6.1.3—Pyrimidines (cytosine, thymine, uracil)

A pyrimidine is a nitrogenous base with a single-ring structure. The pyrimidines in this course are cytosine (C), thymine (T) and uracil (U).

• Cytosine: C is used in both DNA and RNA nucleotides.
• Thymine: T is the pyrimidine used in DNA; it pairs with adenine in DNA.
• Uracil: U replaces T in RNA; it can pair with adenine in an RNA sequence.
• Pairing boundary: C pairs with the purine G in both DNA and RNA. A purine–pyrimidine pair maintains the regular spacing across a paired nucleic-acid strand. The ring classification describes the base, not the complete nucleotide.

The DNA/RNA context determines which pyrimidine is present: DNA uses T, whereas RNA uses U, while C is shared. Therefore the same base category can support different nucleic-acid sequences, but a sequence must be read with its molecule type known before assigning the complementary partner.

Uracil is not an additional DNA base in this course, and thymine is not the usual RNA pyrimidine. Pyrimidine means a single-ring nitrogenous base; it does not mean a whole nucleotide or automatically identify the sugar and phosphate attached to it.