base pairing (A-T, G-C)
/ BAYS PAIR-ing /
Think of two zipper halves: each tooth on one side only fits the matching tooth across from it, so the zipper closes cleanly and predictably. DNA's information rests on the same idea. The 'teeth' are four chemical letters — adenine (A), thymine (T), guanine (G), and cytosine (C) — and they pair off by a strict rule: A only clasps T, and G only clasps C. This pairing is what holds the two strands of the double helix together.
Why these exact partners? Each base has a particular shape and a set of spots that can form weak hydrogen bonds. A and T fit together with two hydrogen bonds; G and C fit with three. A purine (the larger A or G) always pairs with a pyrimidine (the smaller T or C), so every rung of the ladder is the same width and the helix stays smooth. Mismatched pairs (say A with G) don't fit geometrically and don't form the right bonds, so they are unstable and the cell's machinery rejects or repairs them.
This single rule is the deepest reason DNA works as an information molecule. Because each base specifies its partner, the two strands are complementary copies of each other: pull them apart and each one is a perfect template for rebuilding the other. That is the foundation of DNA replication, and it is also how the cell reads DNA into RNA. The same logic — letters that pair by shape — powers lab tools from PCR to DNA sequencing.
Because G-C pairs share three hydrogen bonds while A-T pairs share only two, stretches of DNA rich in G and C are harder to pull apart — which is why such regions stay stable at higher temperatures in the lab.
Three bonds vs two: why G-C pairs grip harder than A-T.
In RNA the rule is almost the same, except uracil (U) takes the place of thymine, so A pairs with U instead of T.