Thesis

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Unformatted text preview: a. UVB, a minor component of sunlight reaching the earth, is experimentally demonstrated to be the most effective radiation to induce skin cancer in animals. UVB can cause DNA damage, particularly cyclobutane pyrimidine dimmers (CPDs) and (6-4) photoproducts, which may induce mutations in the epidermal cells, leading to the development of cancer. UVB is also known to upregulate gene expression through intracellular signal transduction pathways, which may contribute to the development of skin cancer at the tumor promotion stage. In addition, UVB is proven to suppress immune reactions, and to induce tolerance to antigens, which have been applied topically or systemically in experimental animals. These three effects of UVB on the skin are believed to cooperatively contribute to producing skin cancer in humans (8). UVA is the major portion (approximately 95%) of UV light reaching the earth surface and it is reported to lead to benign and malignant tumor formation. UVA penetrates through the epidermis deep into the dermis. UVA-mediated cellular damage occurs primarily through the formation of reactive oxygen species (ROS). After UVA exposure, singlet oxygen, H2O2 (hydrogen peroxide), superoxide and hydroxyl free radicals are generated. These interact and can cause damage to cellular proteins, lipids and saccharides. UVA can also indirectly produce structural damage to the DNA and 87 oxo-guanine is the most common lesion inhibits DNA repair as well as affecting numerous signal transduction pathways and impairing the immune system (9). Figure 2. UV radiation is subdivided into three wavelength bands; UVA (320-400 nm), UVB (290-320 n m) and UVC (220-290 n m). UVA and UVB radiation are proved to produce DNA damag e directly and indirectly through oxidative stress. UVB induces formation of cy clobutane pyrimi dine dimmers and 6-4 photoproducts. The wavelength of UVA is too long to be absorbed by DNA, therefore causing DNA damage via reactive oxygen species (ROS). Modified from http://images....
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This document was uploaded on 03/06/2014.

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