الفهرس 
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Abstract
Pyrimidine ring systems are planar,have high solubility due to polar groups,undergo keto-ennol tautomeric shift and absorb u.v.light.the have been shown to exert pronounced physiological effects.for instance,the replacement of naaturally occurring pyrimidine bases of the dne of microorganisms by unnaturalanalogous pyrimidines is attributed to the similairy in their steric properties.