Sun damage causes malignant transformation of skin by
Correct Answer: Free radical formation
Description: The most significant cause of BCC and SCC is UV exposure whether through direct exposure to sunlight or by aificial UV light sources (tanning beds). Both UVA and UVB can induce DNA damage through free radical formation (UV A) or induction of pyrimidine dimers (UVB). The sun emits energy across the UV spectrum whereas tanning bed equipment typically emits 97% UV A and 3% UVB Thymine Dimers and Skin Cancercauses of skin cancer : Pathophysiology: UV light causes a crosslink in the DNA The absorption of UV light by a molecule promotes an electron from a filled orbital, to an unfilled orbital such as a non-bonding orbital or antibonding orbital. This makes the molecule potentially very reactive, and in ceain situations can result in new bonds being made between different molecules. This is especially impoant for ceain molecules containing pi bonds such as the DNA bases. In paicular, when thymine absorbs UV light, is becomes reactive with an adjacent thymine molecule in the DNA double helix and can lead to formation of a cyclobutane dimer in which the two thymine residues are covalently linked. This makes it impossible for RNA polymerase to make an accurate mRNA from the DNA, so it effectively prevents the cell from being able to use the DNA to make the correct protein or even replicating. Cytosine residues can also form similar crosslinks. The general term used in the literature to describe these dimers is "cyclobutane pyrimidine dimers, or CPDs". Heavy exposure of your skin to UV light causes many such thymine crosslinks to form in the DNA. Founately, we have repair enzymes that can remove the thymine crosslinks and restore the DNA in your skin cells back to the correct sequence. When you get a sunburn, it is because you have caused more thymine crosslinks than can be repaired, so the cells generally just die. This results in the pain and dead skin you get with sunburns. However, in very rare cases, the thymine dimers interupt the funcions of just the right genes so that your skin cells become cancerous. In other words, they sta growing and do not stop like they should. You get tumors, and eventually the cancer cells invade vital organs and prevent an impoant function in your body and you could die. Reference: Harrisons Textbook of Internal Medicine; 19th edition
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