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:
- The yeast cell is at the same time the yeast organism. Therefore, the study of yeast is pertinent to both cellular and organismal aging.
- Because yeast are microbes they divide very rapidly, in a short time generating much material for physiological, biochemical, and molecular analysis.
- Yeast mutants can be created and selected rapidly, again because it is a microbe producing many generations of progeny in a short time.
- Yeast life spans are short, and last as little as a few days.
- Methodologies for life span determination are in place. Several procedures for the bulk preparation of age-synchronized yeast cells are available.
- The basic phenomenology of yeast aging is well established.
- 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.
- Several yeast genetic databases are accessible online, which facilitates functional genome analyses. In addition, cross-referencing databases are online, allowing comparative genomic analyses.
- 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): 795–800.
Jazwinski, S. M. ‘‘Molecular Mechanisms of Yeast Longevity.’’ Trends in Microbiology 7 (1999): 247–252.
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): 2135–2137.
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): 3125–3136.
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): 179–190.
Mortimer, R. K., and Johnston, J. R. ‘‘Life Span of Individual Yeast Cells.’’ Nature 183 (1959): 1751–1752.
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): 47–52.
Sinclair, D. A., and Guarente, L. ‘‘Molecular Mechanisms of Aging.’’ Trends in Biochemical Sciences 23 (1998): 131–134.
Yeast
Yeast
The importance of yeast for humans
Yeasts are single-celled fungi, belonging mainly to the Ascomycetes, that serve as nutrient recyclers in nature, but are also important in industry, biotechnology, and as the agents of disease in humans. The term yeast is generically used in reference to many species of single-celled, budding fungi, including Saccharomyces—used in baking and brewing—and Candida—an infectious yeast common in people with compromised immune systems, such as AIDS victims.
Life cycle
Yeasts secrete enzymes that break down carbohydrates (through fermentation) to yield carbon dioxide and alcohol. The source of carbohydrates are either living hosts or non-living hosts such as rotting vegetation, or the moist body cavities of animals. Yeasts are considered by some scientists to be closely related to the algae, lacking only in photosynthetic capability— perhaps as a result of an evolutionary trend toward a lifestyle dependent upon host nutrition. Ecologically yeasts are decomposers that secrete enzymes which dismantle the complex carbon compounds of plantcell walls and animal tissues, which they convert to sugars for their own growth and sustenance. Yeast reproduction may involve sexual spore production or asexual budding, dependent upon surrounding conditions. Though yeasts are highly tolerant of environmental variations in temperature and acidity, they thrive in warm and moist places high in oxygen and low in carbon dioxide. Whether or not they reproduce through asexual budding depends on the favorability of surrounding conditions: when times are good, yeast clones are produced by budding. In times of environmental stress, yeasts produce spores which are capable of withstanding periods of environmental hardship— perhaps even to lie dormant, until conditions improve and the mingling of genes can take place with the spore of another yeast. This rare version of yeast reproduction provides for genetic variation when conditions demand it, though budding is the predominant mode of yeast reproduction.
The importance of yeast for humans
People have been using yeast in bread baking for centuries, but suffering from its scourges for much longer. The biochemical by-products of yeast sugar metabolism—carbon dioxide and alcohol—are essential in baking and brewing. Bakers yeast (Saccharomyces cerevisiae), when added to baker’s dough, produces carbon dioxide pockets that make bread rise. Brewer’s yeast, another strain of Saccharomyces, takes advantage of the yeasts characteristic of switching to anaerobic fermentation when deprived of oxygen to produce alcohol as a by-product of incomplete sugar breakdown. Yeasts 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.
Yeasts also comprise some of the natural microbial flora residing in and upon animals, including humans. The yeast species Candida albicans is perhaps the most notorious of the yeast inhabitants of the human body, responsible for the affliction Candidiasis, which may take many forms. Through normal health and hygiene, Candida is held in check by the populous and benign bacterial residents of our skin and mucous membranes. But in instances of compromised health, Candida albi-cans can result in skin sores (such as Thrush), urogenital tract infections (such as vaginitis), and internally, endocarditis (heart muscle infection), inflammation of the spleen, liver, kidneys and lungs. Victims of AIDS are particularly susceptible to Candidiasis.
