What Effect Do Ultraviolet Rays Have On Yeast Colony Growth?
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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 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
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.
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References