Final answer:
The nitrogenous base that is characteristic of RNA, as opposed to DNA, is uracil. Uracil is the molecule that pairs with adenine in RNA and replaces thymine, which pairs with adenine in DNA.
Step-by-step explanation:
The characteristic feature of RNA in comparison to DNA is the presence of the nitrogenous base uracil. DNA and RNA both contain the bases adenine, guanine, and cytosine, which are common to each. However, where DNA contains thymine, RNA contains uracil. Uracil pairs with adenine in the RNA strands just as thymine pairs with adenine in DNA. The structures of uracil and thymine are very similar; however, uracil is essentially thymine without a methyl group, making it an unmethylated form of thymine.
Adenine and guanine, both found in RNA, are purines, which are nitrogenous bases with a double ring structure. On the other hand, cytosine and uracil are pyrimidines, characterized by a single ring structure in their molecules. This differentiation is essential for the molecular biology shorthand in which we abbreviate these bases as A, G for purines and C, U for pyrimidines in RNA, whereas DNA contains A, G, C, and T.