Transcript What Effect Do Ultraviolet Rays Have On Yeast Colony Growth?

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
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
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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
Data
Number of Sections Showing Yeast Growth out of 34
Trial #
Control
1 min
Exposed Control
1 min
3 min
Exposed Control
3 min
5 min
Exposed Control
5 min
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
33
29
21
31
22
34
23
Average 22
Data
Average Yeast Growth
45
Number of Sections with growth out of 34
40
35
30
25
20
15
10
5
0
control 1 min
exposed 1 min
control 3 min
exposed 3 min
control 5 min
Amount of time exposed to UV light
exposed 5 min
control 7 min
exposed 7 min
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
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