What Effect Do Ultraviolet Rays Have On Yeast Colony Growth?By Katie SchneiderGrade 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

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

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

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 Biologica• 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

DataNumber 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 34 18 30 34 0 34 34

2 32 30 34 0 26 33 34 0

3 0 34 34 34 34 34 34 34

Average 22 33 29 21 31 22 34 23

Data

control 1 min exposed 1 min control 3 min exposed 3 min control 5 min exposed 5 min control 7 min exposed 7 min0

5

10

15

20

25

30

35

40 Average Yeast Growth

Amount of time exposed to UV light

Num

ber o

f Sec

tions

with

gro

wth

out

of 3

4

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.

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

References• 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