pyrimidinepairing

Pyrimidine pairing is the base-pairing interaction in nucleic acids that involves pyrimidine bases—cytosine (C) and thymine (T) in DNA, and uracil (U) in RNA—forming hydrogen-bonded pairs with complementary purine bases. In the standard Watson-Crick model, cytosine pairs with guanine, and thymine (DNA) or uracil (RNA) pairs with adenine. Thus, pyrimidines pair with purines to create the familiar A–T (or A–U in RNA) and G–C base pairs.

In DNA, the G–C pair contains three hydrogen bonds, contributing greater stability and higher melting temperatures for GC-rich regions. A–T pairs have two hydrogen bonds and are comparatively less stable. In RNA, adenine pairs with uracil, and guanine with cytosine, forming the same general geometry that supports secondary structures such as hairpins and loops.

The pairing geometry produced by pyrimidine–purine base pairs helps maintain the uniform width of the double helix in DNA and the helical-like features in RNA secondary structures. This canonical pairing is essential for accurate replication, transcription, and translation, influencing gene expression and genome organization.

Beyond canonical pairing, noncanonical interactions involving pyrimidines can occur, particularly in RNA, where structural motifs and Hoogsteen or wobble-type contacts contribute to folding and function. The presence of thymine in DNA (as opposed to uracil in RNA) adds a methyl group that can affect recognition by proteins and contribute to DNA stability. Overall, pyrimidine pairing is a central concept in molecular biology, underpinning the fidelity and stability of genetic information.