Direct DNA damage

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Direct DNA damage: The UV-photon is directly absorbed by the DNA (left). One of the possible reactions from the excited state is the formation of a thymine-thymine cyclobutane dimer (right). The direct DNA damage leads to sunburn and it causes an increases melanin production. Thereby it leads to a long lasting tan. Importantly it is responsible for only 8% of all melanoma.
Direct DNA damage: The UV-photon is directly absorbed by the DNA (left). One of the possible reactions from the excited state is the formation of a thymine-thymine cyclobutane dimer (right). The direct DNA damage leads to sunburn and it causes an increases melanin production. Thereby it leads to a long lasting tan. Importantly it is responsible for only 8% of all melanoma.

Direct DNA damage can occur when DNA directly absorbs the UV-B-photon. UVB light causes thymine base pairs next to each other in genetic sequences to bond together into thymine dimers, a disruption in the strand which reproductive enzymes cannot copy. It causes sunburn and it triggers the production of melanin.[1]

Other names for the "direct DNA damage" are[2]:

Due to the excellent photochemical properties of DNA this nature-made molecule is damaged only by a tiny fraction of the absorbed photons. DNA transforms more than 99.9% of the photons into harmless heat[3] (But the damage from the remaining < 0.1% of the photons is still enough to cause sunburn).[1] 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 internal conversion is an extremely powerful photoprotection for DNA. [3]

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 identical to the absorption spectrum of DNA, it is generally accepted that the direct DNA damages are the cause of sunburn.[1] While the human body reacts to direct DNA damages with a painful warning signal,[1]no such warning signal is generated from indirect DNA damage, and the indirect DNA damage is responsible for 92% of all melanoma cases.[4]

Contents

[edit] Genetic analysis and the mechanistic cause of melanoma

A study by Hanson suggests sunscreen which penetrates into the skin and thereby amplifies the amount of free radicals and oxidative stress [5] is one of the reasons for the increased melanoma rate.

[edit] Effect of topical sunscreen and effect of absorbed sunscreen

The direct DNA damage is reduced by sunscreen. This prevents sunburn. When the sunscreen is at the surface of the skin it filters the UV-rays which attenuates the intensity. Even when the sunscreen molecules have penetrated into the skin they protect against the direct DNA damage, because the UV-light is absorbed by the sunscreen and not by the DNA.[6]

Please compare to indirect DNA damage.

[edit] See also

[edit] References

  1. ^ a b c d John A. Parrish, Kurt F. Jaenicke, R. Rox Anderson (1982). "ERYTHEMA AND MELANOGENESIS ACTION SPECTRA OF NORMAL HUMAN SKIN". Photochemistry and Photobiology 36 (2): 187–191. doi:10.1111/j.1751-1097.1982.tb04362.x. 
  2. ^ Effects of Solar Ultraviolet Photons on Mammalian Cell DNA
  3. ^ a b ultrafast internal conversion of DNA. Retrieved on 2008-02-13.
  4. ^ Davies H.; Bignell G. R.; Cox C.; (6 2002). "Mutations of the BRAF gene in human cancer". Nature 417: 949-954. 
  5. ^ Hanson Kerry M.; Gratton Enrico; Bardeen Christopher J. (2006). "Sunscreen enhancement of UV-induced reactive oxygen species in the skin". Free Radical Biology and Medicine 41 (8): 1205-1212. 
  6. ^ Gulston M, Knowland J. (July 1999). "Illumination of human keratinocytes in the presence of the sunscreen ingredient Padimate-O and through an SPF-15 sunscreen reduces direct photodamage to DNA but increases strand breaks.". Mutat Res. 444 (1): 49–60. PMID 10477339. 
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