaction; Genetics: Longevity Assurance; Longevity: Selection.

BIBLIOGRAPHY

Imai, S.-I.; Armstrong, C. M.; Kaeberlein, M.; and Guarente, L. Transcriptional Silencing and Longevity Protein Sir2 Is an NAD-dependent Histone Deacetylase. Nature 403 (2000): 795800.

Jazwinski, S. M. Molecular Mechanisms of Yeast Longevity. Trends in Microbiology 7 (1999): 247252.

Jiang, J. C.; Jaruga, E.; Repnevskaya, M. V.; and Jazwinski, S. M. An Intervention Resembling Caloric Restriction Prolongs Life Span and Retards Aging in Yeast. The FASEB Journal 14 (2000): 21352137.

Kim, S.; Benguria, A.; LAI, C.-Y.; and Jazwinski, S. M. Modulation of Life-span by Histone Deacetylase Genes in Saccharomyces cerevisiae. Molecular Biology of the Cell 10 (1999): 31253136.

Kirchman, P. A.; Kim, S.; LAI, C.-Y.; and Jazwinski, S. M. Interorganelle Signaling Is a Determinant of Longevity in Saccharomyces cerevisiae. Genetics 152 (1999): 179190.

Mortimer, R. K., and Johnston, J. R. Life Span of Individual Yeast Cells. Nature 183 (1959): 17511752.

MÜller, I.; Zimmermann, M.; Becker, D.; and FlÖmer, M. Calendar Life Span Versus Budding Life Span of Saccharomyces cerevisiae. Mechanisms of Ageing and Development 12 (1980): 4752.

Sinclair, D. A., and Guarente, L. Molecular Mechanisms of Aging. Trends in Biochemical Sciences 23 (1998): 131134.

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Yeast

Yeast

Yeast are single-celled eukaryotic organisms related to fungi. The baker's yeast Saccahromyces cerevisiae and the distantly related Schizosaccharomyces pombe are favored model organisms for genetic research. The interest in yeast research stems from the fact that, as eukaryotic organisms, the sub-cellular organization of yeast is similar to that of cells of more complex organisms. Thus, understanding how a particular gene functions in yeast frequently correlates to how similar genes function in mammals, including humans.

Yeast Genetics

Yeast have many advantages as a genetic research tool. First, yeast are nonpathogenic (they do not cause diseases) and are therefore easy and safe to grow. Yeast can divide by simple fission (mitosis) or by budding and, like bacteria, they can be rapidly grown on solid agar plates or in liquid media . After just a few days in culture, a single yeast cell can produce millions of identical copies of itself, giving scientist a large supply of a genetically pure research tool.

Second, yeast grow as either haploids (having only one set of chromosomes) or diploids (with two chromosome sets). Thus, genetically recessive mutations can be readily identified by phenotypic (visually observable) changes in the haploid strain. In addition, complementation can be performed by simply mating two haploid strains, where one does not contain the mutation. The resulting diploid strain contains both the functional and nonfunctional version of a gene responsible for a phenotype. The addition of the functional gene complements for the defect caused by the nonfunctional gene in the haploid strain. Diploid strains can be induced to undergo meiosis, a process in which the cell divides and passes one-half of its chromosomes to each of the resulting cells. After two such divisions, reproductive structures called asci are produced that contain four haploid offspring, called ascospores. The asci can be dissected and each of the ascospores isolated. In this way, scientists can easily mate different yeast strains and obtain new haploid genotypes through sexual reproduction and meiosis.

Third, the genome of yeast is small, about 3.5 times larger than that of bacteria and 200 times smaller than that of mammals. The yeast genome is arranged in 16 linear chromosomes that range from 200 to 2,200 kilobases in length. Unlike mammals, the yeast genome is very compact, with only 12 million base pairs, very few introns , and very little spacer DNA between functional genes. As a result, in 1996 baker's yeast was the first eukaryotic organism to have its entire genome sequenced.

