Final answer:
In RNA, adenine pairs with uracil, whereas in DNA, adenine pairs with thymine. Uracil is structurally similar to thymine but lacks a methyl group present in thymine.
Step-by-step explanation:
In RNA, the nitrogenous base adenine pairs with uracil. This is different from DNA, where adenine pairs with thymine. The structure of uracil is very similar to that of thymine, but uracil lacks a methyl group that is present in thymine. RNA and DNA share the common bases of adenine, guanine, and cytosine. However, thymine is unique to DNA and is replaced by uracil in RNA. Complementary base pairing in RNA follows the same principle as in DNA, involving hydrogen bonding between a purine and a pyrimidine base, with adenine (a purine) pairing with uracil (a pyrimidine) in RNA.