Part C: UV Experiments

Using Yeast to Measure the Intensity of Solar Ultraviolet Radiation

In the experiment Observing the Effects of Solar Ultraviolet Radiation on Cells you observed that when yeast cells are exposed to sunlight, all, some, or none of them may be killed. If you use the serial dilution method to refine the, procedure you can measure the fraction of the irradiated cells that survive. With the surviving fraction data and a standard dose-survival curve you can estimate the amount of UV-B in sunlight. This is a bioassay for solar UV-B and gives a better measure of DNA-damaging UV than most physical meters because it is based directly on UV damage to yeast DNA. Most meters don't measure just those energies of UV.


In this experiment the number of yeast cells that are killed by being exposed to sunlight depends on the intensity of UV-B in the sunlight. Therefore, you can estimate the amount of UV-B by measuring how many cells are killed. You measure the killing indirectly by determining the fraction of cells in a sample that survive a particular exposure time. We assume you have some experience with yeast, we have left some of the details for you to design. You will make a yeast suspension with a known number of cells per mL, and then dilute that suspension by making a set of serial dilution tubes so that the number of cells per mL in each tube is 1/10th of the number in the previous tube (see Figure 2). You will use the diluted yeast suspension in the tubes to plate between 300 and 3000 cells on each plate. That will yield between 30 and 300 survivors for an exposure time that gives a 0.1 surviving fraction. These are the techniques described in Serial Dilutions and Viable Cell Counts. This experiment will require some practice. To select the best dilutions for each dose, you must know the approximate outcome of the experiment in advance. Make your starting guesses using the survival curve in Figure 1. In the first trial, plate different dilutions and try several exposure times. Use the results to make better choices in subsequent trials. After several trials, you'll be getting good results and you will be able to monitor solar UV-B. The goal is to get skilled enough so that you only need two plates each time you take a UV reading. (One control plate and one timed exposure plate) Time Line: Day before:10 min Getting Ready

45 min Discussion of the strategy and objectives
Day 1: 50 min Dilution, Plating and Irradiation of Cells
Day 3 or 4: 50 min Counting Colonies and Analyzing Results


For each student or team:

Common Materials: