Nucleotide: The basic building block of nucleic acids such as DNA and RNA. Each nucleotide consists of a sugar, phosphate group, and nitrogenous base like adenine.
Base Pairing Rule: A rule stating that cytosine pairs with guanine and adenine pairs with thymine in DNA or uracil in RNA.
Purines: A class of organic molecules, including adenine and guanine, which are key components in both DNA and RNA.