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I have read that UV light can damage pure DNA (DNA that has been extracted and purified). Is there a particular frequency threshold where this damage occurs or is it more of a gradient where the high frequency, the more damage. I want to understand how much damage light can do at what frequencies.

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Not only pure DNA, UV radiations are one of the main causes of skin cancer because they damage cellular DNA in skin cells. Talking about frequency range, two different types of UV radiations damage DNA in two different ways:

  • UV-A radiation damages DNA in indirect ways. UV-A radiation easily generates free radicals, such as hydroxyl and oxygen radicals, which in trun cause the main damage1. The damage caused by such radicals is mostly single-stranded breaks in DNA. In fact, it has only recently been established that UV-A radiations do damage DNA, while these were originally considered as not (or less) harmful2. UV-A also has immunosuppressize effects on the entire body and is also mutagenic for keratinocytes in skin.

    Wavelength range: 315 - 400 nm

    Effect: sourceuv-a damage dna

  • UV-B radiation is in fact what causes most damages and deadliest damages to DNA. UV-B radiation directly attacks DNA, it excites atoms of adjacent thymine molecules, which in turn form covalent bonds with each other and lead to formation of thymine dimers. These dimers, because of being bonded side-by-side instead of being bonded to opposite strand base, form a bulge which interrupts structure and function of DNA. Such dimers are removed by a process known as nucleotide excision repair, which involved about 30 diffeeent proteins3, meaning UV-B is more harmful for the body than UV-A. UV-B also causes double-stranded breaks in DNA, which are a major problem for DNA replication and a threat for cell's survival.

    Wavelength range: 280 - 315 nm

    Effect: sourceuv-b damage dna

  • Other effects of UV radiation on the body include damage to collagen fibers and vitamin-A in the body and accelerated ageing effects 4. Apart from this, UV radiations, because of harmful effects on DNA, are used for sterilization as they destroy microbes' DNA and thus prevent them from replication.

    Wavelength range: 10 - 400 nm (complete UV spectrum)

References:

  1. Svobodová AR, Galandáková A, Sianská J, et al. (January 2012). "DNA damage after acute exposure of mice skin to physiological doses of UVB and UVA light"

  2. Halliday GM, Byrne SN, Damian DL (December 2011). "Ultraviolet A radiation: its role in immunosuppression and carcinogenesis"

  3. Bernstein C, Bernstein H, Payne CM, Garewal H (June 2002). "DNA repair/pro-apoptotic dual-role proteins in five major DNA repair pathways: fail-safe protection against carcinogenesis"

  4. Torma, H; Berne, B; Vahlquist, A (1988). "UV irradiation and topical vitamin A modulate retinol esterification in hairless mouse epidermis"

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