The Effect of Sunscreen
on Mutant Yeast
Nathan Reeves
March 20, 2013
Abstract
The purpose of the report and the experiment is to show the effect of different sunscreen SPF
levels on mutant yeast that does not have the ability to repair its own DNA which will cause
death. What happened in the experiment was that sunscreen was spread on 3 plates of mutant
yeast with 3 different levels of sunscreen that were SPF 15, SPF 30, and SPF 50 along with a
control of mutant yeast with no alterations. The results of the experiment were that the control
had no growth, SPF 50 had the most growth, SPF 30 had low growth, due to an unequal
distribution of sunscreen between the plates, and SPF 15 had a little more growth than SPF 30
due to this error but was still less than SPF 50. Without the error mentioned the experiment
would have been correct towards the hypothesis, which was basically as the level of SPF
increases the more growth and protection the mutant yeast would have. When the hypothesis is
compared to the actual results the hypothesis is true for SPF levels 15 and 50 but not 30 because
of the error with unequal speeding of the sunscreen.
Background
Previous research that was done leading up to this experiment was on skin cancer where it was
found that without the ability to repair the cell’s DNA, the cell will damaged by the sun and can
cause mutations that can cause cancer. Research was also done on the difference between sun
block and sunscreen and the differnet types of UV rays which is explained below.
The SPF number is the level of protection the sunscreen provides against UVB and UVA rays.
Sunscreens vary in their ability to protect against these harmful rays. UVB rays is the shorter
wave Uv ray that causes sunburns, skin damage, and this can cause skin cancer. UVA is the
longer wave UV ray that causes skin damage, skin aging and causes skin cancer as well.
According to the Skin cancer foundation they mentioned that it takes 20 minutes for one to turn
red if they're not wearing sunscreen. But it is said that if one uses a sunscreen of SPF 15 prevents
burning for about 5 hours. Most sunscreens with an SPF of 15 or higher protect well against
UVB rays. Only certain sunscreens will protect from the deeper penetrating UVA rays as the spf
level only applies to UVB rays. There is a difference between sunscreen and sunblock.
Sunscreen protects against UVA rays as well as UVB rays when sunblock on the other hand does
not protect against UVA rays.
The goal of this experiment is to show that the higher the spf of you sunscreen the less you are at
risk for developing skin cancer.
Hypothesis
If you increase the spf level of sunscreen then the protectiveness against the suns UV rays will
increase because SPF means sun protection factor which as the sun protection factor increases
then obviously your protection from the sun will increase up to a point. The independent variable
is the level of spf of the sunscreen. The dependant variable is the quality of protection from the
suns UV rays.
Materials and Methods
Materials:
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Computer with Internet access
Laboratory journal
Safety goggles
Latex or nitrile exam gloves
Carolina’s Sunscreen for Yeast and People, Too lab kit:
4 YED plates
Bottle of sterile water
Sterile, capped, plastic test tubes
1-mL individually-wrapped sterile bulb pipets
5-mL individually-wrapped sterile bulb pipets
Small jar of glass beads
inoculating loops
Starter plate with UV light-sensitive yeast strain (mutant yeast strain)
Starter plate with wild-type yeast strain
Aluminum foil squares
30 ˚C Incubator
Sunscreens SPF 15, SPF 30, and SPF 50
Waste bucket
Butter knife
Procedures:
Sun Screen Levels
15 spf
30 spf
50 spf
Yeast Observations
1. Gather all materials needed to proceed in the experiment.
2. Copy the table from above into your lab journal.
3. Working with your partner(s), obtain a Mutant Yeast Strain starter plate from your
teacher.
4. Label the tube Mutant Yeast Strain.
5. Lable all four yed plates one for each spf 15, 30, 50, and control
6. Pick up an inoculating loop. Be careful not to touch one end of the inoculating loops.
7. Using the clean end of the inoculating loop collect a mass of yeast from the starter plate
labeled Mutant yeast strain. The mass of yeast on your inoculating loop should be about
1 mm in diameter.
8. Smear the mass of yeast as far down as you can possibly reach in the test tube labeled
Mutant Yeast Strain.
9. Pull out the 5 Ml pipet from the packet from the bulb end (the side away from the end
used to take up liquid). This will ensure that the pipet remains sterile.
10. Pipet 10 mL of sterile water into the test tube. Make sure to not touch the walls of the
tube with the pipet. You do not want to cross-contaminate your tubes. If you do
accidentally touch the wall of the tube with the pipet, obtain a new pipet from your
teacher before adding water to the second test tube.
11. Shake the tube until the yeast cells are completely re-suspended in the water (the water
should look slightly cloudy).
12. Obtain one YED plates from your teacher. YED stands for Yeast Extract-Dextrose agar, a
growth medium for yeast.
13. Label the plate Mutant Yeast Strain. Also label the plate with your and your partner’s
names. In addition, label the lid of the plate with this information.
14. Obtain a small jar of glass beads.
15. Open each of the plates with the lid-side down. Shake 4 – 5 glass beads onto the lid of the
plates and immediately close and flip the plates back over.
