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

2. 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.

3. 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.

4. 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.

5. 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.

6. 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

7. 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

8. 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

9. 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

10. 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.

11. 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

12. 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. 

13. 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

14. 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.

15. References
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