Nitrogen Base – Nucleotides and nucleic acids are molecules biologically that hold heterocyclic nitrogenous bases as principal components of their structure. Nitrogen base is one of the three components of a nucleotide and originates in two wide-ranging types: pyrimidines and purines. Of the four nitrogen bases, guanine and adenine are purines, while thymine and cytosine are pyrimidines. Thru hydrogen bonding, base pairs link in a complementary nature: adenine with thymine and guanine with cytosine, creating the double-stranded helix of DNA. In RNA, thymine is placed by uracil.