NCIt definition : One of two classes of heterocyclic nitrogenous bases found in the nucleic acids DNA
and RNA: in DNA the pyrimidines are cytosine and thymine, in RNA uracil replaces thymine.;
Alternative definition : NCI-GLOSS: One of two chemical compounds that cells use to make the building blocks
of DNA and RNA. Examples of pyrimidines are cytosine, thymine, and uracil. Cytosine
and thymine are used to make DNA and cytosine and uracil are used to make RNA.;