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Radiation Effects on Yeast

Red and White Yeast: An Introduction to Science as a Process

Transformation of Yeast

Radiation Effects on Yeast

An excellent use of yeast is as a model system to explore the lethal and mutagenic effects of various physical and chemical agents. The experiments described here use ultraviolet light (UV) but are readily adapted to other agents. To demonstrate the lethal effects of UV from sunlight (UV-B) we use a special yeast strain with defects in three repair pathways; if a short wave UV source (UV-C or germicidal UV) is available (see information on UV sources, below), wild type yeast can be used in killing and mutagenesis experiments.

A simple experiment demonstrating that yeast are killed by solar UV

A thick suspension in water of a radiation-sensitive strain of yeast, G948-1C/U, carrying mutations in three different DNA repair pathways, is spread on an agar plate and allowed to soak into the agar. Portions of the plate may be covered with paper or sunscreen or other possible filters; the plate is then exposed to sunlight for about 5 minutes, depending on the time of year and latitude (5 minutes works for summer, midday on a clear day in the middle latitudes of the US; longer times are needed at other times of day, year, or with cloud cover or a more acute sun angle). Following exposure, the plates are incubated at 30 C. or room temperature overnight. The sun-exposed portion of the plate should not grow. Students are free to explore variations on this basic theme which might include effects of exposure time, various filters, and various UV sources.

Examples of possible student research projects

  • exploration of various aspects of solar UV killing with the sensitive strain (G948-1C/U) including dose dependence, effects of time of day, effects of time of year, effects of location, effects of UV blockers including ozone (ozone effects are best modeled using the computer program UV-RISK, which can be downloaded at the link). For a quantitatively accurate project, it would be best to do these studies by making dilutions and plating for single colony formation as described in the GENE Project experiment Using Yeast to Measure the Intensity of Solar Ultraviolet Radiation.
  • comparison of UV sensitivity of strains with various mutations in DNA repair
  • exploration of other potential DNA damaging agents with repair-proficient or repair-deficient strains
  • screening of genetic libraries of other organisms to find cross-complementing genes that compensate for the repair defect of a yeast mutant strain
  • a demonstration that DNA damage directly reduces genetic function using the transformation technique with irradiated plasmid DNA

UV Sources

To build your own UV-C source, complete directions are available at the link UV Radiation Chamber. Germicidal UV (i.e., UV-C) lamps such as are used in tissue culture hoods may also be used to irradiate plates although typically these are high up in a hood and a long way from the working surface. A biological assay for the dose available from such lamps using the sensitive strain G948-1C/U is the easiest way to determine their utility. High intensity "yard lights" available in hardware and discount department stores deliver a significant amount of UV-B and UV-C if the protective cover is swung away from in front of the bulb. These lights also get quite hot, so use them with caution or you may do more damage to the yeast with heat than with UV! With all UV sources, be sure to wear eye protection, and do not unnecessarily expose your skin.

For a discussion of the properties of UV wavelengths of various types see the essays at the GENE project site Ultraviolet radiation in our environment and Biological consequences of UV exposure

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