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LITERATURE REVIEW
Every living thing has cells that constantly replace themselves by going through mitosis, but sometimes these cells can get damaged whether by disease or radiation. Humans have been trying to figure out how to save and restart these damaged cells, especially in cancer and specific genetic disorders like Xeroderma Pigmentosum. Photoreactivation repairs DNA usually because of an enzymatic light-dependant reaction after an Ultraviolet (UV) light exposure
(Photoreactivation, 2010). If enough research is found, doctors and scientists might be able to use photoreactivation to recover a large portion of cells from a fatal dose of UV light (Harris and
Adams 2003). There have been previous experiments using yeast and common bacteria with different variables. Most of the experiments use only one dose of UV light per trial. This experiment is going to compare the difference between two doses of UV light, one long dose and two short doses, using Escherichia coli or E.coli.
UV light is electromagnetic radiation. The wavelengths of UV light are longer than Xrays but shorter then visible light rays. Humans cannot see UV, so it is also known as black light.
There are extremely harmful wavelengths of UV light which are very detrimental to human life.
Many of these rays do not make it through the Earth’s atmosphere because the Earth is protected by the Ozone Layer. The effects of UV light on humans can cause tanning and sunburn, and can eventually lead to skin cancer and cataracts of the eye. UV light can also destroy DNA in bacteria by not allowing them to replicate and replenish themselves. This is where photoreactivation is used to try to restart the damaged and mutated cells (Fleishman, 2010).
Wavelengths in intermediate regions of UV-B light can pass through the ozone layer
(Sparling, 2001) and into DNA and attack it. UV light is absorbed by a double bond in

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, 2 pyrimidine bases, like thymine and cytosine. Commonly, what happens is two new bonds are formed around the neighbor bases which form a four ‘member’ ring. This is so common; in fact, every skin cell has fifty to one hundred reactions every second of sun exposure. Most of these are corrected instantly by nucleotide excision repair (Mullenders, Stary, and Sarasin, 2001).
Most often, even if these mutations are not corrected, they do not cause any damage because they are read and combined correctly. The ‘signature’ mutation is a CC to TT which is when a CC dimer is incorrectly paired with two adenine bases. This leads to an obvious connection to many types of cancer (Goodsell, 2001).
Counting bacteria in a sample can be a very difficult and tedious process because of their small size. There are microscopes that can give you direct count, but it requires high expertise and much time. A much easier way is to carry out a serial dilution. Putting out an undiluted sample would result in a lawn. A lawn, in other words, is a smear, which consists of many cells growing on top of or next to each other. To reduce them number of cells given, they must be repeated over and over. A small amount of the newly diluted samples are put on an agar plate
(Summer, 2005). Agar plates are plates made of a gelatinous material called agar. The agar often contains nutrients for the cells. After put on the agar plate, they are once again diluted (Glossary,
2010). Working backwards, with the ‘dilution factor,’ the number of bacteria on your original plate can be found. That also means, a specific number of colonies can be divided onto many plates so it is constant in an experiment (Summer, 2005).
In March 2002, Hiroyuki Katayama, and Shinichiro Ohgaki (2002) tested photoreactivation in E. coli cells. They used a low-pressure (LP) UV lamp (254 nm), mediumpressure (MP) (220- 580 nm) and a filtered medium-pressure (MPF) lamp (300- 580 nm). It was found that more than 80% of the pyrimidine dimers induced by LP or MPF UV irradiation were

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, 3 repaired after the light reactivation. But, the MP UV light was revealed to play an important role in the inability to reactivate cells. This is probably because it induces damage other than the damage caused to the pyrimidine dimmers (Katayama and Ohgaki, 2002). In 2001, photoreactivation was tested in the airborne Mycobacterium parafortuitum (M. patafortuitum ).
UV inactivation rate of airborne M.
parafortuitum cells decreased by a factor of four as relative humidity (RH) increased from 40% to 95%; however, under identical conditions with visible light present, the UV inactivation rate of airborne cells decreased only by a factor of two. The results show that, unlike in waterborne bacteria, thymine dimers are not the most significant form of UV-induced DNA damage (Peccia & Hernandez, 2001).
In this experiment, the effect of white light on E.coli
that was previously exposed to UV light will be tested. There will be twelve plates of E. coli that have no UV light exposure with lethal dose (LD) of 50, and six of these plates will be exposed to white light. The next twelve plates will be put under UV-B light (LD 50) and half of these plates will be put under the white light. The last group of twelve plates will be put under UV-B light (LD 50). The plates will be taken out for ten minutes, and then put back under for five minutes. Once again, half of these plates will be put under the white light, and half will not. This experiment will be different because instead of the independent variable being the type of bacteria or type of UV light, it will be the amount of time under the UV-B light and whether it was placed under the white light. The reactivated cells will be counted by colonies and the percent of plate coverage. The experiment will only be valid if the temperature, white light wattage, light source and amount of time under
UV-B light always stay the same.
In previous experiments involving yeast and common bacteria, most have only tested one dose of UV light per trial. This experiment is going to compare the difference between two doses

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, 4 of UV light, one long dose and two short doses, using the bacteria Escherichia coli or E.coli.
All living organisms have cells that constantly replace themselves by going through mitosis, but sometimes these cells can get damaged whether by disease or radiation. Scientists are researching methods about how to restore these damaged cells, especially in cancer and specific genetic disorders like Xeroderma Pigmentosum. Photoreactivation is capable of repairing DNA due to an enzymatic light-dependant reaction after an Ultraviolet (UV) light exposure
(Photoreactivation, 2010). If enough research is found, scientists could use photoreactivation to recover a large portion of damaged cells.

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WORKS CITED
Fleishman, S. (2010). Ultraviolet Light. Grolier Multimedia Encyclopedia. Retrieved June 6,
2010, from Grolier Online http://gme.grolier.com/article?assetid=0297770-0.
Glossary: Agar Plate. (2010). Retrieved June 5, 2010, from http://www.acad.carleton.edu/curricular/BIOL/classes/bio125/recdna/glossary.html.
Goodsell, D.S. (2001). The Molecular Perspective: Ultraviolet Light and Pyrimidine Dimers. The
Oncologist. 6(3), 298-299.
Harris, G.D., & Adams, V.D. (2003). Ultraviolet Inactivation of Selected Bacteria and Viruses with Photoreactivation of the Bacteria. Water Research, 21(6), 687-692.
Katayama, H., & Ohgaki, S. (2002). Photoreactivation of Escherichia coli after Low- or
Medium-Pressure UV Disinfection Determined by an Endonuclease Sensitive Site Assay.
Applied and Enviornmental Microbiology, 68(12), 6029-6035.
Mullenders, L.H.F., Stary, A., & Sarasin, A. (2001). Nucleotide Excision Repair. Retrieved
June 6, 2010, from http://atlasgeneticsoncology.org/Deep/ExcisRepairID20014.html.
Peccia, J., Hernandez, M. (2001). Photoreactivation in Airborne Mycobacterium parafortuitum.
Applied and Enviornmental Microbiology, 67(9), 4225-4332.
Photoreactivation. (2010). In Merriam-Webster Online Dictionary. Retrieved June 6, 2010, from http://www.merriam-webster.com/dictionary/photoreactivation.
Sparling, B. (2001). Retrieved June 6, 2010, from http://www.nas.nasa.gov/About/Education/Ozone/radiation.html.
Summer Science Academy Experiment: Estimating the Number of Bacteria on a Solid Surface.
(2005). Retrieved June 5, 2010, from http:// www.waksmanfoundation.org /labs
/Rochester dilution.htm.