Copyright 0 1993 by the Genetics Society of America

Genetics in the Classroom

Programs, Proposals and Experience in Teaching Precollege Genetics Edited by Leland Hartwell

Using Yeast Genetics to Generate a Research Environment

Thomas R. Manney**t and Monta L. Manney*

*Department of Physics and ‘Division ofBiology, Kansas State University, Manhattan, Kansas 66506

ABSTRACT Many of the same features of the yeast Saccharomyces cerevisiae that have made it so useful as a

genetics and molecular biology research organism make it equally useful as a teaching organism. Furthermore, the fact that it is a modern research organism makes it all the more exciting to students and teachers. The unique characteristic of yeast as a unicellular, eukaryotic organism with a complete sexual life cycle is ideal for teaching. A simple monohybrid cross to explore dominance and recessive- ness, a dihybrid cross to demonstrate independent assortment, pigmented adenine auxotrophs for investigating the fundamentals of gene action, and easily measured responses to ultraviolet radiation provide an array of appropriate laboratory tools that put real science in the hands of students and teachers. Direct collaborations between scientists and science teachers bring together complementing knowledge and experience, providing an effective and efficient way to adapt and simplify techniques and procedures to accommodate time and money constraints. Collaborations quickly identify technical and theoretical problems that must be solved for implementation in classrooms. They also provide a continuing stimulus to teachers and students to participate in the research process.

A T the outset, our goal in working with high school teachers was to present yeast genetics as

an interesting way to teach experimental genetics with real research organisms that are safe and economical of time and money. We anticipated that this would contribute to greater understanding of genetics by both students and teachers. While this has occurred, an even more striking outcome has been an increased student enthusiasm for science and changes in the way teachers present science in their classrooms. The Kan- sas State University GENE program (for Genetics Education Networking and Enhancement), sponsored by the National Science Foundation, introduced teachers to Saccharomyces cerevisiae and other research organisms (the flour beetle Tribolium castaneum and Wisconsin Fast Plants, Brassica rapa) to investigate basic principles of Mendelian and molecular genetics.

An example: the yeast life cycle and a simple monohybrid cross

T h e “visible” life cycle of yeast is ideally suited to teaching the sexual cycle at the cellular level. T h e basic yeast life cycle, illustrated by a simple monohy- brid cross, is a good example (MANNEY and MANNEY 1992). Starting with a pair of haploid strains of op- Genetics 134 387-391 (May, 1993)

posite mating types, one carrying a red ade2 mutation (adenine requiring) and the other carrying t r p 5 (tryp- tophan requiring), one can observe the complete sex- ual cycle in a about a week, spending from 10 to 20 min per day on the actual manipulations. With the aid of these simple color and nutritional traits the events are followed at the genetic level, in parallel with cellular events that are easily observed with classroom- grade microscopes. Furthermore, any video camera can be used to view the image through a microscope, so the class and teacher can view and discuss the same objects.

On the first day the students mix the two strains ( a d e 2 and t rp5) together on an agar plate, using a toothpick. The color differences of the yeast growing on the plate will be conspicuous. Through the micro- scope they observe the characteristic shapes and bud- ding pattern of yeast at each stage. If the teacher refrigerates the plate after about three hours, then the students can look at them the next day and see the characteristic peanut-shaped zygotes that will have formed. With the proper choice of strains, they may also see haploid cells that have responded to mating pheromones, becoming pear-shaped “shmoos.” (Both teachers and students are very fond of shmoos.) By

388 T. R. Mannev and M. L. Manney

developing a and a

)L ,mating type cells

germination -J" Sexual of spores 0

$!/8- / reproduction Asexual

formation of ascus

and spores (melosls) ."-

% zygote

/ - diploid colony

(mitosis)

FIGURE I .-The life cycle o f Saccharomyces cereuisiae emphasizing the features that distin- guish it as a representative sexual life cycle (drawing courtesy of BSCS). The artist's ren- dering represents the cells approximately as they appear in the microscope, to aid students in identifying the forms they see in classroom experiments. Students, working collectively as a class, are encouraged to make sketches of the cells they observe at each step in the life cycle, to attempt to construct their own model of the life cycle, and to compare it with the drawing [in BSCS (1 992). Chapter 71.

the next day, the culture is predominantly diploid and has the characteristic cream color of the trp5 parent. At this point the students are primed to discuss pos- sible explanations of this. With a little help, they w i l l reinvent dominance and recessiveness for themselves.

