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Direct DNA damage can occur when DNA directly absorbs a UVB photon, or for numerous other reasons. UVB light causes thymine base pairs next to each other in genetic sequences to bond together into pyrimidine dimers, a disruption in the strand, which reproductive enzymes cannot copy. It causes sunburn and it triggers the production of melanin. Other names for the "direct DNA damage" are:〔(Effects of Solar Ultraviolet Photons on Mammalian Cell DNA )〕 *thymine dimers *pyrimidine dimers *Cyclobutane Pyrimidine Dimers (CPDs) *UV-endonuclease-sensitive-sites (ESS) Due to the excellent photochemical properties of DNA, this nature-made molecule is damaged by only a tiny fraction of the absorbed photons. DNA transforms more than 99.9% of the photons into harmless heat〔 (but the damage from the remaining < 0.1% is still enough to cause sunburn).〔 The transformation of excitation energy into harmless heat occurs via a photochemical process called internal conversion. In DNA, this internal conversion is extremely fast, and therefore efficient. This ultrafast (subpicosecond) internal conversion is a powerful photoprotection provided by single nucleotides.〔(【引用サイトリンク】title=Ultrafast Photodynamics of Nucleic Acids )〕 However, the Ground-State Recovery is much slower (picoseconds) in G·C−DNA duplexes and hairpins.〔Ground-State Recovery Following UV Excitation is Much Slower in G·C−DNA Duplexes and Hairpins Than in Mononucleotides: Carlos E. Crespo-Hernndez†, Kimberly de La Harpe and Bern Kohler. http://pubs.acs.org/doi/abs/10.1021/ja802183s〕 It is presumed to be even slower for double-stranded DNA in conditions of the nucleus. The absorption spectrum of DNA shows a strong absorption for UVB radiation and a much lower absorption for UVA radiation. Since the action spectrum of sunburn is indistinguishable from the absorption spectrum of DNA, it is generally accepted that the direct DNA damages are the cause of sunburn.〔 While the human body reacts to direct DNA damages with a painful warning signal,〔 no such warning signal is generated from indirect DNA damage. ==Sunscreen and melanoma== A study by Hanson suggests sunscreen that penetrates into the skin and thereby amplifies the amount of free radicals and oxidative stress contributes to the formation of melanoma, but this idea has not been validated by other researchers. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Direct DNA damage」の詳細全文を読む スポンサード リンク
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