What Effect Do Ultraviolet Rays Have On Yeast Colony Growth? By Katie Schneider Grade 10

Research UV-A light causes tanning, skin aging, and cataracts UV-B causes sunburn, skin aging and skin cancer UV-C is most effective at killing microorganisms UV-C is the strongest Sunburn is a short-term sign of UV damage, but skin aging and skin cancer are long-term effects of UV damage.

Research UVA radiation causes 2 types of DNA damage: cyclobutane pyrimidine dimers 6-4 photoproducts These 2 types of damage are repaired by nucleotide excision repair These types of damage cause bends in DNA structure, therefore hindering replication and transcription In nucleotide excision repair, the damage is detected, the section of DNA that includes the damage is removed, and it is filled in with new DNA by DNA polymerase.

Research In this experiment, the Saccharomyces cerivisiae is genetically engineered to be DNA-repair-deficient The enzymes that normally would repair DNA damage are knocked out so that the yeast is especially sensitive to UV light This helps to show what effect that gene has in yeast’s lives, and how fatal UV light would be if the DNA damage was not repaired.

Hypothesis If yeast colonies are exposed to UV light for varied amounts of time, a short amount of UV exposure will aid in yeast production, but longer exposure will kill the yeast cells because of the damaging effects of UV light.

Materials UV-sensitive yeast strain purchased from Carolina Biological Sterile dilution tubes Sterile toothpicks Dextrose (YED) Petri dishes Sterile distilled water Pipettes Glass spreading beads Disposable gloves Microwave oven Permanent marker Aluminum foil Stopwatch Cardboard box 3 UV flashlights Transparent sticker with 34 sections

Procedure The agar plates were poured by heating sterile YED agar in a microwave then pouring the agar onto 25 petri dishes The master plate was then streaked with yeast To determine the optimal serial dilution needed for the experiment, 1:1,000 and 1:10,000 dilutions of a yeast suspension made from the growth on the master plate were tested They showed no growth, so they were tested again, but without UV exposure They again showed no growth, so 1:10 and 1:100 dilutions were tested The 1:10 dilution provided growth that was too dense, so 1:100 dilution was used for actual experimentation Plastic test tubes and sterile bulb pipettes were then used to make the 1:100 serial dilution of the yeast suspension

Procedure 2.5 mL of this suspension was pipetted into each petri dish, which was labeled either exposed or control for 1, 3, 5, or 7 minutes, with 3 trials for each 5 glass beads were placed in each petri dish, which were swirled across the plate to spread the yeast suspension Three holes were made across the middle of a cardboard box, and UV flashlights emitting light in the 385 nm range were places in the openings Three petri dishes at a time were then exposed to the light for their indicated times Controls were covered in aluminum foil before being exposed to the UV light The exposed samples were then covered in aluminum foil, and stored in an incubator for 2 days at 30°C To collect the data, a transparent grid containing 34 sections was placed over the petri dish The number of sections containing yeast growth out of 34 was then recorded

Procedure There were 3 trials for each amount of time for control and exposed Independent variable-the amount of time the yeast was exposed to UV light Dependent variable-the amount of yeast growth Control-the yeast not exposed to UV light Constants-amount of yeast in each petri dish, size of the petri dishes

Photos transparent grid used to calculate the amount of yeast growth. UV light set-up with 3 UV flashlights. Test tubes labeled with their serial dilutions.

Number of Sections Showing Yeast Growth out of 34 Data Number of Sections Showing Yeast Growth out of 34 Trial # Control 1 min Exposed 1 min Control 3 min Exposed 3 min Control 5 min Exposed 5 min Control 7 min Exposed 7 min 1 34 18 30 2 32 26 33 3 Average 22 29 21 31 23

Data Out of the 3 trials for the 3, 5 and 7 minute exposed, 1 of the trials showed no growth, while all trials of the 1 minute exposed showed growth. Also, the 1 minute exposed showed more growth than the 1 minute control, while the 3, 5, and 7 minute exposed showed less growth than their respective controls. However, because of the varied results within trials, the standard deviation rates were high. These rates show that the data is not completely reliable. 

Sources of Error and Improvements This project should be conducted in the summer so that sunlight can be used as UV light instead of UV flashlights. Also, in this experiment, the data was measured by determining whether there was growth in each of 34 different sections. Any growth at all in a designated grid section was considered a positive result. Also, when spreading the yeast suspension with the glass beads, it ended up being concentrated primarily around the perimeter of the dishes, so this may have affected the results.   If this experiment were to be performed again, one should use a higher serial dilution level, such as 1:1000 instead of 1:100, and an alternate method for counting the yeast growth should be used that yields more accurate results. Additional trials performed using these changes might result in data that is more reliable. Only one wavelength was used in this experiment, whereas sunlight emits multiple wavelengths through UV-A, B and C light

Conclusion Hypothesis-“a short amount of time will aid in yeast production, but a longer amount of time will kill the yeast cells because of the damaging effects of UV light.” The results support the hypothesis Because of the high standard deviation, however, the results may not be reliable. This experiment has relevance to the world today because UV light is a major cause of skin cancer, and extended exposure can be extremely detrimental to skin health. Other experiments regarding UV light could be conducted testing the strength of UV at different times of day, during different seasons, or for longer periods of time.

