UV Light,Electromagnetic,Non Ionizing Radiations,Multicellular Animals,Pyrimidine Hydrates,Mutagenic Effect,Thymine Dimerization

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	UV Light - Ultraviolet rays are electromagnetic and non ionizing radiations. Because of their lower energy, they penetrate the surface tissues of multicellular animals and plants. UV is readily absorbed by certain substances like purines and pyrimidines, which then enter into a more reactive or excited state. The two major products formed by absorption of UV by pyrimidine are pyrimidine hydrates and thymine dimers. UV acts on any 'C' atom of pyrimidine of DNA in water solution and adds a water molecule across the double bond. But thymine dimerization is probably the major mutagenic effect by UV.
UV light produces thymine dimer mainly by linking adjacent thymine bases in the same strands of DNA via C-C bonds. These thymine dimers block the transcription and replication. Normally thymine dimers are repaired by DNA repair mechanism, and only a small fraction of the dimer results in mutations. The relationship between mutation rate and UV dosage is highly variable depending on the type of mutation, the organism and conditions employed for the study. Electromagnetic radiation with a wavelength of 0.05-10 A are emitted in quanta. X-rays affect the chromosomes by directly breaking them or altering the DNA base pair. Chromosomes "re extremely sensitive to breakage during meiotic prophase.