Uv Exposures And 3 Yeast Colony Survival example essay topic

UV-blocking Efficacy of Sunscreen On Yeast Cells Henry Pan April 17, 2003 INTRODUCTION While having a tan is still desired in this culture, people have become more aware of the dangers that prolonged ultraviolet (UV) radiation exposure presents. Problems ranging from actinic changes such as wrinkling, premature aging of the skin, and irregular thinning of the epidermis, to more serious issues like the development of pre malignancies and skin cancer can result from excessive UV exposure (Campos, 2001). UV radiation affects a cell by damaging its DNA. It promotes the formation of covalent linkage between two adjacent pyrimidine bases, resulting in thymine dimers (Alberts, 1998).

This type of damage can often stall DNA replication machinery at the site of dimerization and can lead to disastrous consequences both for the cell and the organism to which it is part of. It will also interfere with the readout of genetic information and DNA replication (Goldberg, 2000). The recent public concern for UV exposure has brought about a massive selection of sunscreens available to the public in a vast array of strengths that are typically assigned a sun protection factor (SPF). The SPF rating is a multiplier, so whatever amount of time it takes for an individual to sunburn should be multiplied by the SPF factor to get the new amount of time that the individual can sit safely in the sun. Whether or not the SPF factor actually makes a difference is part of the objective of this study.

There are two methods in which sunscreens work. Some work by providing an opaque barrier that scatters and reflects UV such as those creams containing titanium dioxide or zinc oxide, while others absorb UV using a variety of chemicals like para-amino benzoic acid (PABA) (G"artner, 2002). Other common UV absorbing chemicals are cinnamates, which absorb UVB, benzophenone's that absorb UVA and anthranilates that absorb both UVA and UVB (Kim 2000). The effects of sunscreen on humans are well documented. However, its effects on single-celled organisms such as yeast cells have not been as fully addressed. This area is of particular interest because the type of damage should be the same, so any information regarding effectiveness should be applicable to humans as well.

The objectives of this study are, 1) to investigate the effects of UV light on a cells ability to survive, and 2) to determine how effective plain lotion, a high rating SPF sunscreen, and a lower rating SPF sunscreen are at protecting yeast cells from UV induced death. It is the consensus of the experimenters that, 1) with the high SPF sunscreen, more yeast colonies will survive on the plates exposed to UV light, 2) plates with plain lotion will have fewer colonies as compared to those with sunscreen of both high and low SPF, but more than plates with nothing at all for all UV exposures, and 3) yeast colony survival is inversely proportional to time of UV exposure. MATERIALS AND METHODS Yeast Preparation Haploid 'a's train yeast cells at an initial concentration of 2 x 10^8 were used. The stock yeast solution was then diluted to 2 x 10^3, using the protocol outlined in Genetics, an Introduction (Guild, 2003). Sunscreen and Lotion Preparation Walgreen's brand SPF 8 and SPF 30 sunscreens were used for the experiment. Vaseline's Intensive Care brand lotion was also used.

Twenty 0.5-gram servings of each were measured. Plate Preparation Five groups of plates were used - 1) Plate with no lid, 2) Plate with clear lid only, 3) Plate with lid and plain lotion, 4) Plate with lid and SPF 8 sunscreen, and 5) Plate with lid and SPF 30 sunscreen. The sunscreen and lotion were all smeared onto the lids of the plates. Assessment of Sunscreen Effect on Yeast Cells To test the effect of sunscreen and lotion, each of the five groups of plates were exposed to UVB light for 0, 1, 3 and 5 minutes. The plates were then left in an incubator for 3 days. After the incubation period, the plates were collected and the yeast colonies were counted.

RESULTS The following table summarizes the results of the experiment. PLATE TYPE TIME EXPOSED (MIN.) NUMBER OF COLONIES Lotion 0 500 Lotion 1 484 Lotion 3 1536 Lotion 5 1032 No lid 0 756 No lid 1 564 No lid 3 716 No lid 5 692 Clear lid only 0 1440 Clear lid only 1 1252 Clear lid only 3 1296 Clear lid only 5 1324 Sunscreen: SPF 8 0 920 Sunscreen: SPF 8 1 1100 Sunscreen: SPF 8 3 880 Sunscreen: SPF 8 5 1220 Sunscreen: SPF 30 0 1180 Sunscreen: SPF 30 1 756 Sunscreen: SPF 30 3 400 Sunscreen: SPF 30 5 472 Table 1. This table shows the number of yeast colonies counted for each of the three experimental groups and control groups. The Controls The negative control (Clear lid / 0 minutes exposure): The negative control yielded 500 colonies. The positive control (Clear lid / 5 minutes exposure): The positive control yielded 1032 colonies. All plates exposed to UVB radiation yielded large numbers of yeast colonies as is shown in Table 1.

