Fill each rung with the complementary base. Every correct pair raises molecular stability.
A ↔ T · C ↔ G
Purines always bond to pyrimidines. Two hydrogen bonds hold A–T; three hold C–G, so GC-rich strands are stronger.
DNA is a twisted ladder, a double helix. The rungs are pairs of nitrogen bases, and the pairing is never random.
Adenine always bonds with Thymine (A–T). Cytosine always bonds with Guanine (C–G). Read one strand and you can predict the other exactly, the rule that makes copying the genome possible.
A–T pairs share two hydrogen bonds; C–G pairs share three. Strands richer in C–G are held together more tightly, higher stability.
Every rung paired correctly. The helix holds, a stable, replicable strand.