RESULTS
The purpose of this experiment was to determine the relationship between UV blocking ability of Sunscreens with different SPFs and the rate of mutation in UV exposed Saccharomyces cerevisiae cells to find out how effective Sunscreens with different sun protection factors (SPFs) are in blocking and preventing ultraviolet light-induced mutations in S. cerevisiae.
Each plate was inoculated with 10E8 HA2 cells and irradiated using UV dosage in increment of 5 sec. from 5- 35 sec to find a dose of UV light that resulted in a survival rate of 50% (LD 50). The number of surviving colonies decreased significantly from the exposure to UV-C from 5sec to 35 sec. LD 50 was used as a mutagen to irradiate Saccharomyces cerevisiae cells grown on the Petri-dish while being protected by Sunscreens of the same brand (UVA UVB and UVC protection) with SPF 15, SPF 30, SPF 70 (Ordinal IV). S. cerevisiae exposed to no UV light and S. cerevisiae exposed to UV light without sunscreen protection served as a control.
Number of Survivorswas recorded as dependent variable (Continuous DV) which was least (Mean was 128 survivors) in Control (No Sunscreen) and there was significant increase in survivors in all the plates protected with Sunscreen with highest count on the SPF 70 (Mean was 184 survivors). Percentage of revertants (number of white revertants per 100 survivors) for each SPF were calculated.
The null hypothesis states that the Sun protection factor would have no effect on percentage of revertants (number of white revertants per 100 survivors for each SPF) in S. cerevisiae. The data (table 1) did not support the null hypothesis.
THE RELATIONSHIP BETWEEN UV BLOCKING ABILITY OF SUNSCREENS WITH DIFFERENT SPFs AND THE RATE OF MUTATION IN UV EXPOSED YEAST CELLS.
Table 1
The relationship between UV blocking ability of Sunscreens with different SPFs and the rate of mutation in UV exposed yeast cells.Mean/Average / % revertants / Std Deviation
No Sunscreen
Survivors (Control) / 127.5333333 / 29.25226894
No Sunscreen Revertants(Control) / 18.66666667 / 14.63669629 / 4.33699479
SPF 15 Survivors / 161 / 42.19004622
SPF15 Revertants / 10.93333333 / 6.790890269 / 5.311263862
SPF 30 Survivors / 163.0667 / 43.81367
SPF30 Revertants / 5.933333333 / 3.638593622 / 4.02610529
SPF 70 Survivors / 184.3333333 / 29.32494469
SPF 70 Revertants / 2.8666667 / 1.5551537 / 2.8502297
Graph 1Graph 2
There was a statistically significant difference in percentage of revertants between control (no Suncreen) and all the experimental groups (P<0.05). Percent reversions were highest for the control (14.6%). As seen in the graph 2, percentage of revertants for SPF 70 (1.55%), was significantly lower than percentage of revertants for SPF 30 (3.63%) and percentage of revertants for SPF 30, was significantly lower than percentage of revertants for SPF 15 (6.79%). However the variances were large.
The effectiveness of sunscreens with different sun protection factors (SPFs) in preventing ultraviolet light-induced mutations in S. cerevisiaewas determined by comparing SPFs to the control (No Sunscreen), using a t-test. These results are reported in Tables 2-6.
There was a statistically significant difference between all groups at the 0.05 level. UVC-induced mutagenesis in Saccharomyces cerevisiae is inversely proportional to sunscreen SPF rating
CONCLUSION
UV radiation harms cells by causing mutation in the DNA of a gene leading to cancer. Tanning beds contain less UV-B than sunlight and more UV-A which causes premature aging. Proteins in the lens of the eye are changed by UV radiation, leading to the formation of cataracts and partial or complete blindness.
Saccharomyces cerevisiae cellscontain genes for repairing DNA damage caused by UV radiation that are very similar to human genes with the same function. A source of high energy UV-C (germicidal lamp) was used as a mutagen on S. cerevisiae cells to explore effectiveness Sunscreens with different Sun protection factors (SPFs) in blocking and preventing ultraviolet light-induced mutations. Mutations in genes of S. cerevisiae (HA2) blocked the formation of the red pigment resulting in white revertant colonies.
The alternative hypothesis was supported by the statistical analysis of the data obtained as a result of this experiment. SPF 15, 30, 70 showed a statistically significant difference when compared to the control, no Sunscreen, and when compared to each other. Sunscreens either absorb UVB and/or UVA radiation, or scatter and reflect UVA and UVB light. This study appeared to support this as yeast cells protected with Sunscreens showed less reversion as compare to control. In conclusion, this study showed a significant reduction in UV light-induced reversion in yeast when protected by Sunscreens with SPF 70.
Most sunscreens focus on UV-B and SPF relates only to UV-B protection; as most UVC rays are blocked by the ozone layer and there is no standard for UVA protection in the U.S. UV-A is more harmful than initially thought as it penetrates deeper into the skin layers and is responsible for aging our skin among other problems. Further studies need to be carried out to study the effect of UV-A on eukaryotic cells.
There is a limitation to use the results of this study to predict effects of radiation on human cells. For haploid cells, the fraction of cells killed depends on the radiation dose. In diploid cells there is a threshold for radiation killing as it is necessary to damage both copies to kill the cell. For multi-cellular organisms it is necessary to kill more than one cell to kill the organism.
Increasing the number of trials might have reduced variability in this experiment and would have allowed better comparison of different SPFs. In addition to more trials, further studies could address safety of other ingredients in Sunscreen. Recent data shows that chemical Sunscreens have strong hormone disrupting effects, often mimicking the effects of estrogen in the body which can cause reproductive and menstrual problems and may be a trigger for estrogen dependent cancer. The synthetic chemicals are alien to the human body and can accumulate in body fat stores and body has difficulty removing non-biological compounds such as chemical Sunscreens. Even though mineral Sunblock, which sits on the skin, may be safer than chemicals that are absorbed into the skin and generate free radicals is highly recommended, it uses nano-particles of zinc oxide and titanium dioxide. These minute particles can bypass the entire immune system and pass through the blood-brain barrier that protects the brain.
UV light has been known to generate oxidative stress in cells. One of the defenses can be antioxidants which are known to provide significant protection against UV-B induced sunburn, UV-induced immunosuppression and skin cancer.