Chemical modifications in DNA and RNA. (a) Modified nucleotides in DNA. The most common postsynthetic modification to DNA is base methylation. 5-Methyldeoxycytidine occurs in the DNA of animals and higher plants; the other methylated bases shown here are produced by specific enzymes. (b) Modified nucleotides in RNA. Enzyme-catalyzed RNA base modifications are common in tRNA and rRNA, although the function of such alterations is not always clear. The presence of N⁴-acetylcytidine in bacterial tRNAs may enhance protein synthesis.