The data is also graphed in Fig. 1 below. Fig. 1. This graph shows the distribution of times and number of colonies for each of the groups. DISCUSSION Interpretation of Results The experiment has failed to reliably establish the effectiveness of sunscreen on yeast colonies because the data does not support any conclusions. More importantly, the control groups against which the experimental groups should be compared to yielded the wrong results.

The negative control was supposed to have yielded significantly more yeast colonies than the positive one. The positive control should have had very few, if any, surviving yeast colonies. A difference of just 116 colonies was not significant enough to support any conclusions regarding the effectiveness of sunscreen on yeast colonies. The number of colonies was actually very consistent throughout the range of UV exposure times. This seems to indicate that the length of UV exposure was insufficient to generate the desired kill curve.

There were many unexpected results. The lotion group actually increased in number of colonies with the amount of UV exposure, which should not have happened. In addition, all the 0 min. exposures should have had the same number of colonies because they were never exposed to anything that should have generated varying numbers. When they should have been roughly equal, a large difference of 940 was observed. Strangely, the SPF 30 group exhibited a somewhat expected trend - decreasing numbers of colonies with increasing amount of UV exposure. However, the decrease was sharp and should not have been outnumbered by the SPF 8 group by so far a margin, if at all.

It is worth noting here that the SPF 8 group actually increased in number of colonies as UV exposure increased, which should not have happened. It should have, at best, stayed roughly the same. Instead, an increase of 300 colonies, a 32.6 percent change was observed. It is for these reasons that the experimenters cannot reliably draw from the data any other conclusion other than one of inconclusiveness. The Hypotheses Statement 1. With the high SPF sunscreen, more yeast colonies will survive on the plates exposed to UV light.

The data states that less colonies survived. Statement 2. Plates with plain lotion will have fewer colonies as compared to those with sunscreen of both high and low SPF, but more than plates with nothing at all for all UV exposures. At the highest exposure, the data states that plates with lotion had more colonies than SPF 30, fewer colonies than SPF 8 and fewer colonies compared to the nothing-at-all group. Statement 3. Yeast colony survival is inversely proportional to time of UV exposure.

Apparently, this is not true according to the data. Only one of the groups exhibited this trend, two showed an increase, and two showed relatively stable numbers throughout the range of exposures. For the Future There were really several problems with the experiment, ranging from poor dilution technique to an abysmal experimental design. In the future, were the experimenters to run the procedure again, several changes would have to be made in order to obtain reliable data that could yield definitive results. The first flaw was in the serial dilutions and really constitutes two major problems. Most importantly, the experimenters had to produce two separate dilutions in order to achieve enough yeast suspension to plate.

This really opened the door to inconsistencies in concentration, which may be partly responsible for the fact that such varied numbers of colonies were observed. The second major problem is that too many cells were plated, which made it extraordinarily difficult and time consuming to accurately count all the colonies present. In the future, it would be wise to just plate 2 x 10^2 cells, instead of 2 x 10^3. The second major flaw with the experiment was in the experimental design. Too many variables were present. Not only did the experiment have different types of plates, but it also had different amounts of UV exposure.

It would have been both more efficient and simple if only one time was used. That way, one could just look at the effect of the sunscreen and lotion, instead of having time of exposure being an issue as well. Another problem with the experimental design was that there were useless plates. There was absolutely no reason to plate and expose yeast cells with no lid.

This group was completely unnecessary and in the future should be left out entirely. Instead of having a "no-lid" group, there should have been a "ridiculously high SPF" group that would have served as the negative control in which UV would have no effect on the yeast because all of it was effectively absorbed. A better design would be one where the three experimental groups are 1) lotion, 2) SPF 8, and 3) SPF 30 and the controls are 1) clear lid, and 2) SPF∞ . In sum, the experiment was not entirely unsuccessful. Although the experiment failed to meet one of the objectives, the experimenters did investigate with some degree of success the effect of UV light on cellular mechanisms. Even though the data did not work out quite right, it was in the research of the subject that the most was learned.

As a result, the experimenters were at least able to learn something about UV and the formation of pyrimidine dimers.

Bibliography

1. Alberts, Bruce, et al. Essential Cell Biology: An Introduction to the Molecular Biology of the Cell. New York, Garland, 1998.
2. Campos, Maia and L.R. Gaspar. "Rheological behavior and the SPF of sunscreens". International Journal of Pharmaceuticals 250 (2003): 35-44.
3. G"artner, E., et al. "Distribution of sunscreens on skin". Advanced Drug Delivery Reviews 54 (2002): S 157-S 163.
4. Goldberg, Michael L, et al. Genetics: From Genes to Genomes. Boston, McGraw Hill, 2000.
5. Guild, Nancy. Genetics: An Introduction MCD B 2151 Lab Manual. Boulder, University of Colorado, 2003.
6. Kim, Jin Kyu, et al. "Mathematical description of synergistic interaction of UV light and hyperthermia for yeast cells". Journal of Photochemistry and Photo biology 55 (2000): 74-79.