By streaking the parent cultures and the mating mixtures onto minimal medium, students can select a diploid strain and discover genetic complementation and prototroph selection. They can then sporulate the diploid and observe the four-spored asci through the microscope. When they streak the sporulation mix- ture on complete medium, they will see the parental color types reappear among colonies that grow from segregating spores. They will have observed the com- plete cycle at both the cellular and genetic levels.

When students have seen a sexual life cycle for themselves, they are ready to appreciate its role in the life cycles of higher organisms. By relating what they have observed to a diagram of the yeast life cycle (Figure l ) , they can understand the important events of mitotic cell division, conjugation, meiosis and ga- mete formation, and the underlying molecular and biochemical processes. This approach has been adopted by BSCS (Biological Science Curriculum Study) in the latest of one of their high school biology text books (BSCS 1992).

Variations on the theme When basic strains, materials and techniques of

yeast genetics are available in a classroom, the number of possible applications becomes limitless. The follow- ing examples, which are all in use by teachers from the GENE project and their students, illustrate the diversity that can be achieved in working with the yeast system consisting of just three markers in each of the two mating types. All of these experiments have

been developed or refined through collaboration with teachers and their students.

A dihybrid cross: Independent assortment during meiosis is revealed and some basic aspects of gene action are demonstrated. The traditional case of two different traits (ade2lADE2 trp5ITRPS) gives a 9:3:3: 1 segregation during meiosis. Students are pro- vided with haploid strains corresponding to the four different FI gametes in both mating types. By mating them to produce 16 diploids in a Punnett square grid they observe simple, graphical results that relate di- rectly to standard textbook cases. BSCS has also used this experiment to illustrate independent assortment and its basis in meiosis (BSCS 1992). A more interest- ing case of (nearly) identical traits (adeI/ADEI ade2/ ADE2), illustrates the same principles and at the same time provides a puzzle-solving situation that demon- strates a nonclassical genetic phenomenon, comple- mentation. Both adel and ade2 produce a red colony phenotype, but the dihybrid diploid is the wild-type cream color. This grid leads to one case where two red haploid strains form a white diploid, and another where two red strains form a red diploid.

Isolation of auxotrophic mutants: Beadle and Ta- tum are revisited. By isolating UV-induced red adel and ade2 mutants, especially with the aid of a hyper- mutable strain, students can reproduce the classic experiment and pursue their own questions about what genes do. Half the class isolates red mutants in a mating type MATa strain while the other half uses a mating type MATa strain. The mutants obtained in opposite mating types can then be tested for allelism by complementation for the normal cream color, and the students discover that they affect two different genes.

Genetics in the Classroom 389

Genetic-environmental interaction: Gene action and regulation is observed. The dependence of the development of red pigment on a limiting adenine concentration and on aerobic growth is used to dem- onstrate and investigate the regulation of a biosyn- thetic pathway. When a lawn of ade2 or adel cells is spread on minimal medium, there is no growth. But if an adenine-impregnated disk is placed in the center of the plate, the adenine diffuses outward. Near the center, where the concentration is high, the red color is inhibited, but where the adenine concentration becomes growth limiting, the lawn develops the red color. At the edge of the plate there is still no growth. Development of the red color also requires aerobic growth. This can be demonstrated by making a lawn on complete medium and then folding one half over on the other, so the cells are in a sandwich between two layers of agar. The color develops only near the edges. However, if the sandwich is opened after two days, the cells turn red within a few hours.

Actions of UV-radiation on cells: Simple experi- ments reveal lethality, mutagenesis, and DNA repair resulting from radiation. Radiation from a germicidal lamp or sunlight can be used for both qualitative and quantitative experiments. T h e efficacy of sunscreen preparations can be investigated, and lethal UV-radia- tion in sunlight can be monitored using a repair- deficient mutant.

