By Kevin Beck Updated Aug 30, 2022
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Deoxyribonucleic acid, or DNA, is widely referred to as the “genetic code” and the foundation of all life as we know it. It resides in the nuclei of eukaryotic cells—including your own. A related molecule, ribonucleic acid (RNA), carries the genetic instructions from DNA to the ribosome, where proteins are synthesized.
In genetic sequences, you may see strands of letters such as AGCCCTAG… or UCGGGAUC… Each letter represents a different nucleotide, and nucleotides fall into two core categories based on nitrogen content: purines and pyrimidines.
The four biologically significant purines are adenine, guanine, hypoxanthine, and xanthine. Adenine and guanine are incorporated into both DNA and RNA, whereas hypoxanthine and xanthine serve as intermediates in purine metabolism.
The key pyrimidines are cytosine, thymine, uracil, and orotic acid. DNA contains thymine, while RNA replaces thymine with uracil at corresponding positions.
A purine base consists of a fused six‑membered nitrogen ring and a five‑membered nitrogen ring, resembling a hexagon joined to a pentagon. Adenine and guanine exemplify purines in nucleic acids. Purine synthesis begins with a ribose sugar that is modified before the nitrogenous base is attached.
Pyrimidines feature a single six‑membered nitrogen ring. They are smaller and lighter than purines. Cytosine and thymine are the pyrimidines found in DNA; cytosine and uracil are present in RNA. Pyrimidine synthesis typically starts with the free base, which is later incorporated into the nucleotide structure.
DNA is double‑stranded, and its complementary base pairing ensures structural stability. In DNA, adenine pairs with thymine (A‑T) and cytosine pairs with guanine (C‑G). In RNA, uracil substitutes for thymine, so adenine pairs with uracil (A‑U). These purine‑pyrimidine pairs maintain uniform pair size, preventing mismatches that would distort the helix.
Understanding these base pairs is essential for grasping genetic fidelity, transcription, and the biochemical pathways that underpin life.
