Baker's yeast, Saccharomyces cerevisiae, has been genetically studied for years because its cell cycle shows similarities to that of human cells. Scientists determined that studying mutations in yeast can help understand the mutations that cause cancer in human cells. The purpose of our experiment is to observe the effect of UV radiation on yeast cells, as well as calculate percent survival and percent mutation using data compiled by the entire class. A serial dilution was prepared for a yeast culture, bearing the trp1-289 mutant allele. We pipetted 100 μL from the 1:10000 onto 7 different synthetic complete (SC) plates. Each plate was exposed to increasing amounts of UV radiation, ranging from 0-120 seconds, using 20-second intervals. We pipetted 100 μL from the undiluted culture was plated onto 7 different SD plates. Unlike SC plates, SD plates lack tryptophan. The SD plates were exposed to radiation in an identical method as the SC plates. Both sets of plates were then incubated. The following week, we counted the colonies, complied class data, and made the percent survival and percent mutation calculations. The percent survival constantly decreased as length of radiation increased. Percent mutation was a little more surprising because it increased up to 60 seconds, then decreased until 100 seconds, and then very slightly increased until 120 seconds. After reviewing the data, we concluded that UV decreases survival and induces genetic changes in yeast. This experiment raises the awareness of the harmful effects of UV radiation on living cells.
The procedure used in this experiment is from the L113 Biology lab manual (Bonner et al. 2012). A serial dilution set of the baker's yeast was prepared using the following concentrations: 1:10, 1:100, 1:1000, 1:10000. We spread 100 μL of the 1:10000 dilution on to 7 different "synthetic complete" (SC) Petri plates. We also spread 100 μL from the undiluted yeast culture on to seven different "synthetic dropout" (SD) plates. SD plates lack tryptophan, meaning that non-mutant yeast cells should not grow on them. The SD and SC plates were paired up based on the duration of radiation each plate would receive. The first pair would not receive any UV radiation, the second pair would receive 20 seconds of radiation, and each successive pair received an additional 20 seconds of radiation. The final pair would receive 120 seconds of UV radiation. Each pair had their lids replaced by Saran Wrap and was placed equidistantly under a short-wave UV for their designated duration. The lid was put back on immediately following the radiation.. After being exposed to radiation, all seven pairs of plates were incubated for 2 days at 30 degrees Celsius. The following week, the number of colonies on each plate was counted. The entire class data was averaged and percent survival as well as mutation rate were calculated and graphed using Microsoft Excel. Percent survival was calculated using the SC data and this formula:
(# of colonies on plate)/(# of colonies on ""0\" " plate ) x 100
Percent mutation was calculated using the formula:
(# of colonies on SD plate)/(# of colonies on SC plate x 10000) x 100
The main purpose of this experiment was to calculate the percent survival and percent mutation of the yeast. The percent survival of the yeast cells was inversely correlated to the amount of UV exposure received by the yeast cells (Fig. 1). This constant decrease in survival was the expected outcome. The greatest