Transformation: A simple yeast system for trans- formation utilizes the red adenine phenotype. ADEZ and LEU2 on a multicopy plasmid are used to trans- form an adel leu2 strain from red to white. Rapid colony transformation techniques have been stream- lined (with an acceptable loss of efficiency) to be carried out in one class period. Loss of plasmid on nonselective medium produces dramatic cases of white colonies with red sectors, illustrating the segregation of a plasmid-borne gene.

Experimental protocols: Most of these experiments and many others are described in detail in Handbook f o r Using Yeast to Teach Genetics (MANNEY and MANNEY 1991), a guide for teachers used in the GENE work- shops. A copy may be obtained from the authors for $10 to cover printing and postage (prepaid by check made out to Kansas State University).

Teacher feedback Approximately 100 teachers, who attended GENE

workshops, have used these experiments with more than 20,000 students from grades 6 through 12. We have collected feedback through questionnaires and telephone interviews on how they have used yeast experiments and how this has affected their teaching and their students. The feedback has been positive and enthusiastic, and defines a useful role for scien- tists. T h e main “take-home lesson” is that the use of research-based activities has a big effect on the way science is taught. Although we used “cookbook” ac-

tivities to teach workshop participants yeast genetics techniques, we spent considerable time in the labora- tory talking about research results, and how we knew what we were telling them. This enabled many teach- ers to introduce their students to the excitement of research, the excitement of going further than just data collection; many teachers gained confidence in drawing conclusions from their own data and in turn passed that on to students.

T h e most frequently reported changes in these teachers’ classrooms was an increase in the use of hands-on activities, especially those involving living organisms. Quoting from the interviews of teachers:

They are much more interested in genetics, and it’s easier for them to understand. They’re displaying more enthusi- asm towards the subject of genetics, and it’s something that they can see has actually taken place rather than just seeing a model on the chalkboard.

Doing it on the board in chalk is not nearly as interesting as seeing the changes in the yeast; it sparked their enthusiasm.

When students learned to use sterile technique, it changed the care that they took with all aspects o f their work.

Students after graduating have come back and expressed gratitude at having learned sterile technique in the high school classroom. Other students who graduated before GENE was taught likewise have expressed regret that they did NOT get instruction in sterile and micro techniques because it would have been helpful in their college pre-med and med tech courses.

Since GENE, they ask as they come into class each day if they’re going to get to do a lab. The [yeast] sunscreen project, for instance, really caught their attention. The [yeast] sexual cycle also interested them.

Corresponding to these classroom changes, the most frequently reported student changes were increased enthusiasm, interest, and motivation. One reflection of this is a substantial increase in the quantity and quality of students’ independent research projects. More than 2400 independent student projects were reported among 65 teachers interviewed.

Using yeast projects allows individual students to “own” the projects. Cooperative learning is important, but sometimes it’s important for the individual student to have their very own project. This is especially important in [yeast] life cycle, sunscreen projects [with yeast], and fast plants. In some cases they’ve even named their fast plants! They’ve displayed some real enthusiasm, coming in before school. I’ve never had that happen before, where kids are coming in to check on the progress of their lab projects.

Clearly the use of research organisms and open- ended experiments enhances the traditional textbook and lecture approach to teaching genetics. T h e basic biological principles are found in every text book, and included in every course in biology, but the traditional examples are far removed from the experiences of most students, and not amenable to laboratories in the classroom. However, open-ended activities drawn from real science, truly excite students. There is no

390 T. R. Manney and M. L. Manney

greater motivation than the teacher being able to tell the students that no one knows the Outcome of the experiment they are about to do.

In our experience students take to problem solving with much more enthusiasm than to hypothesis test- ing, which is the staple fare of traditional student projects. They do not have enough background to formulate interesting hypotheses, but they can ask questions and become enthusiastic about the idea of being able to find their own answers. For example, one student wanting to study photoreactivation of UV damage to yeast was frustrated by insufficient sunlight in winter at the times he was out of class. The solution he worked out was to use the fluorescent lights in the classroom that were being used to grow plants. In our summer workshops we had always used sunlight, so no one could tell him whether it would work. He made it work and taught the rest of us. Another student was interested in birth defects, which seems far removed from yeast. But her teacher explored this with the GENE staff during the workshop and ended up taking home a set of strains for measuring chro- mosome loss in yeast (courtesy of BETH MONTELONE). She is now exploring environmental effects on this system.

The overwhelming consensus from teacher feed- back is that what really works to get students excited about science is to simply get them involved in real science.

Guidelines for adapting research materials to the classroom

The elements that make the yeast experiment suc- cessful can be reproduced with other genetic systems to illustrate and explore a variety of phenomena. Many of the procedures that are popular in modern genetics research, such as gene transfer, lend them- selves to classroom adaptation because they have been refined to make them rapid, simple, and efficient. The practical issues that have to be addressed include safety, time limitations, money and equipment limi- tations, teaching strategies, and follow-up support.

Involve teachers: An effective way to adapt mate- rials to the classroom is to involve experienced and knowledgeable teachers as collaborators and field- testers. Working through a classroom application with a teacher quickly identifies the important technical problems that have to be solved.

Safety is paramount: Schools, teachers and vendors are preoccupied with risks of student injury and ex- posure to litigation. In many cases safer approaches to traditional activities can be easily developed. For example, yeast geneticists’ standard practice of using of toothpicks instead of traditional flamed loops elim- inates the hazards of fire. Most yeast experiments can be adapted to require no flames whatever. Bakers’ yeast itself is a friendly microbe with a good public image. The use of this organism offers an opportunity

to discuss other yeast that are human pathogens, and the fact that many bacteria are also innocuous.

Yeast has made microbiology experiments tremendously more accessible. Much easier and safer to handle than bacteria. I don’t have to worry about the kids who aren’t interested doing something destructive or dangerous with the cultures because yeast is so safe.

Time limitations: Time often poses the greatest challenge. Fortunately, we find that many steps in the yeast life cycle can be reversibly arrested by refriger- ation. Only the first couple of hours of mating appears to be appreciably inhibited by holding in the refrig- erator. With simple adaptations, including the use of strains selected for rapid sporulation, the yeast exper- iments fit well with the rhythm of the classroom schedule.

The time factor for yeast growth is excellent. It grows up quickly, which fits nicely with the attention span of the non- college prep students. They would never have the patience for fruit flies, which take much longer.

A more difficult time problem is the limited time teachers have available for preparation. Limited prep- aration time was cited as the greatest single barrier to using hands-on activities, ahead of lack of money and difficulties associated with preparing media (contami- nation!).

Money and equipment limitations: Materials for yeast experiments are relatively inexpensive as long as basic media such as yeast extract and yeast nitrogen base are used and expensive reagents are avoided. Use of the red adenine mutants, for instance, is a signifi- cant economy, compared with chromogenic substrates ( e .g . , X-gal, which is used with bacteria) as a way to achieve visible character traits. The cost of supplies is deceiving because even seemingly trivial items become expensive when multiplied by the number of students that one teacher serves in one day.

Cost and availability of materials are closely related, but knowing where to order materials and having administrative support to deal with research supply distributors is a major impediment for many teachers. Experiments should use materials that are available from local stores or school science supply companies. Strains for the life cycle and dihybrid cross experi- ments have been deposited in the Yeast Genetics Stock Center. Strains and media for the yeast genetics ex- periments in the BSCS textbook can be purchased from Wards Natural Science Establishment, Inc.

Particular thought must be given to major equip- ment that is routinely found in the research labora- tory, such as incubators, shakers, centrifuges, water baths, and even microscopes. Some schools are gen- erously equipped with such items and some even enjoy a full complement of modern molecular biology ap- paratus. But most schools have little, and experiments that require the least equipment will reach the greatest number of students. Here again, yeast adapts well.