Biotechnology and yeast
Yeasts are rising stars in the toolbox of biotechnologists. Early in the development of biotechnology, which used cells as recipients of transplanted genes, bacteria were the organism of choice. However, limitations involving differences between bacterial cells and our own have relegated bacteria to a second place behind yeasts. Both yeast cells and human cells are eukaryotic—possessing a nucleus and membrane-bound organelles such as ribosomes and mitochondria. As biotechnology techniques have progressed
KEY TERMS
Candidiasis —Infection of the yeast Candida albi-cans which may take many forms depending upon the site of infection from inflammation of the heart (endocarditis) to throat and lung infections.
Eukaryote —A cell whose genetic material is carried on chromosomes inside a nucleus encased in a membrane. Eukaryotic cells also have organelles that perform specific metabolic tasks and are supported by a cytoskeleton which runs through the cytoplasm, giving the cell form and shape.
Human genome —The entire human gene library, consisting of an estimated 100,000 genes encoded in the tightly woven DNA of 23 chromosomes.
Microbial flora —Benign, naturally occurring bacteria and fungi found on, and in, the human body, including the intestinal tract, the mucous membranes, and the skin.
over the last decade, yeasts have come to the fore as host cells for human gene implantation and as potential surrogate cells for housing human chromosomes. Yeasts have also been subjected to the alteration of their own genes, as biotechnologists attempt to develop strains of yeast more efficient in the metabolism of sugars for food and industrial applications.
By studying yeast genetics, scientists hope to gain insight into how the genes of all eukaryotic cells, including our own, function. In 1993 the first yeast chromosome was completely mapped, and the function of each of the 182 genes has yielded insight into such vital genetic processes as mutation repair, enzyme production, and cellular division regulation. These findings may yield insights into such fundamental human health issues as cancer and the aging process. Yeasts have also been transformed through genetic engineering to produce the first genetically engineered vaccine for hepatitis B. Insulin and hemoglobin are also being commercially manufactured by yeast cells which have been re-coded by inserted human genes. Yeasts are a hopeful host cell for housing the entire human genome in an international effort to decipher the gene library of human cells.
Resources
BOOKS
de Winde, Johannes H. Functional Genetics of Industrial Yeasts. The Netherlands: Springer, 2003.
Krasner, R.I. The Microbial Challenge: Human-Microbe Interactions. Washington: American Society for Microbiology, 2002.
PERIODICALS
Chan, K.C., Csikasz-Nagy, A., et al. “Kinetic Analysis of a Molecular Model of the Budding Yeast Cell Cycle.” Molecular Biology of the Cell. Vol.11 (2000): 369-391.
Jeffrey Weld
Yeast
Yeast
Yeasts are single-celled fungi , belonging mainly to the Ascomycetes, that serve as nutrient recyclers in nature, but are also important in industry, biotechnology , and as the agents of disease in humans. The term yeast is generically used in reference to many species of single-celled, budding fungi, including Saccharomyces—used in baking and brewing —and Candida—an infectious yeast common in people with compromised immune systems, such as AIDS victims.
Life cycle
Yeasts secrete enzymes that break down carbohydrates (through fermentation ) to yield carbon dioxide and alcohol . The source of carbohydrates are either living hosts or non-living hosts such as rotting vegetation, or the moist body cavities of animals. Yeasts are considered by some scientists to be closely related to the algae , lacking only in photosynthetic capability—perhaps as a result of an evolutionary trend toward a lifestyle dependent upon host nutrition . Ecologically yeasts are decomposers that secrete enzymes which dismantle the complex carbon compounds of plant cell walls and animal tissues, which they convert to sugars for their own growth and sustenance. Yeast reproduction may involve sexual spore production or asexual budding, dependent upon surrounding conditions. Though yeasts are highly tolerant of environmental variations in temperature and acidity, they thrive in warm and moist places high in oxygen and low in carbon dioxide. Whether or not they reproduce through asexual budding depends on the favorability of surrounding conditions: when times are good, yeast clones are produced by budding. In times of environmental stress, yeasts produce spores which are capable of withstanding periods of environmental hardship—perhaps even to lie dormant, until conditions improve and the mingling of genes can take place with the spore of another yeast. This rare version of yeast reproduction provides for genetic
variation when conditions demand it, though budding is the predominant mode of yeast reproduction.
The importance of yeast for humans
People have been using yeast in bread baking for centuries, but suffering from its scourges for much longer. The biochemical by-products of yeast sugar metabolism—carbon dioxide and alcohol—are essential in baking and brewing. Bakers yeast (Saccharomyces cerevisiae), when added to baker's dough, produces carbon dioxide pockets that make bread rise. Brewer's yeast, another strain of Saccharomyces, takes advantage of the yeasts characteristic of switching to anaerobic fermentation when deprived of oxygen to produce alcohol as a by-product of incomplete sugar breakdown. Yeasts 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.