Genetic Transformation

Finally, one of the most useful properties of yeast for genetic studies is the ease with which DNA can be introduced into them, in a process called transformation. The introduced DNA can be maintained on self-replicating, circular strands of DNA called plasmids, or it can integrate into the yeast genome. Most importantly, integration usually occurs by a process called homologous recombination, whereby the introduced DNA replaces chromosomal DNA that contains the same sequence. This process permits scientists to readily mutate any yeast gene and replace the native gene in the cells with the mutated version. Since yeast can be grown as haploids, the phenotypic changes caused by the introduced gene can be readily identified. In addition, the function of a cloned piece of DNA (e.g., a gene) can be identified by transforming yeast in which the DNA is carried on a circular plasmid. The introduced gene may either functionally replace a defective gene or cause a phenotypic defect in the cells indicating a function for that gene.

The ability to complement yeast defects with cloned pieces of DNA has been extended to mammalian genes. Recognizing that some genes have similar sequences and functions in both mammals and yeast, scientists sometimes use yeast as a tool to identify the functions of mammalian genes. Not many mammalian genes can directly substitute for a yeast gene, however. More frequently, scientists study the yeast gene itself to understand how its protein functions in the cell. The knowledge gained can often lead to an understanding of how similar genes might function in mammals. Now that the yeast genome has been completely sequenced and the results have been deposited in a public databank for all to use, rapid progress is being made in identifying all yeast genes and their functions.

An important method for studying mammalian genes in yeast is called the two-hybrid system. This system is used to determine if two proteins functionally interact with each other. Both genes are cloned into yeast plasmids and transformed into the cells. A special detection system is used that is active only when both cloned proteins physically contact each other in the cell. When that happens, scientist can identify which proteins need to interact with each other in order to function.

Yeast are also being used in the laboratory and commercial production of important nonyeast proteins. Foreign genes are transformed into yeast and, after transcription and translation, the foreign proteins can be isolated. Because of the ease of growing large quantities of cells, yeast can produce a large amount of the protein. While similar protein production can be performed by bacteria, eukaryotic proteins often do not function when made in bacteria. This is because most eukaryotic proteins are normally altered after translation by the addition of short sugar chains, and these modifications are often required for proper function, but bacteria do not carry out these necessary post-translational modifications. Yeast, however, does permit these modifications, and is thus more likely to produce a functional protein.

see also Cell, Eukaryotic; Cell Cycle; Genome; Human Genome Project; Model Organisms; Plasmid; Post-translational Control; Transformation; Transgenic Animals.

Suzanne Bradshaw

Bibliography

Sherman, Fred. "Getting Started with Yeast." In Methods in Enzymology, vol. 194, Christine Guthrie and Gerald R. Fink, eds. New York: Academic Press, 1991.

Watson, James D., Michael Gilman, Jan Witowski, and Mark Zoller. Recombinant DNA. New York: Scientific American Books, 1992.

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Yeast

Yeast

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.

See also Cell cycle (eukaryotic), genetic regulation of; Chromosomes, eukaryotic; Economic uses and benefits of microorganisms; Yeast artificial chromosome; Yeast, infectious

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Yeast Genetics

Yeast genetics

Yeast genetics provides an excellent model for the study of the genetics of growth in animal and plant cells. The yeast Saccharomyces cerevisiae is similar to animal cells (e.g., similar length to the phases of its cell cycle , similarity of the chromosomal structures called telomeres). Another yeast, Saccharomyces pombe is rather more similar to plant cells (e.g., similarities in their patterns of division, and in organization of their genome).

As well as being a good model system to study the mechanics of eukaryotic cells, yeast is well suited for genetic studies. Yeasts are easy to work with in the laboratory. They have a rapid growth cycle (1.5 to two hours), so that many cycles can be studied in a day. Yeasts that are not a health threat are available, so the researcher is usually not in danger when handling the organisms. Yeasts exist that can be maintained with two copies of their genetic material (diploid state) or one copy (haploid state). Haploid strains can be mated together to produce a diploid that has genetic traits of both "parents." Finally, it is easy to introduce new DNA sequences into the yeast.

