Base Pairing, Purines, and Pyrimidines

DNA has four bases split into two families by ring structure. Adenine and guanine are purines, built from two fused rings, so they're bigger. Thymine and cytosine are pyrimidines, built from a single ring, so they're smaller. If two purines tried to pair, the rung would be too wide; two pyrimidines would leave it too narrow. Pairing one purine with one pyrimidine every time keeps the helix a constant width from end to end, like rungs on a ladder cut to the same length.

Shape alone doesn't explain why A pairs with T specifically, and not with C. The real lock is hydrogen bonding: each base has a precise pattern of hydrogen donors and acceptors, and only A and T line up correctly, only G and C line up correctly. A-T forms two hydrogen bonds; G-C forms three. This is Chargaff's rule: in double-stranded DNA, percent A equals percent T, and percent G equals percent C, because every A on one strand demands a T across from it, and every G demands a C. Since the two pairs account for all the bases, percent A plus percent T plus percent G plus percent C equals 100.

This base-pairing rule is what makes the genetic code readable and copyable. It sets up replication, where each strand serves as a template specifying the exact partner base for the new strand, and it carries forward into transcription, where the same pairing logic builds a complementary RNA strand. Everything downstream in gene expression depends on this strict, predictable pairing.