Yeasts also comprise some of the natural microbial flora residing in and upon animals, including humans. The yeast species Candida albicans is perhaps the most notorious of the yeast inhabitants of the human body, responsible for the affliction Candidiasis, which may take many forms. Through normal health and hygiene, Candida is held in check by the populous and benign bacterial residents of our skin and mucous membranes. But in instances of compromised health, Candida albicans can result in skin sores (such as Thrush), urogenital tract infections (such as vaginitis), and internally, endocarditis (heart muscle infection ), inflammation of the spleen, liver, kidneys and lungs. Victims of AIDS are particularly susceptible to Candidiasis.
Biotechnology and yeast
Yeasts are rising stars in the toolbox of biotechnologists. Early in the development of biotechnology, which used cells as recipients of transplanted genes, bacteria were the organism of choice. However, limitations involving differences between bacterial cells and our own have relegated bacteria to a second place behind yeasts. Both yeast cells and human cells are eukaryotic—possessing a nucleus and membrane-bound organelles such as ribosomes and mitochondria. As biotechnology techniques have progressed over the last decade, yeasts have come to the fore as host cells for human gene implantation and as potential surrogate cells for housing human chromosomes. Yeasts have also been subjected to the alteration of their own genes, as biotechnologists attempt to develop strains of yeast more efficient in the metabolism of sugars for food and industrial applications.
By studying yeast genetics , scientists hope to gain insight into how the genes of all eukaryotic cells, including our own, function. In 1993 the first yeast chromosome was completely mapped, and the function of each of the 182 genes has yielded insight into such vital genetic processes as mutation repair, enzyme production, and cellular division regulation. These findings may yield insights into such fundamental human health issues as cancer and the aging process. Yeasts have also been transformed through genetic engineering to produce the first genetically engineered vaccine for hepatitis B. Insulin and hemoglobin are also being commercially manufactured by yeast cells which have been re-coded by inserted human genes. Yeasts are a hopeful host cell for housing the entire human genome in an international effort to decipher the gene library of human cells.
Resources
books
Davis, Joel. Mapping the Code: The Human Genome Project and the Choices of Modern Science. Wiley, 1990.
Gross, Cynthia S. The New Biotechnology: Putting Microbes to Work. New York: Lerner, 1988.
Hanlin, Richard, and Miguel Ulloa. Atlas of Introductory Mycology. 2nd Ed. H. W. Wilson Co., 1988.
Krasner, R.I. The Microbial Challenge: Human-Microbe Interactions. Washington: American Society for Microbiology, 2002.
Sagan, Dorion. Garden of Microbial Delights: A Practical Guide to the Subvisible World. Harcourt Brace Jovanovich, 1988.
Stephens, R. B., ed. Mycology Guidebook. Mycology Guidebook Committee, Mycological Society of America, University of Washington Press, 1981.
periodicals
Chan, K.C., A. Csikasz-Nagy, et al. "Kinetic Analysis of a Molecular Model of the Budding Yeast Cell Cycle". Molecular Biology of the Cell 11 (2000): 369-391.
Hoffman, M. "Yeast Biology Enters a Surprising New Phase." Science (March 20, 1992): 1210-1511.
Jeffrey Weld
KEY TERMS
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .- Candidiasis
—Infection of the yeast Candida albicans which may take many forms depending upon the site of infection from inflammation of the heart (endocarditis) to throat and lung infections.
- Eukaryote
—A cell whose genetic material is carried on chromosomes inside a nucleus encased in a membrane. Eukaryotic cells also have organelles that perform specific metabolic tasks and are supported by a cytoskeleton which runs through the cytoplasm, giving the cell form and shape.
- Human genome
—The entire human gene library, consisting of an estimated 100,000 genes encoded in the tightly woven DNA of 23 chromosomes.
- Microbial flora
—Benign, naturally occurring of bacteria and fungi found on, and in, the human body, including the intestinal tract, the mucous membranes, and the skin.
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.
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
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 points—and the consequences of the blocks on later events—demonstrated, 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
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 carbohydrates—starches and sugars—to 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 fermentation—carbon dioxide and alcohol—have 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 (1822–1895) 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 ]
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.’
yeast
Some yeasts are pathogenic (especially Candida spp., which cause thrush); many are used in biotechnology for production of hormones and other proteins.