16. Use the 1 mL pipet and pipet 250 μL of each yeast suspension onto the appropriately
labeled plate. Use the diagram below to help you determine the 250 μL mark. Use a
different pipet for each yeast strain. Close the plates as soon as the yeast suspension has
been added.
17.
Spread the yeast cells onto the plate by shaking the glass beads back and forth across the
entire surface of the plate. Do not use a swirling motion, as this will only run the beads
along the edge of the plate.
18. Let the plate sit until the excess liquid has soaked into the agar.
19. Hold the plate vertically over a disposal container and open the plates slightly to allow
the beads to drop into the container. Immediately close the plate.
20. Remove the lid from the plate and quickly cover the plate with plastic wrap. Make sure
the plastic wrap is pulled taut and does not touch the surface of the plate. Note: The
plastic wrap is used instead of the petri dish cover because the petri dish cover might
contain a pigment that absorbs some components of sunlight and will interfere with this
experiment.
21. smear sunscreen SPF 15 on the plate using the strait end of the butter knife
22. Smear SPF 30 on the second plate
23. SPF 50 on the third plate
24. Leave the fourth plate blank
25. put the plates outside for 20minutes.
26. Bring back the plates stack them and wrap in foil
27. Incubate them for 24 hours
28. Pull them out of the incubnator
29. Observe and write the observations in the table
30. Clean up laboratory
Results
The data shows that 50 SPF had the highest level of yeast growth. The plate with 50 SPFwas
heavily covered with yeast. The 30 SPF had a small amount of yeast growth. The plate of SPF 15
showed medium yeast growth, the surface of the plate had more than the 30 SPF but less than the
50 SPF sunscreen. The control did not have any yeast growth as the surface of the control was
blank and showed no signs of yeast growth.
Sun Screen
Levels
15 spf
30 spf
50 spf
Control
no spf
Yeast
Observations
Medium amount of
yeast growth. Medium
amounts of yeast
seemed to have grown
Small amount of yeast
growth. Minimal
amounts of yeast growth
seemed to have grown.
Had the most
growth of yeast (
large amount of
yeast growth)
No
yeast
growth
Discussion
First some errors that could have occurred throughout the experiment could have been exposure
to sunlight, not spreading the sunscreen equally on the plates, and possible contamination. One
of these possible errors did occur which was unequal spreading of the sunscreens on the plates
which would have caused the 30 SPF sunscreen to have lower growth than the 15 SPF sunscreen.
The next error that may have had an effect on the experiment was exposure to UV rays before
spreading the sunscreen on the plates. The third error is least ;likely and probably didn’t happen
but was possible, contamination would have caused odd variance in the results like that of the 30
SPF sunscreen, which was caused by unequal spreading. To improve the experiment and avoid
these errors is to take more care when spreading the sunscreen and making then as equal as
possible. The experiment could also be improved by trying to limit as much UV exposure before
putting on the sunscreen to avoid early exposure and unintended yeast death. To avoid the third
error during the experiment to take very special care to make sure everything is sterile and not
used more than it needs to, in this case one use and whatever is being used is done.
As it was explained before and in the table the 50 SPF had the best results, the 30 SPF had the
worst results, and 15 SPF was in between but also on the lower side. SPF stands for Sun
Protection Factor so what should happen is that the higher the SPF the higher the protection of
the yeast should be, if not for one large error this would be true in this experiment. The
experiment’s results were not in line with this because of unequal spreading of the sunscreen.
The SPF levels of 50 and 15 were where they were supposed to be, however the sunscreen SPF
30 was not spread very well and because of that had the least growth which is not what should
have happened.
One other experiment or addition to this experiment could be to see if the different sunscreen
levels will protect better than the normal wild type yeast without any alterations. The sunscreen
would still be put on mutant yeast. What the experiment would look for is if the sunscreen on the
mutant yeast has better protection or more growth than on the normal wild type yeast without
any alterations.
For the most part the background research supports the experiments results minus one error;
however it was very clear as to what should have happened without that error.
Conclusion
The hypothesis is mostly correct as the SPF levels 15 and 50 were on the mark, and would be
fully correct if, again, the sunscreen was spread equally on the plates.
Citations
"Skin Cancer Foundation." Sunscreens Explained. Cancer Foundation, 2013. Web. 09 Mar.
2013. <http://www.skincancer.org/prevention/sun-protection/sunscreen/sunscreensexplained>.
"Sunblock vs Sunscreen. Which Is Better?" Sunblock vs Sunscreen. Which Is Better?
Family Health and Nutrition, n.d. Web. 09 Mar. 2013. <http://www.family-health-andnutrition.com/sunblock-vs-sunscreen.html>.
"Skin Cancer Facts." Skin Cancer Facts. American Cancer Society, 23 Jan. 2012. Web. 15 Mar.
2013. <http://www.cancer.org/cancer/cancercauses/sunanduvexposure/skin-cancer-facts>.
Staff, Mayo Clinic. "Definition." Mayo Clinic. Mayo Foundation for Medical Education and
Research, 06 June 2012. Web. 15 Mar. 2013. <http://www.mayoclinic.com/health/skincancer/DS00190>