Genetic studies of the yeast cell cycle, the cycle of growth and reproduction, are particularly valuable. For example, the origin of a variety of cancers is a malfunction in some aspect of the cell cycle. Various strains of Saccharomyces cerevisiae and Saccharomyces pombe provide useful models of study because they are also defective in some part of their cell division cycle. In particular, cell division cycle (cdc) mutants are detected when the point in the cell cycle is reached where the particular protein coded for by the defective gene is active. These points where the function of the protein is critical have been dubbed the "execution points." Mutations that affect the cell division cycle tend to be clustered at two points in the cycle. One point is at the end of a phase known as G1. At the end of G1 a yeast cell becomes committed to the manufacture of DNA in the next phase of the cell cycle (S phase). The second cluster of mutations occurs at the beginning of a phase called the M phase, where the yeast cell commits to the separation of the chromosomal material in the process of mitosis.

Lee Hartwell of the University of Washington at Seattle spearheaded the analysis of the various cdc mutants in the 1960s and 1970s. His detailed examination of the blockage of the cell cycle at certain pointsand the consequences of the blocks on later eventsdemonstrated, for example, that the manufacture of DNA was an absolute prerequisite for division of the nuclear material. In contrast the formation of the bud structures by Saccharomyces pombe can occur even when DNA replication is blocked.

Hartwell also demonstrated that the cell cycle depends on the completion of a step that was termed "start." This step is now known to be a central control point, where the cell essentially senses materials available to determine whether the growth rate of the cell will be sufficient to accumulate enough material to permit cell division to occur. Depending on the information, a yeast cell either commits to another cycle of cell growth and division or does not. These events have been confirmed by the analysis of a yeast cell mutant called cdc28. The cdc28 mutant is blocked at start and so does not enter S phase where the synthesis of DNA occurs.

Analysis of this and other cdc mutations has found a myriad of functions associated with the genetic mutations. For example, in yeast cells defective in a gene dubbed cdc2, the protein coded for by the cdc2 gene does not modify various proteins. The absence of these modifications causes defects in the aggregation of the chromosomal material prior to mitosis, the change in the supporting structures of the cell that are necessary for cell division, and the ability of the cell to change shape.

Studies of such cdc mutants has shown that virtually all eukaryotic cells contain a similar control mechanism that governs the ability of a cell to initiate mitosis. This central control point is affected by the activities of other proteins in the cell. A great deal of research effort is devoted to understanding this master control, because scientists presume that knowledge of its operation could help thwart the development of cancers related to a defect in the master control.

See also Cell cycle (eukaryotic), genetic regulation of; Genetic regulation of eukaryotic cells; Molecular biology and molecular genetics

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Yeast

Yeast

Yeast are microscopic, single-celled organisms that are classified in the family Fungi. Individual yeast cells multiply rapidly by the process of budding, in which a new cell begins as a small bulge along the cell wall of a parent cell. In the presence of an abundant food source, huge populations of yeast cells gather. The cells often appear as long chains with newly formed cells still attached to their parent cells, due to the short budding time of two hours.

Yeast are among the few living organisms that do not need oxygen in order to produce energy. This oxygen-independent state is called anaerobic (pronounced a-na-ROE-bik; "without oxygen"). During such anaerobic conditions, yeast convert carbohydratesstarches and sugarsto alcohol and carbon dioxide gas. This process is known as fermentation.

The fermentation process of yeast is caused by enzymes, catalysts in chemical reactions similar to the digestive enzymes in the human body. In fact, the word enzyme means "in yeast." Certain enzymes in yeast act on starch to break down the long chainlike molecules into smaller units of sugar. Then other yeast enzymes convert one kind of sugar molecule to another. Still other enzyme reactions break apart the sugar molecule (composed of carbon, hydrogen, and oxygen atoms) into ethyl alcohol and carbon dioxide. The series of reactions provides the yeast cells with the energy necessary for their growth and division (form of reproduction).

In nature, yeast enzymes break down the complex carbon compounds of plant cell walls and animal tissues, feeding on the sugar produced in the process. In this way, yeast function as natural decomposers in the environment.

Words to Know

Anaerobic: Living or growing in an atmosphere lacking oxygen.

Budding: Process by which a small outgrowth on a simple organism grows into a complete new organism of the same species.

