<em>Nucleotides are typically made up of phosphate group, nitrogenous bases and pentose sugars. They represent the building blocks of nucleic acids.</em>
<em>The nitogenous bases are of two types - purine and pyrimidine. Purine consists of adenine and guanine while pyrimidine consists of cytosine and thymine/uracil. </em>
<em>Both DNA and RNA consist of cytosine, adenine and guanine. However, thymine is only present in DNA and replaced by uracil in RNA.</em>
Hence, the cytosine could have come from either of DNA or RNA.