<P> A nitrogenous base, or nitrogen - containing base, is an organic molecule with a nitrogen atom that has the chemical properties of a base . The main biological function of a nitrogenous base is to bond nucleic acids together . A nitrogenous base owes its basic properties to the lone pair of electrons of a nitrogen atom . </P> <P> Nitrogenous bases are typically classified as the derivatives of two parent compounds, pyrimidine and purine . They are non-polar and due to their aromaticity, planar . Both pyrimidines and purines resemble pyridine and are thus weak bases and relatively unreactive towards electrophilic aromatic substitution . </P> <P> In the biological sciences, nitrogenous bases are increasingly termed nucleobases because of their role in nucleic acids - their flat shape is particularly important when considering their roles as the building blocks of DNA and RNA . A set of five nitrogenous bases is used in the construction of nucleotides, which in turn build up the nucleic acids like DNA and RNA . These nitrogenous bases are adenine (A), uracil (U), guanine (G), thymine (T), and cytosine (C). These nitrogenous bases hydrogen bond between opposing DNA strands to form the rungs of the "twisted ladder" or double helix of DNA or a biological catalyst that is found in the nucleotides . Adenine is always paired with thymine, and guanine is always paired with cytosine . These are known as base pairs . Uracil is only present in RNA, replacing thymine . Pyrimidines include thymine, cytosine, and uracil . They have a single ring structure . Purines include adenine and guanine . They have a double ring structure . </P>

In rna the nitrogenous base that replaces thymine is