References Cells and DNA. (2011, October 17). Retrieved from http://ghr.nlm.nih.gov/‌handbook/‌basics?show=all#gene Clancy, S. (2008). DNA Damage & Repair: Mechanisms for Maintaining DNA Integrity. Retrieved from Nature Education database. Deoxyribonucleic Acid (DNA). (2011, March 23). Retrieved from http://www.genome.gov/‌25520880 DNA damage from deamination and depurination [illustration]. (n.d.). Retrieved from Science Online database. Genetic Engineering. (2011, May 23). Retrieved from http://www.fda.gov/‌AnimalVeterinary/‌DevelopmentApprovalProcess/‌GeneticEngineering/‌default.htm Goodman, B., MA. (2011, October 6). UVA Radiation May Cause DNA Damage in Skin. Retrieved from http://www.webmd.com/‌healthy-beauty/‌news/‌20111006/‌uva-radiation-may-cause-dna-damage-in-skin Highland, J. (n.d.). The Effects of UV Light on Yeast. Retrieved from http://www.livestrong.com/‌article/‌252729-the-effects-of-uv-light-on-yeast/ Hockberger, P. E. (2002). History of Ultraviolet Photobiology. Retrieved from Department of Physiology, Feinberg School of Medicine, Northwestern University website: http://www.photobiology.info/‌Hockberger.html Kirkland, K., PhD. (2007). Infrared and Ultraviolet. In Light and Optics (pp. 106-109). New York: Facts On File, Inc. Liu, S., & Usinger, L. (n.d.). All About Agar. Retrieved from http://www.sciencebuddies.org/‌science-fair-projects/‌project_ideas/‌MicroBio_Agar.shtml MacNeal, R. J., MD. (2007, August). Overview of Sunlight and Skin Damage. Retrieved from Merck website: http://www.merckmanuals.com/‌home/‌skin_disorders/‌sunlight_and_skin_damage/‌overview_of_sunlight_and_skin_damage.html Maczulak, A. (n.d.). Saccharomyces. In Encyclopedia of Microbiology. Retrieved from http://www.fofweb.com/‌activelink2.asp?ItemID=WE40&SID=5&iPin=EMBIO0159&SingleRecord=True Tanner, F. W., & Ryder, E. (1923, May). Action of Ultraviolet Light on Yeast-Like Fungi. Botanical Gazette, 75(3), 309. Retrieved from http://www.jstor.org/‌pss/‌2470221 Twyman, R. (2002, August 28). What are ‘model organisms’? Retrieved from http://genome.wellcome.ac.uk/‌doc_wtd020803.html Ultraviolet radiation and human health. (2009, December). Retrieved from World Health Organization database. Ultraviolet treatment for water suggested. (2010). Biofouling. Retrieved from http://www.fofweb.com/‌activelink2.asp?ItemID=WE40&SID=5&iPin=UPI-1-20100422-140951-bc-israel-watertreatment&SingleRecord=True Ultraviolet (UV) Radiation. (2010, August 17). Retrieved from http://www.fda.gov/‌Radiation-EmittingProducts/‌RadiationEmittingProductsandProcedures/‌Tanning/‌ucm116425.htm Whyte, D. B. (2009, December 4). Exploring DNA Damage: What Effect Do Ultraviolet Rays Have on Yeast Colony Growth? . Wyman, B., & Stevenson, H. L. (n.d.). Ultraviolet radiation–A range (UV-A). In The Facts On File Dictionary of Environmental Science, Third Edition. Retrieved from http://www.fofweb.com/‌activelink2.asp?ItemID=WE40&SID=5&iPin=DEST4675&SingleRecord=True Wyman, B., & Stevenson, H. L. (n.d.). Ultraviolet radiation–B range (UV-B). In The Facts On File Dictionary of Environmental Science, Third Edition. Retrieved from http://www.fofweb.com/‌activelink2.asp?ItemID=WE40&SID=5&iPin=DEST4676&SingleRecord=True Wyman, B., & Stevenson, H. L. (n.d.). Ultraviolet radiation–C range (UV-C). In The Facts On File Dictionary of Environmental Science, Third Edition. Retrieved from http://www.fofweb.com/‌activelink2.asp?ItemID=WE40&SID=5&iPin=DEST4677&SingleRecord=True