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, causing an increase in melanin production, thereby leading to a long-lasting tan. However, 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 pyrimidine 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 (subpicosecond) internal conversion is an extremely powerful photoprotection provided by single nucleotides.[3] However, the Ground-State Recovery is much slower (picoseconds) in G·C−DNA duplexes and hairpins.[4] 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 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.

Sunscreen and melanoma[edit]

A study by Hanson suggests sunscreen that penetrates into the skin and thereby amplifies the amount of free radicals and oxidative stress [5] contributes to the formation of melanoma, but this idea has not been validated by other researchers.

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

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]

See also[edit]


  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. PMID 7122713. 
  2. ^ Effects of Solar Ultraviolet Photons on Mammalian Cell DNA
  3. ^ a b "ultrafast internal conversion of DNA". Retrieved 2008-02-13. 
  4. ^ 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.
  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. doi:10.1016/j.freeradbiomed.2006.06.011. PMID 17015167. 
  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. doi:10.1016/s1383-5718(99)00091-1. PMID 10477339.