Enzyme: An organic compound that speeds up the rate of chemical reactions in living organisms.

Fermentation: Chemical reaction in which enzymes break down complex organic compounds into simpler ones.

The importance of yeast for humans

The by-products of fermentationcarbon dioxide and alcoholhave been used by humans for centuries in the production of breads and alcoholic beverages. Before the mid-nineteenth century, however, bakers and brewers knew very little about the nature of the organisms that helped

make their products. The experiments of French microbiologist Louis Pasteur (18221895) showed that fermentation could only take place in the presence of living yeast cells. He also deduced that anaerobic conditions were necessary for proper fermentation of wine and beer (in the presence of oxygen, yeast convert alcohol to acetic acid [vinegar]).

Brewer's yeast is added to liquids derived from grains and fruits to brew beer and wine. The natural starches and sugars in the liquids provide food for the yeast. Deprived of oxygen during the fermentation process, yeast produce alcohol as a by-product of incomplete sugar breakdown. Yeast that occur naturally on the skins of grapes also play a vital role in fermentation, converting the sugars of grapes into alcohol for wine production.

Baker's yeast, another variety of yeast, are added to a dough made from the starchy portion of ground grains (such as wheat or rye flour). The yeast break down some of the starch and sugar present in the mixture, producing carbon dioxide. The carbon dioxide bubbles through the dough, forming many air holes and causing the bread to rise. Since oxygen is present, no alcohol is produced when the bread is rising. When the bread is baked, the air holes give the bread a lighter texture.

In recent times, yeast have been used to aid in the production of alternative energy sources that do not produce toxic chemicals as byproducts. Yeast are placed in huge vats of corn or other organic material. When fermentation takes place, the yeast convert the organic material into ethanol fuel. Present-day geneticists are working on developing yeast strains that will convert even larger organic biomasses (living material) into ethanol more efficiently.

[See also Brewing; Fermentation; Fungi ]

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yeast

yeast, name applied specifically to a certain group of microscopic fungi and to commercial products consisting of masses of dried yeast cells or of yeast mixed with a starchy material and pressed into yeast cakes. Although a number of fungi are sometimes called yeasts, the true yeasts are unicellular, consist of oval or round cells, and reproduce chiefly by budding. Under certain conditions some yeast cells secrete a thickened wall, and the cytoplasm of the single cell within divides to form four or eight cells, or spores, known as ascospores, which emerge when the wall ruptures. In a few species two cells fuse before undergoing spore formation. There are about 500 species in all.

Yeasts, especially those of the genus Saccharomyces, have long been of commercial importance because they are the chief agents in alcoholic fermentation. Because of this they are essential to the making of beer, wine, and other alcoholic beverages and industrial alcohol. Wild yeasts, those found in nature and probably carried by insects from the soil to fruits, are frequently active in the fermentation process. In breadmaking the yeasts act upon the carbohydrates in the dough, forming carbon dioxide and ethyl alcohol, which are driven off in the baking process; the escaping carbon dioxide causes the bread to rise. Since early times yeast has been used in treating various ailments; brewer's yeast has a high content of thiamine and other vitamins of the B-complex group. Yeasts are classified in the kingdom Fungi, phyla (divisions) Ascomycota and Basidiomycota.

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yeast

yeast / yēst/ • n. a microscopic fungus (genus Saccharomyces, subdivision Ascomycotina) consisting of single oval cells that reproduce by budding, and are capable of converting sugar into alcohol and carbon dioxide. ∎  a grayish-yellow preparation of this obtained chiefly from fermented beer, used as a fermenting agent, to raise bread dough, and as a food supplement. ∎ Biol. any unicellular fungus that reproduces vegetatively by budding or fission, including forms such as candida that can cause disease. DERIVATIVES: yeast·like / -ˌlīk/ adj. ORIGIN: Old English, of Germanic origin; related to Dutch gist and German Gischt ‘froth, yeast,’ from an Indo-European root shared by Greek zein ‘to boil.’

