Direct DNA damage can occur when DNA directly absorbs the UV-B-photon. UVB light causes
thymineThymine is one of the four nucleobases in the nucleic acid of DNA that are represented by the letters G–C–A–T. The others are adenine, guanine, and cytosine. Thymine always pairs with adenine. Thymine is also known as 5-methyluracil, a pyrimidine nucleobase. As the name suggests, thymine may be...
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
sunburnA sunburn is a burn to living tissue such as skin produced by overexposure to ultraviolet radiation, commonly from the sun's rays.Usual mild symptoms in humans and animals are red or reddish skin that is hot to the touch, general fatigue, and mild dizziness. An excess of UV-radiation can be...
and it triggers the production of
melaninMelanin is a class of compounds found in plants, animals, and protists, where it serves predominantly as a pigment. The class of pigments are derivatives of the amino acid tyrosine. Many melanins are insoluble salts and show affinity to water...
.
Other names for the "direct DNA damage" are:
- thymine dimer
A thymine dimer is the covalent bonding of two adjacent thymine residues within a DNA molecule, often catalyzed by ultraviolet radiation or chemical mutagenic agents...
s
- pyrimidine dimers
- Cyclobutane
Cyclobutane is an organic compound with the formula
4. Cyclobutane is a colourless gas and commercially available as a liquefied gas. Derivatives of cyclobutane are called cyclobutanes...
PyrimidinePyrimidine is a heterocyclic aromatic organic compound similar to benzene and pyridine, containing two nitrogen atoms at positions 1 and 3 of the six-member ring...
Dimers (CPD's).
- UV-endonuclease-sensitive-sites (ESS)
Due to the excellent photochemical properties of DNA this nature-made molecule is damaged only by a tiny fraction of the absorbed photons.
Direct DNA damage can occur when DNA directly absorbs the UV-B-photon. UVB light causes
thymineThymine is one of the four nucleobases in the nucleic acid of DNA that are represented by the letters G–C–A–T. The others are adenine, guanine, and cytosine. Thymine always pairs with adenine. Thymine is also known as 5-methyluracil, a pyrimidine nucleobase. As the name suggests, thymine may be...
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
sunburnA sunburn is a burn to living tissue such as skin produced by overexposure to ultraviolet radiation, commonly from the sun's rays.Usual mild symptoms in humans and animals are red or reddish skin that is hot to the touch, general fatigue, and mild dizziness. An excess of UV-radiation can be...
and it triggers the production of
melaninMelanin is a class of compounds found in plants, animals, and protists, where it serves predominantly as a pigment. The class of pigments are derivatives of the amino acid tyrosine. Many melanins are insoluble salts and show affinity to water...
.
Other names for the "direct DNA damage" are:
- thymine dimer
A thymine dimer is the covalent bonding of two adjacent thymine residues within a DNA molecule, often catalyzed by ultraviolet radiation or chemical mutagenic agents...
s
- pyrimidine dimers
- Cyclobutane
Cyclobutane is an organic compound with the formula
4. Cyclobutane is a colourless gas and commercially available as a liquefied gas. Derivatives of cyclobutane are called cyclobutanes...
PyrimidinePyrimidine is a heterocyclic aromatic organic compound similar to benzene and pyridine, containing two nitrogen atoms at positions 1 and 3 of the six-member ring...
Dimers (CPD's).
- UV-endonuclease-sensitive-sites (ESS)
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 (But the damage from the remaining < 0.1% of the photons is still enough to cause
sunburnA sunburn is a burn to living tissue such as skin produced by overexposure to ultraviolet radiation, commonly from the sun's rays.Usual mild symptoms in humans and animals are red or reddish skin that is hot to the touch, general fatigue, and mild dizziness. An excess of UV-radiation can be...
). The transformation of excitation energy into harmless heat occurs via a photochemical process called
internal conversionInternal conversion is a transition from a higher to a lower electronic state in a molecule. It is sometimes called "radiationless de-excitation", because no photons are emitted...
. In DNA this
internal conversionInternal conversion is a transition from a higher to a lower electronic state in a molecule. It is sometimes called "radiationless de-excitation", because no photons are emitted...
is extremely fast - and therefore efficient. This ultrafast (subpicosecond)
internal conversionInternal conversion is a transition from a higher to a lower electronic state in a molecule. It is sometimes called "radiationless de-excitation", because no photons are emitted...
is an extremely powerful
photoprotectionPhotoprotection is a group of mechanisms that nature has developed to minimize the damages that the human body suffers when exposed to UV radiation...
provided by single nucleotides. However, the Ground-State Recovery is much slower (picoseconds) in G·C−DNA duplexes and hairpins. Probably it is 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.
While the human body reacts to direct DNA damages with a painful warning signal,no such warning signal is generated from
indirect DNA damageIndirect DNA damage occurs when a UV-photon is absorbed in the human skin by a chromophore 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. This long lifetime leads to a high probability...
, and the
indirect DNA damageIndirect DNA damage occurs when a UV-photon is absorbed in the human skin by a chromophore 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. This long lifetime leads to a high probability...
is responsible for 92% of all melanoma cases.
Genetic analysis and the mechanistic cause of melanoma
A study by Hanson suggests
sunscreenSunscreen is a lotion, spray, gel or other topical product that absorbs or reflects some of the sun's ultraviolet radiation on the skin exposed to sunlight and thus helps protect against sunburn....
which penetrates into the skin and thereby amplifies the amount of free radicals and
oxidative stressOxidative stress is caused by an imbalance between the production of reactive oxygen and a biological system's ability to readily detoxify the reactive intermediates or easily repair the resulting damage. All forms of life maintain a reducing environment within their cells...
is one of the reasons for the increased melanoma rate.
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.
Please compare to
indirect DNA damageIndirect DNA damage occurs when a UV-photon is absorbed in the human skin by a chromophore 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. This long lifetime leads to a high probability...
.
See also
- DNA repair
DNA repair refers to a collection of processes by which a cell identifies and corrects damage to the DNA molecules that encode its genome. In human cells, both normal metabolic activities and environmental factors such as UV light and Radiation can cause DNA damage, resulting in as many as 1...
- Sunburn
A sunburn is a burn to living tissue such as skin produced by overexposure to ultraviolet radiation, commonly from the sun's rays.Usual mild symptoms in humans and animals are red or reddish skin that is hot to the touch, general fatigue, and mild dizziness. An excess of UV-radiation can be...
- Sunscreen
Sunscreen is a lotion, spray, gel or other topical product that absorbs or reflects some of the sun's ultraviolet radiation on the skin exposed to sunlight and thus helps protect against sunburn....
- Photoprotection
Photoprotection is a group of mechanisms that nature has developed to minimize the damages that the human body suffers when exposed to UV radiation...
- Indirect DNA damage
Indirect DNA damage occurs when a UV-photon is absorbed in the human skin by a chromophore 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. This long lifetime leads to a high probability...