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Indirect DNA damage
 
 
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Indirect DNA
DNA

Deoxyribonucleic acid is a nucleic acid that contains the genetics instructions used in the development and functioning of all known living organisms and some viruses....
 damage occurs when a UV-photon
Ultraviolet

Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than x-rays, in the range 400 nanometer to 10 nm, and energies from 3 Electron volt to 124 eV....
 is absorbed in the human skin by a chromophore
Chromophore

A chromophore is part of a molecule responsible for its color.When a molecule absorbs certain wavelengths of visible spectrum and transmits or reflects others, the molecule has a color....
 that does not have the ability to convert the energy into harmless heat very quickly. Molecules which do not have this ability have a long lived excited state
Excited state

Excitation is an elevation in energy level above an arbitrary baseline energy state. In physics there is a specific technical definition for energy level which is often associated with an atom being excited to an excited state....
. This long lifetime
Fluorescence

Fluorescence is a luminescence that is mostly found as an optical phenomenon in cold bodies, in which the molecular absorption of a photon triggers the emission of a photon with a longer wavelength....
 leads to a high probability for reactions with other molecules - so called bimolecular reactions. Melanin
Melanin

Melanin is a class of compounds found in the plant, animal, and protista kingdom , where it serves predominantly as a pigment. The class of pigments are derivatives of the amino acid tyrosine....
 and DNA have extremely short excited state lifetimes in the range of a few femtoseconds (10-15s) .






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Indirect DNA
DNA

Deoxyribonucleic acid is a nucleic acid that contains the genetics instructions used in the development and functioning of all known living organisms and some viruses....
 damage occurs when a UV-photon
Ultraviolet

Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than x-rays, in the range 400 nanometer to 10 nm, and energies from 3 Electron volt to 124 eV....
 is absorbed in the human skin by a chromophore
Chromophore

A chromophore is part of a molecule responsible for its color.When a molecule absorbs certain wavelengths of visible spectrum and transmits or reflects others, the molecule has a color....
 that does not have the ability to convert the energy into harmless heat very quickly. Molecules which do not have this ability have a long lived excited state
Excited state

Excitation is an elevation in energy level above an arbitrary baseline energy state. In physics there is a specific technical definition for energy level which is often associated with an atom being excited to an excited state....
. This long lifetime
Fluorescence

Fluorescence is a luminescence that is mostly found as an optical phenomenon in cold bodies, in which the molecular absorption of a photon triggers the emission of a photon with a longer wavelength....
 leads to a high probability for reactions with other molecules - so called bimolecular reactions. Melanin
Melanin

Melanin is a class of compounds found in the plant, animal, and protista kingdom , where it serves predominantly as a pigment. The class of pigments are derivatives of the amino acid tyrosine....
 and DNA have extremely short excited state lifetimes in the range of a few femtoseconds (10-15s) . The excited state lifetime of these substances is 1,000 to 1,000,000 times longer than the lifetime of melanin and therefore they may cause damage to living cells which come into contact with them.

The molecule which originally absorbs the UV-photon is called a "chromophore". The bimolecular reactions can either occur between the excited chromophore and DNA, or between the excited chromophore and another species to produce free radicals and Reactive Oxygen Species
Reactive oxygen species

Reactive oxygen species are ions or very small molecules that include oxygen ions, radical , and peroxides, both inorganic and organic peroxide....
. These reactive chemical species can reach DNA by diffusion and the bimolecular reaction will damage the DNA (oxidative stress
Oxidative stress

Oxidative stress is caused by an imbalance between the production of reactive oxygen species and a biological system's ability to readily detoxify the reactive intermediates or easily repair the resulting damage....
). Importantly, indirect DNA damage does not result in any warning signal or pain in the human body.

The mutations which result from direct DNA damage
Direct DNA damage

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....
 and those which result from indirect DNA damage are different, and genetic analysis of melanomas can elucidate which DNA damage has caused each respective skin cancer. Studies using these techniques have found that 92% of all melanoma are caused by indirect DNA damage and only 8% of the melanoma are caused by direct DNA damage.

The bimolecular reactions that cause the indirect DNA damage are illustrated in the figure:

1O2 is reactive harmful singlet oxygen
Singlet oxygen

Singlet oxygen is the common name used for the diamagnetic form of molecular oxygen , which is less stable than the normal triplet oxygen. Because of its unusual properties, singlet oxygen can persist for over an hour at room temperature, depending on the environment....
:

Location of the damage

Direct DNA damage is confined to areas that can be reached by UV-B light. In contrast free radicals can travel through the body and affect other areas - possibly even inner organs. The traveling nature of the indirect DNA damage can be seen in the fact that the malignant melanoma can occur in places that are not directly illuminated by the sun - this is in contrast to basal-cell carcinoma and squamous cell carcinoma
Squamous cell carcinoma

In medicine, squamous cell carcinoma is a form of cancer of the carcinoma type that may occur in many different organs, including the skin, lips, mouth, esophagus, urinary bladder, prostate, lungs, vagina, and cervix....
 which only appear on directly illuminated locations of the body.

Effects of topical vs. absorbed sunscreen

Indirect DNA damage is reduced by the topical sunscreen that stays on the surface of the skin. However, if sunscreen penetrates the epidermal barrier and gets into contact with living tissue, the indirect DNA damage is amplified many times, which causes damage to living tissue even at very low concentrations (10 µmol/L).

See also

  • Sunscreen
    Sunscreen

    Sunscreen is a lotion, spray, gel or other topical product that absorbs or reflects the sun's ultraviolet radiation and protects the skin.Sunscreens contain one or more UV filters of which there are three main types :...
  • photoprotection
    Photoprotection

    The term Photoprotection designates the mechanisms that nature has developed to minimize the damages that the human body suffers when exposed to UV-irradiation....
  • direct DNA damage
    Direct DNA damage

    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....