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yeast

yeast Unicellular organisms, grouped with the fungi; they have more complex subcellular organization than bacteria. Some types are of major importance in the food industry. Saccharomyces cerevisiae and S. carlsbergensis are used in brewing, wine making, and baking. Varieties such as Candida utilis (formerly Torula utilis) are grown on carbohydrate or hydrocarbon media as animal feed and potential human food, since they contain about 50% protein (dry weight) and are very rich in B vitamins.

Some yeasts are pathogenic (especially Candida spp., which cause thrush); many are used in biotechnology for production of hormones and other proteins.

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yeast extract

yeast extract A preparation of the water‐soluble fraction of autolysed brewers' yeast, valuable both as a source of the B vitamins and for its strong savoury flavour. Commercial preparations include Marmite, Yeastrel, Yeatex, and Vegemite, used as a drink or a bread spread. A 9‐g portion (1 teaspoonful, or the amount spread on two slices of bread) is a rich source of vitamin B2, niacin, and folate; a good source of vitamin B1; supplies 15 kcal (60 kJ).

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"yeast extract." A Dictionary of Food and Nutrition. . Retrieved June 25, 2017 from Encyclopedia.com: http://www.encyclopedia.com/education/dictionaries-thesauruses-pictures-and-press-releases/yeast-extract

yeast

yeast (yeest) n. any of a group of fungi in which the body consists of individual cells, which may occur singly, in groups of two or three, or in chains. Baker's yeast (Saccharomyces) ferments carbohydrates to produce alcohol and carbon dioxide and is important in brewing and breadmaking. Some yeasts are a commercial source of proteins and of vitamins of the B complex; others (e.g. Candida, Cryptococcus, Pityrosporum) cause disease.

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yeast

yeast frothy substance produced by fermentation of malt, etc. OE. (Angl.) *ġest, WS. *giest (late ġist), corr. to MLG. gest dregs, dirt, MDu. ghist, ghest (Du. gist, gest yeast), MHG. jist, jest, gist, gest (G. gischt) yeast, froth, ON. jǫstr; IE. *jes- is repr. also by Skr. yásyati, Gr. zeîn boil, W. iās seething.

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"yeast." The Concise Oxford Dictionary of English Etymology. . Encyclopedia.com. 25 Jun. 2017 <http://www.encyclopedia.com>.

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yeast

yeast Any of a group of single-celled microscopic fungi found in all parts of the world in the soil and in organic matter. Yeasts reproduce asexually by budding or fission. Yeasts are also produced commercially for use in baking, brewing and wine-making. They occur naturally as a bloom (white covering) on grapes and other fruit.See also fungus

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"yeast." World Encyclopedia. . Encyclopedia.com. 25 Jun. 2017 <http://www.encyclopedia.com>.

"yeast." World Encyclopedia. . Encyclopedia.com. (June 25, 2017). http://www.encyclopedia.com/environment/encyclopedias-almanacs-transcripts-and-maps/yeast

"yeast." World Encyclopedia. . Retrieved June 25, 2017 from Encyclopedia.com: http://www.encyclopedia.com/environment/encyclopedias-almanacs-transcripts-and-maps/yeast

yeast

yeast A general term for a fungus that can exist in the form of single cells, reproducing by fission (see binary fission) or by budding. Sometimes the name refers more specifically to Saccharomyces cerevisiae.

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"yeast." A Dictionary of Plant Sciences. . Encyclopedia.com. 25 Jun. 2017 <http://www.encyclopedia.com>.

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yeast

yeast in figurative usage, leaven.

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"yeast." The Oxford Dictionary of Phrase and Fable. . Encyclopedia.com. 25 Jun. 2017 <http://www.encyclopedia.com>.

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"yeast." The Oxford Dictionary of Phrase and Fable. . Retrieved June 25, 2017 from Encyclopedia.com: http://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/yeast

yeast

yeastarriviste, artiste, batiste, beast, dirigiste, east, feast, least, Mideast, modiste, northeast, piste, priest, southeast, uncreased, unreleased, yeast •wildebeest • hartebeest • beanfeast •anapaest (US anapest)

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"yeast." Oxford Dictionary of Rhymes. . Encyclopedia.com. 25 Jun. 2017 <http://www.encyclopedia.com>.

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