Ultraviolet, or UV, is a form of electromagnetic radiation similar to visible light except in one respect -- the wavelength of UV is shorter than the wavelength of light. The energy contained in photons -- tiny bundles of electromagnetic radiation -- gets higher as the wavelength gets smaller. That means that UV is a higher energy radiation source than radio waves, infrared and light. When a high energy packet of UV light is absorbed by a material, it can boost the energy so much that the material gets rearranged on a molecular scale. That kind of rearrangement is why UV damages strands of DNA.
Packets of Energy
Electromagnetic radiation, such as x-rays, radio waves and ultraviolet, is absorbed and emitted in little bundles called photons. When an infrared photon, for example, is absorbed by a molecule, the extra energy makes the molecule vibrate, which heats it up. When a photon of light is absorbed by an atom it adds energy to an electron. That energized electron can create or break a chemical bond between two atoms, or it can give up its energy as another photon of light. It can also bump into other electrons and heat things up. When an ultraviolet photon is absorbed, it has so much energy that it moves an electron completely away from its home. Sometimes there's so much energy the electron is ripped away from its home atom. If that electron was holding two atoms together when it absorbed the ultraviolet photon, that chemical bond will be broken.
DNA consists of two long strands of molecules called pyrimidines and purines. They're connected together like beads on a string, then the two strings are twisted around each other. DNA contains the information necessary to build the proteins that allow living organisms to exist. The proteins are built by molecular factories that untwist the DNA then read the individual pyrimidines and purines -- the individual "beads" on the string. It's a serious problem if those molecular factories are unable to separate the strands and read the individual pyrimidines and purines.
UV Damage to DNA
UV-B rays, which have wavelengths from about 290 to 320 nanometers are easily absorbed by DNA. Specifically, the UV photons get absorbed by the pyrimidines in DNA. Even more specifically, they get absorbed by electrons within the pyrimidines. The electrons are given so much energy they "jump" from their normal paths and connect one pyrimidine directly to another. Thinking of the pyrimidines as beads again, it's as if two beads on a string get strongly glued to one another. The connected-together pyrimidines -- called dimers -- distort the string that holds a strand of DNA together, and block the operation of the organic machinery that converts DNA to proteins. That is, the molecular factory can't untwist the distorted DNA molecule, and then it can't separate the two connected pyrimidines to read their information.
Repairing the Damage
UV damage to DNA creates two different types of dimers. Cyclobutane pyrimidine dimers and 6-4 photoproducts result in the same type of damage, but have slightly different shapes. When either one of those dimers forms, they kink the DNA strand. When the kink is detected a repair sequence starts. Nucleotide excision repair is one mechanism, in which a 30-bead-long section of the DNA strand is cut out and replaced. Another way of repairing is with a molecule called photolyase, which absorbs blue light to provide the energy to cut out only the damaged pyrimidines, which can then be individually replaced. But the repair mechanisms are not perfect, so UV can result in permanent, serious health problems.
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