Genetics in the Classroom 39 1

Most basic experiments can be incubated at room temperature by adjusting the schedule to allow longer incubation time. Shakers are especially rare, and are unnecessary if procedures are adapted to use solid media, which also eliminates spills and reduces the impact of contamination.

Teaching strategies: One of the most valuable teaching tools is the video microscope camera, that allows the entire class to observe and discuss the same thing. They are becoming increasingly popular, but are beyond the budget of most schools. However, w e have found that nearly every school has at least one video camera (at least in the athletic department), and virtually any video camera can be used to view through any microscope without special adapters. A camera, focused for infinity, placed above the eye- piece, will produce the same image that an observer sees. I t is only necessary to exclude stray light by wrapping a piece of aluminum foil o r cloth around the junction between the camera lens and the eye- piece. This simple trick is the single most popular technique among the teachers with whom we have worked.

Follow-up support: It is not sufficient to provide teachers with techniques and materials and send then1 back to their isolated classrooms. They need the same interactions that all scientists need.

One of the best things is having a source of information and help when things go wrong. Before, when you were a classroom teacher, the only source of help you’d have was a teacher in the school who had the same experiences as you. GENE has helped eliminate the isolation from times before.

Electronic networking of teachers has become a high priority of the National Science Foundation and other agencies funding science education. Conse- quently, this technology is rapidly becoming available in most schools. For scientists who are already using electronic mail to network with their colleagues, this is a particularly convenient way to make themselves available to teachers and their students.

Conclusions

In the science research community, the value we place on direct involvement in improving how science is taught in our schools is a periodic function of our concern that our own children are not receiving the highest quality, world class education in science and technology. This concern peaked in the post-Sputnik era, waned for a decade o r so, and is again peaking for reasons that are well documented (National Corn- mission on Excellence in Education 1983; National Research Council 1990). T h e overriding theme that

has emerged from the GENE project is that “real science” in the classroom is a highly productive way of invigorating science teaching and learning. The only way this comes about is through direct interac- tions between scientists and classroom teachers and their students. The feedback from the GENE teachers has consistently emphasized that follow-up and logis- tical support by scientists is the single most essential factor necessary for continued use and updating of the information and activities presented in the work- shops. Most teachers are not able to take the products of a summer workshop or research experience and effectively incorporate it into their teaching without continued support from the scientists. Our experience has demonstrated that even a modest commitment on the part of scientists-including research assistants and graduate students-can have a large catalytic effect on teachers and students. But second-hand support by scientists through surrogates, while useful, does not serve the same needs.

We believe that this argues strongly for an emphasis on grass-roots approaches to revitalize science educa- tion, as opposed to major dependence on top-down approaches to systemic solutions. GENE teachers were asked to identify the most important factors that would enable them to continue using yeast genetics and similar activities to teach genetics. T h e most fre- quently cited Factors were (1) a continuing affordable source of yeast strains and supplies, (2) continuing opportunities for networking with scientists and other teachers and (3) new activities. Grass-roots collabora- tions between scientists and teacher, to develop appli- cations of research activities for the classroom are the most direct way to serve these needs.

This work was supported by grants DPE-8219148 and DTE- 89546% from the National Science Foundation. We gratefully acknowledge the major contributions of the GENE Staff: RICHARD BEEMAN. LARRY DAVIS, BOR JOHNSON, BEI‘H MONTELONE, STEVE OLIVER, RICHARD PANNRACKER, L A R R Y WEAVER and DEAN ZOLI.- MAN, and all the teachers i n the Genetics Education Network.

L I T E R A T U R E C I T E D BSCS, 1992 Biological Science An Ecological Approach, Ed. 7. Ken-

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Yeast to Teach Genetics. Kansas State University, Manhattan, Kans.

MANNEY, T. R., and M. L. MANNEY, 1992 Yeast: a research organism for teaching Genetics. Am. Biol. Teacher 54: 426- 431.

NATIONAL COMMISSION ON EXCELLENCE IN EDUCATION, 1985 A Nation at Risk: A n Imperative for Educational Reform. U.S. GOV- ernnient Printing Office, Washington, D.C.

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