Pyrimidines/pyrimidine nucleotides ultraviolet light absorbed

There are a number of distinct enzymes which deaminate purine and pyrimidine nucleosides and nucleotides, as well as the free bases. However deamination of purines and pyrimidines in nucleic acids takes place slowly, if at all. Ammonia and the corresponding hydroxyl analog are the products of these reactions. Deamination is known to be irreversible in some cases and can be followed by changes in ultraviolet light absorbing properties 21). 

Direct Repair Several types of damage are repaired without removing a base or nucleotide. The best-characterized example is direct photoreactivation of cyclobutane pyrimidine dimers, a reaction promoted by DNA photolyases. Pyrimidine dimers result from an ultraviolet light-induced reaction, and photolyases use energy derived from absorbed light to reverse the dam-... 

Another property of pyrimidines and purines is their strong absorbance of ultraviolet (UV) light, which is also a consequence of the aromaticity of their heterocyclic ring structures. Figure 11.8 shows characteristic absorption spectra of several of the common bases of nucleic acids—adenine, uracil, cytosine, and guanine—in their nucleotide forms AMP, UMP, CMP, and GMP (see Section 11.4). This property is particularly useful in quantitative and qualitative analysis of nucleotides and nucleic acids. 

---