Answer: Complementary base pairing.
Explanation:
The DNA is composed of the nitrogenous base pair, sugar and phosphate bond. The pairing of the bases are pre defined and it always binds in such a way that the bonds are strong enough to hold such a long double helix model.
The adenine gets complementary attached with thymine and guanine binds with cytosine.
This is how the complementary base pair in the DNA takes place.
